Assert lack of hazardous buffer locks before possible catalog read.

Commit 0bada39c83 fixed a bug of this kind,
which existed in all branches for six days before detection.  While the
probability of reaching the trouble was low, the disruption was extreme.  No
new backends could start, and service restoration needed an immediate
shutdown.  Hence, add this to catch the next bug like it.

The new check in RelationIdGetRelation() suffices to make autovacuum detect
the bug in commit 243e9b40f1 that led to commit
0bada39.  This also checks in a number of similar places.  It replaces each
Assert(IsTransactionState()) that pertained to a conditional catalog read.

Back-patch to v14 - v17.  This a back-patch of commit
f4ece891fc (from before v18 branched) to
all supported branches, to accompany the back-patch of commits 243e9b4
and 0bada39.  For catalog indexes, the bttextcmp() behavior that
motivated IsCatalogTextUniqueIndexOid() was v18-specific.  Hence, this
back-patch doesn't need that or its correction from commit
4a4ee0c2c1.

Reported-by: Alexander Lakhin <exclusion@gmail.com>
Discussion: https://postgr.es/m/20250410191830.0e.nmisch@google.com
Discussion: https://postgr.es/m/10ec0bc3-5933-1189-6bb8-5dec4114558e@gmail.com
Backpatch-through: 14-17

src/backend/storage/buffer/bufmgr.c

@@ -36,6 +36,9 @@
 #include "access/tableam.h"
 #include "access/xlog.h"
 #include "catalog/catalog.h"
+#ifdef USE_ASSERT_CHECKING
+#include "catalog/pg_tablespace_d.h"
+#endif
 #include "catalog/storage.h"
 #include "executor/instrument.h"
 #include "lib/binaryheap.h"
@@ -487,6 +490,10 @@ static void FindAndDropRelFileNodeBuffers(RelFileNode rnode,
 										  BlockNumber firstDelBlock);
 static void AtProcExit_Buffers(int code, Datum arg);
 static void CheckForBufferLeaks(void);
+#ifdef USE_ASSERT_CHECKING
+static void AssertNotCatalogBufferLock(LWLock *lock, LWLockMode mode,
+									   void *unused_context);
+#endif
 static int	rnode_comparator(const void *p1, const void *p2);
 static inline int buffertag_comparator(const BufferTag *a, const BufferTag *b);
 static inline int ckpt_buforder_comparator(const CkptSortItem *a, const CkptSortItem *b);
@@ -2697,6 +2704,65 @@ CheckForBufferLeaks(void)
 #endif
 }
 
+#ifdef USE_ASSERT_CHECKING
+/*
+ * Check for exclusive-locked catalog buffers.  This is the core of
+ * AssertCouldGetRelation().
+ *
+ * A backend would self-deadlock on LWLocks if the catalog scan read the
+ * exclusive-locked buffer.  The main threat is exclusive-locked buffers of
+ * catalogs used in relcache, because a catcache search on any catalog may
+ * build that catalog's relcache entry.  We don't have an inventory of
+ * catalogs relcache uses, so just check buffers of most catalogs.
+ *
+ * It's better to minimize waits while holding an exclusive buffer lock, so it
+ * would be nice to broaden this check not to be catalog-specific.  However,
+ * bttextcmp() accesses pg_collation, and non-core opclasses might similarly
+ * read tables.  That is deadlock-free as long as there's no loop in the
+ * dependency graph: modifying table A may cause an opclass to read table B,
+ * but it must not cause a read of table A.
+ */
+void
+AssertBufferLocksPermitCatalogRead(void)
+{
+	ForEachLWLockHeldByMe(AssertNotCatalogBufferLock, NULL);
+}
+
+static void
+AssertNotCatalogBufferLock(LWLock *lock, LWLockMode mode,
+						   void *unused_context)
+{
+	BufferDesc *bufHdr;
+	BufferTag	tag;
+	Oid			relid;
+
+	if (mode != LW_EXCLUSIVE)
+		return;
+
+	if (!((BufferDescPadded *) lock > BufferDescriptors &&
+		  (BufferDescPadded *) lock < BufferDescriptors + NBuffers))
+		return;					/* not a buffer lock */
+
+	bufHdr = (BufferDesc *)
+		((char *) lock - offsetof(BufferDesc, content_lock));
+	tag = bufHdr->tag;
+
+	/*
+	 * This relNode==relid assumption holds until a catalog experiences VACUUM
+	 * FULL or similar.  After a command like that, relNode will be in the
+	 * normal (non-catalog) range, and we lose the ability to detect hazardous
+	 * access to that catalog.  Calling RelidByRelfilenode() would close that
+	 * gap, but RelidByRelfilenode() might then deadlock with a held lock.
+	 */
+	relid = tag.rnode.relNode;
+
+	Assert(!IsCatalogRelationOid(relid));
+	/* Shared rels are always catalogs: detect even after VACUUM FULL. */
+	Assert(tag.rnode.spcNode != GLOBALTABLESPACE_OID);
+}
+#endif
+
+
 /*
  * Helper routine to issue warnings when a buffer is unexpectedly pinned
  */

