Refactor libpqwalreceiver

The whole walreceiver API is now wrapped into a struct, like most of our
other loadable module APIs.  The libpq connection is no longer a global
variable in libpqwalreceiver.  Instead, it is encapsulated into a struct
that is passed around the functions.  This allows multiple walreceivers
to run at the same time.

Add some rudimentary support for logical replication connections to
libpqwalreceiver.

These changes are mostly cosmetic and are going to be useful for the
future logical replication patches.

From: Petr Jelinek <petr@2ndquadrant.com>

src/backend/replication/walreceiver.c

@@ -74,16 +74,9 @@ int			wal_receiver_status_interval;
 int			wal_receiver_timeout;
 bool		hot_standby_feedback;
 
-/* libpqreceiver hooks to these when loaded */
-walrcv_connect_type walrcv_connect = NULL;
-walrcv_get_conninfo_type walrcv_get_conninfo = NULL;
-walrcv_identify_system_type walrcv_identify_system = NULL;
-walrcv_startstreaming_type walrcv_startstreaming = NULL;
-walrcv_endstreaming_type walrcv_endstreaming = NULL;
-walrcv_readtimelinehistoryfile_type walrcv_readtimelinehistoryfile = NULL;
-walrcv_receive_type walrcv_receive = NULL;
-walrcv_send_type walrcv_send = NULL;
-walrcv_disconnect_type walrcv_disconnect = NULL;
+/* libpqwalreceiver connection */
+static WalReceiverConn *wrconn = NULL;
+WalReceiverFunctionsType *WalReceiverFunctions = NULL;
 
 #define NAPTIME_PER_CYCLE 100	/* max sleep time between cycles (100ms) */
 
@@ -286,14 +279,7 @@ WalReceiverMain(void)
 
 	/* Load the libpq-specific functions */
 	load_file("libpqwalreceiver", false);
-	if (walrcv_connect == NULL ||
-		walrcv_get_conninfo == NULL ||
-		walrcv_startstreaming == NULL ||
-		walrcv_endstreaming == NULL ||
-		walrcv_identify_system == NULL ||
-		walrcv_readtimelinehistoryfile == NULL ||
-		walrcv_receive == NULL || walrcv_send == NULL ||
-		walrcv_disconnect == NULL)
+	if (WalReceiverFunctions == NULL)
 		elog(ERROR, "libpqwalreceiver didn't initialize correctly");
 
 	/*
@@ -307,14 +293,14 @@ WalReceiverMain(void)
 
 	/* Establish the connection to the primary for XLOG streaming */
 	EnableWalRcvImmediateExit();
-	walrcv_connect(conninfo);
+	wrconn = walrcv_connect(conninfo, false, "walreceiver");
 	DisableWalRcvImmediateExit();
 
 	/*
 	 * Save user-visible connection string.  This clobbers the original
 	 * conninfo, for security.
 	 */
-	tmp_conninfo = walrcv_get_conninfo();
+	tmp_conninfo = walrcv_get_conninfo(wrconn);
 	SpinLockAcquire(&walrcv->mutex);
 	memset(walrcv->conninfo, 0, MAXCONNINFO);
 	if (tmp_conninfo)
@@ -328,12 +314,25 @@ WalReceiverMain(void)
 	first_stream = true;
 	for (;;)
 	{
+		char	   *primary_sysid;
+		char		standby_sysid[32];
+
 		/*
 		 * Check that we're connected to a valid server using the
 		 * IDENTIFY_SYSTEM replication command,
 		 */
 		EnableWalRcvImmediateExit();
-		walrcv_identify_system(&primaryTLI);
+		primary_sysid = walrcv_identify_system(wrconn, &primaryTLI);
+
+		snprintf(standby_sysid, sizeof(standby_sysid), UINT64_FORMAT,
+				 GetSystemIdentifier());
+		if (strcmp(primary_sysid, standby_sysid) != 0)
+		{
+			ereport(ERROR,
+					(errmsg("database system identifier differs between the primary and standby"),
+					 errdetail("The primary's identifier is %s, the standby's identifier is %s.",
+							   primary_sysid, standby_sysid)));
+		}
 		DisableWalRcvImmediateExit();
 
