Allow SIGINT to cancel psql database reconnections.

After installing the SIGINT handler in psql, SIGINT can no longer cancel
database reconnections. For instance, if the user starts a reconnection
and then needs to do some form of interaction (ie psql is polling),
there is no way to cancel the reconnection process currently.

Use PQconnectStartParams() in order to insert a cancel_pressed check
into the polling loop.

Tristan Partin, reviewed by Gurjeet Singh, Heikki Linnakangas, Jelte
Fennema-Nio, and me.

Discussion: http://postgr.es/m/D08WWCPVHKHN.3QELIKZJ2D9RZ@neon.tech

src/bin/psql/command.c

@@ -159,6 +159,7 @@ static void discard_query_text(PsqlScanState scan_state, ConditionalStack cstack
 static bool copy_previous_query(PQExpBuffer query_buf, PQExpBuffer previous_buf);
 static bool do_connect(enum trivalue reuse_previous_specification,
 					   char *dbname, char *user, char *host, char *port);
+static void wait_until_connected(PGconn *conn);
 static bool do_edit(const char *filename_arg, PQExpBuffer query_buf,
 					int lineno, bool discard_on_quit, bool *edited);
 static bool do_shell(const char *command);
@@ -3595,11 +3596,12 @@ do_connect(enum trivalue reuse_previous_specification,
 		values[paramnum] = NULL;
 
 		/* Note we do not want libpq to re-expand the dbname parameter */
-		n_conn = PQconnectdbParams(keywords, values, false);
+		n_conn = PQconnectStartParams(keywords, values, false);
 
 		pg_free(keywords);
 		pg_free(values);
 
+		wait_until_connected(n_conn);
 		if (PQstatus(n_conn) == CONNECTION_OK)
 			break;
 
@@ -3748,6 +3750,74 @@ do_connect(enum trivalue reuse_previous_specification,
 	return true;
 }
 
+/*
+ * Processes the connection sequence described by PQconnectStartParams(). Don't
+ * worry about reporting errors in this function. Our caller will check the
+ * connection's status, and report appropriately.
+ */
+static void
+wait_until_connected(PGconn *conn)
+{
+	bool		forRead = false;
+
+	while (true)
+	{
+		int			rc;
+		int			sock;
+		time_t		end_time;
+
+		/*
+		 * On every iteration of the connection sequence, let's check if the
+		 * user has requested a cancellation.
+		 */
+		if (cancel_pressed)
+			break;
+
+		/*
+		 * Do not assume that the socket remains the same across
+		 * PQconnectPoll() calls.
+		 */
+		sock = PQsocket(conn);
+		if (sock == -1)
+			break;
+
+		/*
+		 * If the user sends SIGINT between the cancel_pressed check, and
+		 * polling of the socket, it will not be recognized. Instead, we will
+		 * just wait until the next step in the connection sequence or forever,
+		 * which might require users to send SIGTERM or SIGQUIT.
+		 *
+		 * Some solutions would include the "self-pipe trick," using
+		 * pselect(2) and ppoll(2), or using a timeout.
+		 *
+		 * The self-pipe trick requires a bit of code to setup. pselect(2) and
+		 * ppoll(2) are not on all the platforms we support. The simplest
+		 * solution happens to just be adding a timeout, so let's wait for 1
+		 * second and check cancel_pressed again.
+		 */
+		end_time = time(NULL) + 1;
+		rc = PQsocketPoll(sock, forRead, !forRead, end_time);
+		if (rc == -1)
+			return;
+
+		switch (PQconnectPoll(conn))
+		{
+			case PGRES_POLLING_OK:
+			case PGRES_POLLING_FAILED:
+				return;
+			case PGRES_POLLING_READING:
+				forRead = true;
+				continue;
+			case PGRES_POLLING_WRITING:
+				forRead = false;
+				continue;
+			case PGRES_POLLING_ACTIVE:
+				pg_unreachable();
+		}
+	}
+
+	pg_unreachable();
+}
 
 void
 connection_warnings(bool in_startup)