Make autovacuum more aggressive to remove orphaned temp tables

Commit dafa084, added in 10, made the removal of temporary orphaned
tables more aggressive.  This commit makes an extra step into the
aggressiveness by adding a flag in each backend's MyProc which tracks
down any temporary namespace currently in use.  The flag is set when the
namespace gets created and can be reset if the temporary namespace has
been created in a transaction or sub-transaction which is aborted.  The
flag value assignment is assumed to be atomic, so this can be done in a
lock-less fashion like other flags already present in PGPROC like
databaseId or backendId, still the fact that the temporary namespace and
table created are still locked until the transaction creating those
commits acts as a barrier for other backends.

This new flag gets used by autovacuum to discard more aggressively
orphaned tables by additionally checking for the database a backend is
connected to as well as its temporary namespace in-use, removing
orphaned temporary relations even if a backend reuses the same slot as
one which created temporary relations in a past session.

The base idea of this patch comes from Robert Haas, has been written in
its first version by Tsunakawa Takayuki, then heavily reviewed by me.

Author: Tsunakawa Takayuki
Reviewed-by: Michael Paquier, Kyotaro Horiguchi, Andres Freund
Discussion: https://postgr.es/m/0A3221C70F24FB45833433255569204D1F8A4DC6@G01JPEXMBYT05
Backpatch: 11-, as PGPROC gains a new flag and we don't want silent ABI
breakages on already released versions.

Author: michaelpq

src/backend/access/transam/twophase.c

@@ -471,6 +471,7 @@ MarkAsPreparingGuts(GlobalTransaction gxact, TransactionId xid, const char *gid,
 	proc->backendId = InvalidBackendId;
 	proc->databaseId = databaseid;
 	proc->roleId = owner;
+	proc->tempNamespaceId = InvalidOid;
 	proc->isBackgroundWorker = false;
 	proc->lwWaiting = false;
 	proc->lwWaitMode = 0;

src/backend/catalog/namespace.c

@@ -47,7 +47,7 @@
 #include "parser/parse_func.h"
 #include "storage/ipc.h"
 #include "storage/lmgr.h"
-#include "storage/sinval.h"
+#include "storage/sinvaladt.h"
 #include "utils/acl.h"
 #include "utils/builtins.h"
 #include "utils/catcache.h"
@@ -3204,6 +3204,46 @@ isOtherTempNamespace(Oid namespaceId)
 	return isAnyTempNamespace(namespaceId);
 }
 
+/*
+ * isTempNamespaceInUse - is the given namespace owned and actively used
+ * by a backend?
+ *
+ * Note: this can be used while scanning relations in pg_class to detect
+ * orphaned temporary tables or namespaces with a backend connected to a
+ * given database.  The result may be out of date quickly, so the caller
+ * must be careful how to handle this information.
+ */
+bool
+isTempNamespaceInUse(Oid namespaceId)
+{
+	PGPROC	   *proc;
+	int			backendId;
+
+	Assert(OidIsValid(MyDatabaseId));
+
+	backendId = GetTempNamespaceBackendId(namespaceId);
+
+	if (backendId == InvalidBackendId ||
+		backendId == MyBackendId)
+		return false;
+
+	/* Is the backend alive? */
+	proc = BackendIdGetProc(backendId);
+	if (proc == NULL)
+		return false;
+
+	/* Is the backend connected to the same database we are looking at? */
+	if (proc->databaseId != MyDatabaseId)
+		return false;
+
+	/* Does the backend own the temporary namespace? */
+	if (proc->tempNamespaceId != namespaceId)
+		return false;
+
+	/* all good to go */
+	return true;
+}
+
 /*
  * GetTempNamespaceBackendId - if the given namespace is a temporary-table
  * namespace (either my own, or another backend's), return the BackendId
@@ -3893,6 +3933,16 @@ InitTempTableNamespace(void)
 	myTempNamespace = namespaceId;
 	myTempToastNamespace = toastspaceId;
 