 		/*
@@ -370,7 +369,7 @@ WalReceiverMain(void)
 		 * on the new timeline.
 		 */
 		ThisTimeLineID = startpointTLI;
-		if (walrcv_startstreaming(startpointTLI, startpoint,
+		if (walrcv_startstreaming(wrconn, startpointTLI, startpoint,
 								  slotname[0] != '\0' ? slotname : NULL))
 		{
 			if (first_stream)
@@ -422,7 +421,7 @@ WalReceiverMain(void)
 				}
 
 				/* See if we can read data immediately */
-				len = walrcv_receive(&buf, &wait_fd);
+				len = walrcv_receive(wrconn, &buf, &wait_fd);
 				if (len != 0)
 				{
 					/*
@@ -453,7 +452,7 @@ WalReceiverMain(void)
 							endofwal = true;
 							break;
 						}
-						len = walrcv_receive(&buf, &wait_fd);
+						len = walrcv_receive(wrconn, &buf, &wait_fd);
 					}
 
 					/* Let the master know that we received some data. */
@@ -570,7 +569,7 @@ WalReceiverMain(void)
 			 * our side, too.
 			 */
 			EnableWalRcvImmediateExit();
-			walrcv_endstreaming(&primaryTLI);
+			walrcv_endstreaming(wrconn, &primaryTLI);
 			DisableWalRcvImmediateExit();
 
 			/*
@@ -726,7 +725,7 @@ WalRcvFetchTimeLineHistoryFiles(TimeLineID first, TimeLineID last)
 							tli)));
 
 			EnableWalRcvImmediateExit();
-			walrcv_readtimelinehistoryfile(tli, &fname, &content, &len);
+			walrcv_readtimelinehistoryfile(wrconn, tli, &fname, &content, &len);
 			DisableWalRcvImmediateExit();
 
 			/*
@@ -778,8 +777,8 @@ WalRcvDie(int code, Datum arg)
 	SpinLockRelease(&walrcv->mutex);
 
 	/* Terminate the connection gracefully. */
-	if (walrcv_disconnect != NULL)
-		walrcv_disconnect();
+	if (wrconn != NULL)
+		walrcv_disconnect(wrconn);
 
 	/* Wake up the startup process to notice promptly that we're gone */
 	WakeupRecovery();
@@ -1150,7 +1149,7 @@ XLogWalRcvSendReply(bool force, bool requestReply)
 		 (uint32) (applyPtr >> 32), (uint32) applyPtr,
 		 requestReply ? " (reply requested)" : "");
 
-	walrcv_send(reply_message.data, reply_message.len);
+	walrcv_send(wrconn, reply_message.data, reply_message.len);
 }
 
 /*
@@ -1228,7 +1227,7 @@ XLogWalRcvSendHSFeedback(bool immed)
 	pq_sendint64(&reply_message, GetCurrentIntegerTimestamp());
 	pq_sendint(&reply_message, xmin, 4);
 	pq_sendint(&reply_message, nextEpoch, 4);
-	walrcv_send(reply_message.data, reply_message.len);
+	walrcv_send(wrconn, reply_message.data, reply_message.len);
 	if (TransactionIdIsValid(xmin))
 		master_has_standby_xmin = true;
 	else

src/include/replication/walreceiver.h

@@ -134,33 +134,64 @@ typedef struct
 
 extern WalRcvData *WalRcv;
 
-/* libpqwalreceiver hooks */
-typedef void (*walrcv_connect_type) (char *conninfo);
-extern PGDLLIMPORT walrcv_connect_type walrcv_connect;
-
-typedef char *(*walrcv_get_conninfo_type) (void);
-extern PGDLLIMPORT walrcv_get_conninfo_type walrcv_get_conninfo;
-
-typedef void (*walrcv_identify_system_type) (TimeLineID *primary_tli);
-extern PGDLLIMPORT walrcv_identify_system_type walrcv_identify_system;
-
-typedef void (*walrcv_readtimelinehistoryfile_type) (TimeLineID tli, char **filename, char **content, int *size);
-extern PGDLLIMPORT walrcv_readtimelinehistoryfile_type walrcv_readtimelinehistoryfile;
-
-typedef bool (*walrcv_startstreaming_type) (TimeLineID tli, XLogRecPtr startpoint, char *slotname);
-extern PGDLLIMPORT walrcv_startstreaming_type walrcv_startstreaming;
+struct WalReceiverConn;
+typedef struct WalReceiverConn WalReceiverConn;
 
