libpq connection timeout mismanagement

The patch that taught libpq about allowing multiple target hosts
modified connectDBComplete() with the intent of making the
connect_timeout (if specified) apply per-host, not to the complete
connection attempt.  It did not do a very good job though, because
the timeout only gets reset when connectDBComplete() itself detects
a timeout.  If PQconnectPoll advances to a new host due to some
other cause, the previous host's timeout continues to run, possibly
causing a premature timeout failure for the new one.

Another thing that I find pretty strange is that it is coded so that,
in event of a timeout detection by connectDBComplete, we give up on the
current connhost entry and advance to the next host, ignoring any
additional addresses we might have for the current hostname.  This seems
at best poorly thought through.  There's no good reason for libpq to
assume that all the addresses returned by DNS point at the same machine,
or share the same network failure points in between.

Attached is an attempt to improve this.  I'll park it in the Sept fest.

            regards, tom lane

diff --git a/src/interfaces/libpq/fe-connect.c b/src/interfaces/libpq/fe-connect.c
index 986f74f..429be74 100644
*** a/src/interfaces/libpq/fe-connect.c
--- b/src/interfaces/libpq/fe-connect.c
*************** connectDBComplete(PGconn *conn)
*** 1905,1910 ****
--- 1905,1912 ----
      PostgresPollingStatusType flag = PGRES_POLLING_WRITING;
      time_t        finish_time = ((time_t) -1);
      int            timeout = 0;
+     int            last_whichhost = -2;    /* certainly different from whichhost */
+     struct addrinfo *last_addr_cur = NULL;

      if (conn == NULL || conn->status == CONNECTION_BAD)
          return 0;
*************** connectDBComplete(PGconn *conn)
*** 1922,1929 ****
               */
              if (timeout < 2)
                  timeout = 2;
-             /* calculate the finish time based on start + timeout */
-             finish_time = time(NULL) + timeout;
          }
      }

--- 1924,1929 ----
*************** connectDBComplete(PGconn *conn)
*** 1932,1937 ****
--- 1932,1952 ----
          int            ret = 0;

          /*
+          * (Re)start the connect_timeout timer if it's active and we are
+          * considering a different host than we were last time through.  If
+          * we've already succeeded, though, needn't recalculate.
+          */
+         if (flag != PGRES_POLLING_OK &&
+             timeout > 0 &&
+             (conn->whichhost != last_whichhost ||
+              conn->addr_cur != last_addr_cur))
+         {
+             finish_time = time(NULL) + timeout;
+             last_whichhost = conn->whichhost;
+             last_addr_cur = conn->addr_cur;
+         }
+
+         /*
           * Wait, if necessary.  Note that the initial state (just after
           * PQconnectStart) is to wait for the socket to select for writing.
           */
*************** connectDBComplete(PGconn *conn)
*** 1975,1992 ****
          if (ret == 1)            /* connect_timeout elapsed */
          {
              /*
!              * Attempt connection to the next host, ignoring any remaining
!              * addresses for the current host.
               */
!             conn->try_next_addr = false;
!             conn->try_next_host = true;
              conn->status = CONNECTION_NEEDED;
-
-             /*
-              * Restart the connect_timeout timer for the new host.
-              */
-             if (timeout > 0)
-                 finish_time = time(NULL) + timeout;
          }

          /*
--- 1990,1999 ----
          if (ret == 1)            /* connect_timeout elapsed */
          {
              /*
!              * Give up on current server/address, try the next one.
               */
!             conn->try_next_addr = true;
              conn->status = CONNECTION_NEEDED;
          }

          /*