src/backend/storage/lmgr/lwlock.c

@@ -1921,6 +1921,21 @@ LWLockReleaseAll(void)
 }
 
 
+/*
+ * ForEachLWLockHeldByMe - run a callback for each held lock
+ *
+ * This is meant as debug support only.
+ */
+void
+ForEachLWLockHeldByMe(void (*callback) (LWLock *, LWLockMode, void *),
+					  void *context)
+{
+	int			i;
+
+	for (i = 0; i < num_held_lwlocks; i++)
+		callback(held_lwlocks[i].lock, held_lwlocks[i].mode, context);
+}
+
 /*
  * LWLockHeldByMe - test whether my process holds a lock in any mode
  *

src/backend/utils/adt/pg_locale.c

@@ -64,6 +64,7 @@
 #include "utils/lsyscache.h"
 #include "utils/memutils.h"
 #include "utils/pg_locale.h"
+#include "utils/relcache.h"
 #include "utils/syscache.h"
 
 #ifdef USE_ICU
@@ -1262,6 +1263,8 @@ lookup_collation_cache(Oid collation, bool set_flags)
 	Assert(OidIsValid(collation));
 	Assert(collation != DEFAULT_COLLATION_OID);
 
+	AssertCouldGetRelation();
+
 	if (collation_cache == NULL)
 	{
 		/* First time through, initialize the hash table */

src/backend/utils/cache/catcache.c

@@ -942,12 +942,41 @@ RehashCatCache(CatCache *cp)
 	cp->cc_bucket = newbucket;
 }
 
+/*
+ *		ConditionalCatalogCacheInitializeCache
+ *
+ * Call CatalogCacheInitializeCache() if not yet done.
+ */
+pg_attribute_always_inline
+static void
+ConditionalCatalogCacheInitializeCache(CatCache *cache)
+{
+#ifdef USE_ASSERT_CHECKING
+	/*
+	 * TypeCacheRelCallback() runs outside transactions and relies on TYPEOID
+	 * for hashing.  This isn't ideal.  Since lookup_type_cache() both
+	 * registers the callback and searches TYPEOID, reaching trouble likely
+	 * requires OOM at an unlucky moment.
+	 *
+	 * InvalidateAttoptCacheCallback() runs outside transactions and likewise
+	 * relies on ATTNUM.  InitPostgres() initializes ATTNUM, so it's reliable.
+	 */
+	if (!(cache->id == TYPEOID || cache->id == ATTNUM) ||
+		IsTransactionState())
+		AssertCouldGetRelation();
+	else
+		Assert(cache->cc_tupdesc != NULL);
+#endif
+
+	if (unlikely(cache->cc_tupdesc == NULL))
+		CatalogCacheInitializeCache(cache);
+}
+
 /*
  *		CatalogCacheInitializeCache
  *
  * This function does final initialization of a catcache: obtain the tuple
- * descriptor and set up the hash and equality function links.  We assume
- * that the relcache entry can be opened at this point!
+ * descriptor and set up the hash and equality function links.
  */
 #ifdef CACHEDEBUG
 #define CatalogCacheInitializeCache_DEBUG1 \
@@ -1082,8 +1111,7 @@ CatalogCacheInitializeCache(CatCache *cache)
 void
 InitCatCachePhase2(CatCache *cache, bool touch_index)
 {
-	if (cache->cc_tupdesc == NULL)
-		CatalogCacheInitializeCache(cache);
+	ConditionalCatalogCacheInitializeCache(cache);
 