-typedef void (*walrcv_endstreaming_type) (TimeLineID *next_tli);
-extern PGDLLIMPORT walrcv_endstreaming_type walrcv_endstreaming;
-
-typedef int (*walrcv_receive_type) (char **buffer, pgsocket *wait_fd);
-extern PGDLLIMPORT walrcv_receive_type walrcv_receive;
-
-typedef void (*walrcv_send_type) (const char *buffer, int nbytes);
-extern PGDLLIMPORT walrcv_send_type walrcv_send;
-
-typedef void (*walrcv_disconnect_type) (void);
-extern PGDLLIMPORT walrcv_disconnect_type walrcv_disconnect;
+/* libpqwalreceiver hooks */
+typedef WalReceiverConn *(*walrcv_connect_fn) (const char *conninfo, bool logical,
+											   const char *appname);
+typedef char *(*walrcv_get_conninfo_fn) (WalReceiverConn *conn);
+typedef char *(*walrcv_identify_system_fn) (WalReceiverConn *conn,
+											TimeLineID *primary_tli);
+typedef void (*walrcv_readtimelinehistoryfile_fn) (WalReceiverConn *conn,
+												   TimeLineID tli,
+												   char **filename,
+												   char **content, int *size);
+typedef bool (*walrcv_startstreaming_fn) (WalReceiverConn *conn,
+										  TimeLineID tli,
+										  XLogRecPtr startpoint,
+										  const char *slotname);
+typedef void (*walrcv_endstreaming_fn) (WalReceiverConn *conn,
+										TimeLineID *next_tli);
+typedef int (*walrcv_receive_fn) (WalReceiverConn *conn, char **buffer,
+								  pgsocket *wait_fd);
+typedef void (*walrcv_send_fn) (WalReceiverConn *conn, const char *buffer,
+								int nbytes);
+typedef void (*walrcv_disconnect_fn) (WalReceiverConn *conn);
+
+typedef struct WalReceiverFunctionsType
+{
+	walrcv_connect_fn					connect;
+	walrcv_get_conninfo_fn				get_conninfo;
+	walrcv_identify_system_fn			identify_system;
+	walrcv_readtimelinehistoryfile_fn	readtimelinehistoryfile;
+	walrcv_startstreaming_fn			startstreaming;
+	walrcv_endstreaming_fn				endstreaming;
+	walrcv_receive_fn					receive;
+	walrcv_send_fn						send;
+	walrcv_disconnect_fn				disconnect;
+} WalReceiverFunctionsType;
+
+extern PGDLLIMPORT WalReceiverFunctionsType *WalReceiverFunctions;
+
+#define walrcv_connect(conninfo, logical, appname) \
+	WalReceiverFunctions->connect(conninfo, logical, appname)
+#define walrcv_get_conninfo(conn) \
+	WalReceiverFunctions->get_conninfo(conn)
+#define walrcv_identify_system(conn, primary_tli) \
+	WalReceiverFunctions->identify_system(conn, primary_tli)
+#define walrcv_readtimelinehistoryfile(conn, tli, filename, content, size) \
+	WalReceiverFunctions->readtimelinehistoryfile(conn, tli, filename, content, size)
+#define walrcv_startstreaming(conn, tli, startpoint, slotname) \
+	WalReceiverFunctions->startstreaming(conn, tli, startpoint, slotname)
+#define walrcv_endstreaming(conn, next_tli) \
+	WalReceiverFunctions->endstreaming(conn, next_tli)
+#define walrcv_receive(conn, buffer, wait_fd) \
+	WalReceiverFunctions->receive(conn, buffer, wait_fd)
+#define walrcv_send(conn, buffer, nbytes) \
+	WalReceiverFunctions->send(conn, buffer, nbytes)
+#define walrcv_disconnect(conn) \
+	WalReceiverFunctions->disconnect(conn)
 
 /* prototypes for functions in walreceiver.c */
 extern void WalReceiverMain(void) pg_attribute_noreturn();