Subject: [PATCH] Support pg_walfile_name on standby

From: Jehan-Guillaume de Rorthais <[email-masked]>
Date: Tue, 23 Jul 2019 17:28:44 +0200

Support executing both SQL functions pg_walfile_name() and
pg_walfile_name_offset() on a standby.
---
 src/backend/access/transam/xlogfuncs.c | 39 +++++++++++++++++---------
 1 file changed, 25 insertions(+), 14 deletions(-)

diff --git a/src/backend/access/transam/xlogfuncs.c b/src/backend/access/transam/xlogfuncs.c
index b35043bf71..86c4d8382b 100644
--- a/src/backend/access/transam/xlogfuncs.c
+++ b/src/backend/access/transam/xlogfuncs.c
@@ -460,13 +460,7 @@ pg_walfile_name_offset(PG_FUNCTION_ARGS)
 	TupleDesc	resultTupleDesc;
 	HeapTuple	resultHeapTuple;
 	Datum		result;
-
-	if (RecoveryInProgress())
-		ereport(ERROR,
-				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
-				 errmsg("recovery is in progress"),
-				 errhint("%s cannot be executed during recovery.",
-						 "pg_walfile_name_offset()")));
+	TimeLineID  tl;
 
 	/*
 	 * Construct a tuple descriptor for the result row.  This must match this
@@ -480,11 +474,24 @@ pg_walfile_name_offset(PG_FUNCTION_ARGS)
 
 	resultTupleDesc = BlessTupleDesc(resultTupleDesc);
 
+	if (RecoveryInProgress())
+	{
+		GetWalRcvWriteRecPtr(NULL, &tl);
+
+		if (!tl)
+		{
+			isnull[0] = isnull[1] = true;
+			goto result;
+		}
+	}
+	else
+		tl = ThisTimeLineID;
+
 	/*
 	 * xlogfilename
 	 */
 	XLByteToPrevSeg(locationpoint, xlogsegno, wal_segment_size);
-	XLogFileName(xlogfilename, ThisTimeLineID, xlogsegno, wal_segment_size);
+	XLogFileName(xlogfilename, tl, xlogsegno, wal_segment_size);
 
 	values[0] = CStringGetTextDatum(xlogfilename);
 	isnull[0] = false;
@@ -497,6 +504,7 @@ pg_walfile_name_offset(PG_FUNCTION_ARGS)
 	values[1] = UInt32GetDatum(xrecoff);
 	isnull[1] = false;
 
+result:
 	/*
 	 * Tuple jam: Having first prepared your Datums, then squash together
 	 */
@@ -517,16 +525,19 @@ pg_walfile_name(PG_FUNCTION_ARGS)
 	XLogSegNo	xlogsegno;
 	XLogRecPtr	locationpoint = PG_GETARG_LSN(0);
 	char		xlogfilename[MAXFNAMELEN];
+	TimeLineID  tl;
 
 	if (RecoveryInProgress())
-		ereport(ERROR,
-				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
-				 errmsg("recovery is in progress"),
-				 errhint("%s cannot be executed during recovery.",
-						 "pg_walfile_name()")));
+	{
+		GetWalRcvWriteRecPtr(NULL, &tl);
+		if (!tl)
+			PG_RETURN_NULL();
+	}
+	else
+		tl = ThisTimeLineID;
 
 	XLByteToPrevSeg(locationpoint, xlogsegno, wal_segment_size);
-	XLogFileName(xlogfilename, ThisTimeLineID, xlogsegno, wal_segment_size);
+	XLogFileName(xlogfilename, tl, xlogsegno, wal_segment_size);
 
 	PG_RETURN_TEXT_P(cstring_to_text(xlogfilename));
 }
-- 
2.20.1

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