Improve the situation for parallel query versus temp relations.

Transmit the leader's temp-namespace state to workers.  This is important
because without it, the workers do not really have the same search path
as the leader.  For example, there is no good reason (and no extant code
either) to prevent a worker from executing a temp function that the
leader created previously; but as things stood it would fail to find the
temp function, and then either fail or execute the wrong function entirely.

We still prohibit a worker from creating a temp namespace on its own.
In effect, a worker can only see the session's temp namespace if the leader
had created it before starting the worker, which seems like the right
semantics.

Also, transmit the leader's BackendId to workers, and arrange for workers
to use that when determining the physical file path of a temp relation
belonging to their session.  While the original intent was to prevent such
accesses entirely, there were a number of holes in that, notably in places
like dbsize.c which assume they can safely access temp rels of other
sessions anyway.  We might as well get this right, as a small down payment
on someday allowing workers to access the leader's temp tables.  (With
this change, directly using "MyBackendId" as a relation or buffer backend
ID is deprecated; you should use BackendIdForTempRelations() instead.
I left a couple of such uses alone though, as they're not going to be
reachable in parallel workers until we do something about localbuf.c.)

Move the thou-shalt-not-access-thy-leader's-temp-tables prohibition down
into localbuf.c, which is where it actually matters, instead of having it
in relation_open().  This amounts to recognizing that access to temp
tables' catalog entries is perfectly safe in a worker, it's only the data
in local buffers that is problematic.

Having done all that, we can get rid of the test in has_parallel_hazard()
that says that use of a temp table's rowtype is unsafe in parallel workers.
That test was unduly expensive, and if we really did need such a
prohibition, that was not even close to being a bulletproof guard for it.
(For example, any user-defined function executed in a parallel worker
might have attempted such access.)

src/backend/access/heap/heapam.c

@@ -1131,13 +1131,7 @@ relation_open(Oid relationId, LOCKMODE lockmode)
 
 	/* Make note that we've accessed a temporary relation */
 	if (RelationUsesLocalBuffers(r))
-	{
-		if (IsParallelWorker())
-			ereport(ERROR,
-					(errcode(ERRCODE_INVALID_TRANSACTION_STATE),
-					 errmsg("cannot access temporary tables during a parallel operation")));
 		MyXactAccessedTempRel = true;
-	}
 
 	pgstat_initstats(r);
 
@@ -1183,13 +1177,7 @@ try_relation_open(Oid relationId, LOCKMODE lockmode)
 
 	/* Make note that we've accessed a temporary relation */
 	if (RelationUsesLocalBuffers(r))
-	{
-		if (IsParallelWorker())
-			ereport(ERROR,
-					(errcode(ERRCODE_INVALID_TRANSACTION_STATE),
-					 errmsg("cannot access temporary tables during a parallel operation")));
 		MyXactAccessedTempRel = true;
-	}
 
 	pgstat_initstats(r);
 

src/backend/access/transam/parallel.c

@@ -17,6 +17,7 @@
 #include "access/xact.h"
 #include "access/xlog.h"
 #include "access/parallel.h"
+#include "catalog/namespace.h"
 #include "commands/async.h"
 #include "libpq/libpq.h"
 #include "libpq/pqformat.h"
@@ -67,6 +68,8 @@ typedef struct FixedParallelState
 	Oid			database_id;
 	Oid			authenticated_user_id;
 	Oid			current_user_id;
+	Oid			temp_namespace_id;
+	Oid			temp_toast_namespace_id;
 	int			sec_context;
 	PGPROC	   *parallel_master_pgproc;
 	pid_t		parallel_master_pid;
@@ -288,6 +291,8 @@ InitializeParallelDSM(ParallelContext *pcxt)
 	fps->database_id = MyDatabaseId;
 	fps->authenticated_user_id = GetAuthenticatedUserId();
 	GetUserIdAndSecContext(&fps->current_user_id, &fps->sec_context);
+	GetTempNamespaceState(&fps->temp_namespace_id,
+						  &fps->temp_toast_namespace_id);
 	fps->parallel_master_pgproc = MyProc;
 	fps->parallel_master_pid = MyProcPid;
 	fps->parallel_master_backend_id = MyBackendId;
@@ -1019,6 +1024,13 @@ ParallelWorkerMain(Datum main_arg)
 	/* Restore user ID and security context. */
 	SetUserIdAndSecContext(fps->current_user_id, fps->sec_context);
 
+	/* Restore temp-namespace state to ensure search path matches leader's. */
+	SetTempNamespaceState(fps->temp_namespace_id,
+						  fps->temp_toast_namespace_id);
+
+	/* Set ParallelMasterBackendId so we know how to address temp relations. */
+	ParallelMasterBackendId = fps->parallel_master_backend_id;
+
 	/*
 	 * We've initialized all of our state now; nothing should change
 	 * hereafter.

