Don't consider a port "handled" by an isolated circuit.

Previously, circuit_stream_is_being_handled incorrectly reported
that (1) an exit port was "handled" by a circuit regardless of
whether the circuit was already isolated in some way, and
(2) that a stream could be "handled" by a circuit even if their
isolation settings were incompatible.

As a result of (1), in Tor Browser, circuit_get_unhandled_ports was
reporting that all ports were handled even though all non-internal
circuits had already been isolated by a SOCKS username+password.

Therefore, circuit_predict_and_launch_new was declining to launch
new exit circuits. Then, when the user visited a new site in Tor
Browser, a stream with new SOCKS credentials would be initiated,
and the stream would have to wait while a new circuit with those
credentials could be built. That wait was making the
time-to-first-byte longer than it needed to be.

Now, clean, not-yet-isolated circuit(s) will be automatically
launched ahead of time and be ready for use whenever a new stream
with new SOCKS credentials (or other isolation criteria) is
initiated.

Fixes bug 18859. Thanks to Nick Mathewson for improvements.

changes/bug18859

@@ -0,0 +1,5 @@
+  o Minor bugfixes (circuit prediction):
+    - Fix circuit_stream_is_being_handled so it correctly reports on circuits
+      with isolation settings. Ports must not be said to be "handled" by
+      already-isolated circuits, and a stream can only be handled by a circuit
+      if their isolation settings are compatible. Fixes bug 18859.

src/or/circuituse.c

@@ -973,7 +973,7 @@ circuit_remove_handled_ports(smartlist_t *needed_ports)
     tor_assert(*port);
     if (circuit_stream_is_being_handled(NULL, *port,
                                         MIN_CIRCUITS_HANDLING_STREAM)) {
-//      log_debug(LD_CIRC,"Port %d is already being handled; removing.", port);
+      log_debug(LD_CIRC,"Port %d is already being handled; removing.", *port);
       smartlist_del(needed_ports, i--);
       tor_free(port);
     } else {
@@ -1010,6 +1010,10 @@ circuit_stream_is_being_handled(entry_connection_t *conn,
         continue;
       if (origin_circ->unusable_for_new_conns)
         continue;
+      if (origin_circ->isolation_values_set &&
+          (conn == NULL ||
+           !connection_edge_compatible_with_circuit(conn, origin_circ)))
+        continue;
 
       exitnode = build_state_get_exit_node(build_state);
       if (exitnode && (!need_uptime || build_state->need_uptime)) {