Use unnamed POSIX semaphores, if available, on Linux and FreeBSD.

We've had support for using unnamed POSIX semaphores instead of System V
semaphores for quite some time, but it was not used by default on any
platform.  Since many systems have rather small limits on the number of
SysV semaphores allowed, it seems desirable to switch to POSIX semaphores
where they're available and don't create performance or kernel resource
problems.  Experimentation by me shows that unnamed POSIX semaphores
are at least as good as SysV semaphores on Linux, and we previously had
a report from Maksym Sobolyev that FreeBSD is significantly worse with
SysV semaphores than POSIX ones.  So adjust those two platforms to use
unnamed POSIX semaphores, if configure can find the necessary library
functions.  If this goes well, we may switch other platforms as well,
but it would be advisable to test them individually first.

It's not currently contemplated that we'd encourage users to select
a semaphore API for themselves, but anyone who wants to experiment
can add PREFERRED_SEMAPHORES=UNNAMED_POSIX (or NAMED_POSIX, or SYSV)
to their configure command line to do so.

I also tweaked configure to report which API it's selected, mainly
so that we can tell that from buildfarm reports.

I did not touch the user documentation's discussion about semaphores;
that will need some adjustment once the dust settles.

Discussion: <8536.1475704230@sss.pgh.pa.us>

configure

@@ -14855,23 +14855,148 @@ fi
 
 # Select semaphore implementation type.
 if test "$PORTNAME" != "win32"; then
+  if test x"$PREFERRED_SEMAPHORES" = x"NAMED_POSIX" ; then
+    # Need sem_open for this
+    { $as_echo "$as_me:${as_lineno-$LINENO}: checking for library containing sem_open" >&5
+$as_echo_n "checking for library containing sem_open... " >&6; }
+if ${ac_cv_search_sem_open+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  ac_func_search_save_LIBS=$LIBS
+cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+
+/* Override any GCC internal prototype to avoid an error.
+   Use char because int might match the return type of a GCC
+   builtin and then its argument prototype would still apply.  */
+#ifdef __cplusplus
+extern "C"
+#endif
+char sem_open ();
+int
+main ()
+{
+return sem_open ();
+  ;
+  return 0;
+}
+_ACEOF
+for ac_lib in '' rt pthread; do
+  if test -z "$ac_lib"; then
+    ac_res="none required"
+  else
+    ac_res=-l$ac_lib
+    LIBS="-l$ac_lib  $ac_func_search_save_LIBS"
+  fi
+  if ac_fn_c_try_link "$LINENO"; then :
+  ac_cv_search_sem_open=$ac_res
+fi
+rm -f core conftest.err conftest.$ac_objext \
+    conftest$ac_exeext
+  if ${ac_cv_search_sem_open+:} false; then :
+  break
+fi
+done
+if ${ac_cv_search_sem_open+:} false; then :
+
+else
+  ac_cv_search_sem_open=no
+fi
+rm conftest.$ac_ext
+LIBS=$ac_func_search_save_LIBS
+fi
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $ac_cv_search_sem_open" >&5
+$as_echo "$ac_cv_search_sem_open" >&6; }
+ac_res=$ac_cv_search_sem_open
+if test "$ac_res" != no; then :
+  test "$ac_res" = "none required" || LIBS="$ac_res $LIBS"
+  USE_NAMED_POSIX_SEMAPHORES=1
+fi
+
+  fi
+  if test x"$PREFERRED_SEMAPHORES" = x"UNNAMED_POSIX" ; then
+    # Need sem_init for this
+    { $as_echo "$as_me:${as_lineno-$LINENO}: checking for library containing sem_init" >&5
+$as_echo_n "checking for library containing sem_init... " >&6; }
+if ${ac_cv_search_sem_init+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  ac_func_search_save_LIBS=$LIBS
+cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+
+/* Override any GCC internal prototype to avoid an error.
+   Use char because int might match the return type of a GCC
+   builtin and then its argument prototype would still apply.  */
+#ifdef __cplusplus
+extern "C"
+#endif
+char sem_init ();
+int
+main ()
+{
+return sem_init ();
+  ;
+  return 0;
+}
+_ACEOF
+for ac_lib in '' rt pthread; do
+  if test -z "$ac_lib"; then
+    ac_res="none required"
+  else
+    ac_res=-l$ac_lib
+    LIBS="-l$ac_lib  $ac_func_search_save_LIBS"
+  fi
+  if ac_fn_c_try_link "$LINENO"; then :
+  ac_cv_search_sem_init=$ac_res
+fi
+rm -f core conftest.err conftest.$ac_objext \
+    conftest$ac_exeext
+  if ${ac_cv_search_sem_init+:} false; then :
+  break
+fi
+done
+if ${ac_cv_search_sem_init+:} false; then :
+
+else
+  ac_cv_search_sem_init=no
+fi
+rm conftest.$ac_ext
+LIBS=$ac_func_search_save_LIBS
+fi
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $ac_cv_search_sem_init" >&5
+$as_echo "$ac_cv_search_sem_init" >&6; }
+ac_res=$ac_cv_search_sem_init
+if test "$ac_res" != no; then :
+  test "$ac_res" = "none required" || LIBS="$ac_res $LIBS"
+  USE_UNNAMED_POSIX_SEMAPHORES=1
+fi
+
+  fi
+  { $as_echo "$as_me:${as_lineno-$LINENO}: checking which semaphore API to use" >&5
+$as_echo_n "checking which semaphore API to use... " >&6; }
   if test x"$USE_NAMED_POSIX_SEMAPHORES" = x"1" ; then
 
