gstdio: Add macOS support to g_fsync()

Apparently, `fsync()` doesn’t actually sync to the spinning disk on
macOS. You need an `fcntl()` for that.

See: https://developer.apple.com/library/archive/documentation/System/Conceptual/ManPages_iPhoneOS/man2/fsync.2.html

Spotted by Christoph Reiter in a comment on !369.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

glib/gstdio.c

@@ -1661,7 +1661,7 @@ g_freopen (const gchar *filename,
  * @fd: a file descriptor
  *
  * A wrapper for the POSIX `fsync()` function. On Windows, `_commit()` will be
- * used.
+ * used. On macOS, `fcntl(F_FULLFSYNC)` will be used.
  * The `fsync()` function is used to synchronize a file's in-core
  * state with that of the disk.
  *
@@ -1679,10 +1679,14 @@ g_fsync (gint fd)
 {
 #ifdef G_OS_WIN32
   return _commit (fd);
-#elif defined(HAVE_FSYNC)
+#elif defined(HAVE_FSYNC) || defined(HAVE_FCNTL_F_FULLFSYNC)
   int retval;
   do
+#ifdef HAVE_FCNTL_F_FULLFSYNC
+    retval = fcntl (fd, F_FULLFSYNC, 0);
+#else
     retval = fsync (fd);
+#endif
   while (G_UNLIKELY (retval < 0 && errno == EINTR));
   return retval;
 #else

meson.build

@@ -875,6 +875,17 @@ if cc.compiles('''#include <fcntl.h>
   glib_conf.set('HAVE_OPEN_O_DIRECTORY', 1)
 endif
 
+# fcntl takes F_FULLFSYNC as an option
+# See https://developer.apple.com/library/archive/documentation/System/Conceptual/ManPages_iPhoneOS/man2/fsync.2.html
+if cc.compiles('''#include <fcntl.h>
+                  #include <sys/types.h>
+                  #include <sys/stat.h>
+                  void some_func (void) {
+                    fcntl(0, F_FULLFSYNC, 0);
+                  }''', name : 'fcntl() option F_FULLFSYNC')
+  glib_conf.set('HAVE_FCNTL_F_FULLFSYNC', 1)
+endif
+
 # Check whether there is a vsnprintf() function with C99 semantics installed.
 # (similar tests to AC_FUNC_VSNPRINTF_C99)
 # Check whether there is a snprintf() function with C99 semantics installed.