Make CLUSTER lock the old table's toast table before copying data.

We must lock out autovacuuming of the old toast table before computing the
OldestXmin horizon we will use.  Otherwise, autovacuum could start on the
toast table later, compute a later OldestXmin horizon, and remove as DEAD
toast tuples that we still need (because we think their parent tuples are
only RECENTLY_DEAD).  Per further thought about bug #5998.

src/backend/commands/cluster.c

@@ -38,6 +38,7 @@
 #include "miscadmin.h"
 #include "optimizer/planner.h"
 #include "storage/bufmgr.h"
+#include "storage/lmgr.h"
 #include "storage/procarray.h"
 #include "storage/smgr.h"
 #include "utils/acl.h"
@@ -750,6 +751,22 @@ copy_heap_data(Oid OIDNewHeap, Oid OIDOldHeap, Oid OIDOldIndex,
 	values = (Datum *) palloc(natts * sizeof(Datum));
 	isnull = (bool *) palloc(natts * sizeof(bool));
 
+	/*
+	 * If the OldHeap has a toast table, get lock on the toast table to keep
+	 * it from being vacuumed.  This is needed because autovacuum processes
+	 * toast tables independently of their main tables, with no lock on the
+	 * latter.  If an autovacuum were to start on the toast table after we
+	 * compute our OldestXmin below, it would use a later OldestXmin, and then
+	 * possibly remove as DEAD toast tuples belonging to main tuples we think
+	 * are only RECENTLY_DEAD.  Then we'd fail while trying to copy those
+	 * tuples.
+	 *
+	 * We don't need to open the toast relation here, just lock it.  The lock
+	 * will be held till end of transaction.
+	 */
+	if (OldHeap->rd_rel->reltoastrelid)
+		LockRelationOid(OldHeap->rd_rel->reltoastrelid, AccessExclusiveLock);
+
 	/*
 	 * We need to log the copied data in WAL iff WAL archiving/streaming is
 	 * enabled AND it's not a WAL-logged rel.