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

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

src/backend/commands/wait.c

+/*-------------------------------------------------------------------------
+ *
+ * wait.c
+ *	  Implements WAIT FOR clause for BEGIN and START TRANSACTION commands.
+ *	  This clause allows waiting for given LSN to be replayed on standby.
+ *
+ * Copyright (c) 2020, PostgreSQL Global Development Group
+ *
+ * IDENTIFICATION
+ *	  src/backend/commands/wait.c
+ *
+ *-------------------------------------------------------------------------
+ */
+#include "postgres.h"
+
+#include <math.h>
+
+#include "access/xlog.h"
+#include "access/xlogdefs.h"
+#include "commands/wait.h"
+#include "funcapi.h"
+#include "miscadmin.h"
+#include "pgstat.h"
+#include "storage/backendid.h"
+#include "storage/pmsignal.h"
+#include "storage/proc.h"
+#include "storage/shmem.h"
+#include "storage/sinvaladt.h"
+#include "storage/spin.h"
+#include "utils/builtins.h"
+#include "utils/pg_lsn.h"
+#include "utils/timestamp.h"
+
+/*
+ * Shared memory structure representing information about LSNs, which backends
+ * are waiting for replay.
+ */
+typedef struct
+{
+	slock_t		mutex;			/* mutex protecting the fields below */
+	int			max_backend_id; /* max backend_id present in lsns[] */
+	pg_atomic_uint64 min_lsn;	/* minimal waited LSN */
+	/* per-backend array of waited LSNs */
+	XLogRecPtr	lsns[FLEXIBLE_ARRAY_MEMBER];
+}			WaitLSNState;
+
+static WaitLSNState * state;
+
+/*
+ * Add the wait event of the current backend to shared memory array
+ */
+static void
+WaitLSNAdd(XLogRecPtr lsn_to_wait)
+{
+	SpinLockAcquire(&state->mutex);
+	if (state->max_backend_id < MyBackendId)
+		state->max_backend_id = MyBackendId;
+
+	state->lsns[MyBackendId] = lsn_to_wait;
+
+	if (lsn_to_wait < state->min_lsn.value)
+		state->min_lsn.value = lsn_to_wait;
+	SpinLockRelease(&state->mutex);
+}
+
+/*
+ * Delete wait event of the current backend from the shared memory array.
+ */
+void
+WaitLSNDelete(void)
+{
+	int			i;
+	XLogRecPtr	deleted_lsn;
+
+	SpinLockAcquire(&state->mutex);
+
+	deleted_lsn = state->lsns[MyBackendId];
+	state->lsns[MyBackendId] = InvalidXLogRecPtr;
+
+	/* If we are deleting the minimal LSN, then choose the next min_lsn */
+	if (!XLogRecPtrIsInvalid(deleted_lsn) &&
+		deleted_lsn == state->min_lsn.value)
+	{
+		state->min_lsn.value = InvalidXLogRecPtr;
+		for (i = 2; i <= state->max_backend_id; i++)
+		{
+			if (!XLogRecPtrIsInvalid(state->lsns[i]) &&
+				(state->lsns[i] < state->min_lsn.value ||
+				 XLogRecPtrIsInvalid(state->min_lsn.value)))
+			{
+				state->min_lsn.value = state->lsns[i];
+			}
+		}
+	}
+
+	/* If deleting from the end of the array, shorten the array's used part */
+	if (state->max_backend_id == MyBackendId)
+	{
+		for (i = (MyBackendId); i >= 2; i--)
+			if (!XLogRecPtrIsInvalid(state->lsns[i]))
+			{
+				state->max_backend_id = i;
+				break;
+			}
+	}
+
+	SpinLockRelease(&state->mutex);
+}
+
+/*
+ * Report amount of shared memory space needed for WaitLSNState
+ */
+Size
+WaitLSNShmemSize(void)
+{
+	Size		size;
+
+	size = offsetof(WaitLSNState, lsns);
+	size = add_size(size, mul_size(MaxBackends + 1, sizeof(XLogRecPtr)));
+	return size;
+}
+
+/*
+ * Initialize an shared memory structure for waiting for LSN
+ */
+void
+WaitLSNShmemInit(void)
+{
+	bool		found;
+	uint32		i;
+
+	state = (WaitLSNState *) ShmemInitStruct("pg_wait_lsn",
+											 WaitLSNShmemSize(),
+											 &found);
+	if (!found)
+	{
+		SpinLockInit(&state->mutex);
+
+		for (i = 0; i < (MaxBackends + 1); i++)
+			state->lsns[i] = InvalidXLogRecPtr;
+
+		state->max_backend_id = 0;
+		pg_atomic_init_u64(&state->min_lsn, InvalidXLogRecPtr);
