Rewrite async-connection loop in libpqwalreceiver.c, once again.

The original coding in commit 1e8a85009 didn't use PQconnectPoll per
spec, and while the rewrite in e434ad39a is closer, it still doesn't
guarantee to wait until the socket is read-ready or write-ready (as
appropriate) before calling PQconnectPoll.  It's not clear whether
that omission is causing the continuing failures on buildfarm member
bowerbird; but given the lack of other explanations meeting the
available facts, let's tighten that up and see what happens.

An independent issue in the same loop was that it had a race condition
whereby it could clear the process's latch without having serviced an
interrupt request, causing failure to respond to a cancel while waiting
for connection (the very problem 1e8a85009 was meant to fix).

Discussion: https://postgr.es/m/7295.1489596949@sss.pgh.pa.us

diff --git a/src/backend/replication/libpqwalreceiver/libpqwalreceiver.c b/src/backend/replication/libpqwalreceiver/libpqwalreceiver.c
index cd2e57867c062acc8132abc8f91656481aaa35c2..65a9e6c81ce82960f9b6837e18a378623fc898e7 100644 (file)
--- a/src/backend/replication/libpqwalreceiver/libpqwalreceiver.c
+++ b/src/backend/replication/libpqwalreceiver/libpqwalreceiver.c
@@ -159,41 +159,41 @@ libpqrcv_connect(const char *conninfo, bool logical, const char *appname,
        /*
         * Poll connection until we have OK or FAILED status.
         *
-        * Note that the initial state after PQconnectStartParams is
-        * PGRES_POLLING_WRITING.
+        * Per spec for PQconnectPoll, first wait till socket is write-ready.
         */
-       for (status = PGRES_POLLING_WRITING;
-                status != PGRES_POLLING_OK && status != PGRES_POLLING_FAILED;
-                status = PQconnectPoll(conn->streamConn))
+       status = PGRES_POLLING_WRITING;
+       do
        {
-               /* Sleep a bit if waiting for socket. */
-               if (status == PGRES_POLLING_READING ||
-                       status == PGRES_POLLING_WRITING)
+               /* Wait for socket ready and/or other events. */
+               int                     io_flag;
+               int                     rc;
+
+               io_flag = (status == PGRES_POLLING_READING
+                                  ? WL_SOCKET_READABLE
+                                  : WL_SOCKET_WRITEABLE);
+
+               rc = WaitLatchOrSocket(&MyProc->procLatch,
+                                                          WL_POSTMASTER_DEATH |
+                                                          WL_LATCH_SET | io_flag,
+                                                          PQsocket(conn->streamConn),
+                                                          0,
+                                                          WAIT_EVENT_LIBPQWALRECEIVER);
+
+               /* Emergency bailout? */
+               if (rc & WL_POSTMASTER_DEATH)
+                       exit(1);
+
+               /* Interrupted? */
+               if (rc & WL_LATCH_SET)
                {
-                       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();
+                       CHECK_FOR_INTERRUPTS();
                }
-       }
+
+               /* If socket is ready, advance the libpq state machine */
+               if (rc & io_flag)
+                       status = PQconnectPoll(conn->streamConn);
+       } while (status != PGRES_POLLING_OK && status != PGRES_POLLING_FAILED);
 
        if (PQstatus(conn->streamConn) != CONNECTION_OK)
        {