warn if we launch too many circuits for a given stream

changes/bug1090-launch-warning

@@ -0,0 +1,5 @@
+  o Minor features:
+    - Keep track of how many times we launch a new circuit to handle
+      a given stream. Too many launches could indicate an inconsistency
+      between our "launch a circuit to handle this stream" logic and our
+      "attach our stream to one of the available circuits" logic.

src/or/circuituse.c

@@ -1409,7 +1409,18 @@ circuit_get_open_circ_or_launch(edge_connection_t *conn,
 
     extend_info_free(extend_info);
 
-    if (desired_circuit_purpose != CIRCUIT_PURPOSE_C_GENERAL) {
+    if (desired_circuit_purpose == CIRCUIT_PURPOSE_C_GENERAL) {
+      /* We just caused a circuit to get built because of this stream.
+       * If this stream has caused a _lot_ of circuits to be built, that's
+       * a bad sign: we should tell the user. */
+      if (conn->num_circuits_launched < NUM_CIRCUITS_LAUNCHED_THRESHOLD &&
+          ++conn->num_circuits_launched == NUM_CIRCUITS_LAUNCHED_THRESHOLD)
+        log_warn(LD_BUG, "The application request to %s:%d has launched "
+                 "%d circuits without finding one it likes.",
+                 escaped_safe_str_client(conn->socks_request->address),
+                 conn->socks_request->port,
+                 conn->num_circuits_launched);
+    } else {
       /* help predict this next time */
       rep_hist_note_used_internal(time(NULL), need_uptime, 1);
       if (circ) {

src/or/or.h

@@ -1151,6 +1151,13 @@ typedef struct edge_connection_t {
    * already retried several times. */
   uint8_t num_socks_retries;
 
+#define NUM_CIRCUITS_LAUNCHED_THRESHOLD 10
+  /** Number of times we've launched a circuit to handle this stream. If
+    * it gets too high, that could indicate an inconsistency between our
+    * "launch a circuit to handle this stream" logic and our "attach our
+    * stream to one of the available circuits" logic. */
+  unsigned int num_circuits_launched:4;
+
   /** True iff this connection is for a DNS request only. */
   unsigned int is_dns_request:1;