 	if (touch_index &&
 		cache->id != AMOID &&
@@ -1262,16 +1290,12 @@ SearchCatCacheInternal(CatCache *cache,
 	dlist_head *bucket;
 	CatCTup    *ct;
 
-	/* Make sure we're in an xact, even if this ends up being a cache hit */
-	Assert(IsTransactionState());
-
 	Assert(cache->cc_nkeys == nkeys);
 
 	/*
 	 * one-time startup overhead for each cache
 	 */
-	if (unlikely(cache->cc_tupdesc == NULL))
-		CatalogCacheInitializeCache(cache);
+	ConditionalCatalogCacheInitializeCache(cache);
 
 #ifdef CATCACHE_STATS
 	cache->cc_searches++;
@@ -1550,8 +1574,7 @@ GetCatCacheHashValue(CatCache *cache,
 	/*
 	 * one-time startup overhead for each cache
 	 */
-	if (cache->cc_tupdesc == NULL)
-		CatalogCacheInitializeCache(cache);
+	ConditionalCatalogCacheInitializeCache(cache);
 
 	/*
 	 * calculate the hash value
@@ -1600,8 +1623,7 @@ SearchCatCacheList(CatCache *cache,
 	/*
 	 * one-time startup overhead for each cache
 	 */
-	if (cache->cc_tupdesc == NULL)
-		CatalogCacheInitializeCache(cache);
+	ConditionalCatalogCacheInitializeCache(cache);
 
 	Assert(nkeys > 0 && nkeys < cache->cc_nkeys);
 
@@ -2226,8 +2248,7 @@ PrepareToInvalidateCacheTuple(Relation relation,
 			continue;
 
 		/* Just in case cache hasn't finished initialization yet... */
-		if (ccp->cc_tupdesc == NULL)
-			CatalogCacheInitializeCache(ccp);
+		ConditionalCatalogCacheInitializeCache(ccp);
 
 		hashvalue = CatalogCacheComputeTupleHashValue(ccp, ccp->cc_nkeys, tuple);
 		dbid = ccp->cc_relisshared ? (Oid) 0 : MyDatabaseId;

src/backend/utils/cache/inval.c

@@ -720,6 +720,12 @@ InvalidateSystemCachesExtended(bool debug_discard)
 void
 AcceptInvalidationMessages(void)
 {
+#ifdef USE_ASSERT_CHECKING
+	/* message handlers shall access catalogs only during transactions */
+	if (IsTransactionState())
+		AssertCouldGetRelation();
+#endif
+
 	ReceiveSharedInvalidMessages(LocalExecuteInvalidationMessage,
 								 InvalidateSystemCaches);
 
@@ -771,7 +777,8 @@ PrepareInvalidationState(void)
 {
 	TransInvalidationInfo *myInfo;
 
-	Assert(IsTransactionState());
+	/* PrepareToInvalidateCacheTuple() needs relcache */
+	AssertCouldGetRelation();
 	/* Can't queue transactional message while collecting inplace messages. */
 	Assert(inplaceInvalInfo == NULL);
 
@@ -807,7 +814,7 @@ PrepareInplaceInvalidationState(void)
 {
 	InvalidationInfo *myInfo;
 
-	Assert(IsTransactionState());
+	AssertCouldGetRelation();
 	/* limit of one inplace update under assembly */
 	Assert(inplaceInvalInfo == NULL);
 
