Add a wrapper for a common networkstatus param pattern

We frequently want to check a networkstatus parameter only when it
isn't overridden from the torrc file.

src/or/networkstatus.c

@@ -2303,6 +2303,25 @@ networkstatus_get_param(const networkstatus_t *ns, const char *param_name,
                                  default_val, min_val, max_val);
 }
 
+/**
+ * As networkstatus_get_param(), but check torrc_value before checking the
+ * consensus. If torrc_value is in-range, then return it instead of the
+ * value from the consensus.
+ */
+int32_t
+networkstatus_get_overridable_param(const networkstatus_t *ns,
+                                    int32_t torrc_value,
+                                    const char *param_name,
+                                    int32_t default_val,
+                                    int32_t min_val, int32_t max_val)
+{
+  if (torrc_value >= min_val && torrc_value <= max_val)
+    return torrc_value;
+  else
+    return networkstatus_get_param(
+                         ns, param_name, default_val, min_val, max_val);
+}
+
 /**
  * Retrieve the consensus parameter that governs the
  * fixed-point precision of our network balancing 'bandwidth-weights'

src/or/networkstatus.h

@@ -111,6 +111,11 @@ int32_t networkstatus_get_param(const networkstatus_t *ns,
                                 const char *param_name,
                                 int32_t default_val, int32_t min_val,
                                 int32_t max_val);
+int32_t networkstatus_get_overridable_param(const networkstatus_t *ns,
+                                            int32_t torrc_value,
+                                            const char *param_name,
+                                            int32_t default_val,
+                                            int32_t min_val, int32_t max_val);
 int getinfo_helper_networkstatus(control_connection_t *conn,
                                  const char *question, char **answer,
                                  const char **errmsg);

src/test/test_dir.c

@@ -1494,6 +1494,15 @@ test_dir_param_voting(void *arg)
   tt_int_op(-8,OP_EQ, networkstatus_get_param(&vote4, "ab", -12, -100, -8));
   tt_int_op(0,OP_EQ, networkstatus_get_param(&vote4, "foobar", 0, -100, 8));
 
+  tt_int_op(100,OP_EQ, networkstatus_get_overridable_param(
+                                        &vote4, -1, "x-yz", 50, 0, 300));
+  tt_int_op(30,OP_EQ, networkstatus_get_overridable_param(
+                                        &vote4, 30, "x-yz", 50, 0, 300));
+  tt_int_op(0,OP_EQ, networkstatus_get_overridable_param(
+                                        &vote4, -101, "foobar", 0, -100, 8));
+  tt_int_op(-99,OP_EQ, networkstatus_get_overridable_param(
+                                        &vote4, -99, "foobar", 0, -100, 8));
+
   smartlist_add(votes, &vote1);
 
   /* Do the first tests without adding all the other votes, for