Avoid marking pthread_atfork() as weak if provided by libc_nonshared.a.

On Linux, pthread_atfork() is provided not by libpthread.so, but by
libc_nonshared.a (for glibc >= 2.28) or by libpthread_nonshared.a
(for glibc <= 2.27), and using "#pragma weak pthread_atfork" will
cause pthread_atfork to be undefined (and have an address of NULL)
even if the relevant lib*_nonshared.a was explicitly specified to
the linker.  This is explained by this paragraph from Chapter 4,
section "Symbol Table", of the System V ABI specification, as pointed
out by Hugo Grostabussiat:

	When the link editor searches archive libraries [see ``Archive
	File'' in Chapter 7], it extracts archive members that contain
	definitions of undefined global symbols. The member's definition
	may be either a global or a weak symbol. The link editor does
	not extract archive members to resolve undefined weak symbols.
	Unresolved weak symbols have a zero value.

Therefore, if either of these lib*_nonshared.a libraries exists, we
should avoid marking pthread_atfork as weak.

We were doing this correctly for glibc <= 2.27, as we were checking
for libpthread_nonshared.a, but not for libc_nonshared.a, so we would
still mark pthread_atfork as a weak symbol on machines that provide
libc_nonshared.a but not libpthread_nonshared.a, that is, on glibc
>= 2.28 machines without a dummy backward compatibility version of
libpthread_nonshared.a installed.

This patch makes us do the right thing for glibc >= 2.28 as well, and
was derived from patches provided by Rolf Eike Beer <[email protected]>
and Antal Nemes <[email protected]>.

Thanks to Hugo Grostabussiat <[email protected]>, Henrik Grindal
Bakken <[email protected]> and Tomáš Mózes <[email protected]> for the
discussion and testing.

Reported-by: Changqing Li <[email protected]>
Signed-off-by: Lennert Buytenhek <[email protected]>

configure.ac

@@ -78,15 +78,31 @@ solaris*)
 esac
 
 #
-# On Linux, linking against libpthread.so.0 doesn't give you
-# pthread_atfork(), as the latter is defined in libpthread_nonshared.a,
-# which doesn't get pulled in if libpthread.so.0 is pulled in
-# indirectly, e.g. via -lrt, but does get pulled in if you link against
-# -lpthread, as libpthread.so is a linker script that references both
-# libpthread.so.0 and libpthread_nonshared.a.  As we can link in
-# libpthread_nonshared.a without pulling in libpthread.so.0, explicitly
-# link in libpthread_nonshared.a if it is available.
+# On Linux, pthread_atfork() is provided not by libpthread.so, but by
+# libc_nonshared.a (for glibc >= 2.28) or by libpthread_nonshared.a
+# (for glibc <= 2.27), and using "#pragma weak pthread_atfork" will
+# cause pthread_atfork to be undefined (and have an address of NULL)
+# even if the relevant lib*_nonshared.a was explicitly specified to
+# the linker.  This is explained by this paragraph from Chapter 4,
+# section "Symbol Table", of the System V ABI specification, as pointed
+# out by Hugo Grostabussiat:
 #
+#	When the link editor searches archive libraries [see ``Archive
+#	File'' in Chapter 7], it extracts archive members that contain
+#	definitions of undefined global symbols. The member's definition
+#	may be either a global or a weak symbol. The link editor does
+#	not extract archive members to resolve undefined weak symbols.
+#	Unresolved weak symbols have a zero value.
+#
+# Therefore, if either of these lib*_nonshared.a libraries exists, we
+# need to make note of this, to avoid marking pthread_atfork as weak.
+# Also, if pthread_atfork is provided by libpthread_nonshared.a, we'll
+# need to pull that library in explicitly, as we are not necessarily
+# linked against libpthread.so.0.
+#
+AC_CHECK_LIB([c_nonshared], [pthread_atfork],
+	     [AC_DEFINE(HAVE_LIBC_NONSHARED, 1,
+	      Define to 1 if you have the `c_nonshared' library.)])
 AC_CHECK_LIB([pthread_nonshared], [pthread_atfork])
 
 # Checks for header files.

src/pthr.h

@@ -37,12 +37,12 @@ static inline int pthreads_available(void)
 #ifdef HAVE_PRAGMA_WEAK
 
 /*
- * On Linux, pthread_atfork() is defined in libpthread_nonshared.a,
- * a static library, and we want to avoid "#pragma weak" for that
- * symbol because that causes it to be undefined even if you link
- * libpthread_nonshared.a in explicitly.
+ * On Linux, pthread_atfork() is defined in libc_nonshared.a (for
+ * glibc >= 2.28) or libpthread_nonshared.a (for glibc <= 2.27), and
+ * we want to avoid "#pragma weak" for that symbol because that causes
+ * it to be undefined even if you link lib*_nonshared.a in explicitly.
  */
-#ifndef HAVE_LIBPTHREAD_NONSHARED
+#if !defined(HAVE_LIBC_NONSHARED) && !defined(HAVE_LIBPTHREAD_NONSHARED)
 #pragma weak pthread_atfork
 #endif