Merge branch 'master' of git+ssh://gnunet.org/gnunet
[oweals/gnunet.git] / src / util / scheduler.c
index 4128969466e72f83c40db5978e234a6305f67717..409a0942f553acfe3fab620e13f8693228f60df1 100644 (file)
@@ -1,10 +1,10 @@
 /*
       This file is part of GNUnet
-      (C) 2009, 2011 Christian Grothoff (and other contributing authors)
+      Copyright (C) 2009-2016 GNUnet e.V.
 
       GNUnet is free software; you can redistribute it and/or modify
       it under the terms of the GNU General Public License as published
-      by the Free Software Foundation; either version 2, or (at your
+      by the Free Software Foundation; either version 3, or (at your
       option) any later version.
 
       GNUnet is distributed in the hope that it will be useful, but
@@ -14,8 +14,8 @@
 
       You should have received a copy of the GNU General Public License
       along with GNUnet; see the file COPYING.  If not, write to the
-      Free Software Foundation, Inc., 59 Temple Place - Suite 330,
-      Boston, MA 02111-1307, USA.
+      Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
+      Boston, MA 02110-1301, USA.
  */
 
 /**
  * @author Christian Grothoff
  */
 #include "platform.h"
-#include "gnunet_common.h"
-#include "gnunet_os_lib.h"
-#include "gnunet_scheduler_lib.h"
-#include "gnunet_signal_lib.h"
-#include "gnunet_time_lib.h"
+#include "gnunet_util_lib.h"
 #include "disk.h"
-#define LOG(kind,...) GNUNET_log_from (kind, "util", __VA_ARGS__)
 
-#define LOG_STRERROR(kind,syscall) GNUNET_log_from_strerror (kind, "util", syscall)
-#ifdef LINUX
-#include "execinfo.h"
+#define LOG(kind,...) GNUNET_log_from (kind, "util-scheduler", __VA_ARGS__)
+
+#define LOG_STRERROR(kind,syscall) GNUNET_log_from_strerror (kind, "util-scheduler", syscall)
 
 
+#if HAVE_EXECINFO_H
+#include "execinfo.h"
 
 /**
  * Use lsof to generate file descriptor reports on select error?
@@ -52,7 +49,7 @@
 /**
  * Check each file descriptor before adding
  */
-#define DEBUG_FDS GNUNET_EXTRA_LOGGING
+#define DEBUG_FDS GNUNET_NO
 
 /**
  * Depth of the traces collected via EXECINFO.
@@ -60,8 +57,6 @@
 #define MAX_TRACE_DEPTH 50
 #endif
 
-#define DEBUG_TASKS GNUNET_EXTRA_LOGGING
-
 /**
  * Should we figure out which tasks are delayed for a while
  * before they are run? (Consider using in combination with EXECINFO).
  */
 #define DELAY_THRESHOLD GNUNET_TIME_UNIT_SECONDS
 
+
 /**
- * Linked list of pending tasks.
+ * Entry in list of pending tasks.
  */
-struct Task
+struct GNUNET_SCHEDULER_Task
 {
   /**
    * This is a linked list.
    */
-  struct Task *next;
+  struct GNUNET_SCHEDULER_Task *next;
+
+  /**
+   * This is a linked list.
+   */
+  struct GNUNET_SCHEDULER_Task *prev;
 
   /**
    * Function to run when ready.
    */
-  GNUNET_SCHEDULER_Task callback;
+  GNUNET_SCHEDULER_TaskCallback callback;
 
   /**
-   * Closure for the callback.
+   * Closure for the @e callback.
    */
   void *callback_cls;
 
@@ -109,19 +110,9 @@ struct Task
    */
   struct GNUNET_NETWORK_FDSet *write_set;
 
-  /**
-   * Unique task identifier.
-   */
-  GNUNET_SCHEDULER_TaskIdentifier id;
-
-  /**
-   * Identifier of a prerequisite task.
-   */
-  GNUNET_SCHEDULER_TaskIdentifier prereq_id;
-
   /**
    * Absolute timeout value for the task, or
-   * GNUNET_TIME_UNIT_FOREVER_ABS for "no timeout".
+   * #GNUNET_TIME_UNIT_FOREVER_ABS for "no timeout".
    */
   struct GNUNET_TIME_Absolute timeout;
 
@@ -160,6 +151,16 @@ struct Task
    */
   int lifeness;
 
+  /**
+   * Is this task run on shutdown?
+   */
+  int on_shutdown;
+
+  /**
+   * Is this task in the ready list?
+   */
+  int in_ready_list;
+
 #if EXECINFO
   /**
    * Array of strings which make up a backtrace from the point when this
@@ -178,9 +179,24 @@ struct Task
 
 
 /**
- * List of tasks waiting for an event.
+ * Head of list of tasks waiting for an event.
+ */
+static struct GNUNET_SCHEDULER_Task *pending_head;
+
+/**
+ * Tail of list of tasks waiting for an event.
+ */
+static struct GNUNET_SCHEDULER_Task *pending_tail;
+
+/**
+ * Head of list of tasks waiting for shutdown.
  */
-static struct Task *pending;
+static struct GNUNET_SCHEDULER_Task *shutdown_head;
+
+/**
+ * Tail of list of tasks waiting for shutdown.
+ */
+static struct GNUNET_SCHEDULER_Task *shutdown_tail;
 
 /**
  * List of tasks waiting ONLY for a timeout event.
@@ -189,38 +205,37 @@ static struct Task *pending;
  * building select sets (we just look at the head
  * to determine the respective timeout ONCE).
  */
-static struct Task *pending_timeout;
+static struct GNUNET_SCHEDULER_Task *pending_timeout_head;
 
 /**
- * Last inserted task waiting ONLY for a timeout event.
- * Used to (heuristically) speed up insertion.
+ * List of tasks waiting ONLY for a timeout event.
+ * Sorted by timeout (earliest first).  Used so that
+ * we do not traverse the list of these tasks when
+ * building select sets (we just look at the head
+ * to determine the respective timeout ONCE).
  */
-static struct Task *pending_timeout_last;
+static struct GNUNET_SCHEDULER_Task *pending_timeout_tail;
 
 /**
- * ID of the task that is running right now.
+ * Last inserted task waiting ONLY for a timeout event.
+ * Used to (heuristically) speed up insertion.
  */
-static struct Task *active_task;
+static struct GNUNET_SCHEDULER_Task *pending_timeout_last;
 
 /**
- * List of tasks ready to run right now,
- * grouped by importance.
+ * ID of the task that is running right now.
  */
-static struct Task *ready[GNUNET_SCHEDULER_PRIORITY_COUNT];
+static struct GNUNET_SCHEDULER_Task *active_task;
 
 /**
- * Identity of the last task queued.  Incremented for each task to
- * generate a unique task ID (it is virtually impossible to start
- * more than 2^64 tasks during the lifetime of a process).
+ * Head of list of tasks ready to run right now, grouped by importance.
  */
-static GNUNET_SCHEDULER_TaskIdentifier last_id;
+static struct GNUNET_SCHEDULER_Task *ready_head[GNUNET_SCHEDULER_PRIORITY_COUNT];
 
 /**
- * Highest number so that all tasks with smaller identifiers
- * have already completed.  Also the lowest number of a task
- * still waiting to be executed.
+ * Tail of list of tasks ready to run right now, grouped by importance.
  */
-static GNUNET_SCHEDULER_TaskIdentifier lowest_pending_id;
+static struct GNUNET_SCHEDULER_Task *ready_tail[GNUNET_SCHEDULER_PRIORITY_COUNT];
 
 /**
  * Number of tasks on the ready list.
@@ -251,20 +266,26 @@ static int current_lifeness;
 
 /**
  * Function to use as a select() in the scheduler.
- * If NULL, we use GNUNET_NETWORK_socket_select ().
+ * If NULL, we use GNUNET_NETWORK_socket_select().
  */
 static GNUNET_SCHEDULER_select scheduler_select;
 
 /**
- * Closure for 'scheduler_select'.
+ * Task context of the current task.
+ */
+static struct GNUNET_SCHEDULER_TaskContext tc;
+
+/**
+ * Closure for #scheduler_select.
  */
 static void *scheduler_select_cls;
 
+
 /**
  * Sets the select function to use in the scheduler (scheduler_select).
  *
  * @param new_select new select function to use
- * @param new_select_cls closure for 'new_select'
+ * @param new_select_cls closure for @a new_select
  * @return previously used select function, NULL for default
  */
 void
@@ -292,60 +313,6 @@ check_priority (enum GNUNET_SCHEDULER_Priority p)
 }
 
 
-/**
- * Is a task with this identifier still pending?  Also updates
- * "lowest_pending_id" as a side-effect (for faster checks in the
- * future), but only if the return value is "GNUNET_NO" (and
- * the "lowest_pending_id" check failed).
- *
- * @param id which task are we checking for
- * @return GNUNET_YES if so, GNUNET_NO if not
- */
-static int
-is_pending (GNUNET_SCHEDULER_TaskIdentifier id)
-{
-  struct Task *pos;
-  enum GNUNET_SCHEDULER_Priority p;
-  GNUNET_SCHEDULER_TaskIdentifier min;
-
-  if (id < lowest_pending_id)
-    return GNUNET_NO;
-  min = -1;                     /* maximum value */
-  pos = pending;
-  while (pos != NULL)
-  {
-    if (pos->id == id)
-      return GNUNET_YES;
-    if (pos->id < min)
-      min = pos->id;
-    pos = pos->next;
-  }
-  pos = pending_timeout;
-  while (pos != NULL)
-  {
-    if (pos->id == id)
-      return GNUNET_YES;
-    if (pos->id < min)
-      min = pos->id;
-    pos = pos->next;
-  }
-  for (p = 0; p < GNUNET_SCHEDULER_PRIORITY_COUNT; p++)
-  {
-    pos = ready[p];
-    while (pos != NULL)
-    {
-      if (pos->id == id)
-        return GNUNET_YES;
-      if (pos->id < min)
-        min = pos->id;
-      pos = pos->next;
-    }
-  }
-  lowest_pending_id = min;
-  return GNUNET_NO;
-}
-
-
 /**
  * Update all sets and timeout for select.
  *
@@ -354,49 +321,42 @@ is_pending (GNUNET_SCHEDULER_TaskIdentifier id)
  * @param timeout next timeout (updated)
  */
 static void
-update_sets (struct GNUNET_NETWORK_FDSet *rs, struct GNUNET_NETWORK_FDSet *ws,
+update_sets (struct GNUNET_NETWORK_FDSet *rs,
+             struct GNUNET_NETWORK_FDSet *ws,
              struct GNUNET_TIME_Relative *timeout)
 {
-  struct Task *pos;
+  struct GNUNET_SCHEDULER_Task *pos;
   struct GNUNET_TIME_Absolute now;
   struct GNUNET_TIME_Relative to;
 
