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, 7 insertions, 0 deletions

diff --git a/posix/unistd.h b/posix/unistd.h
index d9d8929f71..217c6c5363 100644
--- a/posix/unistd.h
+++ b/posix/unistd.h
@@ -781,6 +781,13 @@ extern __pid_t fork (void) __THROWNL;
 extern __pid_t vfork (void) __THROW;
 #endif /* Use misc or XPG < 7. */
 
+#ifdef __USE_GNU
+/* This is similar to fork, however it does not run the atfork handlers
+   neither reinitialize any internal locks in multithread case.
+   Different than fork, _Fork is async-signal-safe.  */
+extern __pid_t _Fork (void) __THROW;
+#endif
+
 
 /* Return the pathname of the terminal FD is open on, or NULL on errors.
    The returned storage is good only until the next call to this function.  */