+	}
+}
+
+/*
+ * Set latches in shared memory to signal that new LSN has been replayed
+ */
+void
+WaitLSNSetLatch(XLogRecPtr cur_lsn)
+{
+	uint32		i;
+	int			max_backend_id;
+	PGPROC	   *backend;
+
+	SpinLockAcquire(&state->mutex);
+	max_backend_id = state->max_backend_id;
+
+	for (i = 2; i <= max_backend_id; i++)
+	{
+		backend = BackendIdGetProc(i);
+
+		if (backend && state->lsns[i] != 0 &&
+			state->lsns[i] <= cur_lsn)
+		{
+			SetLatch(&backend->procLatch);
+		}
+	}
+	SpinLockRelease(&state->mutex);
+}
+
+/*
+ * Get minimal LSN that some backend is waiting for
+ */
+XLogRecPtr
+WaitLSNGetMin(void)
+{
+	return state->min_lsn.value;
+}
+
+/*
+ * On WAIT use a latch to wait till LSN is replayed, postmaster dies or timeout
+ * happens. Timeout is specified in milliseconds.  Returns true if LSN was
+ * reached and false otherwise.
+ */
+bool
+WaitLSNUtility(XLogRecPtr target_lsn, const int timeout_ms)
+{
+	XLogRecPtr	cur_lsn;
+	int			latch_events;
+	float8		endtime;
+	bool		res = false;
+	bool		wait_forever = (timeout_ms <= 0);
+
+	endtime = GetNowFloat() + timeout_ms / 1000.0;
+
+	latch_events = WL_LATCH_SET | WL_TIMEOUT | WL_EXIT_ON_PM_DEATH;
+
+	/* Check if we already reached the needed LSN */
+	cur_lsn = GetXLogReplayRecPtr(NULL);
+	if (cur_lsn >= target_lsn)
+		return true;
+
+	WaitLSNAdd(target_lsn);
+	ResetLatch(MyLatch);
+
+	/* Recheck if LSN was reached while WaitLSNAdd() and ResetLatch() */
+	cur_lsn = GetXLogReplayRecPtr(NULL);
+	if (cur_lsn >= target_lsn)
+		return true;
+
+	for (;;)
+	{
+		int			rc;
+		float8		time_left = 0;
+		long		time_left_ms = 0;
+
+		time_left = endtime - GetNowFloat();
+
+		/* Use 1 second as the default timeout to check for interrupts */
+		if (wait_forever || time_left < 0 || time_left > 1.0)
+			time_left_ms = 1000;
+		else
+			time_left_ms = (long) ceil(time_left * 1000.0);
+
+		/* If interrupt, LockErrorCleanup() will do WaitLSNDelete() for us */
+		CHECK_FOR_INTERRUPTS();
+
+		/* If postmaster dies, finish immediately */
+		if (!PostmasterIsAlive())
+			break;
+
+		rc = WaitLatch(MyLatch, latch_events, time_left_ms,
+					   WAIT_EVENT_CLIENT_READ);
+
+		ResetLatch(MyLatch);
+
+		if (rc & WL_LATCH_SET)
+			cur_lsn = GetXLogReplayRecPtr(NULL);
+
+		if (rc & WL_TIMEOUT)
+		{
+			time_left = endtime - GetNowFloat();
+			/* If the time specified by user has passed, stop waiting */
+			if (!wait_forever && time_left <= 0.0)
+				break;
+			cur_lsn = GetXLogReplayRecPtr(NULL);
+		}
+
+		/* If LSN has been replayed */
+		if (target_lsn <= cur_lsn)
+			break;
+	}
+
+	WaitLSNDelete();
+
+	if (cur_lsn < target_lsn)
+		ereport(WARNING,
+				(errcode(ERRCODE_IN_FAILED_SQL_TRANSACTION),
+				 errmsg("didn't start transaction because LSN was not reached"),
+				 errhint("Try to increase wait timeout.")));
+	else
+		res = true;
+
+	return res;
+}
+
+/*
+ * Implementation of WAIT FOR clause for BEGIN and START TRANSACTION commands
+ */
+int
+WaitLSNMain(WaitClause *stmt, DestReceiver *dest)
+{
+	TupleDesc	tupdesc;
+	TupOutputState *tstate;
+	XLogRecPtr	target_lsn;
+	bool		res = false;
+
+	target_lsn = DatumGetLSN(DirectFunctionCall1(pg_lsn_in,
+												 CStringGetDatum(stmt->lsn)));
+	res = WaitLSNUtility(target_lsn, stmt->timeout);
+
+	/* Need a tuple descriptor representing a single TEXT column */
+	tupdesc = CreateTemplateTupleDesc(1);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 1, "LSN reached", TEXTOID, -1, 0);
+
+	/* Prepare for projection of tuples */
+	tstate = begin_tup_output_tupdesc(dest, tupdesc, &TTSOpsMinimalTuple);
+
+	/* Send the result */
+	do_text_output_oneline(tstate, res ? "t" : "f");
+	end_tup_output(tstate);
+	return res;
+}

