Launch sufficient circuits to satisfy pending isolated streams

Our old "do we need to launch a circuit for stream S" logic was,
more or less, that if we had a pending circuit that could handle S,
we didn't need to launch a new one.

But now that we have streams isolated from one another, we need
something stronger here: It's possible that some pending C can
handle either S1 or S2, but not both.

This patch reuses the existing isolation logic for a simple
solution: when we decide during circuit launching that some pending
C would satisfy stream S1, we "hypothetically" mark C as though S1
had been connected to it.  Now if S2 is incompatible with S1, it
won't be something that can attach to C, and so we'll launch a new
stream.

When the circuit becomes OPEN for the first time (with no streams
attached to it), we reset the circuit's isolation status.  I'm not
too sure about this part: I wanted some way to be sure that, if all
streams that would have used a circuit die before the circuit is
done, the circuit can still get used.  But I worry that this
approach could also lead to us launching too many circuits.  Careful
thought needed here.

1 files changed, 11 insertions, 2 deletions

diff --git a/src/or/circuituse.c b/src/or/circuituse.c
index 19a62344f3..93098e527d 100644
--- a/src/or/circuituse.c
+++ b/src/or/circuituse.c
@@ -1457,12 +1457,20 @@ circuit_get_open_circ_or_launch(edge_connection_t *conn,
           rend_client_rendcirc_has_opened(circ);
       }
     }
-  }
-  if (!circ)
+  } /* endif (!circ) */
+  if (circ) {
+    /* Mark the circuit with the isolation fields for this connection.
+     * When the circuit arrives, we'll clear these flags: this is
+     * just some internal bookkeeping to make sure that we have
+     * launched enough circuits.
+     */
+    connection_edge_update_circuit_isolation(conn, circ, 0);
+  } else {
     log_info(LD_APP,
              "No safe circuit (purpose %d) ready for edge "
              "connection; delaying.",
              desired_circuit_purpose);
+  }
   *circp = circ;
   return 0;
 }
@@ -1509,6 +1517,7 @@ link_apconn_to_circ(edge_connection_t *apconn, origin_circuit_t *circ,
     apconn->cpath_layer = circ->cpath->prev;
   }
 
+  circ->isolation_any_streams_attached = 1;
   connection_edge_update_circuit_isolation(apconn, circ, 0);
 }