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, 2 insertions, 1 deletions

diff --git a/posix/Makefile b/posix/Makefile
index 3bd7d605df..e91ea25ba1 100644
--- a/posix/Makefile
+++ b/posix/Makefile
@@ -130,7 +130,7 @@ test-srcs	:= globtest
 tests           += wordexp-test tst-exec tst-spawn tst-spawn2 tst-spawn3
 endif
 ifeq (yesyes,$(build-shared)$(have-thread-library))
-tests		+= tst-getopt-cancel
+tests		+= tst-getopt-cancel tst-_Fork
 endif
 tests-static	= tst-exec-static tst-spawn-static
 tests		+= $(tests-static)
@@ -299,6 +299,7 @@ $(objpfx)ptestcases.h: PTESTS PTESTS2C.sed
 $(objpfx)runptests.o: $(objpfx)ptestcases.h
 
 $(objpfx)tst-getopt-cancel: $(shared-thread-library)
+$(objpfx)tst-_Fork: $(shared-thread-library)
 
 test-xfail-annexc = yes
 $(objpfx)annexc.out: $(objpfx)annexc