From c3b5992b91c4b0d2c4f4eab0fb856f34854c129d Mon Sep 17 00:00:00 2001 From: Andres Freund Date: Sat, 8 Oct 2022 09:33:23 -0700 Subject: [PATCH] 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 Author: Andres Freund Reviewed-by: Masahiko Sawada Reviewed-by: Kyotaro Horiguchi Discussion: https://postgr.es/m/YxfagaTXUNa9ggLb@ahch-to Backpatch: 15-, where the bug was introduced in 5891c7a8ed8f --- src/backend/replication/slot.c | 28 ++++++++++ src/backend/utils/activity/pgstat.c | 2 +- src/backend/utils/activity/pgstat_replslot.c | 56 +++++++++----------- src/include/pgstat.h | 6 ++- src/include/replication/slot.h | 1 + src/include/utils/pgstat_internal.h | 5 +- 6 files changed, 64 insertions(+), 34 deletions(-) diff --git a/src/backend/replication/slot.c b/src/backend/replication/slot.c index 850b74936fe..329599e99de 100644 --- a/src/backend/replication/slot.c +++ b/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. * diff --git a/src/backend/utils/activity/pgstat.c b/src/backend/utils/activity/pgstat.c index 168231e71d6..d58a299d3e6 100644 --- a/src/backend/utils/activity/pgstat.c +++ b/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); diff --git a/src/backend/utils/activity/pgstat_replslot.c b/src/backend/utils/activity/pgstat_replslot.c index b77c05ab5fa..1a8473bcd45 100644 --- a/src/backend/utils/activity/pgstat_replslot.c +++ b/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; - PgStatShared_ReplSlot *shstatent; - 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); + pgstat_get_entry_ref(PGSTAT_KIND_REPLSLOT, InvalidOid, + ReplicationSlotIndex(slot), true, NULL); } /* @@ -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 diff --git a/src/include/pgstat.h b/src/include/pgstat.h index ac28f813b4e..dbb2074ed4a 100644 --- a/src/include/pgstat.h +++ b/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; diff --git a/src/include/replication/slot.h b/src/include/replication/slot.h index 8c9f3321d50..deba2c4e499 100644 --- a/src/include/replication/slot.h +++ b/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); diff --git a/src/include/utils/pgstat_internal.h b/src/include/utils/pgstat_internal.h index 901d2041d66..4b65dfef7d4 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 *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);