posix: Add _Fork [BZ #4737]

Austin Group issue 62 [1] dropped the async-signal-safe requirement
for fork and provided a async-signal-safe _Fork replacement that
does not run the atfork handlers.  It will be included in the next
POSIX standard.

It allow to close a long standing issue to make fork AS-safe (BZ#4737).
As indicated on the bug, besides the internal lock for the atfork
handlers itself; there is no guarantee that the handlers itself will
not introduce more AS-safe issues.

The idea is synchronize fork with the required internal locks to allow
children in multithread processes to use mostly of standard function
(even though POSIX states only AS-safe function should be used).  On
signal handles, _Fork should be used intead and only AS-safe functions
should be used.

For testing, the new tst-_Fork only check basic usage.  I also added
a new tst-mallocfork3 which uses the same strategy to check for
deadlock of tst-mallocfork2 but using threads instead of subprocesses
(and it does deadlock if it replaces _Fork with fork).

[1] https://austingroupbugs.net/view.php?id=62

1 files changed, 8 insertions, 0 deletions

diff --git a/NEWS b/NEWS
index 6931135..60933bd 100644
--- a/NEWS
+++ b/NEWS
@@ -52,6 +52,14 @@ Major new features:
 * On Linux, a new tunable, glibc.pthread.stack_cache_size, can be used
   to configure the size of the thread stack cache.
 
+* The function _Fork has been added as an async-signal-safe fork replacement
+  since Austin Group issue 62 droped the async-signal-safe requirement for
+  fork (and it will be included in the future POSIX standard).  The new _Fork
+  function does not run any atfork function neither resets any internal state
+  or lock (such as the malloc one), and only sets up a minimal state required
+  to call async-signal-safe functions (such as raise or execve).  This function
+  is currently a GNU extension.
+
 Deprecated and removed features, and other changes affecting compatibility:
 
 * The function pthread_mutex_consistent_np has been deprecated; programs