Make our usage of memset_s() conform strictly to the C11 standard.

Per the letter of the C11 standard, one must #define
__STDC_WANT_LIB_EXT1__ as 1 before including <string.h> in order to
have access to memset_s().  It appears that many platforms are lenient
about this, because we weren't doing it and yet the code appeared to
work anyway.  But we now find that with -std=c11, macOS is strict and
doesn't declare memset_s, leading to compile failures since we try to
use it anyway.  (Given the lack of prior reports, perhaps this is new
behavior in the latest SDK?  No matter, we're clearly in the wrong.)

In addition to the immediate problem, which could be fixed merely by
adding the needed #define to explicit_bzero.c, it seems possible that
our configure-time probe for memset_s() could fail in case a platform
implements the function in some odd way due to this spec requirement.
This concern can be fixed in largely the same way that we dealt with
strchrnul() in 6da2ba1d8: switch to using a declaration-based
configure probe instead of a does-it-link probe.

Back-patch to v13 where we started using memset_s().

Reported-by: Lakshmi Narayana Velayudam <dev.narayana.v@gmail.com>
Author: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/CAA4pTnLcKGG78xeOjiBr5yS7ZeE-Rh=FaFQQGOO=nPzA1L8yEA@mail.gmail.com
Backpatch-through: 13

configure

@@ -15616,7 +15616,7 @@ fi
 LIBS_including_readline="$LIBS"
 LIBS=`echo "$LIBS" | sed -e 's/-ledit//g' -e 's/-lreadline//g'`
 
-for ac_func in backtrace_symbols copyfile copy_file_range elf_aux_info getauxval getifaddrs getpeerucred inet_pton kqueue localeconv_l mbstowcs_l memset_s posix_fallocate ppoll pthread_is_threaded_np setproctitle setproctitle_fast strsignal syncfs sync_file_range uselocale wcstombs_l
+for ac_func in backtrace_symbols copyfile copy_file_range elf_aux_info getauxval getifaddrs getpeerucred inet_pton kqueue localeconv_l mbstowcs_l posix_fallocate ppoll pthread_is_threaded_np setproctitle setproctitle_fast strsignal syncfs sync_file_range uselocale wcstombs_l
 do :
   as_ac_var=`$as_echo "ac_cv_func_$ac_func" | $as_tr_sh`
 ac_fn_c_check_func "$LINENO" "$ac_func" "$as_ac_var"
@@ -16192,6 +16192,19 @@ cat >>confdefs.h <<_ACEOF
 #define HAVE_DECL_STRCHRNUL $ac_have_decl
 _ACEOF
 
+ac_fn_c_check_decl "$LINENO" "memset_s" "ac_cv_have_decl_memset_s" "#define __STDC_WANT_LIB_EXT1__ 1
+#include <string.h>
+"
+if test "x$ac_cv_have_decl_memset_s" = xyes; then :
+  ac_have_decl=1
+else
+  ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_MEMSET_S $ac_have_decl
+_ACEOF
+
 
 # This is probably only present on macOS, but may as well check always
 ac_fn_c_check_decl "$LINENO" "F_FULLFSYNC" "ac_cv_have_decl_F_FULLFSYNC" "#include <fcntl.h>

configure.ac

@@ -1792,7 +1792,6 @@ AC_CHECK_FUNCS(m4_normalize([
 	kqueue
 	localeconv_l
 	mbstowcs_l
-	memset_s
 	posix_fallocate
 	ppoll
 	pthread_is_threaded_np
@@ -1838,6 +1837,8 @@ AC_CHECK_DECLS([strlcat, strlcpy, strnlen, strsep, timingsafe_bcmp])
 AC_CHECK_DECLS([preadv], [], [], [#include <sys/uio.h>])
 AC_CHECK_DECLS([pwritev], [], [], [#include <sys/uio.h>])
 AC_CHECK_DECLS([strchrnul], [], [], [#include <string.h>])
+AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
+#include <string.h>])
 
 # This is probably only present on macOS, but may as well check always
 AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])

meson.build

@@ -2654,6 +2654,7 @@ decl_checks += [
   ['preadv', 'sys/uio.h'],
   ['pwritev', 'sys/uio.h'],
   ['strchrnul', 'string.h'],
+  ['memset_s', 'string.h', '#define __STDC_WANT_LIB_EXT1__ 1'],
 ]
 
 # Check presence of some optional LLVM functions.
@@ -2667,21 +2668,23 @@ endif
 foreach c : decl_checks
   func = c.get(0)
   header = c.get(1)
-  args = c.get(2, {})
+  prologue = c.get(2, '')
+  args = c.get(3, {})
   varname = 'HAVE_DECL_' + func.underscorify().to_upper()
 
   found = cc.compiles('''
-#include <@0@>
+@0@
+#include <@1@>
 
 int main()
 {
-#ifndef @1@
-    (void) @1@;
+#ifndef @2@
+    (void) @2@;
 #endif
 
 return 0;
 }
-'''.format(header, func),
+'''.format(prologue, header, func),
     name: 'test whether @0@ is declared'.format(func),
     # need to add cflags_warn to get at least
     # -Werror=unguarded-availability-new if applicable
@@ -2880,7 +2883,6 @@ func_checks = [
   ['kqueue'],
   ['localeconv_l'],
   ['mbstowcs_l'],
-  ['memset_s'],
   ['mkdtemp'],
   ['posix_fadvise'],
   ['posix_fallocate'],

src/include/pg_config.h.in

@@ -91,6 +91,10 @@
    `LLVMCreatePerfJITEventListener', and to 0 if you don't. */
 #undef HAVE_DECL_LLVMCREATEPERFJITEVENTLISTENER
 
+/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
+   don't. */
+#undef HAVE_DECL_MEMSET_S
+
 /* Define to 1 if you have the declaration of `posix_fadvise', and to 0 if you
    don't. */
 #undef HAVE_DECL_POSIX_FADVISE
@@ -291,9 +295,6 @@
 /* Define to 1 if you have the <memory.h> header file. */
 #undef HAVE_MEMORY_H
 
-/* Define to 1 if you have the `memset_s' function. */
-#undef HAVE_MEMSET_S
-
 /* Define to 1 if you have the `mkdtemp' function. */
 #undef HAVE_MKDTEMP
 

src/port/explicit_bzero.c

@@ -12,9 +12,11 @@
  *-------------------------------------------------------------------------
  */
 
+#define __STDC_WANT_LIB_EXT1__ 1	/* needed to access memset_s() */
+
 #include "c.h"
 
-#if defined(HAVE_MEMSET_S)
+#if HAVE_DECL_MEMSET_S
 
 void
 explicit_bzero(void *buf, size_t len)