   now = GNUNET_TIME_absolute_get ();
-  pos = pending_timeout;
-  if (pos != NULL)
+  pos = pending_timeout_head;
+  if (NULL != pos)
   {
     to = GNUNET_TIME_absolute_get_difference (now, pos->timeout);
-    if (timeout->rel_value > to.rel_value)
+    if (timeout->rel_value_us > to.rel_value_us)
       *timeout = to;
-    if (pos->reason != 0)
+    if (0 != pos->reason)
       *timeout = GNUNET_TIME_UNIT_ZERO;
   }
-  pos = pending;
-  while (pos != NULL)
+  for (pos = pending_head; NULL != pos; pos = pos->next)
   {
-    if ((pos->prereq_id != GNUNET_SCHEDULER_NO_TASK) &&
-        (GNUNET_YES == is_pending (pos->prereq_id)))
-    {
-      pos = pos->next;
-      continue;
-    }
-    if (pos->timeout.abs_value != GNUNET_TIME_UNIT_FOREVER_ABS.abs_value)
+    if (pos->timeout.abs_value_us != GNUNET_TIME_UNIT_FOREVER_ABS.abs_value_us)
     {
       to = GNUNET_TIME_absolute_get_difference (now, pos->timeout);
-      if (timeout->rel_value > to.rel_value)
+      if (timeout->rel_value_us > to.rel_value_us)
         *timeout = to;
     }
-    if (pos->read_fd != -1)
+    if (-1 != pos->read_fd)
       GNUNET_NETWORK_fdset_set_native (rs, pos->read_fd);
-    if (pos->write_fd != -1)
+    if (-1 != pos->write_fd)
       GNUNET_NETWORK_fdset_set_native (ws, pos->write_fd);
-    if (pos->read_set != NULL)
+    if (NULL != pos->read_set)
       GNUNET_NETWORK_fdset_add (rs, pos->read_set);
-    if (pos->write_set != NULL)
+    if (NULL != pos->write_set)
       GNUNET_NETWORK_fdset_add (ws, pos->write_set);
-    if (pos->reason != 0)
+    if (0 != pos->reason)
       *timeout = GNUNET_TIME_UNIT_ZERO;
-    pos = pos->next;
   }
 }
 
@@ -404,11 +364,11 @@ update_sets (struct GNUNET_NETWORK_FDSet *rs, struct GNUNET_NETWORK_FDSet *ws,
 /**
  * Check if the ready set overlaps with the set we want to have ready.
  * If so, update the want set (set all FDs that are ready).  If not,
- * return GNUNET_NO.
+ * return #GNUNET_NO.
  *
  * @param ready set that is ready
  * @param want set that we want to be ready
- * @return GNUNET_YES if there was some overlap
+ * @return #GNUNET_YES if there was some overlap
  */
 static int
 set_overlaps (const struct GNUNET_NETWORK_FDSet *ready,
@@ -435,17 +395,18 @@ set_overlaps (const struct GNUNET_NETWORK_FDSet *ready,
  * @param now the current time
  * @param rs set of FDs ready for reading
  * @param ws set of FDs ready for writing
- * @return GNUNET_YES if we can run it, GNUNET_NO if not.
+ * @return #GNUNET_YES if we can run it, #GNUNET_NO if not.
  */
 static int
-is_ready (struct Task *task, struct GNUNET_TIME_Absolute now,
+is_ready (struct GNUNET_SCHEDULER_Task *task,
+          struct GNUNET_TIME_Absolute now,
           const struct GNUNET_NETWORK_FDSet *rs,
           const struct GNUNET_NETWORK_FDSet *ws)
 {
   enum GNUNET_SCHEDULER_Reason reason;
 
   reason = task->reason;
-  if (now.abs_value >= task->timeout.abs_value)
+  if (now.abs_value_us >= task->timeout.abs_value_us)
     reason |= GNUNET_SCHEDULER_REASON_TIMEOUT;
   if ((0 == (reason & GNUNET_SCHEDULER_REASON_READ_READY)) &&
       (((task->read_fd != -1) &&
@@ -457,17 +418,9 @@ is_ready (struct Task *task, struct GNUNET_TIME_Absolute now,
         (GNUNET_YES == GNUNET_NETWORK_fdset_test_native (ws, task->write_fd)))
        || (set_overlaps (ws, task->write_set))))
     reason |= GNUNET_SCHEDULER_REASON_WRITE_READY;
-  if (reason == 0)
+  if (0 == reason)
     return GNUNET_NO;           /* not ready */
-  if (task->prereq_id != GNUNET_SCHEDULER_NO_TASK)
-  {
-    if (GNUNET_YES == is_pending (task->prereq_id))
-    {
-      task->reason = reason;
-      return GNUNET_NO;         /* prereq waiting */
-    }
-    reason |= GNUNET_SCHEDULER_REASON_PREREQ_DONE;
-  }
+  reason |= GNUNET_SCHEDULER_REASON_PREREQ_DONE;
   task->reason = reason;
   return GNUNET_YES;
 }
@@ -479,14 +432,14 @@ is_ready (struct Task *task, struct GNUNET_TIME_Absolute now,
  * @param task task ready for execution
  */
 static void
-queue_ready_task (struct Task *task)
+queue_ready_task (struct GNUNET_SCHEDULER_Task *task)
 {
-  enum GNUNET_SCHEDULER_Priority p = task->priority;
+  enum GNUNET_SCHEDULER_Priority p = check_priority (task->priority);
 
-  if (0 != (task->reason & GNUNET_SCHEDULER_REASON_SHUTDOWN))
-    p = GNUNET_SCHEDULER_PRIORITY_SHUTDOWN;
-  task->next = ready[check_priority (p)];
-  ready[check_priority (p)] = task;
+  GNUNET_CONTAINER_DLL_insert (ready_head[p],
+                               ready_tail[p],
+                               task);
+  task->in_ready_list = GNUNET_YES;
   ready_count++;
 }
 
@@ -502,93 +455,58 @@ static void
 check_ready (const struct GNUNET_NETWORK_FDSet *rs,
              const struct GNUNET_NETWORK_FDSet *ws)
 {
-  struct Task *pos;
-  struct Task *prev;
-  struct Task *next;
+  struct GNUNET_SCHEDULER_Task *pos;
+  struct GNUNET_SCHEDULER_Task *next;
   struct GNUNET_TIME_Absolute now;
 
   now = GNUNET_TIME_absolute_get ();
-  prev = NULL;
-  pos = pending_timeout;
-  while (pos != NULL)
+  while (NULL != (pos = pending_timeout_head))
   {
-    next = pos->next;
-    if (now.abs_value >= pos->timeout.abs_value)
+    if (now.abs_value_us >= pos->timeout.abs_value_us)
       pos->reason |= GNUNET_SCHEDULER_REASON_TIMEOUT;
     if (0 == pos->reason)
       break;
-    pending_timeout = next;
+    GNUNET_CONTAINER_DLL_remove (pending_timeout_head,
+                                 pending_timeout_tail,
+                                 pos);
     if (pending_timeout_last == pos)
       pending_timeout_last = NULL;
     queue_ready_task (pos);
-    pos = next;
   }
-  pos = pending;
-  while (pos != NULL)
+  pos = pending_head;
+  while (NULL != pos)
   {
-#if DEBUG_TASKS
-    LOG (GNUNET_ERROR_TYPE_DEBUG, "Checking readiness of task: %llu / %p\n",
-         pos->id, pos->callback_cls);
-#endif
     next = pos->next;
     if (GNUNET_YES == is_ready (pos, now, rs, ws))
     {
-      if (prev == NULL)
-        pending = next;
-      else
-        prev->next = next;
+      GNUNET_CONTAINER_DLL_remove (pending_head,
+                                   pending_tail,
+                                   pos);
       queue_ready_task (pos);
-      pos = next;
-      continue;
     }
-    prev = pos;
     pos = next;
   }
 }
 
 
 /**
- * Request the shutdown of a scheduler.  Marks all currently
- * pending tasks as ready because of shutdown.  This will
- * cause all tasks to run (as soon as possible, respecting
- * priorities and prerequisite tasks).  Note that tasks
- * scheduled AFTER this call may still be delayed arbitrarily.
+ * Request the shutdown of a scheduler.  Marks all tasks
+ * awaiting shutdown as ready. Note that tasks
+ * scheduled with #GNUNET_SCHEDULER_add_shutdown() AFTER this call
+ * will be delayed until the next shutdown signal.
  */
 void
 GNUNET_SCHEDULER_shutdown ()
 {
-  struct Task *pos;
-  int i;
+  struct GNUNET_SCHEDULER_Task *pos;
 
-  pos = pending_timeout;
-  while (pos != NULL)
+  while (NULL != (pos = shutdown_head))
   {
+    GNUNET_CONTAINER_DLL_remove (shutdown_head,
+                                 shutdown_tail,
+                                 pos);
     pos->reason |= GNUNET_SCHEDULER_REASON_SHUTDOWN;
-    /* we don't move the task into the ready queue yet; check_ready
-     * will do that later, possibly adding additional
-     * readiness-factors */
-    pos = pos->next;
-  }
-  pos = pending;
-  while (pos != NULL)
-  {
-    pos->reason |= GNUNET_SCHEDULER_REASON_SHUTDOWN;
-    /* we don't move the task into the ready queue yet; check_ready
-     * will do that later, possibly adding additional
-     * readiness-factors */
-    pos = pos->next;
-  }
-  for (i = 0; i < GNUNET_SCHEDULER_PRIORITY_COUNT; i++)
-  {
-    pos = ready[i];
-    while (pos != NULL)
-    {
-      pos->reason |= GNUNET_SCHEDULER_REASON_SHUTDOWN;
-      /* we don't move the task into the ready queue yet; check_ready
-       * will do that later, possibly adding additional
-       * readiness-factors */
-      pos = pos->next;
-    }
+    queue_ready_task (pos);
   }
 }
 
@@ -599,7 +517,7 @@ GNUNET_SCHEDULER_shutdown ()
  * @param t task to destroy
  */
 static void
-destroy_task (struct Task *t)
+destroy_task (struct GNUNET_SCHEDULER_Task *t)
 {
   if (NULL != t->read_set)
     GNUNET_NETWORK_fdset_destroy (t->read_set);
@@ -612,6 +530,27 @@ destroy_task (struct Task *t)
 }
 
