[PATCH] Provide pthread_atfork in libc_nonshared.a and libc.a.

["Carlos O'Donell" <carlos at redhat dot com>]

Community,

The standard design pattern for making it optional to link against
libpthread is to mark the function weak, test if the function
address is non-zero and call the function, otherwise use a fallback.
The problem with pthread_atfork is that there is no viable
fallback.  If you need to do something during fork it has to be done
via pthread_atfork.  This makes having libpthread optional and using
pthread_atfork impossible.  We make it possible by providing
pthread_atfork in libc_nonshared.a.  The real work of pthread_atfork
is done by __register_atfork which is already provided in
libc_nonshared.a.  It's included in libc_nonshared.a because
__dso_handle has to be unique to each DSO such that unloading the DSO
can unregister the atfork handlers.  We build pthread_atfork again
under a different file name and include it into libc_nonshared.a and
libc.a. We keep pthread_atfork in libpthread_nonshared.a and
libpthread.a for compatibility and completeness.

Applications that can't rely on a new glibc should use the following
code to optionally include libpthread and still register a function
via pthread_atfork i.e. use __register_atfork directly:

extern void *__dso_handle __attribute__ ((__weak__, __visibility__ ("hidden")));
extern int __register_atfork (void (*) (void), void (*) (void), void (*) (void), void *);

static int __app_atfork (void (*prepare) (void), void (*parent) (void), void (*child) (void))
{
  return __register_atfork (prepare, parent, child,
			    &__dso_handle == NULL ? NULL : __dso_handle);
}

This code requires glibc 2.3.2 or newer. Previous to 2.3.2 no such
interfaces exist and at that point is is impossible to have an
optional libpthread and call pthread_atfork.

This code adds no more ABI requirements than already exist since
__dso_handle and __register_atfork are already part of the ABI.

Tested on x86-64 with no regressions.

There are no versioning requirements since this is all static libraries.

Verified pthread_atfork is available in libc_nonshared.a and libc.a
afterwards and that it's available for linking without libpthread.

What kind of testcase, if any, should go along with this?

Before patch:
[carlos@koi glibc]$ gcc -o /home/carlos/testatfork /home/carlos/testatfork.c 
/tmp/ccmX2uMU.o: In function `main':
testatfork.c:(.text+0x24): undefined reference to `pthread_atfork'
collect2: error: ld returned 1 exit status

After patch the compilation and execution succeeds and the linker map shows:
/home/carlos/install-glibc/usr/lib64/libc_nonshared.a(libc_pthread_atfork.oS)
                              ./testatfork.o (pthread_atfork)
...
 .text          0x00000000004006a0       0x1c /home/carlos/install-glibc/usr/lib64/libc_nonshared.a(libc_pthread_atfork.oS)
                0x00000000004006a0                __pthread_atfork
                0x00000000004006a0                pthread_atfork
...

Comments?

nptl/

2013-10-02  Carlos O'Donell  <carlos@redhat.com>

	* sysdeps/unix/sysv/linux/Makefile (sysdep_routines): Add libc_pthread_atfork.
	(static-only-routines): Likewise.
	* sysdeps/unix/sysv/linux/libc_pthread_atfork.c: New file.

diff --git a/nptl/sysdeps/unix/sysv/linux/Makefile b/nptl/sysdeps/unix/sysv/linux/Makefile
index 6078e2d..36fd50b 100644
--- a/nptl/sysdeps/unix/sysv/linux/Makefile
+++ b/nptl/sysdeps/unix/sysv/linux/Makefile
@@ -18,7 +18,9 @@
 
 ifeq ($(subdir),nptl)
 sysdep_routines += register-atfork unregister-atfork libc_pthread_init \
-		   libc_multiple_threads
+		   libc_multiple_threads libc_pthread_atfork
+
+static-only-routines += libc_pthread_atfork
 
 libpthread-sysdep_routines += pt-fork pthread_mutex_cond_lock
 
diff --git a/nptl/sysdeps/unix/sysv/linux/libc_pthread_atfork.c b/nptl/sysdeps/unix/sysv/linux/libc_pthread_atfork.c
new file mode 100644
index 0000000..667049a
--- /dev/null
+++ b/nptl/sysdeps/unix/sysv/linux/libc_pthread_atfork.c
@@ -0,0 +1,54 @@
+/* Copyright (C) 2013 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+   Contributed by Ulrich Drepper <drepper@redhat.com>, 2002.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+/* The standard design pattern for making it optional to link against
+   libpthread is to mark the function weak, test if the function
+   address is non-zero and call the function, otherwise use a fallback.
+   The problem with pthread_atfork is that there is no viable
+   fallback.  If you need to do something during fork it has to be done
+   via pthread_atfork.  This makes having libpthread optional and using
+   pthread_atfork impossible.  We make it possible by providing
+   pthread_atfork in libc_nonshared.a.  The real work of pthread_atfork
+   is done by __register_atfork which is already provided in
+   libc_nonshared.a.  It's included in libc_nonshared.a because
+   __dso_handle has to be unique to each DSO such that unloading the DSO
+   can unregister the atfork handlers.  We build pthread_atfork again
+   under a different file name and include it into libc_nonshared.a and
+   libc.a. We keep pthread_atfork in libpthread_nonshared.a and
+   libpthread.a for compatibility and completeness.
+
+   Applications that can't rely on a new glibc should use the following
+   code to optionally include libpthread and still register a function
+   via pthread_atfork i.e. use __register_atfork directly:
+
+   extern void *__dso_handle __attribute__ ((__weak__, __visibility__ ("hidden")));
+   extern int __register_atfork (void (*) (void), void (*) (void), void (*) (void), void *);
+
+   static int __app_atfork (void (*prepare) (void), void (*parent) (void), void (*child) (void))
+     {
+       return __register_atfork (prepare, parent, child,
+				 &__dso_handle == NULL ? NULL : __dso_handle);
+     }
+
+   This code requires glibc 2.3.2 or newer. Previous to 2.3.2 no such
+   interfaces exist and at that point is is impossible to have an
+   optional libpthread and call pthread_atfork.
+
+   This code adds no more ABI requirements than already exist since
+   __dso_handle and __register_atfork are already part of the ABI.  */
+#include <pthread_atfork.c>
---

Cheers,
Carlos.