src/backend/nodes/copyfuncs.c

@@ -3748,10 +3748,22 @@ _copyTransactionStmt(const TransactionStmt *from)
 	COPY_STRING_FIELD(savepoint_name);
 	COPY_STRING_FIELD(gid);
 	COPY_SCALAR_FIELD(chain);
+	COPY_NODE_FIELD(wait);
 
 	return newnode;
 }
 
+static WaitClause *
+_copyWaitClause(const WaitClause *from)
+{
+	WaitClause *newnode = makeNode(WaitClause);
+
+	COPY_STRING_FIELD(lsn);
+	COPY_SCALAR_FIELD(timeout);
+
+	return newnode;
+};
+
 static CompositeTypeStmt *
 _copyCompositeTypeStmt(const CompositeTypeStmt *from)
 {
@@ -5339,6 +5351,9 @@ copyObjectImpl(const void *from)
 		case T_TransactionStmt:
 			retval = _copyTransactionStmt(from);
 			break;
+		case T_WaitClause:
+			retval = _copyWaitClause(from);
+			break;
 		case T_CompositeTypeStmt:
 			retval = _copyCompositeTypeStmt(from);
 			break;

src/backend/nodes/equalfuncs.c

@@ -1541,6 +1541,16 @@ _equalTransactionStmt(const TransactionStmt *a, const TransactionStmt *b)
 	COMPARE_STRING_FIELD(savepoint_name);
 	COMPARE_STRING_FIELD(gid);
 	COMPARE_SCALAR_FIELD(chain);
+	COMPARE_NODE_FIELD(wait);
+
+	return true;
+}
+
+static bool
+_equalWaitClause(const WaitClause *a, const WaitClause *b)
+{
+	COMPARE_STRING_FIELD(lsn);
+	COMPARE_SCALAR_FIELD(timeout);
 
 	return true;
 }
@@ -3391,6 +3401,9 @@ equal(const void *a, const void *b)
 		case T_TransactionStmt:
 			retval = _equalTransactionStmt(a, b);
 			break;
+		case T_WaitClause:
+			retval = _equalWaitClause(a, b);
+			break;
 		case T_CompositeTypeStmt:
 			retval = _equalCompositeTypeStmt(a, b);
 			break;

src/backend/nodes/outfuncs.c

@@ -2784,6 +2784,28 @@ _outDefElem(StringInfo str, const DefElem *node)
 	WRITE_LOCATION_FIELD(location);
 }
 
