Add some internal documentation for some thread related functions

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/9186)

diff --git a/doc/internal/man3/ossl_init_thread_deregister.pod b/doc/internal/man3/ossl_init_thread_deregister.pod
new file mode 100644 (file)
index 0000000..4481d0c
--- /dev/null
+++ b/doc/internal/man3/ossl_init_thread_deregister.pod
@@ -0,0 +1,64 @@
+=pod
+
+=head1 NAME
+OSSL_thread_stop_handler_fn,
+ossl_init_thread_start,
+ossl_init_thread_deregister
+- internal thread routines
+=head1 SYNOPSIS
+
+ #include "internal/cryptlib_int.h"
+ #include <openssl/core.h>
+
+ typedef void (*OSSL_thread_stop_handler_fn)(void *arg);
+
+ int ossl_init_thread_start(const void *index, void *arg,
+                            OSSL_thread_stop_handler_fn handfn);
+ int ossl_init_thread_deregister(void *index);
+
+=head1 DESCRIPTION
+
+Thread aware code may be informed about when a thread is stopping, typically to
+perform some cleanup operation.
+Thread stop events may be detected by OpenSSL either automatically (using the
+capabilities of the underlying threading library) where possible or explicitly
+by the application calling OPENSSL_thread_stop() or OPENSSL_thread_stop_ex().
+
+Thread aware code registers a "stop handler" for each new thread that it uses.
+Typically, when a new thread is being used, code will add a new value to some
+thread local variable and then register a stop handler. When the thread is
+stopping the stop handler is called (while on that thread) and the code can
+clean up the value stored in the thread local variable.
+
+A new stop handler is registerd using the function ossl_init_thread_start().
+The B<index> parameter should be a unique value that can be used to identify a
+set of common stop handlers and is passed in a later call to
+ossl_init_thread_deregister. If no later call to ossl_init_thread_deregister is
+made then NULL can be passed for this parameter. The B<arg> parameter is passed
+back as an argument to the stop handler when it is later invoked. Finally the
+B<handfn> is a function pointer to the stop handler itself.
+
+In the event that previously registered stop handlers need to be deregistered
+then this can be done using the function ossl_init_thread_deregister().
+This will deregister all stop handlers (no matter which thread they were
+registered for) which the same B<index> value.
+
+=head1 RETURN VALUES
+
+ossl_init_thread_start() and ossl_init_thread_deregister() return 1 for success
+or 0 on error.
+
+=head1 HISTORY
+
+The functions described here were all added in OpenSSL 3.0.
+
+=head1 COPYRIGHT
+
+Copyright 2019 The OpenSSL Project Authors. All Rights Reserved.
+
+Licensed under the Apache License 2.0 (the "License").  You may not use
+this file except in compliance with the License.  You can obtain a copy
+in the file LICENSE in the source distribution or at
+L<https://www.openssl.org/source/license.html>.
+
+=cut