Add cmux support for inter-cmux comparisons

src/or/circuitmux.c

@@ -1951,3 +1951,51 @@ circuitmux_count_queued_destroy_cells(const channel_t *chan,
   return n_destroy_cells;
 }
 
+/**
+ * Compare cmuxes to see which is more preferred; return < 0 if
+ * cmux_1 has higher priority (i.e., cmux_1 < cmux_2 in the scheduler's
+ * sort order), > 0 if cmux_2 has higher priority, or 0 if they are
+ * equally preferred.
+ *
+ * If the cmuxes have different cmux policies or the policy does not
+ * support the cmp_cmux method, return 0.
+ */
+
+int
+circuitmux_compare_muxes(circuitmux_t *cmux_1, circuitmux_t *cmux_2)
+{
+  const circuitmux_policy_t *policy;
+
+  tor_assert(cmux_1);
+  tor_assert(cmux_2);
+
+  if (cmux_1 == cmux_2) {
+    /* Equivalent because they're the same cmux */
+    return 0;
+  }
+
+  if (cmux_1->policy && cmux_2->policy) {
+    if (cmux_1->policy == cmux_2->policy) {
+      policy = cmux_1->policy;
+
+      if (policy->cmp_cmux) {
+        /* Okay, we can compare! */
+        return policy->cmp_cmux(cmux_1, cmux_1->policy_data,
+                                cmux_2, cmux_2->policy_data);
+      } else {
+        /*
+         * Equivalent because the policy doesn't know how to compare between
+         * muxes.
+         */
+        return 0;
+      }
+    } else {
+      /* Equivalent because they have different policies */
+      return 0;
+    }
+  } else {
+    /* Equivalent because one or both are missing a policy */
+    return 0;
+  }
+}
+

src/or/circuitmux.h

@@ -57,6 +57,9 @@ struct circuitmux_policy_s {
   /* Choose a circuit */
   circuit_t * (*pick_active_circuit)(circuitmux_t *cmux,
                                      circuitmux_policy_data_t *pol_data);
+  /* Optional: channel comparator for use by the scheduler */
+  int (*cmp_cmux)(circuitmux_t *cmux_1, circuitmux_policy_data_t *pol_data_1,
+                  circuitmux_t *cmux_2, circuitmux_policy_data_t *pol_data_2);
 };
 
 /*
@@ -148,5 +151,8 @@ void circuitmux_append_destroy_cell(channel_t *chan,
 void circuitmux_mark_destroyed_circids_usable(circuitmux_t *cmux,
                                               channel_t *chan);
 
+/* Optional interchannel comparisons for scheduling */
+int circuitmux_compare_muxes(circuitmux_t *cmux_1, circuitmux_t *cmux_2);
+
 #endif /* TOR_CIRCUITMUX_H */