Add more LOG messages when starting and ending recovery from a backup

Three LOG messages are added in the recovery code paths, providing
information that can be useful to track corruption issues depending on
the state of the cluster, telling that:
- Recovery has started from a backup_label.
- Recovery is restarting from a backup start LSN, without a
backup_label.
- Recovery has completed from a backup.

Author: Andres Freund
Reviewed-by: David Steele, Laurenz Albe, Michael Paquier
Discussion: https://postgr.es/m/20231117041811.vz4vgkthwjnwp2pp@awork3.anarazel.de

src/backend/access/transam/xlogrecovery.c

@@ -603,6 +603,22 @@ InitWalRecovery(ControlFileData *ControlFile, bool *wasShutdown_ptr,
 		if (StandbyModeRequested)
 			EnableStandbyMode();
 
+		/*
+		 * Omitting backup_label when creating a new replica, PITR node etc.
+		 * unfortunately is a common cause of corruption.  Logging that
+		 * backup_label was used makes it a bit easier to exclude that as the
+		 * cause of observed corruption.
+		 *
+		 * Do so before we try to read the checkpoint record (which can fail),
+		 * as otherwise it can be hard to understand why a checkpoint other
+		 * than ControlFile->checkPoint is used.
+		 */
+		ereport(LOG,
+				(errmsg("starting backup recovery with redo LSN %X/%X, checkpoint LSN %X/%X, on timeline ID %u",
+						LSN_FORMAT_ARGS(RedoStartLSN),
+						LSN_FORMAT_ARGS(CheckPointLoc),
+						CheckPointTLI)));
+
 		/*
 		 * When a backup_label file is present, we want to roll forward from
 		 * the checkpoint it identifies, rather than using pg_control.
@@ -742,6 +758,16 @@ InitWalRecovery(ControlFileData *ControlFile, bool *wasShutdown_ptr,
 				EnableStandbyMode();
 		}
 
+		/*
+		 * For the same reason as when starting up with backup_label present,
+		 * emit a log message when we continue initializing from a base
+		 * backup.
+		 */
+		if (!XLogRecPtrIsInvalid(ControlFile->backupStartPoint))
+			ereport(LOG,
+					(errmsg("restarting backup recovery with redo LSN %X/%X",
+							LSN_FORMAT_ARGS(ControlFile->backupStartPoint))));
+
 		/* Get the last valid checkpoint record. */
 		CheckPointLoc = ControlFile->checkPoint;
 		CheckPointTLI = ControlFile->checkPointCopy.ThisTimeLineID;
@@ -2155,6 +2181,9 @@ CheckRecoveryConsistency(void)
 	if (!XLogRecPtrIsInvalid(backupEndPoint) &&
 		backupEndPoint <= lastReplayedEndRecPtr)
 	{
+		XLogRecPtr	saveBackupStartPoint = backupStartPoint;
+		XLogRecPtr	saveBackupEndPoint = backupEndPoint;
+
 		elog(DEBUG1, "end of backup reached");
 
 		/*
@@ -2165,6 +2194,11 @@ CheckRecoveryConsistency(void)
 		backupStartPoint = InvalidXLogRecPtr;
 		backupEndPoint = InvalidXLogRecPtr;
 		backupEndRequired = false;
+
+		ereport(LOG,
+				(errmsg("completed backup recovery with redo LSN %X/%X and end LSN %X/%X",
+						LSN_FORMAT_ARGS(saveBackupStartPoint),
+						LSN_FORMAT_ARGS(saveBackupEndPoint))));
 	}
 
 	/*