From c117838597b1a28f6f0feb4a41adff1a7e5e0bc9 Mon Sep 17 00:00:00 2001
From: Tom Lane <tgl@sss.pgh.pa.us>
Date: Thu, 2 Jun 2011 14:46:22 -0400
Subject: [PATCH] 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 | 39 +++++++++++++++++++++++++++++++++
 1 file changed, 39 insertions(+)

diff --git a/src/backend/executor/execMain.c b/src/backend/executor/execMain.c
index c477314efa..bbe67ed8bb 100644
--- a/src/backend/executor/execMain.c
+++ b/src/backend/executor/execMain.c
@@ -722,6 +722,45 @@ InitPlan(QueryDesc *queryDesc, int eflags)
 				break;
 		}
 
+		/*
+		 * Check that relation is a legal target for marking.
+		 *
+		 * In most cases parser and/or planner should have noticed this
+		 * already, but they don't cover all cases.
+		 */
+		if (relation)
+		{
+			switch (relation->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(relation))));
+					break;
+				case RELKIND_TOASTVALUE:
+					/* This will be disallowed in 9.1, but for now OK */
+					break;
+				case RELKIND_VIEW:
+					/* Should not get here */
+					ereport(ERROR,
+							(errcode(ERRCODE_WRONG_OBJECT_TYPE),
+							 errmsg("cannot lock rows in view \"%s\"",
+									RelationGetRelationName(relation))));
+					break;
+				default:
+					ereport(ERROR,
+							(errcode(ERRCODE_WRONG_OBJECT_TYPE),
+							 errmsg("cannot lock rows in relation \"%s\"",
+									RelationGetRelationName(relation))));
+					break;
+			}
+		}
+
 		erm = (ExecRowMark *) palloc(sizeof(ExecRowMark));
 		erm->relation = relation;
 		erm->rti = rc->rti;