+static void
+_outTransactionStmt(StringInfo str, const TransactionStmt *node)
+{
+	WRITE_NODE_TYPE("TRANSACTIONSTMT");
+
+	WRITE_STRING_FIELD(savepoint_name);
+	WRITE_STRING_FIELD(gid);
+	WRITE_NODE_FIELD(options);
+	WRITE_BOOL_FIELD(chain);
+	WRITE_ENUM_FIELD(kind, TransactionStmtKind);
+	WRITE_NODE_FIELD(wait);
+}
+
+static void
+_outWaitClause(StringInfo str, const WaitClause *node)
+{
+	WRITE_NODE_TYPE("WAITCLAUSE");
+
+	WRITE_STRING_FIELD(lsn);
+	WRITE_UINT_FIELD(timeout);
+}
+
 static void
 _outTableLikeClause(StringInfo str, const TableLikeClause *node)
 {
@@ -4334,6 +4356,12 @@ outNode(StringInfo str, const void *obj)
 			case T_PartitionRangeDatum:
 				_outPartitionRangeDatum(str, obj);
 				break;
+			case T_TransactionStmt:
+				_outTransactionStmt(str, obj);
+				break;
+			case T_WaitClause:
+				_outWaitClause(str, obj);
+				break;
 
 			default:
 

src/backend/parser/gram.y

@@ -601,6 +601,8 @@ static Node *makeRecursiveViewSelect(char *relname, List *aliases, Node *query);
 %type <partboundspec> PartitionBoundSpec
 %type <list>		hash_partbound
 %type <defelt>		hash_partbound_elem
+%type <ival>		wait_time
+%type <node>		wait_for
 
 /*
  * Non-keyword token types.  These are hard-wired into the "flex" lexer.
@@ -670,7 +672,7 @@ static Node *makeRecursiveViewSelect(char *relname, List *aliases, Node *query);
 
 	LABEL LANGUAGE LARGE_P LAST_P LATERAL_P
 	LEADING LEAKPROOF LEAST LEFT LEVEL LIKE LIMIT LISTEN LOAD LOCAL
-	LOCALTIME LOCALTIMESTAMP LOCATION LOCK_P LOCKED LOGGED
+	LOCALTIME LOCALTIMESTAMP LOCATION LOCK_P LOCKED LOGGED LSN
 
 	MAPPING MATCH MATERIALIZED MAXVALUE METHOD MINUTE_P MINVALUE MODE MONTH_P MOVE
 
@@ -701,7 +703,7 @@ static Node *makeRecursiveViewSelect(char *relname, List *aliases, Node *query);
 	SUBSCRIPTION SUBSTRING SUPPORT SYMMETRIC SYSID SYSTEM_P
 
 	TABLE TABLES TABLESAMPLE TABLESPACE TEMP TEMPLATE TEMPORARY TEXT_P THEN
-	TIES TIME TIMESTAMP TO TRAILING TRANSACTION TRANSFORM
+	TIES TIME TIMEOUT TIMESTAMP TO TRAILING TRANSACTION TRANSFORM
 	TREAT TRIGGER TRIM TRUE_P
 	TRUNCATE TRUSTED TYPE_P TYPES_P
 
