Use correct LSN in GetPage@LSN calls when nothing has been evicted yet.

If no page has been evicted from page cache yet, use the latest flushed
LSN.

Commit 153dee9, "Request special lsn during bootstrap", changed this
but it is not clear to me why. As far as I can see, this isn't related
to bootstrapping, and using the latest flushed LSN should always work.

While we're at it, rearrange the code slightly for readability, and add
comments.

Author: hlinnaka

src/backend/storage/smgr/pagestore_smgr.c

@@ -337,21 +337,32 @@ zenith_get_request_lsn(bool nonrel)
 	}
 	else
 	{
-		lsn = GetLastWrittenPageLSN();
+		XLogRecPtr flushlsn = GetFlushRecPtr();
 
+		/*
+		 * Use the latest LSN that was evicted from the buffer cache. Any
+		 * pages modified by later WAL records must still in the buffer cache,
+		 * so our request cannot concern those.
+		 */
+		lsn = GetLastWrittenPageLSN();
 		elog(DEBUG1, "zenith_get_request_lsn GetLastWrittenPageLSN lsn %X/%X ",
 			(uint32) ((lsn) >> 32), (uint32) (lsn));
-
-		if (lsn > GetFlushRecPtr())
-			XLogFlush(lsn);
 		if (lsn == InvalidXLogRecPtr)
 		{
-			/* we haven't evicted anything yet since the server was started */
-			lsn = GetFlushRecPtr();
-			elog(DEBUG1, "zenith_get_request_lsn GetFlushRecPtr lsn %X/%X request 0",
-			(uint32) ((lsn) >> 32), (uint32) (lsn));
-			lsn = InvalidXLogRecPtr;
+			/*
+			 * We haven't evicted anything yet since the server was
+			 * started. Then just use the latest flushed LSN. That's always
+			 * safe, using the latest evicted LSN is really just an
+			 * optimization.
+			 */
+			lsn = flushlsn;
+			elog(DEBUG1, "zenith_get_request_lsn GetFlushRecPtr lsn %X/%X",
+				 (uint32) ((lsn) >> 32), (uint32) (lsn));
 		}
+
+		/* The record should've been flushed already, since it was evicted, but let's be safe */
+		if (lsn > flushlsn)
+			XLogFlush(lsn);
 	}
 	return lsn;
 }