pgstat: Prevent stats reset from corrupting slotname by removing slotname

Previously PgStat_StatReplSlotEntry contained the slotname, which was mainly
used when writing out the stats during shutdown, to identify the slot in the
serialized data (at runtime the index in ReplicationSlotCtl->replication_slots
is used, but that can change during a restart). Unfortunately the slotname was
overwritten when the slot's stats were reset.

That turned out to only cause "real" problems if the slot was active during
the reset, triggering an assertion failure at the next
pgstat_report_replslot(). In other paths the stats were re-initialized during
pgstat_acquire_replslot().

Fix this by removing slotname from PgStat_StatReplSlotEntry. Instead we can
get the slot's name from the slot itself. Besides fixing a bug, this also is
architecturally cleaner (a name is not really statistics). This is safe
because stats, for a slot removed while shut down, will not be restored at
startup.

In 15 the slotname is not removed, but renamed, to avoid changing the stats
format. In master, bump PGSTAT_FILE_FORMAT_ID.

This commit does not contain a test for the fix. I think this can only be
tested by a tap test starting pg_recvlogical in the background and checking
pg_recvlogical's output. That type of test is notoriously hard to be reliable,
so committing it shortly before the release is wrapped seems like a bad idea.

Reported-by: Jaime Casanova <jcasanov@systemguards.com.ec>
Author: Andres Freund <andres@anarazel.de>
Reviewed-by: Masahiko Sawada <sawada.mshk@gmail.com>
Reviewed-by: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Discussion: https://postgr.es/m/YxfagaTXUNa9ggLb@ahch-to
Backpatch: 15-, where the bug was introduced in 5891c7a8ed8f

3 files changed, 5 insertions, 4 deletions

diff --git a/src/include/pgstat.h b/src/include/pgstat.h
index ad7334a0d2..cc1d1dcb7d 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -242,7 +242,7 @@ typedef struct PgStat_TableXactStatus
  * ------------------------------------------------------------
  */
 
-#define PGSTAT_FILE_FORMAT_ID	0x01A5BCA7
+#define PGSTAT_FILE_FORMAT_ID	0x01A5BCA8
 
 typedef struct PgStat_ArchiverStats
 {
@@ -321,7 +321,6 @@ typedef struct PgStat_StatFuncEntry
 
 typedef struct PgStat_StatReplSlotEntry
 {
-	NameData	slotname;
 	PgStat_Counter spill_txns;
 	PgStat_Counter spill_count;
 	PgStat_Counter spill_bytes;
diff --git a/src/include/replication/slot.h b/src/include/replication/slot.h
index 8d5e764aef..65f2c74239 100644
--- a/src/include/replication/slot.h
+++ b/src/include/replication/slot.h
@@ -218,6 +218,7 @@ extern void ReplicationSlotsDropDBSlots(Oid dboid);
 extern bool InvalidateObsoleteReplicationSlots(XLogSegNo oldestSegno);
 extern ReplicationSlot *SearchNamedReplicationSlot(const char *name, bool need_lock);
 extern int	ReplicationSlotIndex(ReplicationSlot *slot);
+extern bool ReplicationSlotName(int index, Name name);
 extern void ReplicationSlotNameForTablesync(Oid suboid, Oid relid, char *syncslotname, Size szslot);
 extern void ReplicationSlotDropAtPubNode(WalReceiverConn *wrconn, char *slotname, bool missing_ok);
 
diff --git a/src/include/utils/pgstat_internal.h b/src/include/utils/pgstat_internal.h
index 40a3602855..627c1389e4 100644
--- a/src/include/utils/pgstat_internal.h
+++ b/src/include/utils/pgstat_internal.h
@@ -242,7 +242,8 @@ typedef struct PgStat_KindInfo
 	/*
 	 * For variable-numbered stats with named_on_disk. Optional.
 	 */
-	void		(*to_serialized_name) (const PgStatShared_Common *header, NameData *name);
+	void		(*to_serialized_name) (const PgStat_HashKey *key,
+									   const PgStatShared_Common *header, NameData *name);
 	bool		(*from_serialized_name) (const NameData *name, PgStat_HashKey *key);
 
 	/*
@@ -567,7 +568,7 @@ extern void pgstat_relation_delete_pending_cb(PgStat_EntryRef *entry_ref);
  */
 
 extern void pgstat_replslot_reset_timestamp_cb(PgStatShared_Common *header, TimestampTz ts);
-extern void pgstat_replslot_to_serialized_name_cb(const PgStatShared_Common *header, NameData *name);
+extern void pgstat_replslot_to_serialized_name_cb(const PgStat_HashKey *key, const PgStatShared_Common *header, NameData *name);
 extern bool pgstat_replslot_from_serialized_name_cb(const NameData *name, PgStat_HashKey *key);