Use asynchronous connect API in libpqwalreceiver

This makes the connection attempt from CREATE SUBSCRIPTION and from
WalReceiver interruptable by the user in case the libpq connection is
hanging.  The previous coding required immediate shutdown (SIGQUIT) of
PostgreSQL in that situation.

From: Petr Jelinek <petr.jelinek@2ndquadrant.com>
Tested-by: Thom Brown <thom@linux.com>

src/backend/postmaster/pgstat.c

@@ -3340,8 +3340,8 @@ pgstat_get_wait_client(WaitEventClient w)
 		case WAIT_EVENT_WAL_RECEIVER_WAIT_START:
 			event_name = "WalReceiverWaitStart";
 			break;
-		case WAIT_EVENT_LIBPQWALRECEIVER_READ:
-			event_name = "LibPQWalReceiverRead";
+		case WAIT_EVENT_LIBPQWALRECEIVER:
+			event_name = "LibPQWalReceiver";
 			break;
 		case WAIT_EVENT_WAL_SENDER_WAIT_WAL:
 			event_name = "WalSenderWaitForWAL";

src/backend/replication/libpqwalreceiver/libpqwalreceiver.c

@@ -113,6 +113,7 @@ libpqrcv_connect(const char *conninfo, bool logical, const char *appname,
 				 char **err)
 {
 	WalReceiverConn *conn;
+	PostgresPollingStatusType status;
 	const char *keys[5];
 	const char *vals[5];
 	int			i = 0;
@@ -146,7 +147,51 @@ libpqrcv_connect(const char *conninfo, bool logical, const char *appname,
 	Assert(i < sizeof(keys));
 
 	conn = palloc0(sizeof(WalReceiverConn));
-	conn->streamConn = PQconnectdbParams(keys, vals, /* expand_dbname = */ true);
+	conn->streamConn = PQconnectStartParams(keys, vals,
+											/* expand_dbname = */ true);
+	if (PQstatus(conn->streamConn) == CONNECTION_BAD)
+	{
+		*err = pchomp(PQerrorMessage(conn->streamConn));
+		return NULL;
+	}
+
+	/* Poll connection. */
+	do
+	{
+		/* Determine current state of the connection. */
+		status = PQconnectPoll(conn->streamConn);
+
+		/* Sleep a bit if waiting for socket. */
+		if (status == PGRES_POLLING_READING ||
+			status == PGRES_POLLING_WRITING)
+		{
+			int		extra_flag;
+			int		rc;
+
+			extra_flag = (status == PGRES_POLLING_READING
+						  ? WL_SOCKET_READABLE
+						  : WL_SOCKET_WRITEABLE);
+
+			ResetLatch(&MyProc->procLatch);
+			rc = WaitLatchOrSocket(&MyProc->procLatch,
+								   WL_POSTMASTER_DEATH |
+								   WL_LATCH_SET | extra_flag,
+								   PQsocket(conn->streamConn),
+								   0,
+								   WAIT_EVENT_LIBPQWALRECEIVER);
+
+			/* Emergency bailout. */
+			if (rc & WL_POSTMASTER_DEATH)
+				exit(1);
+
+			/* Interrupted. */
+			if (rc & WL_LATCH_SET)
+				CHECK_FOR_INTERRUPTS();
+		}
+
+		/* Otherwise loop until we have OK or FAILED status. */
+	} while (status != PGRES_POLLING_OK && status != PGRES_POLLING_FAILED);
+
 	if (PQstatus(conn->streamConn) != CONNECTION_OK)
 	{
 		*err = pchomp(PQerrorMessage(conn->streamConn));
@@ -529,7 +574,7 @@ libpqrcv_PQexec(PGconn *streamConn, const char *query)
 								   WL_LATCH_SET,
 								   PQsocket(streamConn),
 								   0,
-								   WAIT_EVENT_LIBPQWALRECEIVER_READ);
+								   WAIT_EVENT_LIBPQWALRECEIVER);
 			if (rc & WL_POSTMASTER_DEATH)
 				exit(1);
 

src/include/pgstat.h

@@ -764,7 +764,7 @@ typedef enum
 	WAIT_EVENT_CLIENT_WRITE,
 	WAIT_EVENT_SSL_OPEN_SERVER,
 	WAIT_EVENT_WAL_RECEIVER_WAIT_START,
-	WAIT_EVENT_LIBPQWALRECEIVER_READ,
+	WAIT_EVENT_LIBPQWALRECEIVER,
 	WAIT_EVENT_WAL_SENDER_WAIT_WAL,
 	WAIT_EVENT_WAL_SENDER_WRITE_DATA
 } WaitEventClient;