use !cbt_disabled in place of LearnCBT to avoid needless circs

This would make us do testing circuits "even when cbt is disabled by
consensus, or when we're a directory authority, or when we've failed
to write cbt history to our state file lately." (Roger's words.)

This is a fix for 9671 and an improvement in our fix for 5049.
The original misbehavior was in 0.2.2.14-alpha; the incomplete
fix was in 0.2.3.17-beta.

changes/bug9671_023

@@ -0,0 +1,5 @@
+  o Major bugfixes:
+    - If the circuit build timeout logic is disabled (via the consensus,
+      or because we are an authority), then don't build testing circuits.
+      Fixes bug 9657; bugfix on 0.2.2.14-alpha.
+

src/or/circuitbuild.c

@@ -147,7 +147,7 @@ static void pathbias_count_success(origin_circuit_t *circ);
  *  3. If we are a directory authority
  *  4. If we fail to write circuit build time history to our state file.
  */
-static int
+int
 circuit_build_times_disabled(void)
 {
   if (unit_tests) {

src/or/circuitbuild.h

@@ -32,6 +32,7 @@ char *circuit_list_path_for_controller(origin_circuit_t *circ);
 void circuit_log_path(int severity, unsigned int domain,
                       origin_circuit_t *circ);
 void circuit_rep_hist_note_result(origin_circuit_t *circ);
+int circuit_build_times_disabled(void);
 origin_circuit_t *origin_circuit_init(uint8_t purpose, int flags);
 origin_circuit_t *circuit_establish_circuit(uint8_t purpose,
                                             extend_info_t *exit,

src/or/circuituse.c

@@ -746,7 +746,7 @@ circuit_predict_and_launch_new(void)
    * want, don't do another -- we want to leave a few slots open so
    * we can still build circuits preemptively as needed. */
   if (num < MAX_UNUSED_OPEN_CIRCUITS-2 &&
-      get_options()->LearnCircuitBuildTimeout &&
+      ! circuit_build_times_disabled() &&
       circuit_build_times_needs_circuits_now(&circ_times)) {
     flags = CIRCLAUNCH_NEED_CAPACITY;
     log_info(LD_CIRC,
@@ -882,7 +882,7 @@ circuit_expire_old_circuits_clientside(void)
   tor_gettimeofday(&now);
   cutoff = now;
 
-  if (get_options()->LearnCircuitBuildTimeout &&
+  if (! circuit_build_times_disabled() &&
       circuit_build_times_needs_circuits(&circ_times)) {
     /* Circuits should be shorter lived if we need more of them
      * for learning a good build timeout */