Implement sharable row-level locks, and use them for foreign key references

to eliminate unnecessary deadlocks.  This commit adds SELECT ... FOR SHARE
paralleling SELECT ... FOR UPDATE.  The implementation uses a new SLRU
data structure (managed much like pg_subtrans) to represent multiple-
transaction-ID sets.  When more than one transaction is holding a shared
lock on a particular row, we create a MultiXactId representing that set
of transactions and store its ID in the row's XMAX.  This scheme allows
an effectively unlimited number of row locks, just as we did before,
while not costing any extra overhead except when a shared lock actually
has to be shared.   Still TODO: use the regular lock manager to control
the grant order when multiple backends are waiting for a row lock.

Alvaro Herrera and Tom Lane.

src/backend/catalog/index.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/catalog/index.c,v 1.252 2005/04/14 20:03:23 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/catalog/index.c,v 1.253 2005/04/28 21:47:11 tgl Exp $
  *
  *
  * INTERFACE ROUTINES
@@ -471,7 +471,7 @@ index_create(Oid heapRelationId,
 	int			i;
 
 	/*
-	 * Only SELECT ... FOR UPDATE are allowed while doing this
+	 * Only SELECT ... FOR UPDATE/SHARE are allowed while doing this
 	 */
 	heapRelation = heap_open(heapRelationId, ShareLock);
 
@@ -1460,6 +1460,7 @@ IndexBuildHeapScan(Relation heapRelation,
 					 * a system catalog, because we often release lock on
 					 * system catalogs before committing.
 					 */
+					Assert(!(heapTuple->t_data->t_infomask & HEAP_XMAX_IS_MULTI));
 					if (!TransactionIdIsCurrentTransactionId(
 							   HeapTupleHeaderGetXmax(heapTuple->t_data))
 						&& !IsSystemRelation(heapRelation))