@@ -711,7 +713,8 @@ static Node *makeRecursiveViewSelect(char *relname, List *aliases, Node *query);
 	VACUUM VALID VALIDATE VALIDATOR VALUE_P VALUES VARCHAR VARIADIC VARYING
 	VERBOSE VERSION_P VIEW VIEWS VOLATILE
 
-	WHEN WHERE WHITESPACE_P WINDOW WITH WITHIN WITHOUT WORK WRAPPER WRITE
+	WAIT WHEN WHERE WHITESPACE_P WINDOW
+	WITH WITHIN WITHOUT WORK WRAPPER WRITE
 
 	XML_P XMLATTRIBUTES XMLCONCAT XMLELEMENT XMLEXISTS XMLFOREST XMLNAMESPACES
 	XMLPARSE XMLPI XMLROOT XMLSERIALIZE XMLTABLE
@@ -9955,18 +9958,20 @@ TransactionStmt:
 					n->chain = $3;
 					$$ = (Node *)n;
 				}
-			| BEGIN_P opt_transaction transaction_mode_list_or_empty
+			| BEGIN_P opt_transaction transaction_mode_list_or_empty wait_for
 				{
 					TransactionStmt *n = makeNode(TransactionStmt);
 					n->kind = TRANS_STMT_BEGIN;
 					n->options = $3;
+					n->wait = $4;
 					$$ = (Node *)n;
 				}
-			| START TRANSACTION transaction_mode_list_or_empty
+			| START TRANSACTION transaction_mode_list_or_empty wait_for
 				{
 					TransactionStmt *n = makeNode(TransactionStmt);
 					n->kind = TRANS_STMT_START;
 					n->options = $3;
+					n->wait = $4;
 					$$ = (Node *)n;
 				}
 			| COMMIT opt_transaction opt_transaction_chain
@@ -14240,6 +14245,25 @@ xml_passing_mech:
 			| BY VALUE_P
 		;
 
+/*
+ * WAIT FOR clause of BEGIN and START TRANSACTION statements
+ */
+wait_for:
+			WAIT FOR LSN Sconst wait_time
+				{
+					WaitClause *n = makeNode(WaitClause);
+					n->lsn = $4;
+					n->timeout = $5;
+					$$ = (Node *)n;
+				}
+			| /* EMPTY */		{ $$ = NULL; }
+		;
+
+wait_time:
+			TIMEOUT Iconst		{ $$ = $2; }
+			| /* EMPTY */		{ $$ = 0; }
+		;
+
 
 /*
  * Aggregate decoration clauses
@@ -15391,6 +15415,7 @@ unreserved_keyword:
 			| LOCK_P
 			| LOCKED
 			| LOGGED
+			| LSN
 			| MAPPING
 			| MATCH
 			| MATERIALIZED
@@ -15518,6 +15543,7 @@ unreserved_keyword:
 			| TEMPORARY
 			| TEXT_P
 			| TIES
+			| TIMEOUT
 			| TRANSACTION
 			| TRANSFORM
 			| TRIGGER
@@ -15544,6 +15570,7 @@ unreserved_keyword:
 			| VIEW
 			| VIEWS
 			| VOLATILE
+			| WAIT
 			| WHITESPACE_P
 			| WITHIN
 			| WITHOUT

src/backend/storage/ipc/ipci.c

@@ -22,6 +22,7 @@
 #include "access/subtrans.h"
 #include "access/twophase.h"
 #include "commands/async.h"
+#include "commands/wait.h"
 #include "miscadmin.h"
 #include "pgstat.h"
 #include "postmaster/autovacuum.h"
@@ -147,6 +148,7 @@ CreateSharedMemoryAndSemaphores(void)
 		size = add_size(size, BTreeShmemSize());
 		size = add_size(size, SyncScanShmemSize());
 		size = add_size(size, AsyncShmemSize());
+		size = add_size(size, WaitLSNShmemSize());
 #ifdef EXEC_BACKEND
 		size = add_size(size, ShmemBackendArraySize());
 #endif
@@ -264,6 +266,11 @@ CreateSharedMemoryAndSemaphores(void)
 	SyncScanShmemInit();
 	AsyncShmemInit();
 
