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

src/backend/replication/slot.c

@@ -411,6 +411,34 @@ ReplicationSlotIndex(ReplicationSlot *slot)
 	return slot - ReplicationSlotCtl->replication_slots;
 }
 
+/*
+ * If the slot at 'index' is unused, return false. Otherwise 'name' is set to
+ * the slot's name and true is returned.
+ *
+ * This likely is only useful for pgstat_replslot.c during shutdown, in other
+ * cases there are obvious TOCTOU issues.
+ */
+bool
+ReplicationSlotName(int index, Name name)
+{
+	ReplicationSlot *slot;
+	bool		found;
+
+	slot = &ReplicationSlotCtl->replication_slots[index];
+
+	/*
+	 * Ensure that the slot cannot be dropped while we copy the name. Don't
+	 * need the spinlock as the name of an existing slot cannot change.
+	 */
+	LWLockAcquire(ReplicationSlotControlLock, LW_SHARED);
+	found = slot->in_use;
+	if (slot->in_use)
+		namestrcpy(name, NameStr(slot->data.name));
+	LWLockRelease(ReplicationSlotControlLock);
+
+	return found;
+}
+
 /*
  * Find a previously created slot and mark it as used by this process.
  *

src/backend/utils/activity/pgstat.c

@@ -1367,7 +1367,7 @@ pgstat_write_statsfile(void)
 			/* stats entry identified by name on disk (e.g. slots) */
 			NameData	name;
 
-			kind_info->to_serialized_name(shstats, &name);
+			kind_info->to_serialized_name(&ps->key, shstats, &name);
 
 			fputc('N', fpout);
 			write_chunk_s(fpout, &ps->key.kind);

src/backend/utils/activity/pgstat_replslot.c

@@ -69,6 +69,10 @@ pgstat_reset_replslot(const char *name)
 
 /*
  * Report replication slot statistics.
+ *
+ * We can rely on the stats for the slot to exist and to belong to this
+ * slot. We can only get here if pgstat_create_replslot() or
+ * pgstat_acquire_replslot() have already been called.
  */
 void
 pgstat_report_replslot(ReplicationSlot *slot, const PgStat_StatReplSlotEntry *repSlotStat)
@@ -82,12 +86,6 @@ pgstat_report_replslot(ReplicationSlot *slot, const PgStat_StatReplSlotEntry *re
 	shstatent = (PgStatShared_ReplSlot *) entry_ref->shared_stats;
 	statent = &shstatent->stats;
 
-	/*
-	 * Any mismatch should have been fixed in pgstat_create_replslot() or
-	 * pgstat_acquire_replslot().
-	 */
-	Assert(namestrcmp(&statent->slotname, NameStr(slot->data.name)) == 0);
-
 	/* Update the replication slot statistics */
 #define REPLSLOT_ACC(fld) statent->fld += repSlotStat->fld
 	REPLSLOT_ACC(spill_txns);
@@ -124,38 +122,29 @@ pgstat_create_replslot(ReplicationSlot *slot)
 	 * if we previously crashed after dropping a slot.
 	 */
 	memset(&shstatent->stats, 0, sizeof(shstatent->stats));
-	namestrcpy(&shstatent->stats.slotname, NameStr(slot->data.name));
 
 	pgstat_unlock_entry(entry_ref);
 }
 
 /*
  * Report replication slot has been acquired.
+ *
+ * This guarantees that a stats entry exists during later
+ * pgstat_report_replslot() calls.
+ *
+ * If we previously crashed, no stats data exists. But if we did not crash,
+ * the stats do belong to this slot:
+ * - the stats cannot belong to a dropped slot, pgstat_drop_replslot() would
+ *   have been called
+ * - if the slot was removed while shut down,
+ *   pgstat_replslot_from_serialized_name_cb() returning false would have
+ *   caused the stats to be dropped
  */
 void
 pgstat_acquire_replslot(ReplicationSlot *slot)
 {
-	PgStat_EntryRef *entry_ref;
+	pgstat_get_entry_ref(PGSTAT_KIND_REPLSLOT, InvalidOid,
-	PgStatShared_ReplSlot *shstatent;
+						 ReplicationSlotIndex(slot), true, NULL);
-	PgStat_StatReplSlotEntry *statent;
-
-	entry_ref = pgstat_get_entry_ref_locked(PGSTAT_KIND_REPLSLOT, InvalidOid,
-											ReplicationSlotIndex(slot), false);
-	shstatent = (PgStatShared_ReplSlot *) entry_ref->shared_stats;
-	statent = &shstatent->stats;
-
-	/*
-	 * NB: need to accept that there might be stats from an older slot, e.g.
-	 * if we previously crashed after dropping a slot.
-	 */
-	if (NameStr(statent->slotname)[0] == 0 ||
-		namestrcmp(&statent->slotname, NameStr(slot->data.name)) != 0)
-	{
-		memset(statent, 0, sizeof(*statent));
-		namestrcpy(&statent->slotname, NameStr(slot->data.name));
-	}
-
-	pgstat_unlock_entry(entry_ref);
 }
 
 /*
@@ -185,9 +174,16 @@ pgstat_fetch_replslot(NameData slotname)
 }
 
 void
-pgstat_replslot_to_serialized_name_cb(const PgStatShared_Common *header, NameData *name)
+pgstat_replslot_to_serialized_name_cb(const PgStat_HashKey *key, const PgStatShared_Common *header, NameData *name)
 {
-	namestrcpy(name, NameStr(((PgStatShared_ReplSlot *) header)->stats.slotname));
+	/*
+	 * This is only called late during shutdown. The set of existing slots
+	 * isn't allowed to change at this point, we can assume that a slot exists
+	 * at the offset.
+	 */
+	if (!ReplicationSlotName(key->objoid, name))
+		elog(ERROR, "could not find name for replication slot index %u",
+			 key->objoid);
 }
 
 bool

src/include/pgstat.h

@@ -321,7 +321,11 @@ typedef struct PgStat_StatFuncEntry
 
 typedef struct PgStat_StatReplSlotEntry
 {
-	NameData	slotname;
+	/*
+	 * In PG 15 this field is unused, but not removed, to avoid changing
+	 * PGSTAT_FILE_FORMAT_ID.
+	 */
+	NameData	slotname_unused;
 	PgStat_Counter spill_txns;
 	PgStat_Counter spill_count;
 	PgStat_Counter spill_bytes;

src/include/replication/slot.h

@@ -217,6 +217,7 @@ 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, int szslot);
 extern void ReplicationSlotDropAtPubNode(WalReceiverConn *wrconn, char *slotname, bool missing_ok);
 

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 *tmp, 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);