Be more specific about OOM in XLogReaderAllocate

A couple of spots can benefit from an added errdetail(), which matches
what we were already doing in other places; and those that cannot
withstand errdetail() can get a more descriptive primary message.

Author: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Reviewed-by: Julien Rouhaud <rjuju123@gmail.com>
Discussion: https://postgr.es/m/CALj2ACV+cX1eM03GfcA=ZMLXh5fSn1X1auJLz3yuS1duPSb9QA@mail.gmail.com

src/backend/access/transam/xlog.c

@@ -1265,7 +1265,7 @@ XLogInsertRecord(XLogRecData *rdata,
 
 		if (!debug_reader)
 		{
-			appendStringInfoString(&buf, "error decoding record: out of memory");
+			appendStringInfoString(&buf, "error decoding record: out of memory while allocating a WAL reading processor");
 		}
 		else if (!DecodeXLogRecord(debug_reader, (XLogRecord *) recordBuf.data,
 								   &errormsg))

src/backend/replication/logical/logical.c

@@ -202,7 +202,8 @@ StartupDecodingContext(List *output_plugin_options,
 	if (!ctx->reader)
 		ereport(ERROR,
 				(errcode(ERRCODE_OUT_OF_MEMORY),
-				 errmsg("out of memory")));
+				 errmsg("out of memory"),
+				 errdetail("Failed while allocating a WAL reading processor.")));
 
 	ctx->reorder = ReorderBufferAllocate();
 	ctx->snapshot_builder =

src/backend/replication/walsender.c

@@ -681,7 +681,8 @@ StartReplication(StartReplicationCmd *cmd)
 	if (!xlogreader)
 		ereport(ERROR,
 				(errcode(ERRCODE_OUT_OF_MEMORY),
-				 errmsg("out of memory")));
+				 errmsg("out of memory"),
+				 errdetail("Failed while allocating a WAL reading processor.")));
 
 	/*
 	 * We assume here that we're logging enough information in the WAL for

src/bin/pg_rewind/parsexlog.c

@@ -74,7 +74,7 @@ extractPageMap(const char *datadir, XLogRecPtr startpoint, int tliIndex,
 									XL_ROUTINE(.page_read = &SimpleXLogPageRead),
 									&private);
 	if (xlogreader == NULL)
-		pg_fatal("out of memory");
+		pg_fatal("out of memory while allocating a WAL reading processor");
 
 	XLogBeginRead(xlogreader, startpoint);
 	do
@@ -132,7 +132,7 @@ readOneRecord(const char *datadir, XLogRecPtr ptr, int tliIndex,
 									XL_ROUTINE(.page_read = &SimpleXLogPageRead),
 									&private);
 	if (xlogreader == NULL)
-		pg_fatal("out of memory");
+		pg_fatal("out of memory while allocating a WAL reading processor");
 
 	XLogBeginRead(xlogreader, ptr);
 	record = XLogReadRecord(xlogreader, &errormsg);
@@ -192,7 +192,7 @@ findLastCheckpoint(const char *datadir, XLogRecPtr forkptr, int tliIndex,
 									XL_ROUTINE(.page_read = &SimpleXLogPageRead),
 									&private);
 	if (xlogreader == NULL)
-		pg_fatal("out of memory");
+		pg_fatal("out of memory while allocating a WAL reading processor");
 
 	searchptr = forkptr;
 	for (;;)

src/bin/pg_waldump/pg_waldump.c

@@ -1061,7 +1061,7 @@ main(int argc, char **argv)
 									  .segment_close = WALDumpCloseSegment),
 						   &private);
 	if (!xlogreader_state)
-		fatal_error("out of memory");
+		fatal_error("out of memory while allocating a WAL reading processor");
 
 	/* first find a valid recptr to start from */
 	first_record = XLogFindNextRecord(xlogreader_state, private.startptr);