 
+/**
+ * Output stack trace of task @a t.
+ *
+ * @param t task to dump stack trace of
+ */
+static void
+dump_backtrace (struct GNUNET_SCHEDULER_Task *t)
+{
+#if EXECINFO
+  unsigned int i;
+
+  for (i = 0; i < t->num_backtrace_strings; i++)
+    LOG (GNUNET_ERROR_TYPE_DEBUG,
+        "Task %p trace %u: %s\n",
+        t,
+        i,
+        t->backtrace_strings[i]);
+#endif
+}
+
+
 /**
  * Run at least one task in the highest-priority queue that is not
  * empty.  Keep running tasks until we are either no longer running
@@ -623,79 +562,71 @@ destroy_task (struct Task *t)
  * @param ws FDs ready for writing
  */
 static void
-run_ready (struct GNUNET_NETWORK_FDSet *rs, struct GNUNET_NETWORK_FDSet *ws)
+run_ready (struct GNUNET_NETWORK_FDSet *rs,
+           struct GNUNET_NETWORK_FDSet *ws)
 {
   enum GNUNET_SCHEDULER_Priority p;
-  struct Task *pos;
-  struct GNUNET_SCHEDULER_TaskContext tc;
+  struct GNUNET_SCHEDULER_Task *pos;
 
   max_priority_added = GNUNET_SCHEDULER_PRIORITY_KEEP;
   do
   {
-    if (ready_count == 0)
+    if (0 == ready_count)
       return;
-    GNUNET_assert (ready[GNUNET_SCHEDULER_PRIORITY_KEEP] == NULL);
+    GNUNET_assert (NULL == ready_head[GNUNET_SCHEDULER_PRIORITY_KEEP]);
     /* yes, p>0 is correct, 0 is "KEEP" which should
      * always be an empty queue (see assertion)! */
     for (p = GNUNET_SCHEDULER_PRIORITY_COUNT - 1; p > 0; p--)
     {
-      pos = ready[p];
-      if (pos != NULL)
+      pos = ready_head[p];
+      if (NULL != pos)
         break;
     }
-    GNUNET_assert (pos != NULL);        /* ready_count wrong? */
-    ready[p] = pos->next;
+    GNUNET_assert (NULL != pos);        /* ready_count wrong? */
+    GNUNET_CONTAINER_DLL_remove (ready_head[p],
+                                 ready_tail[p],
+                                 pos);
     ready_count--;
-    if (current_priority != pos->priority)
-    {
-      current_priority = pos->priority;
-      (void) GNUNET_OS_set_process_priority (GNUNET_OS_process_current (),
-                                             pos->priority);
-    }
+    current_priority = pos->priority;
     current_lifeness = pos->lifeness;
     active_task = pos;
 #if PROFILE_DELAYS
-    if (GNUNET_TIME_absolute_get_duration (pos->start_time).rel_value >
-        DELAY_THRESHOLD.rel_value)
+    if (GNUNET_TIME_absolute_get_duration (pos->start_time).rel_value_us >
+        DELAY_THRESHOLD.rel_value_us)
     {
-      LOG (GNUNET_ERROR_TYPE_ERROR, "Task %llu took %llums to be scheduled\n",
-           pos->id,
-           (unsigned long long)
-           GNUNET_TIME_absolute_get_duration (pos->start_time).rel_value);
+      LOG (GNUNET_ERROR_TYPE_DEBUG,
+          "Task %p took %s to be scheduled\n",
+           pos,
+           GNUNET_STRINGS_relative_time_to_string (GNUNET_TIME_absolute_get_duration (pos->start_time),
+                                                   GNUNET_YES));
     }
 #endif
     tc.reason = pos->reason;
-    tc.read_ready = (pos->read_set == NULL) ? rs : pos->read_set;
-    if ((pos->read_fd != -1) &&
+    tc.read_ready = (NULL == pos->read_set) ? rs : pos->read_set;
+    if ((-1 != pos->read_fd) &&
         (0 != (pos->reason & GNUNET_SCHEDULER_REASON_READ_READY)))
       GNUNET_NETWORK_fdset_set_native (rs, pos->read_fd);
-    tc.write_ready = (pos->write_set == NULL) ? ws : pos->write_set;
-    if ((pos->write_fd != -1) &&
+    tc.write_ready = (NULL == pos->write_set) ? ws : pos->write_set;
+    if ((-1 != pos->write_fd) &&
         (0 != (pos->reason & GNUNET_SCHEDULER_REASON_WRITE_READY)))
       GNUNET_NETWORK_fdset_set_native (ws, pos->write_fd);
-    if (((tc.reason & GNUNET_SCHEDULER_REASON_WRITE_READY) != 0) &&
-        (pos->write_fd != -1) &&
+    if ((0 != (tc.reason & GNUNET_SCHEDULER_REASON_WRITE_READY)) &&
+        (-1 != pos->write_fd) &&
         (!GNUNET_NETWORK_fdset_test_native (ws, pos->write_fd)))
-      GNUNET_abort ();          // added to ready in previous select loop!
-#if DEBUG_TASKS
-    LOG (GNUNET_ERROR_TYPE_DEBUG, "Running task: %llu / %p\n", pos->id,
-         pos->callback_cls);
-#endif
-    pos->callback (pos->callback_cls, &tc);
-#if EXECINFO
-    int i;
-
-    for (i = 0; i < pos->num_backtrace_strings; i++)
-      LOG (GNUNET_ERROR_TYPE_ERROR, "Task %llu trace %d: %s\n", pos->id, i,
-           pos->backtrace_strings[i]);
-#endif
+      GNUNET_assert (0);          // added to ready in previous select loop!
+    LOG (GNUNET_ERROR_TYPE_DEBUG,
+        "Running task: %p\n",
+         pos);
+    pos->callback (pos->callback_cls);
+    dump_backtrace (pos);
     active_task = NULL;
     destroy_task (pos);
     tasks_run++;
   }
-  while ((pending == NULL) || (p >= max_priority_added));
+  while ((NULL == pending_head) || (p >= max_priority_added));
 }
 
+
 /**
  * Pipe used to communicate shutdown via signal.
  */
@@ -717,6 +648,26 @@ sighandler_pipe ()
   return;
 }
 #endif
+
+
+/**
+ * Wait for a short time.
+ * Sleeps for @a ms ms (as that should be long enough for virtually all
+ * modern systems to context switch and allow another process to do
+ * some 'real' work).
+ *
+ * @param ms how many ms to wait
+ */
+static void
+short_wait (unsigned int ms)
+{
+  struct GNUNET_TIME_Relative timeout;
+
+  timeout = GNUNET_TIME_relative_multiply (GNUNET_TIME_UNIT_MILLISECONDS, ms);
+  (void) GNUNET_NETWORK_socket_select (NULL, NULL, NULL, timeout);
+}
+
+
 /**
  * Signal handler called for signals that should cause us to shutdown.
  */
@@ -737,26 +688,29 @@ sighandler_shutdown ()
 
 
 /**
- * Check if the system is still life. Trigger shutdown if we
+ * Check if the system is still alive. Trigger shutdown if we
  * have tasks, but none of them give us lifeness.
  *
- * @return GNUNET_OK to continue the main loop,
- *         GNUNET_NO to exit
+ * @return #GNUNET_OK to continue the main loop,
+ *         #GNUNET_NO to exit
  */
 static int
 check_lifeness ()
 {
-  struct Task *t;
+  struct GNUNET_SCHEDULER_Task *t;
 
   if (ready_count > 0)
     return GNUNET_OK;
-  for (t = pending; NULL != t; t = t->next)
+  for (t = pending_head; NULL != t; t = t->next)
+    if (t->lifeness == GNUNET_YES)
+      return GNUNET_OK;
+  for (t = shutdown_head; NULL != t; t = t->next)
     if (t->lifeness == GNUNET_YES)
       return GNUNET_OK;
-  for (t = pending_timeout; NULL != t; t = t->next)
+  for (t = pending_timeout_head; NULL != t; t = t->next)
     if (t->lifeness == GNUNET_YES)
       return GNUNET_OK;
-  if ((NULL != pending) || (NULL != pending_timeout))
+  if (NULL != shutdown_head)
   {
     GNUNET_SCHEDULER_shutdown ();
     return GNUNET_OK;
@@ -768,19 +722,20 @@ check_lifeness ()
 /**
  * Initialize and run scheduler.  This function will return when all
  * tasks have completed.  On systems with signals, receiving a SIGTERM
- * (and other similar signals) will cause "GNUNET_SCHEDULER_shutdown"
+ * (and other similar signals) will cause #GNUNET_SCHEDULER_shutdown()
  * to be run after the active task is complete.  As a result, SIGTERM
  * causes all active tasks to be scheduled with reason
- * "GNUNET_SCHEDULER_REASON_SHUTDOWN".  (However, tasks added
+ * #GNUNET_SCHEDULER_REASON_SHUTDOWN.  (However, tasks added
  * afterwards will execute normally!). Note that any particular signal
  * will only shut down one scheduler; applications should always only
  * create a single scheduler.
  *
  * @param task task to run immediately
- * @param task_cls closure of task
+ * @param task_cls closure of @a task
  */
 void
-GNUNET_SCHEDULER_run (GNUNET_SCHEDULER_Task task, void *task_cls)
+GNUNET_SCHEDULER_run (GNUNET_SCHEDULER_TaskCallback task,
+                      void *task_cls)
 {
   struct GNUNET_NETWORK_FDSet *rs;
   struct GNUNET_NETWORK_FDSet *ws;
@@ -788,6 +743,9 @@ GNUNET_SCHEDULER_run (GNUNET_SCHEDULER_Task task, void *task_cls)
   int ret;
   struct GNUNET_SIGNAL_Context *shc_int;
   struct GNUNET_SIGNAL_Context *shc_term;
+#if (SIGTERM != GNUNET_TERM_SIG)
+  struct GNUNET_SIGNAL_Context *shc_gterm;
+#endif
 
 #ifndef MINGW
   struct GNUNET_SIGNAL_Context *shc_quit;
@@ -799,34 +757,48 @@ GNUNET_SCHEDULER_run (GNUNET_SCHEDULER_Task task, void *task_cls)
   const struct GNUNET_DISK_FileHandle *pr;
   char c;
 