src/backend/catalog/catalog.c

@@ -390,7 +390,7 @@ GetNewRelFileNode(Oid reltablespace, Relation pg_class, char relpersistence)
 	switch (relpersistence)
 	{
 		case RELPERSISTENCE_TEMP:
-			backend = MyBackendId;
+			backend = BackendIdForTempRelations();
 			break;
 		case RELPERSISTENCE_UNLOGGED:
 		case RELPERSISTENCE_PERMANENT:

src/backend/catalog/namespace.c

@@ -3073,6 +3073,51 @@ GetTempToastNamespace(void)
 }
 
 
+/*
+ * GetTempNamespaceState - fetch status of session's temporary namespace
+ *
+ * This is used for conveying state to a parallel worker, and is not meant
+ * for general-purpose access.
+ */
+void
+GetTempNamespaceState(Oid *tempNamespaceId, Oid *tempToastNamespaceId)
+{
+	/* Return namespace OIDs, or 0 if session has not created temp namespace */
+	*tempNamespaceId = myTempNamespace;
+	*tempToastNamespaceId = myTempToastNamespace;
+}
+
+/*
+ * SetTempNamespaceState - set status of session's temporary namespace
+ *
+ * This is used for conveying state to a parallel worker, and is not meant for
+ * general-purpose access.  By transferring these namespace OIDs to workers,
+ * we ensure they will have the same notion of the search path as their leader
+ * does.
+ */
+void
+SetTempNamespaceState(Oid tempNamespaceId, Oid tempToastNamespaceId)
+{
+	/* Worker should not have created its own namespaces ... */
+	Assert(myTempNamespace == InvalidOid);
+	Assert(myTempToastNamespace == InvalidOid);
+	Assert(myTempNamespaceSubID == InvalidSubTransactionId);
+
+	/* Assign same namespace OIDs that leader has */
+	myTempNamespace = tempNamespaceId;
+	myTempToastNamespace = tempToastNamespaceId;
+
+	/*
+	 * It's fine to leave myTempNamespaceSubID == InvalidSubTransactionId.
+	 * Even if the namespace is new so far as the leader is concerned, it's
+	 * not new to the worker, and we certainly wouldn't want the worker trying
+	 * to destroy it.
+	 */
+
+	baseSearchPathValid = false;	/* may need to rebuild list */
+}
+
+
 /*
  * GetOverrideSearchPath - fetch current search path definition in form
  * used by PushOverrideSearchPath.

src/backend/catalog/storage.c

@@ -85,7 +85,7 @@ RelationCreateStorage(RelFileNode rnode, char relpersistence)
 	switch (relpersistence)
 	{
 		case RELPERSISTENCE_TEMP:
-			backend = MyBackendId;
+			backend = BackendIdForTempRelations();
 			needs_wal = false;
 			break;
 		case RELPERSISTENCE_UNLOGGED:

src/backend/optimizer/util/clauses.c

@@ -115,7 +115,6 @@ static bool has_parallel_hazard_walker(Node *node,
 						   has_parallel_hazard_arg *context);
 static bool parallel_too_dangerous(char proparallel,
 					   has_parallel_hazard_arg *context);
-static bool typeid_is_temp(Oid typeid);
 static bool contain_nonstrict_functions_walker(Node *node, void *context);
 static bool contain_leaked_vars_walker(Node *node, void *context);
 static Relids find_nonnullable_rels_walker(Node *node, bool top_level);
@@ -1409,49 +1408,6 @@ has_parallel_hazard_walker(Node *node, has_parallel_hazard_arg *context)
 		return has_parallel_hazard_walker((Node *) rinfo->clause, context);
 	}
 
-	/*
-	 * It is an error for a parallel worker to touch a temporary table in any
-	 * way, so we can't handle nodes whose type is the rowtype of such a
-	 * table.
-	 */
-	if (!context->allow_restricted)
-	{
-		switch (nodeTag(node))
-		{
-			case T_Var:
-			case T_Const:
-			case T_Param:
-			case T_Aggref:
-			case T_WindowFunc:
-			case T_ArrayRef:
-			case T_FuncExpr:
-			case T_NamedArgExpr:
-			case T_OpExpr:
-			case T_DistinctExpr:
-			case T_NullIfExpr:
-			case T_FieldSelect:
-			case T_FieldStore:
-			case T_RelabelType:
-			case T_CoerceViaIO:
-			case T_ArrayCoerceExpr:
-			case T_ConvertRowtypeExpr:
-			case T_CaseExpr:
-			case T_CaseTestExpr:
-			case T_ArrayExpr:
-			case T_RowExpr:
-			case T_CoalesceExpr:
-			case T_MinMaxExpr:
-			case T_CoerceToDomain:
-			case T_CoerceToDomainValue:
-			case T_SetToDefault:
-				if (typeid_is_temp(exprType(node)))
-					return true;
-				break;
-			default:
-				break;
-		}
-	}
-
 	/*
 	 * For each node that might potentially call a function, we need to
 	 * examine the pg_proc.proparallel marking for that function to see
@@ -1558,17 +1514,6 @@ parallel_too_dangerous(char proparallel, has_parallel_hazard_arg *context)
 		return proparallel != PROPARALLEL_SAFE;
 }
 
