From 2b3c948b8dc43cfd189312039447a874ff46532e Mon Sep 17 00:00:00 2001
From: Michael Zhilin <mizhka@FreeBSD.org>
Date: Sun, 31 Dec 2023 17:48:20 +0300
Subject: [PATCH v2] Avoid deadlock and concurrency during orphan temp table
 removal

If temp tables contains autogenerated objects (for example, sequences),
then deadlock with client worker and autovacuum worker is possible at
the moment of orphan temp table removal after unexpected server restart.

The fix idea is to avoid concurrency by conditional locking of namespace
in autovacuum worker.
---
 src/backend/postmaster/autovacuum.c | 20 +++++++++++++++
 src/backend/storage/lmgr/lmgr.c     | 38 +++++++++++++++++++++++++++++
 src/include/storage/lmgr.h          |  2 ++
 3 files changed, 60 insertions(+)

diff --git a/src/backend/postmaster/autovacuum.c b/src/backend/postmaster/autovacuum.c
index 71e8a6f2584..3f4f507d99f 100644
--- a/src/backend/postmaster/autovacuum.c
+++ b/src/backend/postmaster/autovacuum.c
@@ -76,6 +76,7 @@
 #include "catalog/dependency.h"
 #include "catalog/namespace.h"
 #include "catalog/pg_database.h"
+#include "catalog/pg_namespace.h"
 #include "commands/dbcommands.h"
 #include "commands/vacuum.h"
 #include "common/int.h"
@@ -2175,6 +2176,19 @@ do_autovacuum(void)
 			continue;
 		}
 
+		/*
+		 * Try to lock the table namespace.  As explained above, we try to avoid
+		 * concurrency on table removals. If backend or another autovacuum
+		 * worker tries to drop orphan temp tables, let it do its job.
+		 */
+		if (!ConditionalLockDatabaseObject(NamespaceRelationId,
+										   classForm->relnamespace, 0,
+										   AccessShareLock))
+		{
+			UnlockRelationOid(relid, AccessExclusiveLock);
+			continue;
+		}
+
 		/* OK, let's delete it */
 		ereport(LOG,
 				(errmsg("autovacuum: dropping orphan temp table \"%s.%s.%s\"",
@@ -2201,6 +2215,12 @@ do_autovacuum(void)
 		MemoryContextSwitchTo(AutovacMemCxt);
 	}
 
+	if (orphan_oids != NIL) {
+		ereport(LOG,
+				(errmsg("autovacuum: dropping orphan temp tables completed")));
+	}
+
+
 	/*
 	 * Optionally, create a buffer access strategy object for VACUUM to use.
 	 * We use the same BufferAccessStrategy object for all tables VACUUMed by
diff --git a/src/backend/storage/lmgr/lmgr.c b/src/backend/storage/lmgr/lmgr.c
index 41fd856c654..c881b36a44c 100644
--- a/src/backend/storage/lmgr/lmgr.c
+++ b/src/backend/storage/lmgr/lmgr.c
@@ -1018,6 +1018,44 @@ LockDatabaseObject(Oid classid, Oid objid, uint16 objsubid,
 	AcceptInvalidationMessages();
 }
 
+/*
+ *		ConditionalLockDatabaseObject
+ *
+ * As above, but only lock if we can get the lock without blocking.
+ * Returns true iff the lock was acquired.
+ */
+bool
+ConditionalLockDatabaseObject(Oid classid, Oid objid, uint16 objsubid,
+		   LOCKMODE lockmode)
+{
+	LOCKTAG		tag;
+	LOCALLOCK  *locallock;
+	LockAcquireResult res;
+
+	SET_LOCKTAG_OBJECT(tag,
+					   MyDatabaseId,
+					   classid,
+					   objid,
+					   objsubid);
+
+	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock);
+
+	if (res == LOCKACQUIRE_NOT_AVAIL)
+		return false;
+
+	/*
+	 * Now that we have the lock, check for invalidation messages; see notes
+	 * in LockRelationOid.
+	 */
+	if (res != LOCKACQUIRE_ALREADY_CLEAR)
+	{
+		AcceptInvalidationMessages();
+		MarkLockClear(locallock);
+	}
+
+	return true;
+}
+
 /*
  *		UnlockDatabaseObject
  */
diff --git a/src/include/storage/lmgr.h b/src/include/storage/lmgr.h
index e8bd71ba689..22b7856ef1f 100644
--- a/src/include/storage/lmgr.h
+++ b/src/include/storage/lmgr.h
@@ -93,6 +93,8 @@ extern void SpeculativeInsertionWait(TransactionId xid, uint32 token);
 /* Lock a general object (other than a relation) of the current database */
 extern void LockDatabaseObject(Oid classid, Oid objid, uint16 objsubid,
 							   LOCKMODE lockmode);
+extern bool ConditionalLockDatabaseObject(Oid classid, Oid objid,
+										  uint16 objsubid, LOCKMODE lockmode);
 extern void UnlockDatabaseObject(Oid classid, Oid objid, uint16 objsubid,
 								 LOCKMODE lockmode);
 
-- 
2.42.0