-  GNUNET_assert (active_task == NULL);
+  GNUNET_assert (NULL == active_task);
   rs = GNUNET_NETWORK_fdset_create ();
   ws = GNUNET_NETWORK_fdset_create ();
-  GNUNET_assert (shutdown_pipe_handle == NULL);
-  shutdown_pipe_handle = GNUNET_DISK_pipe (GNUNET_NO, GNUNET_NO, GNUNET_NO, GNUNET_NO);
-  GNUNET_assert (shutdown_pipe_handle != NULL);
+  GNUNET_assert (NULL == shutdown_pipe_handle);
+  shutdown_pipe_handle = GNUNET_DISK_pipe (GNUNET_NO,
+                                           GNUNET_NO,
+                                           GNUNET_NO,
+                                           GNUNET_NO);
+  GNUNET_assert (NULL != shutdown_pipe_handle);
   pr = GNUNET_DISK_pipe_handle (shutdown_pipe_handle,
                                 GNUNET_DISK_PIPE_END_READ);
-  GNUNET_assert (pr != NULL);
+  GNUNET_assert (NULL != pr);
   my_pid = getpid ();
-  shc_int = GNUNET_SIGNAL_handler_install (SIGINT, &sighandler_shutdown);
-  shc_term = GNUNET_SIGNAL_handler_install (SIGTERM, &sighandler_shutdown);
+  LOG (GNUNET_ERROR_TYPE_DEBUG,
+       "Registering signal handlers\n");
+  shc_int = GNUNET_SIGNAL_handler_install (SIGINT,
+                                          &sighandler_shutdown);
+  shc_term = GNUNET_SIGNAL_handler_install (SIGTERM,
+                                           &sighandler_shutdown);
+#if (SIGTERM != GNUNET_TERM_SIG)
+  shc_gterm = GNUNET_SIGNAL_handler_install (GNUNET_TERM_SIG,
+                                            &sighandler_shutdown);
+#endif
 #ifndef MINGW
-  shc_pipe = GNUNET_SIGNAL_handler_install (SIGPIPE, &sighandler_pipe);
-  shc_quit = GNUNET_SIGNAL_handler_install (SIGQUIT, &sighandler_shutdown);
-  shc_hup = GNUNET_SIGNAL_handler_install (SIGHUP, &sighandler_shutdown);
+  shc_pipe = GNUNET_SIGNAL_handler_install (SIGPIPE,
+                                           &sighandler_pipe);
+  shc_quit = GNUNET_SIGNAL_handler_install (SIGQUIT,
+                                           &sighandler_shutdown);
+  shc_hup = GNUNET_SIGNAL_handler_install (SIGHUP,
+                                          &sighandler_shutdown);
 #endif
   current_priority = GNUNET_SCHEDULER_PRIORITY_DEFAULT;
   current_lifeness = GNUNET_YES;
-  GNUNET_SCHEDULER_add_continuation (task, task_cls,
-                                     GNUNET_SCHEDULER_REASON_STARTUP);
-#if ENABLE_WINDOWS_WORKAROUNDS
+  GNUNET_SCHEDULER_add_with_reason_and_priority (task,
+                                                 task_cls,
+                                                 GNUNET_SCHEDULER_REASON_STARTUP,
+                                                 GNUNET_SCHEDULER_PRIORITY_DEFAULT);
   active_task = (void *) (long) -1;     /* force passing of sanity check */
   GNUNET_SCHEDULER_add_now_with_lifeness (GNUNET_NO,
                                           &GNUNET_OS_install_parent_control_handler,
                                           NULL);
   active_task = NULL;
-#endif
   last_tr = 0;
   busy_wait_warning = 0;
   while (GNUNET_OK == check_lifeness ())
@@ -842,9 +814,16 @@ GNUNET_SCHEDULER_run (GNUNET_SCHEDULER_Task task, void *task_cls)
       timeout = GNUNET_TIME_UNIT_ZERO;
     }
     if (NULL == scheduler_select)
-      ret = GNUNET_NETWORK_socket_select (rs, ws, NULL, timeout);
+      ret = GNUNET_NETWORK_socket_select (rs,
+                                          ws,
+                                          NULL,
+                                          timeout);
     else
-      ret = scheduler_select (scheduler_select_cls, rs, ws, NULL, timeout);
+      ret = scheduler_select (scheduler_select_cls,
+                              rs,
+                              ws,
+                              NULL,
+                              timeout);
     if (ret == GNUNET_SYSERR)
     {
       if (errno == EINTR)
@@ -859,16 +838,50 @@ GNUNET_SCHEDULER_run (GNUNET_SCHEDULER_Task task, void *task_cls)
       (void) close (1);
       (void) dup2 (2, 1);
       if (0 != system (lsof))
-        LOG_STRERROR (GNUNET_ERROR_TYPE_WARNING, "system");
+        LOG_STRERROR (GNUNET_ERROR_TYPE_WARNING,
+                      "system");
+#endif
 #endif
+#if DEBUG_FDS
+      struct GNUNET_SCHEDULER_Task *t;
+
+      for (t = pending_head; NULL != t; t = t->next)
+      {
+        if (-1 != t->read_fd)
+        {
+          int flags = fcntl (t->read_fd, F_GETFD);
+          if ((flags == -1) && (errno == EBADF))
+            {
+              LOG (GNUNET_ERROR_TYPE_ERROR,
+                   "Got invalid file descriptor %d!\n",
+                   t->read_fd);
+             dump_backtrace (t);
+            }
+        }
+        if (-1 != t->write_fd)
+          {
+            int flags = fcntl (t->write_fd, F_GETFD);
+            if ((flags == -1) && (errno == EBADF))
+              {
+                LOG (GNUNET_ERROR_TYPE_ERROR,
+                     "Got invalid file descriptor %d!\n",
+                     t->write_fd);
+               dump_backtrace (t);
+              }
+          }
+      }
 #endif
-      GNUNET_abort ();
+      GNUNET_assert (0);
       break;
     }
-    if ((ret == 0) && (timeout.rel_value == 0) && (busy_wait_warning > 16))
+
+    if ( (0 == ret) &&
+         (0 == timeout.rel_value_us) &&
+         (busy_wait_warning > 16) )
     {
-      LOG (GNUNET_ERROR_TYPE_WARNING, _("Looks like we're busy waiting...\n"));
-      sleep (1);                /* mitigate */
+      LOG (GNUNET_ERROR_TYPE_WARNING,
+           "Looks like we're busy waiting...\n");
+      short_wait (100);                /* mitigate */
     }
     check_ready (rs, ws);
     run_ready (rs, ws);
@@ -881,6 +894,7 @@ GNUNET_SCHEDULER_run (GNUNET_SCHEDULER_Task task, void *task_cls)
     }
     if (last_tr == tasks_run)
     {
+      short_wait (1);
       busy_wait_warning++;
     }
     else
@@ -891,6 +905,9 @@ GNUNET_SCHEDULER_run (GNUNET_SCHEDULER_Task task, void *task_cls)
   }
   GNUNET_SIGNAL_handler_uninstall (shc_int);
   GNUNET_SIGNAL_handler_uninstall (shc_term);
+#if (SIGTERM != GNUNET_TERM_SIG)
+  GNUNET_SIGNAL_handler_uninstall (shc_gterm);
+#endif
 #ifndef MINGW
   GNUNET_SIGNAL_handler_uninstall (shc_pipe);
   GNUNET_SIGNAL_handler_uninstall (shc_quit);
@@ -904,17 +921,16 @@ GNUNET_SCHEDULER_run (GNUNET_SCHEDULER_Task task, void *task_cls)
 
 
 /**
- * Obtain the reason code for why the current task was
- * started.  Will return the same value as
- * the GNUNET_SCHEDULER_TaskContext's reason field.
+ * Obtain the task context, giving the reason why the current task was
+ * started.
  *
- * @return reason(s) why the current task is run
+ * @return current tasks' scheduler context
  */
-enum GNUNET_SCHEDULER_Reason
-GNUNET_SCHEDULER_get_reason ()
+const struct GNUNET_SCHEDULER_TaskContext *
+GNUNET_SCHEDULER_get_task_context ()
 {
-  GNUNET_assert (active_task != NULL);
-  return active_task->reason;
+  GNUNET_assert (NULL != active_task);
+  return &tc;
 }
 
 
@@ -930,21 +946,17 @@ GNUNET_SCHEDULER_get_reason ()
 unsigned int
 GNUNET_SCHEDULER_get_load (enum GNUNET_SCHEDULER_Priority p)
 {
-  struct Task *pos;
+  struct GNUNET_SCHEDULER_Task *pos;
   unsigned int ret;
 
-  GNUNET_assert (active_task != NULL);
+  GNUNET_assert (NULL != active_task);
   if (p == GNUNET_SCHEDULER_PRIORITY_COUNT)
     return ready_count;
   if (p == GNUNET_SCHEDULER_PRIORITY_KEEP)
     p = current_priority;
   ret = 0;
-  pos = ready[check_priority (p)];
-  while (pos != NULL)
-  {
-    pos = pos->next;
+  for (pos = ready_head[check_priority (p)]; NULL != pos; pos = pos->next)
     ret++;
-  }
   return ret;
 }
 
@@ -957,185 +969,215 @@ GNUNET_SCHEDULER_get_load (enum GNUNET_SCHEDULER_Priority p)
  * @return original closure of the task
  */
 void *
-GNUNET_SCHEDULER_cancel (GNUNET_SCHEDULER_TaskIdentifier task)
+GNUNET_SCHEDULER_cancel (struct GNUNET_SCHEDULER_Task *task)
 {
-  struct Task *t;
-  struct Task *prev;
   enum GNUNET_SCHEDULER_Priority p;
-  int to;
   void *ret;
 
-  GNUNET_assert (active_task != NULL);
-  to = 0;
-  prev = NULL;
-  t = pending;
-  while (t != NULL)
-  {
-    if (t->id == task)
-      break;
-    prev = t;
-    t = t->next;
-  }
-  if (t == NULL)
+  GNUNET_assert ( (NULL != active_task) ||
+                 (GNUNET_NO == task->lifeness) );
+  if (! task->in_ready_list)
   {
-    prev = NULL;
-    to = 1;
-    t = pending_timeout;
-    while (t != NULL)
+    if ( (-1 == task->read_fd) &&
+         (-1 == task->write_fd) &&
+         (NULL == task->read_set) &&
+         (NULL == task->write_set) )
     {
-      if (t->id == task)
-        break;
-      prev = t;
-      t = t->next;
-    }
-    if (pending_timeout_last == t)
-      pending_timeout_last = NULL;
-  }
-  p = 0;
-  while (t == NULL)
-  {
-    p++;
-    if (p >= GNUNET_SCHEDULER_PRIORITY_COUNT)
-    {
-      LOG (GNUNET_ERROR_TYPE_ERROR, _("Attempt to cancel dead task %llu!\n"),
-           (unsigned long long) task);
-      GNUNET_assert (0);
-    }
-    prev = NULL;
-    t = ready[p];
-    while (t != NULL)
-    {
-      if (t->id == task)
-      {
-        ready_count--;
-        break;
-      }
-      prev = t;
-      t = t->next;
-    }
-  }
-  if (prev == NULL)
-  {
-    if (p == 0)
-    {
-      if (to == 0)
-      {
-        pending = t->next;
-      }
+      if (GNUNET_YES == task->on_shutdown)
+       GNUNET_CONTAINER_DLL_remove (shutdown_head,
+                                    shutdown_tail,
+                                    task);
       else
-      {
-        pending_timeout = t->next;
-      }
+       GNUNET_CONTAINER_DLL_remove (pending_timeout_head,
+                                    pending_timeout_tail,
+                                    task);
+      if (task == pending_timeout_last)
+        pending_timeout_last = NULL;
     }
     else
     {
-      ready[p] = t->next;
+      GNUNET_CONTAINER_DLL_remove (pending_head,
+                                   pending_tail,
+                                   task);
     }
   }
   else
   {
-    prev->next = t->next;
+    p = check_priority (task->priority);
+    GNUNET_CONTAINER_DLL_remove (ready_head[p],
+                                 ready_tail[p],
+                                 task);
+    ready_count--;
   }
-  ret = t->callback_cls;
-#if DEBUG_TASKS
-  LOG (GNUNET_ERROR_TYPE_DEBUG, "Canceling task: %llu / %p\n", task,
-       t->callback_cls);
-#endif
-  destroy_task (t);
+  ret = task->callback_cls;
+  LOG (GNUNET_ERROR_TYPE_DEBUG,
+       "Canceling task %p\n",
+       task);
+  destroy_task (task);
   return ret;
 }
 
