Implement WAIT FOR command

WAIT FOR is to be used on standby and specifies waiting for
the specific WAL location to be replayed.  This option is useful when
the user makes some data changes on primary and needs a guarantee to see
these changes are on standby.

WAIT FOR needs to wait without any snapshot held.  Otherwise, the snapshot
could prevent the replay of WAL records, implying a kind of self-deadlock.
This is why separate utility command seems appears to be the most robust
way to implement this functionality.  It's not possible to implement this as
a function.  Previous experience shows that stored procedures also have
limitation in this aspect.

Discussion: https://www.postgresql.org/message-id/flat/CAPpHfdsjtZLVzxjGT8rJHCYbM0D5dwkO+BBjcirozJ6nYbOW8Q@mail.gmail.com
Discussion: https://www.postgresql.org/message-id/flat/CABPTF7UNft368x-RgOXkfj475OwEbp%2BVVO-wEXz7StgjD_%3D6sw%40mail.gmail.com
Author: Kartyshov Ivan <i.kartyshov@postgrespro.ru>
Author: Alexander Korotkov <aekorotkov@gmail.com>
Author: Xuneng Zhou <xunengzhou@gmail.com>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Reviewed-by: Peter Eisentraut <peter.eisentraut@enterprisedb.com>
Reviewed-by: Dilip Kumar <dilipbalaut@gmail.com>
Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
Reviewed-by: Alexander Lakhin <exclusion@gmail.com>
Reviewed-by: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>
Reviewed-by: Euler Taveira <euler@eulerto.com>
Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Reviewed-by: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Reviewed-by: jian he <jian.universality@gmail.com>
Reviewed-by: Álvaro Herrera <alvherre@kurilemu.de>
Reviewed-by: Xuneng Zhou <xunengzhou@gmail.com>

2 files changed, 29 insertions, 5 deletions

diff --git a/src/backend/tcop/pquery.c b/src/backend/tcop/pquery.c
index 74179139fa9..fde78c55160 100644
--- a/src/backend/tcop/pquery.c
+++ b/src/backend/tcop/pquery.c
@@ -1158,10 +1158,11 @@ PortalRunUtility(Portal portal, PlannedStmt *pstmt,
 	MemoryContextSwitchTo(portal->portalContext);
 
 	/*
-	 * Some utility commands (e.g., VACUUM) pop the ActiveSnapshot stack from
-	 * under us, so don't complain if it's now empty.  Otherwise, our snapshot
-	 * should be the top one; pop it.  Note that this could be a different
-	 * snapshot from the one we made above; see EnsurePortalSnapshotExists.
+	 * Some utility commands (e.g., VACUUM, WAIT FOR) pop the ActiveSnapshot
+	 * stack from under us, so don't complain if it's now empty.  Otherwise,
+	 * our snapshot should be the top one; pop it.  Note that this could be a
+	 * different snapshot from the one we made above; see
+	 * EnsurePortalSnapshotExists.
 	 */
 	if (portal->portalSnapshot != NULL && ActiveSnapshotSet())
 	{
@@ -1738,7 +1739,8 @@ PlannedStmtRequiresSnapshot(PlannedStmt *pstmt)
 		IsA(utilityStmt, ListenStmt) ||
 		IsA(utilityStmt, NotifyStmt) ||
 		IsA(utilityStmt, UnlistenStmt) ||
-		IsA(utilityStmt, CheckPointStmt))
+		IsA(utilityStmt, CheckPointStmt) ||
+		IsA(utilityStmt, WaitStmt))
 		return false;
 
 	return true;
diff --git a/src/backend/tcop/utility.c b/src/backend/tcop/utility.c
index 918db53dd5e..082967c0a86 100644
--- a/src/backend/tcop/utility.c
+++ b/src/backend/tcop/utility.c
@@ -56,6 +56,7 @@
 #include "commands/user.h"
 #include "commands/vacuum.h"
 #include "commands/view.h"
+#include "commands/wait.h"
 #include "miscadmin.h"
 #include "parser/parse_utilcmd.h"
 #include "postmaster/bgwriter.h"
@@ -266,6 +267,7 @@ ClassifyUtilityCommandAsReadOnly(Node *parsetree)
 		case T_PrepareStmt:
 		case T_UnlistenStmt:
 		case T_VariableSetStmt:
+		case T_WaitStmt:
 			{
 				/*
 				 * These modify only backend-local state, so they're OK to run
@@ -1055,6 +1057,12 @@ standard_ProcessUtility(PlannedStmt *pstmt,
 				break;
 			}
 
+		case T_WaitStmt:
+			{
+				ExecWaitStmt(pstate, (WaitStmt *) parsetree, dest);
+			}
+			break;
+
 		default:
 			/* All other statement types have event trigger support */
 			ProcessUtilitySlow(pstate, pstmt, queryString,
@@ -2059,6 +2067,9 @@ UtilityReturnsTuples(Node *parsetree)
 		case T_VariableShowStmt:
 			return true;
 
+		case T_WaitStmt:
+			return true;
+
 		default:
 			return false;
 	}
@@ -2114,6 +2125,9 @@ UtilityTupleDescriptor(Node *parsetree)
 				return GetPGVariableResultDesc(n->name);
 			}
 
+		case T_WaitStmt:
+			return WaitStmtResultDesc((WaitStmt *) parsetree);
+
 		default:
 			return NULL;
 	}
@@ -3091,6 +3105,10 @@ CreateCommandTag(Node *parsetree)
 			}
 			break;
 
+		case T_WaitStmt:
+			tag = CMDTAG_WAIT;
+			break;
+
 			/* already-planned queries */
 		case T_PlannedStmt:
 			{
@@ -3689,6 +3707,10 @@ GetCommandLogLevel(Node *parsetree)
 			lev = LOGSTMT_DDL;
 			break;
 
+		case T_WaitStmt:
+			lev = LOGSTMT_ALL;
+			break;
+
 			/* already-planned queries */
 		case T_PlannedStmt:
 			{