Implement waiting for given lsn at transaction start

This commit adds following optional clause to BEGIN and START TRANSACTION
commands.

  WAIT FOR LSN lsn [ TIMEOUT timeout ]

New clause pospones transaction start till given lsn is applied on standby.
This clause allows user be sure, that changes previously made on primary would
be visible on standby.

New shared memory struct is used to track awaited lsn per backend.  Recovery
process wakes up backend once required lsn is applied.

Author: Ivan Kartyshov, Anna Akenteva
Reviewed-by: Craig Ringer, Thomas Munro, Robert Haas, Kyotaro Horiguchi
Reviewed-by: Masahiko Sawada, Ants Aasma, Dmitry Ivanov, Simon Riggs
Reviewed-by: Amit Kapila, Alexander Korotkov
Discussion: https://postgr.es/m/0240c26c-9f84-30ea-fca9-93ab2df5f305%40postgrespro.ru

Author: akorotkov

doc/src/sgml/ref/begin.sgml

@@ -21,7 +21,7 @@ PostgreSQL documentation
 
  <refsynopsisdiv>
 <synopsis>
-BEGIN [ WORK | TRANSACTION ] [ <replaceable class="parameter">transaction_mode</replaceable> [, ...] ]
+BEGIN [ WORK | TRANSACTION ] [ <replaceable class="parameter">transaction_mode</replaceable> [, ...] ] [ WAIT FOR LSN <replaceable class="parameter">lsn_value</replaceable> [TIMEOUT <replaceable class="parameter">number_of_milliseconds</replaceable> ] ]
 
 <phrase>where <replaceable class="parameter">transaction_mode</replaceable> is one of:</phrase>
 
@@ -63,6 +63,17 @@ BEGIN [ WORK | TRANSACTION ] [ <replaceable class="parameter">transaction_mode</
    <xref linkend="sql-set-transaction"/>
    was executed.
   </para>
+
+  <para>
+   The <literal>WAIT FOR</literal> clause allows to wait for the target log
+   sequence number (<acronym>LSN</acronym>) to be replayed on standby before
+   starting the transaction in <productname>PostgreSQL</productname> databases
+   with master-standby asynchronous replication. Wait time can be limited by
+   specifying a timeout, which is measured in milliseconds and must be a positive
+   integer. If <acronym>LSN</acronym> was not reached before timeout, transaction
+   doesn't begin. Waiting can be interrupted by cancelling
+   <literal>BEGIN</literal> command.
+  </para>
  </refsect1>
 
  <refsect1>
@@ -146,6 +157,10 @@ BEGIN;
    different purpose in embedded SQL. You are advised to be careful
    about the transaction semantics when porting database applications.
   </para>
+
+  <para>
+   There is no <literal>WAIT FOR</literal> clause in the SQL standard.
+  </para>
  </refsect1>
 
  <refsect1>

doc/src/sgml/ref/start_transaction.sgml

@@ -21,7 +21,7 @@ PostgreSQL documentation
 
  <refsynopsisdiv>
 <synopsis>
-START TRANSACTION [ <replaceable class="parameter">transaction_mode</replaceable> [, ...] ]
+START TRANSACTION [ <replaceable class="parameter">transaction_mode</replaceable> [, ...] ] [ WAIT FOR LSN <replaceable class="parameter">lsn_value</replaceable> [TIMEOUT <replaceable class="parameter">number_of_milliseconds</replaceable> ] ]
 
 <phrase>where <replaceable class="parameter">transaction_mode</replaceable> is one of:</phrase>
 
@@ -40,6 +40,17 @@ START TRANSACTION [ <replaceable class="parameter">transaction_mode</replaceable
    characteristics, as if <xref linkend="sql-set-transaction"/> was executed. This is the same
    as the <xref linkend="sql-begin"/> command.
   </para>
+
+  <para>
+   The <literal>WAIT FOR</literal> clause allows to wait for the target log
+   sequence number (<acronym>LSN</acronym>) to be replayed on standby before
+   starting the transaction in <productname>PostgreSQL</productname> databases
+   with master-standby asynchronous replication. Wait time can be limited by
+   specifying a timeout, which is measured in milliseconds and must be a positive
+   integer. If <acronym>LSN</acronym> was not reached before timeout, transaction
+   doesn't begin. Waiting can be interrupted by cancelling
+   <literal>START TRANSACTION</literal> command.
+  </para>
  </refsect1>
 
  <refsect1>
@@ -78,6 +89,10 @@ START TRANSACTION [ <replaceable class="parameter">transaction_mode</replaceable
    omitted.
   </para>
 
+  <para>
+   There is no <literal>WAIT FOR</literal> clause in the SQL standard.
+  </para>
+
   <para>
    See also the compatibility section of <xref linkend="sql-set-transaction"/>.
   </para>

src/backend/access/transam/xlog.c

@@ -42,6 +42,7 @@
 #include "catalog/pg_database.h"
 #include "commands/progress.h"
 #include "commands/tablespace.h"
+#include "commands/wait.h"
 #include "common/controldata_utils.h"
 #include "executor/instrument.h"
 #include "miscadmin.h"
@@ -7154,6 +7155,7 @@ StartupXLOG(void)
 			do
 			{
 				bool		switchedTLI = false;
+				XLogRecPtr	minWaitedLSN;
 
 #ifdef WAL_DEBUG
 				if (XLOG_DEBUG ||
@@ -7357,6 +7359,17 @@ StartupXLOG(void)
 					break;
 				}
 
+				/*
+				 * If we replayed an LSN that someone was waiting for, set
+				 * latches in shared memory array to notify the waiter.
+				 */
+				minWaitedLSN = WaitLSNGetMin();
+				if (!XLogRecPtrIsInvalid(minWaitedLSN) &&
+					minWaitedLSN <= XLogCtl->lastReplayedEndRecPtr)
+				{
+					WaitLSNSetLatch(XLogCtl->lastReplayedEndRecPtr);
+				}
+
 				/* Else, try to fetch the next WAL record */
 				record = ReadRecord(xlogreader, LOG, false);
 			} while (record != NULL);

src/backend/commands/Makefile

@@ -57,6 +57,7 @@ OBJS = \
 	user.o \
 	vacuum.o \
 	variable.o \
-	view.o
+	view.o \
+	wait.o
 
 include $(top_srcdir)/src/backend/common.mk