Unify two isLogSwitch tests in XLogInsertRecord.

An upcoming patch wants to introduce an additional special case in
this function. To keep that as cheap as possible, minimize the amount
of branching that we do based on whether this is an XLOG_SWITCH
record.

Additionally, and also in the interest of keeping the overhead of
special-case code paths as low as possible, apply likely() to the
non-XLOG_SWITCH case, since only a very tiny fraction of WAL records
will be XLOG_SWITCH records.

Patch by me, reviewed by Dilip Kumar, Amit Kapila, Andres Freund,
and Michael Paquier.

Discussion: http://postgr.es/m/CA+TgmoYy-Vc6G9QKcAKNksCa29cv__czr+N9X_QCxEfQVpp_8w@mail.gmail.com

src/backend/access/transam/xlog.c

@@ -792,59 +792,74 @@ XLogInsertRecord(XLogRecData *rdata,
 	 *----------
 	 */
 	START_CRIT_SECTION();
-	if (isLogSwitch)
-		WALInsertLockAcquireExclusive();
-	else
-		WALInsertLockAcquire();
 
-	/*
-	 * Check to see if my copy of RedoRecPtr is out of date. If so, may have
-	 * to go back and have the caller recompute everything. This can only
-	 * happen just after a checkpoint, so it's better to be slow in this case
-	 * and fast otherwise.
-	 *
-	 * Also check to see if fullPageWrites was just turned on or there's a
-	 * running backup (which forces full-page writes); if we weren't already
-	 * doing full-page writes then go back and recompute.
-	 *
-	 * If we aren't doing full-page writes then RedoRecPtr doesn't actually
-	 * affect the contents of the XLOG record, so we'll update our local copy
-	 * but not force a recomputation.  (If doPageWrites was just turned off,
-	 * we could recompute the record without full pages, but we choose not to
-	 * bother.)
-	 */
-	if (RedoRecPtr != Insert->RedoRecPtr)
+	if (likely(!isLogSwitch))
 	{
-		Assert(RedoRecPtr < Insert->RedoRecPtr);
-		RedoRecPtr = Insert->RedoRecPtr;
-	}
-	doPageWrites = (Insert->fullPageWrites || Insert->runningBackups > 0);
+		WALInsertLockAcquire();
 
-	if (doPageWrites &&
-		(!prevDoPageWrites ||
-		 (fpw_lsn != InvalidXLogRecPtr && fpw_lsn <= RedoRecPtr)))
-	{
 		/*
-		 * Oops, some buffer now needs to be backed up that the caller didn't
-		 * back up.  Start over.
+		 * Check to see if my copy of RedoRecPtr is out of date. If so, may
+		 * have to go back and have the caller recompute everything. This can
+		 * only happen just after a checkpoint, so it's better to be slow in
+		 * this case and fast otherwise.
+		 *
+		 * Also check to see if fullPageWrites was just turned on or there's a
+		 * running backup (which forces full-page writes); if we weren't
+		 * already doing full-page writes then go back and recompute.
+		 *
+		 * If we aren't doing full-page writes then RedoRecPtr doesn't
+		 * actually affect the contents of the XLOG record, so we'll update
+		 * our local copy but not force a recomputation.  (If doPageWrites was
+		 * just turned off, we could recompute the record without full pages,
+		 * but we choose not to bother.)
 		 */
-		WALInsertLockRelease();
-		END_CRIT_SECTION();
-		return InvalidXLogRecPtr;
-	}
+		if (RedoRecPtr != Insert->RedoRecPtr)
+		{
+			Assert(RedoRecPtr < Insert->RedoRecPtr);
+			RedoRecPtr = Insert->RedoRecPtr;
+		}
+		doPageWrites = (Insert->fullPageWrites || Insert->runningBackups > 0);
 
-	/*
-	 * Reserve space for the record in the WAL. This also sets the xl_prev
-	 * pointer.
-	 */
-	if (isLogSwitch)
-		inserted = ReserveXLogSwitch(&StartPos, &EndPos, &rechdr->xl_prev);
-	else
-	{
+		if (doPageWrites &&
+			(!prevDoPageWrites ||
+			 (fpw_lsn != InvalidXLogRecPtr && fpw_lsn <= RedoRecPtr)))
+		{
+			/*
+			 * Oops, some buffer now needs to be backed up that the caller
+			 * didn't back up.  Start over.
+			 */
+			WALInsertLockRelease();
+			END_CRIT_SECTION();
+			return InvalidXLogRecPtr;
+		}
+
+		/*
+		 * Reserve space for the record in the WAL. This also sets the xl_prev
+		 * pointer.
+		 */
 		ReserveXLogInsertLocation(rechdr->xl_tot_len, &StartPos, &EndPos,
 								  &rechdr->xl_prev);
+
+		/* Normal records are always inserted. */
 		inserted = true;
 	}
+	else
+	{
+		/*
+		 * In order to insert an XLOG_SWITCH record, we need to hold all of
+		 * the WAL insertion locks, not just one, so that no one else can
+		 * begin inserting a record until we've figured out how much space
+		 * remains in the current WAL segment and claimed all of it.
+		 *
+		 * Nonetheless, this case is simpler than the normal cases handled
+		 * above, which must check for changes in doPageWrites and RedoRecPtr.
+		 * Those checks are only needed for records that can contain
+		 * full-pages images, and an XLOG_SWITCH record never does.
+		 */
+		Assert(fpw_lsn == InvalidXLogRecPtr);
+		WALInsertLockAcquireExclusive();
+		inserted = ReserveXLogSwitch(&StartPos, &EndPos, &rechdr->xl_prev);
+	}
 
 	if (inserted)
 	{