+	/*
+	 * Mark MyProc as owning this namespace which other processes can use to
+	 * decide if a temporary namespace is in use or not.  We assume that
+	 * assignment of namespaceId is an atomic operation.  Even if it is not,
+	 * the temporary relation which resulted in the creation of this temporary
+	 * namespace is still locked until the current transaction commits, so it
+	 * would not be accessible yet, acting as a barrier.
+	 */
+	MyProc->tempNamespaceId = namespaceId;
+
 	/* It should not be done already. */
 	AssertState(myTempNamespaceSubID == InvalidSubTransactionId);
 	myTempNamespaceSubID = GetCurrentSubTransactionId();
@@ -3923,6 +3973,15 @@ AtEOXact_Namespace(bool isCommit, bool parallel)
 			myTempNamespace = InvalidOid;
 			myTempToastNamespace = InvalidOid;
 			baseSearchPathValid = false;	/* need to rebuild list */
+
+			/*
+			 * Reset the temporary namespace flag in MyProc.  We assume that
+			 * this operation is atomic.  Even if it is not, the temporary
+			 * table which created this namespace is still locked until this
+			 * transaction aborts so it would not be visible yet, acting as a
+			 * barrier.
+			 */
+			MyProc->tempNamespaceId = InvalidOid;
 		}
 		myTempNamespaceSubID = InvalidSubTransactionId;
 	}
@@ -3975,6 +4034,15 @@ AtEOSubXact_Namespace(bool isCommit, SubTransactionId mySubid,
 			myTempNamespace = InvalidOid;
 			myTempToastNamespace = InvalidOid;
 			baseSearchPathValid = false;	/* need to rebuild list */
+
+			/*
+			 * Reset the temporary namespace flag in MyProc.  We assume that
+			 * this operation is atomic.  Even if it is not, the temporary
+			 * table which created this namespace is still locked until this
+			 * transaction aborts so it would not be visible yet, acting as a
+			 * barrier.
+			 */
+			MyProc->tempNamespaceId = InvalidOid;
 		}
 	}
 

src/backend/postmaster/autovacuum.c

@@ -2084,14 +2084,11 @@ do_autovacuum(void)
 		 */
 		if (classForm->relpersistence == RELPERSISTENCE_TEMP)
 		{
-			int			backendID;
-
-			backendID = GetTempNamespaceBackendId(classForm->relnamespace);
-
-			/* We just ignore it if the owning backend is still active */
-			if (backendID != InvalidBackendId &&
-				(backendID == MyBackendId ||
-				 BackendIdGetProc(backendID) == NULL))
+			/*
+			 * We just ignore it if the owning backend is still active and
+			 * using the temporary schema.
+			 */
+			if (!isTempNamespaceInUse(classForm->relnamespace))
 			{
 				/*
 				 * The table seems to be orphaned -- although it might be that
@@ -2219,7 +2216,6 @@ do_autovacuum(void)
 	{
 		Oid			relid = lfirst_oid(cell);
 		Form_pg_class classForm;
-		int			backendID;
 		ObjectAddress object;
 
 		/*
@@ -2261,10 +2257,8 @@ do_autovacuum(void)
 			UnlockRelationOid(relid, AccessExclusiveLock);
 			continue;
 		}
-		backendID = GetTempNamespaceBackendId(classForm->relnamespace);
-		if (!(backendID != InvalidBackendId &&
-			  (backendID == MyBackendId ||
-			   BackendIdGetProc(backendID) == NULL)))
+
+		if (isTempNamespaceInUse(classForm->relnamespace))
 		{
 			UnlockRelationOid(relid, AccessExclusiveLock);
 			continue;

src/backend/storage/lmgr/proc.c

@@ -371,6 +371,7 @@ InitProcess(void)
 	MyProc->backendId = InvalidBackendId;
 	MyProc->databaseId = InvalidOid;
 	MyProc->roleId = InvalidOid;
+	MyProc->tempNamespaceId = InvalidOid;
 	MyProc->isBackgroundWorker = IsBackgroundWorker;
 	MyPgXact->delayChkpt = false;
 	MyPgXact->vacuumFlags = 0;
@@ -552,6 +553,7 @@ InitAuxiliaryProcess(void)
 	MyProc->backendId = InvalidBackendId;
 	MyProc->databaseId = InvalidOid;
 	MyProc->roleId = InvalidOid;
+	MyProc->tempNamespaceId = InvalidOid;
 	MyProc->isBackgroundWorker = IsBackgroundWorker;
 	MyPgXact->delayChkpt = false;
 	MyPgXact->vacuumFlags = 0;

src/include/catalog/namespace.h

@@ -137,6 +137,7 @@ extern bool isTempToastNamespace(Oid namespaceId);
 extern bool isTempOrTempToastNamespace(Oid namespaceId);
 extern bool isAnyTempNamespace(Oid namespaceId);
 extern bool isOtherTempNamespace(Oid namespaceId);
+extern bool isTempNamespaceInUse(Oid namespaceId);
 extern int	GetTempNamespaceBackendId(Oid namespaceId);
 extern Oid	GetTempToastNamespace(void);
 extern void GetTempNamespaceState(Oid *tempNamespaceId,

src/include/storage/proc.h

@@ -114,6 +114,9 @@ struct PGPROC
 	Oid			databaseId;		/* OID of database this backend is using */
 	Oid			roleId;			/* OID of role using this backend */
 
+	Oid			tempNamespaceId;	/* OID of temp schema this backend is
+									 * using */
+
 	bool		isBackgroundWorker; /* true if background worker. */
 
 	/*