Our version was different from the upstream version in that we tried to use
all possible pthread-related flags that the compiler accepts, rather than
just the first one that works. That change was made in commit
e48322a6d6cfce1ec52ab303441df329ddbc04d1, to work-around a bug affecting GCC
versions 3.2 and below (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=8888),
although we didn't realize that it was a GCC bug at the time. We hardly care
about that old GCC versions anymore, so we no longer need that workaround.

This fixes the macro for compilers that print warnings with the chosen
flags. That's pretty annoying on its own right, but it also inconspicuously
disabled thread-safety, because we refused to use any pthread-related flags
if the compiler produced warnings. Max Filippov reported that problem when
linking with uClibc and OpenSSL. The warnings-check was added because the
workaround for the GCC bug caused warnings otherwise, so it's no longer
needed either. We can just use the upstream version as is.

If you really want to compile with GCC version 3.2 or older, you can still
work-around it manually by setting PTHREAD_CFLAGS="-pthread -lpthread"
manually on the configure command line.

Backpatch to 9.5. I don't want to unnecessarily rock the boat on stable
branches, but 9.5 seems like fair game.

AC_TRY_COMPILE(...) -> AC_COMPILE_IFELSE([AC_LANG_PROGRAM(...)])
AC_TRY_LINK(...) -> AC_LINK_IFELSE([AC_LANG_PROGRAM(...)])
AC_TRY_RUN(...) -> AC_RUN_IFELSE([AC_LANG_PROGRAM(...)])
AC_LANG_SAVE/RESTORE -> AC_LANG_PUSH/POP
AC_DECL_SYS_SIGLIST -> AC_CHECK_DECLS(...) (per snippet in autoconf manual)

Also use AC_LANG_SOURCE instead of AC_LANG_PROGRAM, where the main()
function is not needed.

With these changes, autoconf -Wall doesn't complain anymore.

Andreas Karlsson

Almost ten years ago, commit e48322a6d6cfce1ec52ab303441df329ddbc04d1 broke
the logic in ACX_PTHREAD by looping through all the possible flags rather
than stopping with the first one that would work.  This meant that
$acx_pthread_ok was no longer meaningful after the loop; it would usually
be "no", whether or not we'd found working thread flags.  The reason nobody
noticed is that Postgres doesn't actually use any of the symbols set up
by the code after the loop.  Rather than complicate things some more to
make it work as designed, let's just remove all that dead code, and thereby
save a few cycles in each configure run.

When testing the stderr produced by various thread-support flags, also
run a compilation in addition to a link, because clang warns on
certain flags when compiling but not when linking.

will use pthreadGC2.

Dave Page

compiler emits any warnings, the test program had better be 100%
correct, not only 90% correct.  The recent addition of -Wold-style-definition
broke thread-safety detection on every platform that has that switch,
because the test program used an old-style definition.

than /dev/null, which Win32 doesn't have.