Fix detection and handling of strchrnul() for macOS 15.4.

As of 15.4, macOS has strchrnul(), but access to it is blocked behind
a check for MACOSX_DEPLOYMENT_TARGET >= 15.4.  But our does-it-link
configure check finds it, so we try to use it, and fail with the
present default deployment target (namely 15.0).  This accounts for
today's buildfarm failures on indri and sifaka.

This is the identical problem that we faced some years ago when Apple
introduced preadv and pwritev in the same way.  We solved that in
commit f014b1b9b by using AC_CHECK_DECLS instead of AC_CHECK_FUNCS
to check the functions' availability.  So do the same now for
strchrnul().  Interestingly, we already had a workaround for
"the link check doesn't agree with <string.h>" cases with glibc,
which we no longer need since only the header declaration is being
checked.

Testing this revealed that the meson version of this check has never
worked, because it failed to use "-Werror=unguarded-availability-new".
(Apparently nobody's tried to build with meson on macOS versions that
lack preadv/pwritev as standard.)  Adjust that while at it.  Also,
we had never put support for "-Werror=unguarded-availability-new"
into v13, but we need that now.

Co-authored-by: Tom Lane <tgl@sss.pgh.pa.us>
Co-authored-by: Peter Eisentraut <peter@eisentraut.org>
Discussion: https://postgr.es/m/385134.1743523038@sss.pgh.pa.us
Backpatch-through: 13

5 files changed, 49 insertions, 24 deletions

diff --git a/configure b/configure
index 30d949c3c46..3d0e701c745 100755
--- a/configure
+++ b/configure
@@ -15401,7 +15401,7 @@ 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 strchrnul 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 memset_s 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"
@@ -15955,6 +15955,18 @@ cat >>confdefs.h <<_ACEOF
 #define HAVE_DECL_PWRITEV $ac_have_decl
 _ACEOF
 
+ac_fn_c_check_decl "$LINENO" "strchrnul" "ac_cv_have_decl_strchrnul" "#include <string.h>
+"
+if test "x$ac_cv_have_decl_strchrnul" = xyes; then :
+  ac_have_decl=1
+else
+  ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_STRCHRNUL $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>
diff --git a/configure.ac b/configure.ac
index 25cdfcf65af..47a287926bc 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1772,7 +1772,6 @@ AC_CHECK_FUNCS(m4_normalize([
 	pthread_is_threaded_np
 	setproctitle
 	setproctitle_fast
-	strchrnul
 	strsignal
 	syncfs
 	sync_file_range
@@ -1812,6 +1811,7 @@ AC_CHECK_DECLS([strlcat, strlcpy, strnlen, strsep])
 # won't handle deployment target restrictions on macOS
 AC_CHECK_DECLS([preadv], [], [], [#include <sys/uio.h>])
 AC_CHECK_DECLS([pwritev], [], [], [#include <sys/uio.h>])
+AC_CHECK_DECLS([strchrnul], [], [], [#include <string.h>])
 
 # This is probably only present on macOS, but may as well check always
 AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
diff --git a/meson.build b/meson.build
index b8da4966297..ba7916d1493 100644
--- a/meson.build
+++ b/meson.build
@@ -2577,6 +2577,7 @@ decl_checks = [
 decl_checks += [
   ['preadv', 'sys/uio.h'],
   ['pwritev', 'sys/uio.h'],
+  ['strchrnul', 'string.h'],
 ]
 
 # Check presence of some optional LLVM functions.
@@ -2593,8 +2594,23 @@ foreach c : decl_checks
   args = c.get(2, {})
   varname = 'HAVE_DECL_' + func.underscorify().to_upper()
 
-  found = cc.has_header_symbol(header, func,
-    args: test_c_args, include_directories: postgres_inc,
+  found = cc.compiles('''
+#include <@0@>
+
+int main()
+{
+#ifndef @1@
+    (void) @1@;
+#endif
+
+return 0;
+}
+'''.format(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
+    args: test_c_args + cflags_warn,
+    include_directories: postgres_inc,
     kwargs: args)
   cdata.set10(varname, found, description:
 '''Define to 1 if you have the declaration of `@0@', and to 0 if you
@@ -2802,7 +2818,6 @@ func_checks = [
   ['shm_unlink', {'dependencies': [rt_dep], 'define': false}],
   ['shmget', {'dependencies': [cygipc_dep], 'define': false}],
   ['socket', {'dependencies': [socket_dep], 'define': false}],
-  ['strchrnul'],
   ['strerror_r', {'dependencies': [thread_dep]}],
   ['strlcat'],
   ['strlcpy'],
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92f0616c400..2ac61575883 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -103,6 +103,10 @@
    don't. */
 #undef HAVE_DECL_PWRITEV
 
+/* Define to 1 if you have the declaration of `strchrnul', and to 0 if you
+   don't. */
+#undef HAVE_DECL_STRCHRNUL
+
 /* Define to 1 if you have the declaration of `strlcat', and to 0 if you
    don't. */
 #undef HAVE_DECL_STRLCAT
@@ -373,9 +377,6 @@
 /* Define to 1 if you have the <stdlib.h> header file. */
 #undef HAVE_STDLIB_H
 
-/* Define to 1 if you have the `strchrnul' function. */
-#undef HAVE_STRCHRNUL
-
 /* Define to 1 if you have the `strerror_r' function. */
 #undef HAVE_STRERROR_R
 
diff --git a/src/port/snprintf.c b/src/port/snprintf.c
index f8f2018ea0c..d7f18b42d19 100644
--- a/src/port/snprintf.c
+++ b/src/port/snprintf.c
@@ -338,13 +338,22 @@ static void leading_pad(int zpad, int signvalue, int *padlen,
 static void trailing_pad(int padlen, PrintfTarget *target);
 
 /*
- * If strchrnul exists (it's a glibc-ism), it's a good bit faster than the
- * equivalent manual loop.  If it doesn't exist, provide a replacement.
+ * If strchrnul exists (it's a glibc-ism, but since adopted by some other
+ * platforms), it's a good bit faster than the equivalent manual loop.
+ * Use it if possible, and if it doesn't exist, use this replacement.
  *
  * Note: glibc declares this as returning "char *", but that would require
  * casting away const internally, so we don't follow that detail.
+ *
+ * Note: macOS has this too as of Sequoia 15.4, but it's hidden behind
+ * a deployment-target check that causes compile errors if the deployment
+ * target isn't high enough.  So !HAVE_DECL_STRCHRNUL may mean "yes it's
+ * declared, but it doesn't compile".  To avoid failing in that scenario,
+ * use a macro to avoid matching <string.h>'s name.
  */
-#ifndef HAVE_STRCHRNUL
+#if !HAVE_DECL_STRCHRNUL
+
+#define strchrnul pg_strchrnul
 
 static inline const char *
 strchrnul(const char *s, int c)
@@ -354,19 +363,7 @@ strchrnul(const char *s, int c)
 	return s;
 }
 
-#else
-
-/*
- * glibc's <string.h> declares strchrnul only if _GNU_SOURCE is defined.
- * While we typically use that on glibc platforms, configure will set
- * HAVE_STRCHRNUL whether it's used or not.  Fill in the missing declaration
- * so that this file will compile cleanly with or without _GNU_SOURCE.
- */
-#ifndef _GNU_SOURCE
-extern char *strchrnul(const char *s, int c);
-#endif
-
-#endif							/* HAVE_STRCHRNUL */
+#endif							/* !HAVE_DECL_STRCHRNUL */
 
 
 /*