To improve the code, move the error check in logical_read_xlog_page().

Commit 0fdab27 changed the code to wait for WAL to be available before
determining the timeline but forgot to move the failure check.

This change is to make the related code easier to understand and enhance
otherwise there is no bug in the current code.

In the passing, improve the nearby comments to explain why we determine
am_cascading_walsender after waiting for the required WAL.

Author: Peter Smith
Reviewed-by: Bertrand Drouvot, Amit Kapila
Discussion: https://postgr.es/m/CAHut+PvqX49fusLyXspV1Mmd_EekPtXG0oT146vZjcb9XDvNgw@mail.gmail.com

Author: Amit Kapila

src/backend/replication/walsender.c

@@ -1057,16 +1057,21 @@ logical_read_xlog_page(XLogReaderState *state, XLogRecPtr targetPagePtr, int req
 
 	/*
 	 * Make sure we have enough WAL available before retrieving the current
-	 * timeline. This is needed to determine am_cascading_walsender accurately
-	 * which is needed to determine the current timeline.
+	 * timeline.
 	 */
 	flushptr = WalSndWaitForWal(targetPagePtr + reqLen);
 
+	/* Fail if not enough (implies we are going to shut down) */
+	if (flushptr < targetPagePtr + reqLen)
+		return -1;
+
 	/*
 	 * Since logical decoding is also permitted on a standby server, we need
 	 * to check if the server is in recovery to decide how to get the current
-	 * timeline ID (so that it also cover the promotion or timeline change
-	 * cases).
+	 * timeline ID (so that it also covers the promotion or timeline change
+	 * cases). We must determine am_cascading_walsender after waiting for the
+	 * required WAL so that it is correct when the walsender wakes up after a
+	 * promotion.
 	 */
 	am_cascading_walsender = RecoveryInProgress();
 
@@ -1081,10 +1086,6 @@ logical_read_xlog_page(XLogReaderState *state, XLogRecPtr targetPagePtr, int req
 	sendTimeLineValidUpto = state->currTLIValidUntil;
 	sendTimeLineNextTLI = state->nextTLI;
 
-	/* fail if not (implies we are going to shut down) */
-	if (flushptr < targetPagePtr + reqLen)
-		return -1;
-
 	if (targetPagePtr + XLOG_BLCKSZ <= flushptr)
 		count = XLOG_BLCKSZ;	/* more than one block available */
 	else