-static bool
-typeid_is_temp(Oid typeid)
-{
-	Oid			relid = get_typ_typrelid(typeid);
-
-	if (!OidIsValid(relid))
-		return false;
-
-	return (get_rel_persistence(relid) == RELPERSISTENCE_TEMP);
-}
-
 /*****************************************************************************
  *		Check clauses for nonstrict functions
  *****************************************************************************/

src/backend/storage/buffer/localbuf.c

@@ -15,6 +15,7 @@
  */
 #include "postgres.h"
 
+#include "access/parallel.h"
 #include "catalog/catalog.h"
 #include "executor/instrument.h"
 #include "storage/buf_internals.h"
@@ -412,6 +413,19 @@ InitLocalBuffers(void)
 	HASHCTL		info;
 	int			i;
 
+	/*
+	 * Parallel workers can't access data in temporary tables, because they
+	 * have no visibility into the local buffers of their leader.  This is a
+	 * convenient, low-cost place to provide a backstop check for that.  Note
+	 * that we don't wish to prevent a parallel worker from accessing catalog
+	 * metadata about a temp table, so checks at higher levels would be
+	 * inappropriate.
+	 */
+	if (IsParallelWorker())
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_TRANSACTION_STATE),
+				 errmsg("cannot access temporary tables during a parallel operation")));
+
 	/* Allocate and zero buffer headers and auxiliary arrays */
 	LocalBufferDescriptors = (BufferDesc *) calloc(nbufs, sizeof(BufferDesc));
 	LocalBufferBlockPointers = (Block *) calloc(nbufs, sizeof(Block));

src/backend/utils/adt/dbsize.c

@@ -1004,7 +1004,7 @@ pg_relation_filepath(PG_FUNCTION_ARGS)
 			break;
 		case RELPERSISTENCE_TEMP:
 			if (isTempOrTempToastNamespace(relform->relnamespace))
-				backend = MyBackendId;
+				backend = BackendIdForTempRelations();
 			else
 			{
 				/* Do it the hard way. */

src/backend/utils/cache/relcache.c

@@ -993,7 +993,7 @@ RelationBuildDesc(Oid targetRelId, bool insertIt)
 		case RELPERSISTENCE_TEMP:
 			if (isTempOrTempToastNamespace(relation->rd_rel->relnamespace))
 			{
-				relation->rd_backend = MyBackendId;
+				relation->rd_backend = BackendIdForTempRelations();
 				relation->rd_islocaltemp = true;
 			}
 			else
@@ -2970,7 +2970,7 @@ RelationBuildLocalRelation(const char *relname,
 			break;
 		case RELPERSISTENCE_TEMP:
 			Assert(isTempOrTempToastNamespace(relnamespace));
-			rel->rd_backend = MyBackendId;
+			rel->rd_backend = BackendIdForTempRelations();
 			rel->rd_islocaltemp = true;
 			break;
 		default:

src/backend/utils/init/globals.c

@@ -71,6 +71,8 @@ char		postgres_exec_path[MAXPGPATH];		/* full path to backend */
 
 BackendId	MyBackendId = InvalidBackendId;
 
+BackendId	ParallelMasterBackendId = InvalidBackendId;
+
 Oid			MyDatabaseId = InvalidOid;
 
 Oid			MyDatabaseTableSpace = InvalidOid;

src/include/catalog/namespace.h

@@ -125,6 +125,10 @@ extern bool isAnyTempNamespace(Oid namespaceId);
 extern bool isOtherTempNamespace(Oid namespaceId);
 extern int	GetTempNamespaceBackendId(Oid namespaceId);
 extern Oid	GetTempToastNamespace(void);
+extern void GetTempNamespaceState(Oid *tempNamespaceId,
+					  Oid *tempToastNamespaceId);
+extern void SetTempNamespaceState(Oid tempNamespaceId,
+					  Oid tempToastNamespaceId);
 extern void ResetTempTableNamespace(void);
 
 extern OverrideSearchPath *GetOverrideSearchPath(MemoryContext context);

src/include/storage/backendid.h

@@ -24,4 +24,14 @@ typedef int BackendId;			/* unique currently active backend identifier */
 
 extern PGDLLIMPORT BackendId MyBackendId;		/* backend id of this backend */
 
+/* backend id of our parallel session leader, or InvalidBackendId if none */
+extern PGDLLIMPORT BackendId ParallelMasterBackendId;
+
+/*
+ * The BackendId to use for our session's temp relations is normally our own,
+ * but parallel workers should use their leader's ID.
+ */
+#define BackendIdForTempRelations() \
+	(ParallelMasterBackendId == InvalidBackendId ? MyBackendId : ParallelMasterBackendId)
+
 #endif   /* BACKENDID_H */