Remove wal_sync_method=fsync_writethrough on Windows.

The "fsync" level already flushes drive write caches on Windows (as does
"fdatasync"), so it only confuses matters to have an apparently higher
level that isn't actually different at all.

That leaves "fsync_writethrough" only for macOS, where it actually does
something different.

Reviewed-by: Magnus Hagander <magnus@hagander.net>
Discussion: https://postgr.es/m/CA%2BhUKGJ2CG2SouPv2mca2WCTOJxYumvBARRcKPraFMB6GSEMcA%40mail.gmail.com

Author: macdice

doc/src/sgml/wal.sgml

@@ -109,9 +109,8 @@
         <literal>open_datasync</literal> (the default), write caching can be disabled
         by unchecking <literal>My Computer\Open\<replaceable>disk drive</replaceable>\Properties\Hardware\Properties\Policies\Enable write caching on the disk</literal>.
         Alternatively, set <varname>wal_sync_method</varname> to
-        <literal>fdatasync</literal> (NTFS only), <literal>fsync</literal> or
-        <literal>fsync_writethrough</literal>, which prevent
-        write caching.
+        <literal>fdatasync</literal> (NTFS only) or <literal>fsync</literal>,
+        which prevent write caching.
       </para>
     </listitem>
 

src/backend/storage/file/fd.c

@@ -399,7 +399,7 @@ pg_fsync(int fd)
 #endif
 
 	/* #if is to skip the sync_method test if there's no need for it */
-#if defined(HAVE_FSYNC_WRITETHROUGH) && !defined(FSYNC_WRITETHROUGH_IS_FSYNC)
+#if defined(HAVE_FSYNC_WRITETHROUGH)
 	if (sync_method == SYNC_METHOD_FSYNC_WRITETHROUGH)
 		return pg_fsync_writethrough(fd);
 	else
@@ -437,9 +437,7 @@ pg_fsync_writethrough(int fd)
 {
 	if (enableFsync)
 	{
-#ifdef WIN32
-		return _commit(fd);
-#elif defined(F_FULLFSYNC)
+#if defined(F_FULLFSYNC)
 		return (fcntl(fd, F_FULLFSYNC, 0) == -1) ? -1 : 0;
 #else
 		errno = ENOSYS;

src/bin/pg_test_fsync/pg_test_fsync.c

@@ -605,9 +605,7 @@ signal_cleanup(SIGNAL_ARGS)
 static int
 pg_fsync_writethrough(int fd)
 {
-#ifdef WIN32
-	return _commit(fd);
-#elif defined(F_FULLFSYNC)
+#if defined(F_FULLFSYNC)
 	return (fcntl(fd, F_FULLFSYNC, 0) == -1) ? -1 : 0;
 #else
 	errno = ENOSYS;

src/include/port/win32_port.h

@@ -84,14 +84,6 @@
 /* Windows doesn't have fsync() as such, use _commit() */
 #define fsync(fd) _commit(fd)
 
-/*
- * For historical reasons, we allow setting wal_sync_method to
- * fsync_writethrough on Windows, even though it's really identical to fsync
- * (both code paths wind up at _commit()).
- */
-#define HAVE_FSYNC_WRITETHROUGH
-#define FSYNC_WRITETHROUGH_IS_FSYNC
-
 #define USES_WINSOCK
 
 /*