Add hidden torrc option to always try CC negotiation.

This is for public network testing and for sbws. Should not otherwise be used,
hence it is an undocumented __option.

The option deliberately does not allow force-disabling congestion control, as
this is bad for queueing and fairness.

3 files changed, 42 insertions, 8 deletions

diff --git a/src/app/config/config.c b/src/app/config/config.c
index 8df5275cc6..05bd96fc6a 100644
--- a/src/app/config/config.c
+++ b/src/app/config/config.c
@@ -675,6 +675,7 @@ static const config_var_t option_vars_[] = {
   V(VanguardsLiteEnabled,        AUTOBOOL, "auto"),
   V(UseMicrodescriptors,         AUTOBOOL, "auto"),
   OBSOLETE("UseNTorHandshake"),
+  VAR("__AlwaysCongestionControl",  BOOL, AlwaysCongestionControl, "0"),
   V_IMMUTABLE(User,              STRING,   NULL),
   OBSOLETE("UserspaceIOCPBuffers"),
   OBSOLETE("V1AuthoritativeDirectory"),
diff --git a/src/app/config/or_options_st.h b/src/app/config/or_options_st.h
index 3a1acad044..a1ef7a8cf8 100644
--- a/src/app/config/or_options_st.h
+++ b/src/app/config/or_options_st.h
@@ -601,6 +601,9 @@ struct or_options_t {
   /** Boolean: Switch to toggle the vanguards-lite subsystem */
   int VanguardsLiteEnabled;
 
+  /** Boolean: Switch to override consensus to enable congestion control */
+  int AlwaysCongestionControl;
+
   int RephistTrackTime; /**< How many seconds do we keep rephist info? */
   /** Should we always fetch our dir info on the mirror schedule (which
    * means directly from the authorities) no matter our other config? */
diff --git a/src/core/or/congestion_control_common.c b/src/core/or/congestion_control_common.c
index d4b933a032..7e38510814 100644
--- a/src/core/or/congestion_control_common.c
+++ b/src/core/or/congestion_control_common.c
@@ -26,6 +26,7 @@
 #include "core/or/trace_probes_cc.h"
 #include "lib/time/compat_time.h"
 #include "feature/nodelist/networkstatus.h"
+#include "app/config/config.h"
 
 /* Consensus parameter defaults.
  *
@@ -34,6 +35,7 @@
 #define CIRCWINDOW_INIT (500)
 #define SENDME_INC_DFLT (50)
 #define CC_ALG_DFLT (CC_ALG_SENDME)
+#define CC_ALG_DFLT_ALWAYS (CC_ALG_VEGAS)
 
 #define CWND_INC_DFLT (50)
 #define CWND_INC_PCT_SS_DFLT (100)
@@ -161,6 +163,7 @@ static void
 congestion_control_init_params(congestion_control_t *cc,
                                const circuit_params_t *params)
 {
+  const or_options_t *opts = get_options();
   cc->sendme_inc = params->sendme_inc_cells;
 
 #define CWND_INIT_MIN 100
@@ -219,13 +222,14 @@ congestion_control_init_params(congestion_control_t *cc,
         BWE_SENDME_MIN_MIN,
         BWE_SENDME_MIN_MAX);
 
-#define CC_ALG_MIN 0
-#define CC_ALG_MAX (NUM_CC_ALGS-1)
-  cc->cc_alg =
-    networkstatus_get_param(NULL, "cc_alg",
-        cc_alg,
-        CC_ALG_MIN,
-        CC_ALG_MAX);
+  /* If the consensus says to use OG sendme, but torrc has
+   * always-enabled, use the default "always" alg (vegas),
+   * else use cached conensus alg. */
+  if (cc_alg == CC_ALG_SENDME && opts->AlwaysCongestionControl) {
+    cc->cc_alg = CC_ALG_DFLT_ALWAYS;
+  } else {
+    cc->cc_alg = cc_alg;
+  }
 
   bdp_alg_t default_bdp_alg = 0;
 
@@ -262,10 +266,36 @@ congestion_control_init_params(congestion_control_t *cc,
 }
 
 /** Returns true if congestion control is enabled in the most recent
- * consensus */
+ * consensus, or if __AlwaysCongestionControl is set to true.
+ *
+ * Note that this function (and many many other functions) should not
+ * be called from the CPU worker threads when handling congestion
+ * control negotiation. Relevant values are marshaled into the
+ * `circuit_params_t` struct, in order to be used in worker threads
+ * without touching global state. Use those values in CPU worker
+ * threads, instead of calling this function.
+ *
+ * The danger is still present, in your time, as it was in ours.
+ */
 bool
 congestion_control_enabled(void)
 {
+  const or_options_t *opts = NULL;
+
+  tor_assert_nonfatal_once(in_main_thread());
+
+  opts = get_options();
+
+  /* If the user has set "__AlwaysCongesttionControl",
+   * then always try to negotiate congestion control, regardless
+   * of consensus param. This is to be used for testing and sbws.
+   *
+   * Note that we do *not* allow disabling congestion control
+   * if the consensus says to use it, as this is bad for queueing
+   * and fairness. */
+  if (opts->AlwaysCongestionControl)
+    return 1;
+
   return cc_alg != CC_ALG_SENDME;
 }