 
+/**
+ * Initialize backtrace data for task @a t
+ *
+ * @param t task to initialize
+ */
+static void
+init_backtrace (struct GNUNET_SCHEDULER_Task *t)
+{
+#if EXECINFO
+  void *backtrace_array[MAX_TRACE_DEPTH];
+
+  t->num_backtrace_strings
+    = backtrace (backtrace_array, MAX_TRACE_DEPTH);
+  t->backtrace_strings =
+      backtrace_symbols (backtrace_array,
+                        t->num_backtrace_strings);
+  dump_backtrace (t);
+#endif
+}
+
+
 /**
  * Continue the current execution with the given function.  This is
  * similar to the other "add" functions except that there is no delay
  * and the reason code can be specified.
  *
  * @param task main function of the task
- * @param task_cls closure for 'main'
+ * @param task_cls closure for @a task
  * @param reason reason for task invocation
  * @param priority priority to use for the task
  */
 void
-GNUNET_SCHEDULER_add_continuation_with_priority (GNUNET_SCHEDULER_Task task, void *task_cls,
-                                                enum GNUNET_SCHEDULER_Reason reason,
-                                                enum GNUNET_SCHEDULER_Priority priority)
+GNUNET_SCHEDULER_add_with_reason_and_priority (GNUNET_SCHEDULER_TaskCallback task,
+                                               void *task_cls,
+                                               enum GNUNET_SCHEDULER_Reason reason,
+                                               enum GNUNET_SCHEDULER_Priority priority)
 {
-  struct Task *t;
-
-#if EXECINFO
-  void *backtrace_array[50];
-#endif
+  struct GNUNET_SCHEDULER_Task *t;
 
   GNUNET_assert (NULL != task);
-  GNUNET_assert ((active_task != NULL) ||
-                 (reason == GNUNET_SCHEDULER_REASON_STARTUP));
-  t = GNUNET_malloc (sizeof (struct Task));
-#if EXECINFO
-  t->num_backtrace_strings = backtrace (backtrace_array, 50);
-  t->backtrace_strings =
-      backtrace_symbols (backtrace_array, t->num_backtrace_strings);
-#endif
+  GNUNET_assert ((NULL != active_task) ||
+                 (GNUNET_SCHEDULER_REASON_STARTUP == reason));
+  t = GNUNET_new (struct GNUNET_SCHEDULER_Task);
   t->read_fd = -1;
   t->write_fd = -1;
   t->callback = task;
   t->callback_cls = task_cls;
-  t->id = ++last_id;
 #if PROFILE_DELAYS
   t->start_time = GNUNET_TIME_absolute_get ();
 #endif
   t->reason = reason;
   t->priority = priority;
   t->lifeness = current_lifeness;
-#if DEBUG_TASKS
-  LOG (GNUNET_ERROR_TYPE_DEBUG, "Adding continuation task: %llu / %p\n", t->id,
-       t->callback_cls);
-#endif
+  LOG (GNUNET_ERROR_TYPE_DEBUG,
+       "Adding continuation task %p\n",
+       t);
+  init_backtrace (t);
   queue_ready_task (t);
 }
 
 
 /**
- * Continue the current execution with the given function.  This is
- * similar to the other "add" functions except that there is no delay
- * and the reason code can be specified.
+ * Schedule a new task to be run at the specified time.  The task
+ * will be scheduled for execution at time @a at.
  *
+ * @param at time when the operation should run
+ * @param priority priority to use for the task
  * @param task main function of the task
- * @param task_cls closure for 'main'
- * @param reason reason for task invocation
+ * @param task_cls closure of @a task
+ * @return unique task identifier for the job
+ *         only valid until @a task is started!
  */
-void
-GNUNET_SCHEDULER_add_continuation (GNUNET_SCHEDULER_Task task, void *task_cls,
-                                   enum GNUNET_SCHEDULER_Reason reason)
+struct GNUNET_SCHEDULER_Task *
+GNUNET_SCHEDULER_add_at_with_priority (struct GNUNET_TIME_Absolute at,
+                                       enum GNUNET_SCHEDULER_Priority priority,
+                                       GNUNET_SCHEDULER_TaskCallback task,
+                                       void *task_cls)
 {
-  GNUNET_SCHEDULER_add_continuation_with_priority (task, task_cls,
-                                                  reason,
-                                                  GNUNET_SCHEDULER_PRIORITY_DEFAULT);
+  struct GNUNET_SCHEDULER_Task *t;
+  struct GNUNET_SCHEDULER_Task *pos;
+  struct GNUNET_SCHEDULER_Task *prev;
+
+  GNUNET_assert (NULL != active_task);
+  GNUNET_assert (NULL != task);
+  t = GNUNET_new (struct GNUNET_SCHEDULER_Task);
+  t->callback = task;
+  t->callback_cls = task_cls;
+  t->read_fd = -1;
+  t->write_fd = -1;
+#if PROFILE_DELAYS
+  t->start_time = GNUNET_TIME_absolute_get ();
+#endif
+  t->timeout = at;
+  t->priority = priority;
+  t->lifeness = current_lifeness;
+  /* try tail first (optimization in case we are
+   * appending to a long list of tasks with timeouts) */
+  if ( (NULL == pending_timeout_head) ||
+       (at.abs_value_us < pending_timeout_head->timeout.abs_value_us) )
+  {
+    GNUNET_CONTAINER_DLL_insert (pending_timeout_head,
+                                 pending_timeout_tail,
+                                 t);
+  }
+  else
+  {
+    /* first move from heuristic start backwards to before start time */
+    prev = pending_timeout_last;
+    while ( (NULL != prev) &&
+            (prev->timeout.abs_value_us > t->timeout.abs_value_us) )
+      prev = prev->prev;
+    /* now, move from heuristic start (or head of list) forward to insertion point */
+    if (NULL == prev)
+      pos = pending_timeout_head;
+    else
+      pos = prev->next;
+    while ( (NULL != pos) &&
+            ( (pos->timeout.abs_value_us <= t->timeout.abs_value_us) ||
+              (0 != pos->reason) ) )
+    {
+      prev = pos;
+      pos = pos->next;
+    }
+    GNUNET_CONTAINER_DLL_insert_after (pending_timeout_head,
+                                       pending_timeout_tail,
+                                       prev,
+                                       t);
+  }
+  /* finally, update heuristic insertion point to last insertion... */
+  pending_timeout_last = t;
+
+  LOG (GNUNET_ERROR_TYPE_DEBUG,
+       "Adding task: %p\n",
+       t);
+  init_backtrace (t);
+  return t;
 }
 
 
 /**
- * Schedule a new task to be run after the specified prerequisite task
- * has completed. It will be run with the DEFAULT priority.
+ * Schedule a new task to be run with a specified delay.  The task
+ * will be scheduled for execution once the delay has expired.
  *
- * @param prerequisite_task run this task after the task with the given
- *        task identifier completes (and any of our other
- *        conditions, such as delay, read or write-readiness
- *        are satisfied).  Use  GNUNET_SCHEDULER_NO_TASK to not have any dependency
- *        on completion of other tasks (this will cause the task to run as
- *        soon as possible).
+ * @param delay when should this operation time out?
+ * @param priority priority to use for the task
  * @param task main function of the task
- * @param task_cls closure of task
+ * @param task_cls closure of @a task
  * @return unique task identifier for the job
- *         only valid until "task" is started!
+ *         only valid until @a task is started!
  */
-GNUNET_SCHEDULER_TaskIdentifier
-GNUNET_SCHEDULER_add_after (GNUNET_SCHEDULER_TaskIdentifier prerequisite_task,
-                            GNUNET_SCHEDULER_Task task, void *task_cls)
+struct GNUNET_SCHEDULER_Task *
+GNUNET_SCHEDULER_add_delayed_with_priority (struct GNUNET_TIME_Relative delay,
+                                           enum GNUNET_SCHEDULER_Priority priority,
+                                           GNUNET_SCHEDULER_TaskCallback task,
+                                            void *task_cls)
 {
-  return GNUNET_SCHEDULER_add_select (GNUNET_SCHEDULER_PRIORITY_DEFAULT,
-                                      prerequisite_task, GNUNET_TIME_UNIT_ZERO,
-                                      NULL, NULL, task, task_cls);
+  return GNUNET_SCHEDULER_add_at_with_priority (GNUNET_TIME_relative_to_absolute (delay),
+                                                priority,
+                                                task,
+                                                task_cls);
 }
 
 
@@ -1144,107 +1186,42 @@ GNUNET_SCHEDULER_add_after (GNUNET_SCHEDULER_TaskIdentifier prerequisite_task,
  *
  * @param prio how important is the new task?
  * @param task main function of the task
- * @param task_cls closure of task
+ * @param task_cls closure of @a task
  * @return unique task identifier for the job
- *         only valid until "task" is started!
+ *         only valid until @a task is started!
  */
-GNUNET_SCHEDULER_TaskIdentifier
+struct GNUNET_SCHEDULER_Task *
 GNUNET_SCHEDULER_add_with_priority (enum GNUNET_SCHEDULER_Priority prio,
-                                    GNUNET_SCHEDULER_Task task, void *task_cls)
+                                    GNUNET_SCHEDULER_TaskCallback task,
+                                    void *task_cls)
 {
-  return GNUNET_SCHEDULER_add_select (prio, GNUNET_SCHEDULER_NO_TASK,
-                                      GNUNET_TIME_UNIT_ZERO, NULL, NULL, task,
-                                      task_cls);
+  return GNUNET_SCHEDULER_add_delayed_with_priority (GNUNET_TIME_UNIT_ZERO,
+                                                     prio,
+                                                     task,
+                                                     task_cls);
 }
 