+	/*
+	 * Init array of Latches in shared memory for WAIT
+	 */
+	WaitLSNShmemInit();
+
 #ifdef EXEC_BACKEND
 
 	/*

src/backend/storage/lmgr/proc.c

@@ -38,6 +38,7 @@
 #include "access/transam.h"
 #include "access/twophase.h"
 #include "access/xact.h"
+#include "commands/wait.h"
 #include "miscadmin.h"
 #include "pgstat.h"
 #include "postmaster/autovacuum.h"
@@ -717,6 +718,9 @@ LockErrorCleanup(void)
 
 	AbortStrongLockAcquire();
 
+	/* If BEGIN WAIT FOR LSN was interrupted, then stop waiting for that LSN */
+	WaitLSNDelete();
+
 	/* Nothing to do if we weren't waiting for a lock */
 	if (lockAwaited == NULL)
 	{

src/backend/tcop/utility.c

@@ -57,6 +57,7 @@
 #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"
@@ -591,6 +592,18 @@ standard_ProcessUtility(PlannedStmt *pstmt,
 					case TRANS_STMT_START:
 						{
 							ListCell   *lc;
+							WaitClause *waitstmt = (WaitClause *) stmt->wait;
+
+							/* WAIT FOR cannot be used on master */
+							if (stmt->wait && !RecoveryInProgress())
+								ereport(ERROR,
+										(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+										 errmsg("WAIT FOR can only be "
+												"used on standby")));
+
+							/* If needed to WAIT FOR something but failed */
+							if (stmt->wait && WaitLSNMain(waitstmt, dest) == 0)
+								break;
 
 							BeginTransactionBlock();
 							foreach(lc, stmt->options)

src/backend/utils/adt/misc.c

@@ -372,8 +372,6 @@ pg_sleep(PG_FUNCTION_ARGS)
 	 * less than the specified time when WaitLatch is terminated early by a
 	 * non-query-canceling signal such as SIGHUP.
 	 */
-#define GetNowFloat()	((float8) GetCurrentTimestamp() / 1000000.0)
-
 	endtime = GetNowFloat() + secs;
 
 	for (;;)

src/include/commands/wait.h

+/*-------------------------------------------------------------------------
+ *
+ * wait.h
+ *	  prototypes for commands/wait.c
+ *
+ * Copyright (c) 2020, PostgreSQL Global Development Group
+ *
+ * src/include/commands/wait.h
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef WAIT_H
+#define WAIT_H
+
+#include "tcop/dest.h"
+#include "nodes/parsenodes.h"
+
+extern bool WaitLSNUtility(XLogRecPtr lsn, const int timeout_ms);
+extern Size WaitLSNShmemSize(void);
+extern void WaitLSNShmemInit(void);
+extern void WaitLSNSetLatch(XLogRecPtr cur_lsn);
+extern XLogRecPtr WaitLSNGetMin(void);
+extern int	WaitLSNMain(WaitClause *stmt, DestReceiver *dest);
+extern void WaitLSNDelete(void);
+
+#endif							/* WAIT_H */

src/include/nodes/nodes.h

@@ -492,6 +492,7 @@ typedef enum NodeTag
 	T_StartReplicationCmd,
 	T_TimeLineHistoryCmd,
 	T_SQLCmd,
+	T_WaitClause,
 
 	/*
 	 * TAGS FOR RANDOM OTHER STUFF

src/include/nodes/parsenodes.h

@@ -1431,6 +1431,17 @@ typedef struct OnConflictClause
 	int			location;		/* token location, or -1 if unknown */
 } OnConflictClause;
 
