Disallow SELECT FOR UPDATE/SHARE on sequences.

We can't allow this because such an operation stores its transaction XID
into the sequence tuple's xmax.  Because VACUUM doesn't process sequences
(and we don't want it to start doing so), such an xmax value won't get
frozen, meaning it will eventually refer to nonexistent pg_clog storage,
and even wrap around completely.  Since the row lock is ignored by nextval
and setval, the usefulness of the operation is highly debatable anyway.
Per reports of trouble with pgpool 3.0, which had ill-advisedly started
using such commands as a form of locking.

In HEAD, also disallow SELECT FOR UPDATE/SHARE on toast tables.  Although
this does work safely given the current implementation, there seems no
good reason to allow it.  I refrained from changing that behavior in
back branches, however.

src/backend/executor/execMain.c

@@ -74,6 +74,7 @@ ExecutorCheckPerms_hook_type ExecutorCheckPerms_hook = NULL;
 
 /* decls for local routines only used within this module */
 static void InitPlan(QueryDesc *queryDesc, int eflags);
+static void CheckValidRowMarkRel(Relation rel, RowMarkType markType);
 static void ExecPostprocessPlan(EState *estate);
 static void ExecEndPlan(PlanState *planstate, EState *estate);
 static void ExecutePlan(EState *estate, PlanState *planstate,
@@ -837,12 +838,9 @@ InitPlan(QueryDesc *queryDesc, int eflags)
 				break;
 		}
 
-		/* if foreign table, tuples can't be locked */
-		if (relation && relation->rd_rel->relkind == RELKIND_FOREIGN_TABLE)
-			ereport(ERROR,
-					(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
-					 errmsg("SELECT FOR UPDATE/SHARE cannot be used with foreign table \"%s\"",
-							RelationGetRelationName(relation))));
+		/* Check that relation is a legal target for marking */
+		if (relation)
+			CheckValidRowMarkRel(relation, rc->markType);
 
 		erm = (ExecRowMark *) palloc(sizeof(ExecRowMark));
 		erm->relation = relation;
@@ -977,6 +975,9 @@ InitPlan(QueryDesc *queryDesc, int eflags)
  * In most cases parser and/or planner should have noticed this already, but
  * let's make sure.  In the view case we do need a test here, because if the
  * view wasn't rewritten by a rule, it had better have an INSTEAD trigger.
+ *
+ * Note: when changing this function, you probably also need to look at
+ * CheckValidRowMarkRel.
  */
 void
 CheckValidResultRel(Relation resultRel, CmdType operation)
@@ -1047,6 +1048,57 @@ CheckValidResultRel(Relation resultRel, CmdType operation)
 	}
 }
 
+/*
+ * Check that a proposed rowmark target relation is a legal target
+ *
+ * In most cases parser and/or planner should have noticed this already, but
+ * they don't cover all cases.
+ */
+static void
+CheckValidRowMarkRel(Relation rel, RowMarkType markType)
+{
+	switch (rel->rd_rel->relkind)
+	{
+		case RELKIND_RELATION:
+			/* OK */
+			break;
+		case RELKIND_SEQUENCE:
+			/* Must disallow this because we don't vacuum sequences */
+			ereport(ERROR,
+					(errcode(ERRCODE_WRONG_OBJECT_TYPE),
+					 errmsg("cannot lock rows in sequence \"%s\"",
+							RelationGetRelationName(rel))));
+			break;
+		case RELKIND_TOASTVALUE:
+			/* We could allow this, but there seems no good reason to */
+			ereport(ERROR,
+					(errcode(ERRCODE_WRONG_OBJECT_TYPE),
+					 errmsg("cannot lock rows in TOAST relation \"%s\"",
+							RelationGetRelationName(rel))));
+			break;
+		case RELKIND_VIEW:
+			/* Should not get here */
+			ereport(ERROR,
+					(errcode(ERRCODE_WRONG_OBJECT_TYPE),
+					 errmsg("cannot lock rows in view \"%s\"",
+							RelationGetRelationName(rel))));
+			break;
+		case RELKIND_FOREIGN_TABLE:
+			/* Perhaps we can support this someday, but not today */
+			ereport(ERROR,
+					(errcode(ERRCODE_WRONG_OBJECT_TYPE),
+					 errmsg("cannot lock rows in foreign table \"%s\"",
+							RelationGetRelationName(rel))));
+			break;
+		default:
+			ereport(ERROR,
+					(errcode(ERRCODE_WRONG_OBJECT_TYPE),
+					 errmsg("cannot lock rows in relation \"%s\"",
+							RelationGetRelationName(rel))));
+			break;
+	}
+}
+
 /*
  * Initialize ResultRelInfo data for one result relation
  *