 
-
 /**
- * Schedule a new task to be run with a specified delay.  The task
- * will be scheduled for execution once the delay has expired.
+ * Schedule a new task to be run at the specified time.  The task
+ * will be scheduled for execution once specified time has been
+ * reached. It will be run with the DEFAULT priority.
  *
- * @param delay when should this operation time out? Use
- *        GNUNET_TIME_UNIT_FOREVER_REL for "on shutdown"
- * @param priority priority to use for the task
+ * @param at time at which this operation should run
  * @param task main function of the task
- * @param task_cls closure of task
+ * @param task_cls closure of @a task
  * @return unique task identifier for the job
- *         only valid until "task" is started!
+ *         only valid until @a task is started!
  */
-GNUNET_SCHEDULER_TaskIdentifier
-GNUNET_SCHEDULER_add_delayed_with_priority (struct GNUNET_TIME_Relative delay,
-                                           enum GNUNET_SCHEDULER_Priority priority,
-                                           GNUNET_SCHEDULER_Task task, void *task_cls)
+struct GNUNET_SCHEDULER_Task *
+GNUNET_SCHEDULER_add_at (struct GNUNET_TIME_Absolute at,
+                         GNUNET_SCHEDULER_TaskCallback task,
+                         void *task_cls)
 {
-  struct Task *t;
-  struct Task *pos;
-  struct Task *prev;
-
-#if EXECINFO
-  void *backtrace_array[MAX_TRACE_DEPTH];
-#endif
-
-  GNUNET_assert (active_task != NULL);
-  GNUNET_assert (NULL != task);
-  t = GNUNET_malloc (sizeof (struct Task));
-  t->callback = task;
-  t->callback_cls = task_cls;
-#if EXECINFO
-  t->num_backtrace_strings = backtrace (backtrace_array, MAX_TRACE_DEPTH);
-  t->backtrace_strings =
-      backtrace_symbols (backtrace_array, t->num_backtrace_strings);
-#endif
-  t->read_fd = -1;
-  t->write_fd = -1;
-  t->id = ++last_id;
-#if PROFILE_DELAYS
-  t->start_time = GNUNET_TIME_absolute_get ();
-#endif
-  t->timeout = GNUNET_TIME_relative_to_absolute (delay);
-  t->priority = priority;
-  t->lifeness = current_lifeness;
-  /* try tail first (optimization in case we are
-   * appending to a long list of tasks with timeouts) */
-  prev = pending_timeout_last;
-  if (prev != NULL)
-  {
-    if (prev->timeout.abs_value > t->timeout.abs_value)
-      prev = NULL;
-    else
-      pos = prev->next;         /* heuristic success! */
-  }
-  if (prev == NULL)
-  {
-    /* heuristic failed, do traversal of timeout list */
-    pos = pending_timeout;
-  }
-  while ((pos != NULL) &&
-         ((pos->timeout.abs_value <= t->timeout.abs_value) ||
-          (pos->reason != 0)))
-  {
-    prev = pos;
-    pos = pos->next;
-  }
-  if (prev == NULL)
-    pending_timeout = t;
-  else
-    prev->next = t;
-  t->next = pos;
-  /* hyper-optimization... */
-  pending_timeout_last = t;
-
-#if DEBUG_TASKS
-  LOG (GNUNET_ERROR_TYPE_DEBUG, "Adding task: %llu / %p\n", t->id,
-       t->callback_cls);
-#endif
-#if EXECINFO
-  int i;
-
-  for (i = 0; i < t->num_backtrace_strings; i++)
-    LOG (GNUNET_ERROR_TYPE_DEBUG, "Task %llu trace %d: %s\n", t->id, i,
-         t->backtrace_strings[i]);
-#endif
-  return t->id;
+  return GNUNET_SCHEDULER_add_at_with_priority (at,
+                                                GNUNET_SCHEDULER_PRIORITY_DEFAULT,
+                                                task,
+                                                task_cls);
 }
 
 
@@ -1253,36 +1230,87 @@ GNUNET_SCHEDULER_add_delayed_with_priority (struct GNUNET_TIME_Relative delay,
  * will be scheduled for execution once the delay has expired. It
  * will be run with the DEFAULT priority.
  *
- * @param delay when should this operation time out? Use
- *        GNUNET_TIME_UNIT_FOREVER_REL for "on shutdown"
+ * @param delay when should this operation time out?
  * @param task main function of the task
- * @param task_cls closure of task
+ * @param task_cls closure of @a task
  * @return unique task identifier for the job
- *         only valid until "task" is started!
+ *         only valid until @a task is started!
  */
-GNUNET_SCHEDULER_TaskIdentifier
+struct GNUNET_SCHEDULER_Task *
 GNUNET_SCHEDULER_add_delayed (struct GNUNET_TIME_Relative delay,
-                              GNUNET_SCHEDULER_Task task, void *task_cls)
+                              GNUNET_SCHEDULER_TaskCallback task,
+                             void *task_cls)
 {
   return GNUNET_SCHEDULER_add_delayed_with_priority (delay,
                                                     GNUNET_SCHEDULER_PRIORITY_DEFAULT,
-                                                    task, task_cls);
+                                                    task,
+                                                     task_cls);
 }
 
 
 /**
- * Schedule a new task to be run as soon as possible. The task
- * will be run with the DEFAULT priority.
+ * Schedule a new task to be run as soon as possible.  Note that this
+ * does not guarantee that this will be the next task that is being
+ * run, as other tasks with higher priority (or that are already ready
+ * to run) might get to run first.  Just as with delays, clients must
+ * not rely on any particular order of execution between tasks
+ * scheduled concurrently.
+ *
+ * The task will be run with the DEFAULT priority.
  *
  * @param task main function of the task
- * @param task_cls closure of task
+ * @param task_cls closure of @a task
  * @return unique task identifier for the job
- *         only valid until "task" is started!
+ *         only valid until @a task is started!
+ */
+struct GNUNET_SCHEDULER_Task *
+GNUNET_SCHEDULER_add_now (GNUNET_SCHEDULER_TaskCallback task,
+                         void *task_cls)
+{
+  return GNUNET_SCHEDULER_add_delayed (GNUNET_TIME_UNIT_ZERO,
+                                      task,
+                                      task_cls);
+}
+
+
+/**
+ * Schedule a new task to be run on shutdown, that is when a CTRL-C
+ * signal is received, or when #GNUNET_SCHEDULER_shutdown() is being
+ * invoked.
+ *
+ * @param task main function of the task
+ * @param task_cls closure of @a task
+ * @return unique task identifier for the job
+ *         only valid until @a task is started!
  */
-GNUNET_SCHEDULER_TaskIdentifier
-GNUNET_SCHEDULER_add_now (GNUNET_SCHEDULER_Task task, void *task_cls)
+struct GNUNET_SCHEDULER_Task *
+GNUNET_SCHEDULER_add_shutdown (GNUNET_SCHEDULER_TaskCallback task,
+                              void *task_cls)
 {
-  return GNUNET_SCHEDULER_add_delayed (GNUNET_TIME_UNIT_ZERO, task, task_cls);
+  struct GNUNET_SCHEDULER_Task *t;
+
+  GNUNET_assert (NULL != active_task);
+  GNUNET_assert (NULL != task);
+  t = GNUNET_new (struct GNUNET_SCHEDULER_Task);
+  t->callback = task;
+  t->callback_cls = task_cls;
+  t->read_fd = -1;
+  t->write_fd = -1;
+#if PROFILE_DELAYS
+  t->start_time = GNUNET_TIME_absolute_get ();
+#endif
+  t->timeout = GNUNET_TIME_UNIT_FOREVER_ABS;
+  t->priority = GNUNET_SCHEDULER_PRIORITY_SHUTDOWN;
+  t->on_shutdown = GNUNET_YES;
+  t->lifeness = GNUNET_YES;
+  GNUNET_CONTAINER_DLL_insert (shutdown_head,
+                              shutdown_tail,
+                              t);
+  LOG (GNUNET_ERROR_TYPE_DEBUG,
+       "Adding task: %p\n",
+       t);
+  init_backtrace (t);
+  return t;
 }
 
 
@@ -1291,29 +1319,24 @@ GNUNET_SCHEDULER_add_now (GNUNET_SCHEDULER_Task task, void *task_cls)
  * (transitive) ignore-shutdown flag either explicitly set or
  * explicitly enabled.  This task (and all tasks created from it,
  * other than by another call to this function) will either count or
- * not count for the 'lifeness' of the process.  This API is only
+ * not count for the "lifeness" of the process.  This API is only
  * useful in a few special cases.
  *
- * @param lifeness GNUNET_YES if the task counts for lifeness, GNUNET_NO if not.
+ * @param lifeness #GNUNET_YES if the task counts for lifeness, #GNUNET_NO if not.
  * @param task main function of the task
- * @param task_cls closure of task
+ * @param task_cls closure of @a task
  * @return unique task identifier for the job
- *         only valid until "task" is started!
+ *         only valid until @a task is started!
  */
-GNUNET_SCHEDULER_TaskIdentifier
+struct GNUNET_SCHEDULER_Task *
 GNUNET_SCHEDULER_add_now_with_lifeness (int lifeness,
-                                        GNUNET_SCHEDULER_Task task,
+                                        GNUNET_SCHEDULER_TaskCallback task,
                                         void *task_cls)
 {
-  GNUNET_SCHEDULER_TaskIdentifier ret;
-
-  ret =
-      GNUNET_SCHEDULER_add_select (GNUNET_SCHEDULER_PRIORITY_DEFAULT,
-                                   GNUNET_SCHEDULER_NO_TASK,
-                                   GNUNET_TIME_UNIT_ZERO, NULL, NULL, task,
-                                   task_cls);
-  GNUNET_assert (pending->id == ret);
-  pending->lifeness = lifeness;
+  struct GNUNET_SCHEDULER_Task *ret;
+
+  ret = GNUNET_SCHEDULER_add_now (task, task_cls);
+  ret->lifeness = lifeness;
   return ret;
 }
 
@@ -1331,43 +1354,34 @@ GNUNET_SCHEDULER_add_now_with_lifeness (int lifeness,
  * (prerequisite-run)
  * && (delay-ready
  *     || any-rs-ready
- *     || any-ws-ready
- *     || shutdown-active )
+ *     || any-ws-ready)
  * </code>
  *
- * @param delay how long should we wait? Use GNUNET_TIME_UNIT_FOREVER_REL for "forever",
- *        which means that the task will only be run after we receive SIGTERM
+ * @param delay how long should we wait?
  * @param priority priority to use
  * @param rfd file descriptor we want to read (can be -1)
  * @param wfd file descriptors we want to write (can be -1)
  * @param task main function of the task
- * @param task_cls closure of task
+ * @param task_cls closure of @a task
  * @return unique task identifier for the job
- *         only valid until "task" is started!
+ *         only valid until @a task is started!
  */
 #ifndef MINGW
-static GNUNET_SCHEDULER_TaskIdentifier
-add_without_sets (struct GNUNET_TIME_Relative delay, 
+static struct GNUNET_SCHEDULER_Task *
+add_without_sets (struct GNUNET_TIME_Relative delay,
                  enum GNUNET_SCHEDULER_Priority priority,
-                 int rfd, int wfd,
-                  GNUNET_SCHEDULER_Task task, void *task_cls)
+                 int rfd,
+                  int wfd,
+                  GNUNET_SCHEDULER_TaskCallback task,
+                  void *task_cls)
 {
-  struct Task *t;
-
-#if EXECINFO
-  void *backtrace_array[MAX_TRACE_DEPTH];
-#endif
+  struct GNUNET_SCHEDULER_Task *t;
 