+/*
+ * WaitClause -
+ *		representation of WAIT FOR clause for BEGIN and START TRANSACTION.
+ */
+typedef struct WaitClause
+{
+	NodeTag		type;
+	char	   *lsn;			/* LSN to wait for */
+	int			timeout;		/* Number of milliseconds to limit wait time */
+} WaitClause;
+
 /*
  * CommonTableExpr -
  *	   representation of WITH list element
@@ -3060,6 +3071,7 @@ typedef struct TransactionStmt
 	char	   *savepoint_name; /* for savepoint commands */
 	char	   *gid;			/* for two-phase-commit related commands */
 	bool		chain;			/* AND CHAIN option */
+	Node	   *wait;			/* WAIT FOR clause */
 } TransactionStmt;
 
 /* ----------------------

src/include/parser/kwlist.h

@@ -243,6 +243,7 @@ PG_KEYWORD("location", LOCATION, UNRESERVED_KEYWORD)
 PG_KEYWORD("lock", LOCK_P, UNRESERVED_KEYWORD)
 PG_KEYWORD("locked", LOCKED, UNRESERVED_KEYWORD)
 PG_KEYWORD("logged", LOGGED, UNRESERVED_KEYWORD)
+PG_KEYWORD("lsn", LSN, UNRESERVED_KEYWORD)
 PG_KEYWORD("mapping", MAPPING, UNRESERVED_KEYWORD)
 PG_KEYWORD("match", MATCH, UNRESERVED_KEYWORD)
 PG_KEYWORD("materialized", MATERIALIZED, UNRESERVED_KEYWORD)
@@ -410,6 +411,7 @@ PG_KEYWORD("text", TEXT_P, UNRESERVED_KEYWORD)
 PG_KEYWORD("then", THEN, RESERVED_KEYWORD)
 PG_KEYWORD("ties", TIES, UNRESERVED_KEYWORD)
 PG_KEYWORD("time", TIME, COL_NAME_KEYWORD)
+PG_KEYWORD("timeout", TIMEOUT, UNRESERVED_KEYWORD)
 PG_KEYWORD("timestamp", TIMESTAMP, COL_NAME_KEYWORD)
 PG_KEYWORD("to", TO, RESERVED_KEYWORD)
 PG_KEYWORD("trailing", TRAILING, RESERVED_KEYWORD)
@@ -450,6 +452,7 @@ PG_KEYWORD("version", VERSION_P, UNRESERVED_KEYWORD)
 PG_KEYWORD("view", VIEW, UNRESERVED_KEYWORD)
 PG_KEYWORD("views", VIEWS, UNRESERVED_KEYWORD)
 PG_KEYWORD("volatile", VOLATILE, UNRESERVED_KEYWORD)
+PG_KEYWORD("wait", WAIT, UNRESERVED_KEYWORD)
 PG_KEYWORD("when", WHEN, RESERVED_KEYWORD)
 PG_KEYWORD("where", WHERE, RESERVED_KEYWORD)
 PG_KEYWORD("whitespace", WHITESPACE_P, UNRESERVED_KEYWORD)

src/include/utils/timestamp.h

@@ -109,4 +109,6 @@ extern int	date2isoyearday(int year, int mon, int mday);
 
 extern bool TimestampTimestampTzRequiresRewrite(void);
 
+#define GetNowFloat() ((float8) GetCurrentTimestamp() / 1000000.0)
+
 #endif							/* TIMESTAMP_H */