@@ -1248,6 +1255,9 @@ CacheInvalidateHeapTupleCommon(Relation relation,
 	Oid			databaseId;
 	Oid			relationId;
 
+	/* PrepareToInvalidateCacheTuple() needs relcache */
+	AssertCouldGetRelation();
+
 	/* Do nothing during bootstrap */
 	if (IsBootstrapProcessingMode())
 		return;

src/backend/utils/cache/relcache.c

@@ -2020,6 +2020,23 @@ formrdesc(const char *relationName, Oid relationReltype,
 	relation->rd_isvalid = true;
 }
 
+#ifdef USE_ASSERT_CHECKING
+/*
+ *		AssertCouldGetRelation
+ *
+ *		Check safety of calling RelationIdGetRelation().
+ *
+ *		In code that reads catalogs in the event of a cache miss, call this
+ *		before checking the cache.
+ */
+void
+AssertCouldGetRelation(void)
+{
+	Assert(IsTransactionState());
+	AssertBufferLocksPermitCatalogRead();
+}
+#endif
+
 
 /* ----------------------------------------------------------------
  *				 Relation Descriptor Lookup Interface
@@ -2047,8 +2064,7 @@ RelationIdGetRelation(Oid relationId)
 {
 	Relation	rd;
 
-	/* Make sure we're in an xact, even if this ends up being a cache hit */
-	Assert(IsTransactionState());
+	AssertCouldGetRelation();
 
 	/*
 	 * first try to find reldesc in the cache

src/backend/utils/mb/mbutils.c

@@ -39,6 +39,7 @@
 #include "mb/pg_wchar.h"
 #include "utils/builtins.h"
 #include "utils/memutils.h"
+#include "utils/relcache.h"
 #include "utils/syscache.h"
 
 /*
@@ -310,7 +311,7 @@ InitializeClientEncoding(void)
 	{
 		Oid			utf8_to_server_proc;
 
-		Assert(IsTransactionState());
+		AssertCouldGetRelation();
 		utf8_to_server_proc =
 			FindDefaultConversionProc(PG_UTF8,
 									  current_server_encoding);

src/include/storage/bufmgr.h

@@ -196,6 +196,9 @@ extern void InitBufferPool(void);
 extern void InitBufferPoolAccess(void);
 extern void InitBufferPoolBackend(void);
 extern void AtEOXact_Buffers(bool isCommit);
+#ifdef USE_ASSERT_CHECKING
+extern void AssertBufferLocksPermitCatalogRead(void);
+#endif
 extern void PrintBufferLeakWarning(Buffer buffer);
 extern void CheckPointBuffers(int flags);
 extern BlockNumber BufferGetBlockNumber(Buffer buffer);

src/include/storage/lwlock.h

@@ -127,6 +127,8 @@ extern bool LWLockAcquireOrWait(LWLock *lock, LWLockMode mode);
 extern void LWLockRelease(LWLock *lock);
 extern void LWLockReleaseClearVar(LWLock *lock, uint64 *valptr, uint64 val);
 extern void LWLockReleaseAll(void);
+extern void ForEachLWLockHeldByMe(void (*callback) (LWLock *, LWLockMode, void *),
+								  void *context);
 extern bool LWLockHeldByMe(LWLock *lock);
 extern bool LWLockAnyHeldByMe(LWLock *lock, int nlocks, size_t stride);
 extern bool LWLockHeldByMeInMode(LWLock *lock, LWLockMode mode);

src/include/utils/relcache.h

@@ -36,6 +36,14 @@ typedef Relation *RelationPtr;
 /*
  * Routines to open (lookup) and close a relcache entry
  */
+#ifdef USE_ASSERT_CHECKING
+extern void AssertCouldGetRelation(void);
+#else
+static inline void
+AssertCouldGetRelation(void)
+{
+}
+#endif
 extern Relation RelationIdGetRelation(Oid relationId);
 extern void RelationClose(Relation relation);