-  GNUNET_assert (active_task != NULL);
+  GNUNET_assert (NULL != active_task);
   GNUNET_assert (NULL != task);
-  t = GNUNET_malloc (sizeof (struct Task));
+  t = GNUNET_new (struct GNUNET_SCHEDULER_Task);
   t->callback = task;
   t->callback_cls = task_cls;
-#if EXECINFO
-  t->num_backtrace_strings = backtrace (backtrace_array, MAX_TRACE_DEPTH);
-  t->backtrace_strings =
-      backtrace_symbols (backtrace_array, t->num_backtrace_strings);
-#endif
 #if DEBUG_FDS
   if (-1 != rfd)
   {
@@ -1375,13 +1389,10 @@ add_without_sets (struct GNUNET_TIME_Relative delay,
 
     if ((flags == -1) && (errno == EBADF))
     {
-      LOG (GNUNET_ERROR_TYPE_ERROR, "Got invalid file descriptor %d!\n", rfd);
-#if EXECINFO
-      int i;
-
-      for (i = 0; i < t->num_backtrace_strings; i++)
-        LOG (GNUNET_ERROR_TYPE_DEBUG, "Trace: %s\n", t->backtrace_strings[i]);
-#endif
+      LOG (GNUNET_ERROR_TYPE_ERROR,
+           "Got invalid file descriptor %d!\n",
+           rfd);
+      init_backtrace (t);
       GNUNET_assert (0);
     }
   }
@@ -1391,13 +1402,10 @@ add_without_sets (struct GNUNET_TIME_Relative delay,
 
     if (flags == -1 && errno == EBADF)
     {
-      LOG (GNUNET_ERROR_TYPE_ERROR, "Got invalid file descriptor %d!\n", wfd);
-#if EXECINFO
-      int i;
-
-      for (i = 0; i < t->num_backtrace_strings; i++)
-        LOG (GNUNET_ERROR_TYPE_DEBUG, "Trace: %s\n", t->backtrace_strings[i]);
-#endif
+      LOG (GNUNET_ERROR_TYPE_ERROR,
+           "Got invalid file descriptor %d!\n",
+           wfd);
+      init_backtrace (t);
       GNUNET_assert (0);
     }
   }
@@ -1405,34 +1413,26 @@ add_without_sets (struct GNUNET_TIME_Relative delay,
   t->read_fd = rfd;
   GNUNET_assert (wfd >= -1);
   t->write_fd = wfd;
-  t->id = ++last_id;
 #if PROFILE_DELAYS
   t->start_time = GNUNET_TIME_absolute_get ();
 #endif
-  t->prereq_id = GNUNET_SCHEDULER_NO_TASK;
   t->timeout = GNUNET_TIME_relative_to_absolute (delay);
   t->priority = check_priority ((priority == GNUNET_SCHEDULER_PRIORITY_KEEP) ? current_priority : priority);
   t->lifeness = current_lifeness;
-  t->next = pending;
-  pending = t;
-  max_priority_added = GNUNET_MAX (max_priority_added, t->priority);
-#if DEBUG_TASKS
-  LOG (GNUNET_ERROR_TYPE_DEBUG, "Adding task: %llu / %p\n", t->id,
-       t->callback_cls);
-#endif
-#if EXECINFO
-  int i;
-
-  for (i = 0; i < t->num_backtrace_strings; i++)
-    LOG (GNUNET_ERROR_TYPE_DEBUG, "Task %llu trace %d: %s\n", t->id, i,
-         t->backtrace_strings[i]);
-#endif
-  return t->id;
+  GNUNET_CONTAINER_DLL_insert (pending_head,
+                               pending_tail,
+                               t);
+  max_priority_added = GNUNET_MAX (max_priority_added,
+                                   t->priority);
+  LOG (GNUNET_ERROR_TYPE_DEBUG,
+       "Adding task %p\n",
+       t);
+  init_backtrace (t);
+  return t;
 }
 #endif
 
 
-
 /**
  * Schedule a new task to be run with a specified delay or when the
  * specified file descriptor is ready for reading.  The delay can be
@@ -1440,38 +1440,53 @@ add_without_sets (struct GNUNET_TIME_Relative delay,
  * scheduled for execution once either the delay has expired or the
  * socket operation is ready.  It will be run with the DEFAULT priority.
  *
- * @param delay when should this operation time out? Use
- *        GNUNET_TIME_UNIT_FOREVER_REL for "on shutdown"
+ * @param delay when should this operation time out?
  * @param rfd read file-descriptor
  * @param task main function of the task
- * @param task_cls closure of task
+ * @param task_cls closure of @a task
  * @return unique task identifier for the job
- *         only valid until "task" is started!
+ *         only valid until @a task is started!
  */
-GNUNET_SCHEDULER_TaskIdentifier
+struct GNUNET_SCHEDULER_Task *
 GNUNET_SCHEDULER_add_read_net (struct GNUNET_TIME_Relative delay,
                                struct GNUNET_NETWORK_Handle *rfd,
-                               GNUNET_SCHEDULER_Task task, void *task_cls)
+                               GNUNET_SCHEDULER_TaskCallback task,
+                               void *task_cls)
 {
-#if MINGW
-  struct GNUNET_NETWORK_FDSet *rs;
-  GNUNET_SCHEDULER_TaskIdentifier ret;
+  return GNUNET_SCHEDULER_add_read_net_with_priority (delay,
+                                                     GNUNET_SCHEDULER_PRIORITY_DEFAULT,
+                                                     rfd, task, task_cls);
+}
 
-  GNUNET_assert (rfd != NULL);
-  rs = GNUNET_NETWORK_fdset_create ();
-  GNUNET_NETWORK_fdset_set (rs, rfd);
-  ret =
-    GNUNET_SCHEDULER_add_select (GNUNET_SCHEDULER_PRIORITY_DEFAULT,
-                                GNUNET_SCHEDULER_NO_TASK, delay, rs, NULL,
-                                task, task_cls);
-  GNUNET_NETWORK_fdset_destroy (rs);
-  return ret;
-#else
-  return add_without_sets (delay, 
-                          GNUNET_SCHEDULER_PRIORITY_DEFAULT,
-                          GNUNET_NETWORK_get_fd (rfd), -1, task,
-                           task_cls);
-#endif
+
+/**
+ * Schedule a new task to be run with a specified priority and to be
+ * run after the specified delay or when the specified file descriptor
+ * is ready for reading.  The delay can be used as a timeout on the
+ * socket being ready.  The task will be scheduled for execution once
+ * either the delay has expired or the socket operation is ready.  It
+ * will be run with the DEFAULT priority.
+ *
+ * @param delay when should this operation time out?
+ * @param priority priority to use for the task
+ * @param rfd read file-descriptor
+ * @param task main function of the task
+ * @param task_cls closure of @a task
+ * @return unique task identifier for the job
+ *         only valid until @a task is started!
+ */
+struct GNUNET_SCHEDULER_Task *
+GNUNET_SCHEDULER_add_read_net_with_priority (struct GNUNET_TIME_Relative delay,
+                                            enum GNUNET_SCHEDULER_Priority priority,
+                                            struct GNUNET_NETWORK_Handle *rfd,
+                                            GNUNET_SCHEDULER_TaskCallback task,
+                                             void *task_cls)
+{
+  return GNUNET_SCHEDULER_add_net_with_priority (delay, priority,
+                                                 rfd,
+                                                 GNUNET_YES,
+                                                 GNUNET_NO,
+                                                 task, task_cls);
 }
 
 
@@ -1483,38 +1498,72 @@ GNUNET_SCHEDULER_add_read_net (struct GNUNET_TIME_Relative delay,
  * socket operation is ready.  It will be run with the priority of
  * the calling task.
  *
- * @param delay when should this operation time out? Use
- *        GNUNET_TIME_UNIT_FOREVER_REL for "on shutdown"
+ * @param delay when should this operation time out?
  * @param wfd write file-descriptor
  * @param task main function of the task
- * @param task_cls closure of task
+ * @param task_cls closure of @a task
  * @return unique task identifier for the job
- *         only valid until "task" is started!
+ *         only valid until @a task is started!
  */
-GNUNET_SCHEDULER_TaskIdentifier
+struct GNUNET_SCHEDULER_Task *
 GNUNET_SCHEDULER_add_write_net (struct GNUNET_TIME_Relative delay,
                                 struct GNUNET_NETWORK_Handle *wfd,
-                                GNUNET_SCHEDULER_Task task, void *task_cls)
+                                GNUNET_SCHEDULER_TaskCallback task,
+                                void *task_cls)
 {
-#if MINGW
-  struct GNUNET_NETWORK_FDSet *ws;
-  GNUNET_SCHEDULER_TaskIdentifier ret;
+  return GNUNET_SCHEDULER_add_net_with_priority (delay,
+                                                 GNUNET_SCHEDULER_PRIORITY_DEFAULT,
+                                                 wfd,
+                                                 GNUNET_NO, GNUNET_YES,
+                                                 task, task_cls);
+}
 
-  GNUNET_assert (wfd != NULL);
-  ws = GNUNET_NETWORK_fdset_create ();
-  GNUNET_NETWORK_fdset_set (ws, wfd);
-  ret =
-    GNUNET_SCHEDULER_add_select (GNUNET_SCHEDULER_PRIORITY_DEFAULT,
-                                GNUNET_SCHEDULER_NO_TASK, delay, NULL, ws,
-                                   task, task_cls);
-  GNUNET_NETWORK_fdset_destroy (ws);
+/**
+ * Schedule a new task to be run with a specified delay or when the
+ * specified file descriptor is ready.  The delay can be
+ * used as a timeout on the socket being ready.  The task will be
+ * scheduled for execution once either the delay has expired or the
+ * socket operation is ready.
+ *
+ * @param delay when should this operation time out?
+ * @param priority priority of the task
+ * @param fd file-descriptor
+ * @param on_read whether to poll the file-descriptor for readability
+ * @param on_write whether to poll the file-descriptor for writability
+ * @param task main function of the task
+ * @param task_cls closure of task
+ * @return unique task identifier for the job
+ *         only valid until "task" is started!
+ */
+struct GNUNET_SCHEDULER_Task *
+GNUNET_SCHEDULER_add_net_with_priority  (struct GNUNET_TIME_Relative delay,
+                                         enum GNUNET_SCHEDULER_Priority priority,
+                                         struct GNUNET_NETWORK_Handle *fd,
+                                         int on_read,
+                                         int on_write,
+                                         GNUNET_SCHEDULER_TaskCallback task,
+                                         void *task_cls)
+{
+#if MINGW
+  struct GNUNET_NETWORK_FDSet *s;
+  struct GNUNET_SCHEDULER_Task * ret;
+
+  GNUNET_assert (NULL != fd);
+  s = GNUNET_NETWORK_fdset_create ();
+  GNUNET_NETWORK_fdset_set (s, fd);
+  ret = GNUNET_SCHEDULER_add_select (
+      priority, delay,
+      on_read  ? s : NULL,
+      on_write ? s : NULL,
+      task, task_cls);
+  GNUNET_NETWORK_fdset_destroy (s);
   return ret;
 #else
-  GNUNET_assert (GNUNET_NETWORK_get_fd (wfd) >= 0);
-  return add_without_sets (delay, 
-                          GNUNET_SCHEDULER_PRIORITY_DEFAULT,
-                          -1, GNUNET_NETWORK_get_fd (wfd), task,
-                           task_cls);
+  GNUNET_assert (GNUNET_NETWORK_get_fd (fd) >= 0);
+  return add_without_sets (delay, priority,
+                           on_read  ? GNUNET_NETWORK_get_fd (fd) : -1,
+                           on_write ? GNUNET_NETWORK_get_fd (fd) : -1,
+                           task, task_cls);
 #endif
 }
 
