uloop: Fix race condition in SIGCHLD handling

When uloop_process_add() is called outside of uloop_run(), i.e. not
from a callback (which is the case of at least utrace and ujail),
child events can be missed. The reason is that when SIGCHILD handler
is installed in uloop_run(), after the uloop_process_add() is called,
then an initial signal could be missed.

Commit 4e3a47a ("uloop: use a waker for notifying sigchld and loop
cancel events", 2016-06-09) solved a similar problem and introduced
uloop_init() but forgot to move a call to uloop_setup_signals() there.
This is what this commit does.

Now, uloop_process_add() can be called any time after uloop_init()
without missing any event.

Signed-off-by: Michal Sojka <sojkam1@fel.cvut.cz>
Acked-by: Yousong Zhou <yszhou4tech@gmail.com>

diff --git a/uloop.c b/uloop.c
index d2f41bbfd1c8cc995f5416f287597774341a6190..3813e188522d87c03877072ea820a48f5526eafe 100644 (file)
--- a/uloop.c
+++ b/uloop.c
@@ -116,6 +116,8 @@ static int waker_init(void)
        return 0;
 }
 
+static void uloop_setup_signals(bool add);
+
 int uloop_init(void)
 {
        if (uloop_init_pollfd() < 0)
@@ -126,6 +128,8 @@ int uloop_init(void)
                return -1;
        }
 
+       uloop_setup_signals(true);
+
        return 0;
 }
 
@@ -525,12 +529,7 @@ int uloop_run_timeout(int timeout)
        int next_time = 0;
        struct timeval tv;
 
-       /*
-        * Handlers are only updated for the first call to uloop_run() (and restored
-        * when this call is done).
-        */
-       if (!uloop_run_depth++)
-               uloop_setup_signals(true);
+       uloop_run_depth++;
 
        uloop_status = 0;
        uloop_cancelled = false;
@@ -553,14 +552,15 @@ int uloop_run_timeout(int timeout)
                uloop_run_events(next_time);
        }
 
-       if (!--uloop_run_depth)
-               uloop_setup_signals(false);
+       --uloop_run_depth;
 
        return uloop_status;
 }
 
 void uloop_done(void)
 {
+       uloop_setup_signals(false);
+
        if (poll_fd >= 0) {
                close(poll_fd);
                poll_fd = -1;