 $as_echo "#define USE_NAMED_POSIX_SEMAPHORES 1" >>confdefs.h
 
     SEMA_IMPLEMENTATION="src/backend/port/posix_sema.c"
+    sematype="named POSIX"
   else
     if test x"$USE_UNNAMED_POSIX_SEMAPHORES" = x"1" ; then
 
 $as_echo "#define USE_UNNAMED_POSIX_SEMAPHORES 1" >>confdefs.h
 
       SEMA_IMPLEMENTATION="src/backend/port/posix_sema.c"
+      sematype="unnamed POSIX"
     else
 
 $as_echo "#define USE_SYSV_SEMAPHORES 1" >>confdefs.h
 
       SEMA_IMPLEMENTATION="src/backend/port/sysv_sema.c"
+      sematype="System V"
     fi
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: $sematype" >&5
+$as_echo "$sematype" >&6; }
   fi
 else
 

configure.in

@@ -1939,17 +1939,30 @@ AC_SUBST(PG_CRC32C_OBJS)
 
 # Select semaphore implementation type.
 if test "$PORTNAME" != "win32"; then
+  if test x"$PREFERRED_SEMAPHORES" = x"NAMED_POSIX" ; then
+    # Need sem_open for this
+    AC_SEARCH_LIBS(sem_open, [rt pthread], [USE_NAMED_POSIX_SEMAPHORES=1])
+  fi
+  if test x"$PREFERRED_SEMAPHORES" = x"UNNAMED_POSIX" ; then
+    # Need sem_init for this
+    AC_SEARCH_LIBS(sem_init, [rt pthread], [USE_UNNAMED_POSIX_SEMAPHORES=1])
+  fi
+  AC_MSG_CHECKING([which semaphore API to use])
   if test x"$USE_NAMED_POSIX_SEMAPHORES" = x"1" ; then
     AC_DEFINE(USE_NAMED_POSIX_SEMAPHORES, 1, [Define to select named POSIX semaphores.])
     SEMA_IMPLEMENTATION="src/backend/port/posix_sema.c"
+    sematype="named POSIX"
   else
     if test x"$USE_UNNAMED_POSIX_SEMAPHORES" = x"1" ; then
       AC_DEFINE(USE_UNNAMED_POSIX_SEMAPHORES, 1, [Define to select unnamed POSIX semaphores.])
       SEMA_IMPLEMENTATION="src/backend/port/posix_sema.c"
+      sematype="unnamed POSIX"
     else
       AC_DEFINE(USE_SYSV_SEMAPHORES, 1, [Define to select SysV-style semaphores.])
       SEMA_IMPLEMENTATION="src/backend/port/sysv_sema.c"
+      sematype="System V"
     fi
+  AC_MSG_RESULT([$sematype])
   fi
 else
   AC_DEFINE(USE_WIN32_SEMAPHORES, 1, [Define to select Win32-style semaphores.])

src/template/freebsd

@@ -1 +1,6 @@
 # src/template/freebsd
+
+# Prefer unnamed POSIX semaphores if available, unless user overrides choice
+if test x"$PREFERRED_SEMAPHORES" = x"" ; then
+  PREFERRED_SEMAPHORES=UNNAMED_POSIX
+fi

src/template/linux

@@ -1,5 +1,10 @@
 # src/template/linux
 
+# Prefer unnamed POSIX semaphores if available, unless user overrides choice
+if test x"$PREFERRED_SEMAPHORES" = x"" ; then
+  PREFERRED_SEMAPHORES=UNNAMED_POSIX
+fi
+
 # Force _GNU_SOURCE on; plperl is broken with Perl 5.8.0 otherwise
 CPPFLAGS="$CPPFLAGS -D_GNU_SOURCE"