Check transition of circuit purpose from INTRO->GENERAL if nodes are constrained

This looked at first like another fun way around our node selection
logic: if we had introduction circuits, and we wound up building too
many, we would turn extras into general-purpose circuits.  But when we
did so, we wouldn't necessarily check whether the general-purpose
circuits conformed to our node constraints.  For example, the last
node could totally be in ExcludedExitNodes and we wouldn't have cared...

...except that the circuit should already be internal, so it won't get user
streams attached to it, so the transition should generally be allowed.
Add an assert to make sure we're right about this, and have it not
check whether ExitNodes is set, since that's irrelevant to internal
circuits.

src/or/rendservice.c

@@ -1347,17 +1347,26 @@ rend_service_intro_has_opened(origin_circuit_t *circuit)
   }
 
   /* If we already have enough introduction circuits for this service,
-   * redefine this one as a general circuit. */
+   * redefine this one as a general circuit or close it, depending. */
   if (count_established_intro_points(serviceid) > NUM_INTRO_POINTS) {
-    log_info(LD_CIRC|LD_REND, "We have just finished an introduction "
-             "circuit, but we already have enough. Redefining purpose to "
-             "general.");
-    /* XXX022-1090: This can wind up violating ExcludeNodes/
-     * ExitNodes/ExcludeExitNodes restrictions.
-     */
-    TO_CIRCUIT(circuit)->purpose = CIRCUIT_PURPOSE_C_GENERAL;
-    circuit_has_opened(circuit);
-    return;
+    or_options_t *options = get_options();
+    if (options->ExcludeNodes) {
+      /* XXXX in some future version, we can test whether the transition is
+         allowed or not given the actual nodes in the circuit.  But for now,
+         this case, we might as well close the thing. */
+      log_info(LD_CIRC|LD_REND, "We have just finished an introduction "
+               "circuit, but we already have enough.  Closing it.");
+      circuit_mark_for_close(TO_CIRCUIT(circuit), END_CIRC_REASON_NONE);
+      return;
+    } else {
+      tor_assert(circuit->build_state->is_internal);
+      log_info(LD_CIRC|LD_REND, "We have just finished an introduction "
+               "circuit, but we already have enough. Redefining purpose to "
+               "general; leaving as internal.");
+      TO_CIRCUIT(circuit)->purpose = CIRCUIT_PURPOSE_C_GENERAL;
+      circuit_has_opened(circuit);
+      return;
+    }
   }
 
   log_info(LD_REND,