Allow read only connections during recovery, known as Hot Standby.

Enabled by recovery_connections = on (default) and forcing archive recovery using a recovery.conf. Recovery processing now emulates the original transactions as they are replayed, providing full locking and MVCC behaviour for read only queries. Recovery must enter consistent state before connections are allowed, so there is a delay, typically short, before connections succeed. Replay of recovering transactions can conflict and in some cases deadlock with queries during recovery; these result in query cancellation after max_standby_delay seconds have expired. Infrastructure changes have minor effects on normal running, though introduce four new types of WAL record.

New test mode "make standbycheck" allows regression tests of static command behaviour on a standby server while in recovery. Typical and extreme dynamic behaviours have been checked via code inspection and manual testing. Few port specific behaviours have been utilised, though primary testing has been on Linux only so far.

This commit is the basic patch. Additional changes will follow in this release to enhance some aspects of behaviour, notably improved handling of conflicts, deadlock detection and query cancellation. Changes to VACUUM FULL are also required.

Simon Riggs, with significant and lengthy review by Heikki Linnakangas, including streamlined redesign of snapshot creation and two-phase commit.

Important contributions from Florian Pflug, Mark Kirkwood, Merlin Moncure, Greg Stark, Gianni Ciolli, Gabriele Bartolini, Hannu Krosing, Robert Haas, Tatsuo Ishii, Hiroyuki Yamada plus support and feedback from many other community members.

src/backend/access/index/genam.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/access/index/genam.c,v 1.77 2009/12/07 05:22:21 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/access/index/genam.c,v 1.78 2009/12/19 01:32:32 sriggs Exp $
  *
  * NOTES
  *	  many of the old access method routines have been turned into
@@ -91,8 +91,19 @@ RelationGetIndexScan(Relation indexRelation,
 	else
 		scan->keyData = NULL;
 
+	/*
+	 * During recovery we ignore killed tuples and don't bother to kill them
+	 * either. We do this because the xmin on the primary node could easily
+	 * be later than the xmin on the standby node, so that what the primary
+	 * thinks is killed is supposed to be visible on standby. So for correct
+	 * MVCC for queries during recovery we must ignore these hints and check
+	 * all tuples. Do *not* set ignore_killed_tuples to true when running
+	 * in a transaction that was started during recovery.
+	 * xactStartedInRecovery should not be altered by index AMs.
+	 */
 	scan->kill_prior_tuple = false;
-	scan->ignore_killed_tuples = true;	/* default setting */
+	scan->xactStartedInRecovery = TransactionStartedDuringRecovery();
+	scan->ignore_killed_tuples = !scan->xactStartedInRecovery;
 
 	scan->opaque = NULL;
 

src/backend/access/index/indexam.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/access/index/indexam.c,v 1.115 2009/07/29 20:56:18 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/access/index/indexam.c,v 1.116 2009/12/19 01:32:32 sriggs Exp $
  *
  * INTERFACE ROUTINES
  *		index_open		- open an index relation by relation OID
@@ -455,9 +455,12 @@ index_getnext(IndexScanDesc scan, ScanDirection direction)
 
 			/*
 			 * If we scanned a whole HOT chain and found only dead tuples,
-			 * tell index AM to kill its entry for that TID.
+			 * tell index AM to kill its entry for that TID. We do not do
+			 * this when in recovery because it may violate MVCC to do so.
+			 * see comments in RelationGetIndexScan().
 			 */
-			scan->kill_prior_tuple = scan->xs_hot_dead;
+			if (!scan->xactStartedInRecovery)
+				scan->kill_prior_tuple = scan->xs_hot_dead;
 
 			/*
 			 * The AM's gettuple proc finds the next index entry matching the