Enable logical slots to follow timeline switches

When decoding from a logical slot, it's necessary for xlog reading to be
able to read xlog from historical (i.e. not current) timelines;
otherwise, decoding fails after failover, because the archives are in
the historical timeline.  This is required to make "failover logical
slots" possible; it currently has no other use, although theoretically
it could be used by an extension that creates a slot on a standby and
continues to replay from the slot when the standby is promoted.

This commit includes a module in src/test/modules with functions to
manipulate the slots (which is not otherwise possible in SQL code) in
order to enable testing, and a new test in src/test/recovery to ensure
that the behavior is as expected.

Author: Craig Ringer
Reviewed-By: Oleksii Kliukin, Andres Freund, Petr Jelínek

1 files changed, 9 insertions, 0 deletions

diff --git a/src/backend/access/transam/xlogreader.c b/src/backend/access/transam/xlogreader.c
index 018fdf3d34e..b67da2ee925 100644
--- a/src/backend/access/transam/xlogreader.c
+++ b/src/backend/access/transam/xlogreader.c
@@ -118,6 +118,11 @@ XLogReaderAllocate(XLogPageReadCB pagereadfunc, void *private_data)
 		return NULL;
 	}
 
+#ifndef FRONTEND
+	/* Will be loaded on first read */
+	state->timelineHistory = NIL;
+#endif
+
 	return state;
 }
 
@@ -137,6 +142,10 @@ XLogReaderFree(XLogReaderState *state)
 	pfree(state->errormsg_buf);
 	if (state->readRecordBuf)
 		pfree(state->readRecordBuf);
+#ifndef FRONTEND
+	if (state->timelineHistory)
+		list_free_deep(state->timelineHistory);
+#endif
 	pfree(state->readBuf);
 	pfree(state);
 }