@@ -1526,41 +1575,22 @@ GNUNET_SCHEDULER_add_write_net (struct GNUNET_TIME_Relative delay,
  * scheduled for execution once either the delay has expired or the
  * socket operation is ready. It will be run with the DEFAULT priority.
  *
- * @param delay when should this operation time out? Use
- *        GNUNET_TIME_UNIT_FOREVER_REL for "on shutdown"
+ * @param delay when should this operation time out?
  * @param rfd read file-descriptor
  * @param task main function of the task
- * @param task_cls closure of task
+ * @param task_cls closure of @a task
  * @return unique task identifier for the job
- *         only valid until "task" is started!
+ *         only valid until @a task is started!
  */
-GNUNET_SCHEDULER_TaskIdentifier
+struct GNUNET_SCHEDULER_Task *
 GNUNET_SCHEDULER_add_read_file (struct GNUNET_TIME_Relative delay,
                                 const struct GNUNET_DISK_FileHandle *rfd,
-                                GNUNET_SCHEDULER_Task task, void *task_cls)
+                                GNUNET_SCHEDULER_TaskCallback task, void *task_cls)
 {
-#if MINGW
-  struct GNUNET_NETWORK_FDSet *rs;
-  GNUNET_SCHEDULER_TaskIdentifier ret;
-
-  GNUNET_assert (rfd != NULL);
-  rs = GNUNET_NETWORK_fdset_create ();
-  GNUNET_NETWORK_fdset_handle_set (rs, rfd);
-  ret =
-      GNUNET_SCHEDULER_add_select (GNUNET_SCHEDULER_PRIORITY_DEFAULT,
-                                   GNUNET_SCHEDULER_NO_TASK, delay, rs, NULL,
-                                   task, task_cls);
-  GNUNET_NETWORK_fdset_destroy (rs);
-  return ret;
-#else
-  int fd;
-
-  GNUNET_DISK_internal_file_handle_ (rfd, &fd, sizeof (int));
-  return add_without_sets (delay, 
-                          GNUNET_SCHEDULER_PRIORITY_DEFAULT,
-                          fd, -1, task, task_cls);
-
-#endif
+  return GNUNET_SCHEDULER_add_file_with_priority (
+      delay, GNUNET_SCHEDULER_PRIORITY_DEFAULT,
+      rfd, GNUNET_YES, GNUNET_NO,
+      task, task_cls);
 }
 
 
@@ -1571,46 +1601,77 @@ GNUNET_SCHEDULER_add_read_file (struct GNUNET_TIME_Relative delay,
  * scheduled for execution once either the delay has expired or the
  * socket operation is ready. It will be run with the DEFAULT priority.
  *
- * @param delay when should this operation time out? Use
- *        GNUNET_TIME_UNIT_FOREVER_REL for "on shutdown"
+ * @param delay when should this operation time out?
  * @param wfd write file-descriptor
  * @param task main function of the task
- * @param task_cls closure of task
+ * @param task_cls closure of @a task
  * @return unique task identifier for the job
- *         only valid until "task" is started!
+ *         only valid until @a task is started!
  */
-GNUNET_SCHEDULER_TaskIdentifier
+struct GNUNET_SCHEDULER_Task *
 GNUNET_SCHEDULER_add_write_file (struct GNUNET_TIME_Relative delay,
                                  const struct GNUNET_DISK_FileHandle *wfd,
-                                 GNUNET_SCHEDULER_Task task, void *task_cls)
+                                 GNUNET_SCHEDULER_TaskCallback task, void *task_cls)
 {
-#if MINGW
-  struct GNUNET_NETWORK_FDSet *ws;
-  GNUNET_SCHEDULER_TaskIdentifier ret;
+  return GNUNET_SCHEDULER_add_file_with_priority (
+      delay, GNUNET_SCHEDULER_PRIORITY_DEFAULT,
+      wfd, GNUNET_NO, GNUNET_YES,
+      task, task_cls);
+}
 
-  GNUNET_assert (wfd != NULL);
-  ws = GNUNET_NETWORK_fdset_create ();
-  GNUNET_NETWORK_fdset_handle_set (ws, wfd);
-  ret =
-      GNUNET_SCHEDULER_add_select (GNUNET_SCHEDULER_PRIORITY_DEFAULT,
-                                   GNUNET_SCHEDULER_NO_TASK, delay, NULL, ws,
-                                   task, task_cls);
-  GNUNET_NETWORK_fdset_destroy (ws);
+
+/**
+ * Schedule a new task to be run with a specified delay or when the
+ * specified file descriptor is ready.  The delay can be
+ * used as a timeout on the socket being ready.  The task will be
+ * scheduled for execution once either the delay has expired or the
+ * socket operation is ready.
+ *
+ * @param delay when should this operation time out?
+ * @param priority priority of the task
+ * @param fd file-descriptor
+ * @param on_read whether to poll the file-descriptor for readability
+ * @param on_write whether to poll the file-descriptor for writability
+ * @param task main function of the task
+ * @param task_cls closure of @a task
+ * @return unique task identifier for the job
+ *         only valid until @a task is started!
+ */
+struct GNUNET_SCHEDULER_Task *
+GNUNET_SCHEDULER_add_file_with_priority (struct GNUNET_TIME_Relative delay,
+                                         enum GNUNET_SCHEDULER_Priority priority,
+                                         const struct GNUNET_DISK_FileHandle *fd,
+                                         int on_read, int on_write,
+                                         GNUNET_SCHEDULER_TaskCallback task, void *task_cls)
+{
+#if MINGW
+  struct GNUNET_NETWORK_FDSet *s;
+  struct GNUNET_SCHEDULER_Task * ret;
+
+  GNUNET_assert (NULL != fd);
+  s = GNUNET_NETWORK_fdset_create ();
+  GNUNET_NETWORK_fdset_handle_set (s, fd);
+  ret = GNUNET_SCHEDULER_add_select (
+      priority, delay,
+      on_read  ? s : NULL,
+      on_write ? s : NULL,
+      task, task_cls);
+  GNUNET_NETWORK_fdset_destroy (s);
   return ret;
 #else
-  int fd;
-
-  GNUNET_DISK_internal_file_handle_ (wfd, &fd, sizeof (int));
-  GNUNET_assert (fd >= 0);
-  return add_without_sets (delay, 
-                          GNUNET_SCHEDULER_PRIORITY_DEFAULT,
-                          -1, fd, task, task_cls);
-
+  int real_fd;
+
+  GNUNET_DISK_internal_file_handle_ (fd, &real_fd, sizeof (int));
+  GNUNET_assert (real_fd >= 0);
+  return add_without_sets (
+      delay, priority,
+      on_read  ? real_fd : -1,
+      on_write ? real_fd : -1,
+      task, task_cls);
 #endif
 }
 
 
-
 /**
  * Schedule a new task to be run with a specified delay or when any of
  * the specified file descriptor sets is ready.  The delay can be used
@@ -1624,87 +1685,69 @@ GNUNET_SCHEDULER_add_write_file (struct GNUNET_TIME_Relative delay,
  * (prerequisite-run)
  * && (delay-ready
  *     || any-rs-ready
- *     || any-ws-ready
- *     || (shutdown-active && run-on-shutdown) )
+ *     || any-ws-ready) )
  * </code>
  *
  * @param prio how important is this task?
- * @param prerequisite_task run this task after the task with the given
- *        task identifier completes (and any of our other
- *        conditions, such as delay, read or write-readiness
- *        are satisfied).  Use GNUNET_SCHEDULER_NO_TASK to not have any dependency
- *        on completion of other tasks.
- * @param delay how long should we wait? Use GNUNET_TIME_UNIT_FOREVER_REL for "forever",
- *        which means that the task will only be run after we receive SIGTERM
+ * @param delay how long should we wait?
  * @param rs set of file descriptors we want to read (can be NULL)
  * @param ws set of file descriptors we want to write (can be NULL)
  * @param task main function of the task
- * @param task_cls closure of task
+ * @param task_cls closure of @a task
  * @return unique task identifier for the job
- *         only valid until "task" is started!
+ *         only valid until @a task is started!
  */
-GNUNET_SCHEDULER_TaskIdentifier
+struct GNUNET_SCHEDULER_Task *
 GNUNET_SCHEDULER_add_select (enum GNUNET_SCHEDULER_Priority prio,
-                             GNUNET_SCHEDULER_TaskIdentifier prerequisite_task,
                              struct GNUNET_TIME_Relative delay,
                              const struct GNUNET_NETWORK_FDSet *rs,
                              const struct GNUNET_NETWORK_FDSet *ws,
-                             GNUNET_SCHEDULER_Task task, void *task_cls)
+                             GNUNET_SCHEDULER_TaskCallback task,
+                             void *task_cls)
 {
-  struct Task *t;
-
-#if EXECINFO
-  void *backtrace_array[MAX_TRACE_DEPTH];
-#endif
-
-  GNUNET_assert (active_task != NULL);
+  struct GNUNET_SCHEDULER_Task *t;
+
+  if ( (NULL == rs) &&
+       (NULL == ws) )
+    return GNUNET_SCHEDULER_add_delayed_with_priority (delay,
+                                                       prio,
+                                                       task,
+                                                       task_cls);
+  GNUNET_assert (NULL != active_task);
   GNUNET_assert (NULL != task);
-  t = GNUNET_malloc (sizeof (struct Task));
+  t = GNUNET_new (struct GNUNET_SCHEDULER_Task);
   t->callback = task;
   t->callback_cls = task_cls;
-#if EXECINFO
-  t->num_backtrace_strings = backtrace (backtrace_array, MAX_TRACE_DEPTH);
-  t->backtrace_strings =
-      backtrace_symbols (backtrace_array, t->num_backtrace_strings);
-#endif
   t->read_fd = -1;
   t->write_fd = -1;
-  if (rs != NULL)
+  if (NULL != rs)
   {
     t->read_set = GNUNET_NETWORK_fdset_create ();
     GNUNET_NETWORK_fdset_copy (t->read_set, rs);
   }
-  if (ws != NULL)
+  if (NULL != ws)
   {
     t->write_set = GNUNET_NETWORK_fdset_create ();
     GNUNET_NETWORK_fdset_copy (t->write_set, ws);
   }
-  t->id = ++last_id;
 #if PROFILE_DELAYS
   t->start_time = GNUNET_TIME_absolute_get ();
 #endif
-  t->prereq_id = prerequisite_task;
   t->timeout = GNUNET_TIME_relative_to_absolute (delay);
   t->priority =
       check_priority ((prio ==
                        GNUNET_SCHEDULER_PRIORITY_KEEP) ? current_priority :
                       prio);
   t->lifeness = current_lifeness;
-  t->next = pending;
-  pending = t;
+  GNUNET_CONTAINER_DLL_insert (pending_head,
+                               pending_tail,
+                               t);
   max_priority_added = GNUNET_MAX (max_priority_added, t->priority);
-#if DEBUG_TASKS
-  LOG (GNUNET_ERROR_TYPE_DEBUG, "Adding task: %llu / %p\n", t->id,
-       t->callback_cls);
-#endif
-#if EXECINFO
-  int i;
-
-  for (i = 0; i < t->num_backtrace_strings; i++)
-    LOG (GNUNET_ERROR_TYPE_DEBUG, "Task %llu trace %d: %s\n", t->id, i,
-         t->backtrace_strings[i]);
-#endif
-  return t->id;
+  LOG (GNUNET_ERROR_TYPE_DEBUG,
+       "Adding task %p\n",
+       t);
+  init_backtrace (t);
+  return t;
 }
 
 /* end of scheduler.c */