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

11 files changed, 422 insertions, 8 deletions

diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index ec55d68d272..1651e15e898 100644
--- a/src/backend/access/transam/xlog.c
+++ b/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);
diff --git a/src/backend/commands/Makefile b/src/backend/commands/Makefile
index d4815d3ce65..9b310926c12 100644
--- a/src/backend/commands/Makefile
+++ b/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
diff --git a/src/backend/commands/wait.c b/src/backend/commands/wait.c
new file mode 100644
index 00000000000..b7aee5b794b
--- /dev/null
+++ b/src/backend/commands/wait.c
@@ -0,0 +1,295 @@
+/*-------------------------------------------------------------------------
+ *
+ * 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;
+}
diff --git a/src/backend/nodes/copyfuncs.c b/src/backend/nodes/copyfuncs.c
index 1525c0de725..db179becab5 100644
--- a/src/backend/nodes/copyfuncs.c
+++ b/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;
diff --git a/src/backend/nodes/equalfuncs.c b/src/backend/nodes/equalfuncs.c
index 4f34189ab5c..854d484f603 100644
--- a/src/backend/nodes/equalfuncs.c
+++ b/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;
diff --git a/src/backend/nodes/outfuncs.c b/src/backend/nodes/outfuncs.c
index 5b826509ebe..47753b42c69 100644
--- a/src/backend/nodes/outfuncs.c
+++ b/src/backend/nodes/outfuncs.c
@@ -2785,6 +2785,28 @@ _outDefElem(StringInfo str, const DefElem *node)
 }
 
 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)
 {
 	WRITE_NODE_TYPE("TABLELIKECLAUSE");
@@ -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:
 
diff --git a/src/backend/parser/gram.y b/src/backend/parser/gram.y
index 1219ac8c264..ea1084fa3cf 100644
--- a/src/backend/parser/gram.y
+++ b/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
diff --git a/src/backend/storage/ipc/ipci.c b/src/backend/storage/ipc/ipci.c
index 427b0d59cde..417840a8f11 100644
--- a/src/backend/storage/ipc/ipci.c
+++ b/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
 
 	/*
diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index 9938cddb570..baecb39787f 100644
--- a/src/backend/storage/lmgr/proc.c
+++ b/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)
 	{
diff --git a/src/backend/tcop/utility.c b/src/backend/tcop/utility.c
index b1f7f6e2d01..f516bd22eae 100644
--- a/src/backend/tcop/utility.c
+++ b/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)
diff --git a/src/backend/utils/adt/misc.c b/src/backend/utils/adt/misc.c
index ee340fb0f02..03f997cba70 100644
--- a/src/backend/utils/adt/misc.c
+++ b/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 (;;)