src/test/recovery/t/020_begin_wait.pl

+# Checks for BEGIN WAIT FOR LSN
+use strict;
+use warnings;
+
+use PostgresNode;
+use TestLib;
+use Test::More tests => 8;
+
+# Initialize master node
+my $node_master = get_new_node('master');
+$node_master->init(allows_streaming => 1);
+$node_master->start;
+
+# And some content and take a backup
+$node_master->safe_psql('postgres',
+	"CREATE TABLE wait_test AS SELECT generate_series(1,10) AS a");
+my $backup_name = 'my_backup';
+$node_master->backup($backup_name);
+
+# Using the backup, create a streaming standby with a 1 second delay
+my $node_standby = get_new_node('standby');
+my $delay        = 1;
+$node_standby->init_from_backup($node_master, $backup_name,
+	has_streaming => 1);
+$node_standby->append_conf('postgresql.conf', qq[
+	recovery_min_apply_delay = '${delay}s'
+]);
+$node_standby->start;
+
+
+# Check that timeouts make us wait for the specified time (1s here)
+my $current_time = $node_standby->safe_psql('postgres', "SELECT now()");
+my $two_seconds = 2000; # in milliseconds
+my $start_time = time();
+$node_standby->safe_psql('postgres',
+	"BEGIN WAIT FOR LSN '0/FFFFFFFF' TIMEOUT $two_seconds");
+my $time_waited = (time() - $start_time) * 1000; # convert to milliseconds
+ok($time_waited >= $two_seconds, "WAIT FOR TIMEOUT waits for enough time");
+
+
+# Check that timeouts let us stop waiting right away, before reaching target LSN
+$node_master->safe_psql('postgres',
+	"INSERT INTO wait_test VALUES (generate_series(11, 20))");
+my $lsn1 = $node_master->safe_psql('postgres', "SELECT pg_current_wal_lsn()");
+my ($ret, $out, $err) = $node_standby->psql('postgres',
+	"BEGIN WAIT FOR LSN '$lsn1' TIMEOUT 1");
+
+ok($ret == 0, "zero return value when failed to WAIT FOR LSN on standby");
+ok($err =~ /WARNING:  didn't start transaction because LSN was not reached/,
+	"correct error message when failed to WAIT FOR LSN on standby");
+ok($out eq "f", "if given too little wait time, WAIT doesn't reach target LSN");
+
+
+# Check that WAIT FOR works fine and reaches target LSN if given no timeout
+
+# Add data on master, memorize master's last LSN
+$node_master->safe_psql('postgres',
+	"INSERT INTO wait_test VALUES (generate_series(21, 30))");
+my $lsn2 = $node_master->safe_psql('postgres', "SELECT pg_current_wal_lsn()");
+
+# Wait for it to appear on replica, memorize replica's last LSN
+$node_standby->safe_psql('postgres',
+	"BEGIN WAIT FOR LSN '$lsn2'");
+my $reached_lsn = $node_standby->safe_psql('postgres',
+	"SELECT pg_last_wal_replay_lsn()");
+
+# Make sure that master's and replica's LSNs are the same after WAIT
+my $compare_lsns = $node_standby->safe_psql('postgres',
+	"SELECT pg_lsn_cmp('$reached_lsn'::pg_lsn, '$lsn2'::pg_lsn)");
+ok($compare_lsns eq 0,
+	"standby reached the same LSN as master before starting transaction");
+
+
+# Make sure that it's not allowed to use WAIT FOR on master
+($ret, $out, $err) = $node_master->psql('postgres',
+	"BEGIN WAIT FOR LSN '0/FFFFFFFF'");
+
+ok($ret != 0, "non-zero return value when trying to WAIT FOR LSN on master");
+ok($err =~ /ERROR:  WAIT FOR can only be used on standby/,
+	"correct error message when trying to WAIT FOR LSN on master");
+ok($out eq '', "empty output when trying to WAIT FOR LSN on master");
+
+
+$node_standby->stop;
+$node_master->stop;

src/tools/pgindent/typedefs.list

@@ -2621,6 +2621,7 @@ WSABUF
 WSADATA
 WSANETWORKEVENTS
 WSAPROTOCOL_INFO
+WaitClause
 WaitEvent
 WaitEventActivity
 WaitEventClient
@@ -2628,6 +2629,7 @@ WaitEventIO
 WaitEventIPC
 WaitEventSet
 WaitEventTimeout
+WaitLSNState
 WaitPMResult
 WalCloseMethod
 WalLevel