Factor out warn_early_consensus()

Factor out the early consensus warning code from
networkstatus_set_current_consensus() into a new function
warn_early_consensus().

src/or/networkstatus.c

@@ -1751,6 +1751,48 @@ handle_missing_protocol_warning(const networkstatus_t *c,
     handle_missing_protocol_warning_impl(c, 1);
 }
 
+/**
+ * Check whether we received a consensus that appears to be coming
+ * from the future.  Because we implicitly trust the directory
+ * authorities' idea of the current time, we produce a warning if we
+ * get an early consensus.
+ *
+ * If we got a consensus that is time stamped far in the past, that
+ * could simply have come from a stale cache.  Possible ways to get a
+ * consensus from the future can include:
+ *
+ * - enough directory authorities have wrong clocks
+ * - directory authorities collude to produce misleading time stamps
+ * - our own clock is wrong (this is by far the most likely)
+ *
+ * We neglect highly improbable scenarios that involve actual time
+ * travel.
+ */
+STATIC void
+warn_early_consensus(const networkstatus_t *c, const char *flavor,
+                     time_t now)
+{
+/** If a consensus appears more than this many seconds before its declared
+ * valid-after time, declare that our clock is skewed. */
+#define EARLY_CONSENSUS_NOTICE_SKEW 60
+
+  if (now < c->valid_after - EARLY_CONSENSUS_NOTICE_SKEW) {
+    char tbuf[ISO_TIME_LEN+1];
+    char dbuf[64];
+    long delta = now - c->valid_after;
+    char *flavormsg = NULL;
+    format_iso_time(tbuf, c->valid_after);
+    format_time_interval(dbuf, sizeof(dbuf), delta);
+    log_warn(LD_GENERAL, "Our clock is %s behind the time published in the "
+             "consensus network status document (%s UTC).  Tor needs an "
+             "accurate clock to work correctly. Please check your time and "
+             "date settings!", dbuf, tbuf);
+    tor_asprintf(&flavormsg, "%s flavor consensus", flavor);
+    clock_skew_warning(NULL, delta, 1, LD_GENERAL, flavormsg, "CONSENSUS");
+    tor_free(flavormsg);
+  }
+}
+
 /** Try to replace the current cached v3 networkstatus with the one in
  * <b>consensus</b>.  If we don't have enough certificates to validate it,
  * store it in consensus_waiting_for_certs and launch a certificate fetch.
@@ -2053,25 +2095,7 @@ networkstatus_set_current_consensus(const char *consensus,
     write_str_to_file(consensus_fname, consensus, 0);
   }
 
-/** If a consensus appears more than this many seconds before its declared
- * valid-after time, declare that our clock is skewed. */
-#define EARLY_CONSENSUS_NOTICE_SKEW 60
-
-  if (now < c->valid_after - EARLY_CONSENSUS_NOTICE_SKEW) {
-    char tbuf[ISO_TIME_LEN+1];
-    char dbuf[64];
-    long delta = now - c->valid_after;
-    char *flavormsg = NULL;
-    format_iso_time(tbuf, c->valid_after);
-    format_time_interval(dbuf, sizeof(dbuf), delta);
-    log_warn(LD_GENERAL, "Our clock is %s behind the time published in the "
-             "consensus network status document (%s UTC).  Tor needs an "
-             "accurate clock to work correctly. Please check your time and "
-             "date settings!", dbuf, tbuf);
-    tor_asprintf(&flavormsg, "%s flavor consensus", flavor);
-    clock_skew_warning(NULL, delta, 1, LD_GENERAL, flavormsg, "CONSENSUS");
-    tor_free(flavormsg);
-  }
+  warn_early_consensus(c, flavor, now);
 
   /* We got a new consesus. Reset our md fetch fail cache */
   microdesc_reset_outdated_dirservers_list();

src/or/networkstatus.h

@@ -153,6 +153,8 @@ int any_client_port_set(const or_options_t *options);
 #ifdef TOR_UNIT_TESTS
 STATIC int networkstatus_set_current_consensus_from_ns(networkstatus_t *c,
                                                 const char *flavor);
+STATIC void warn_early_consensus(const networkstatus_t *c, const char *flavor,
+                                 time_t now);
 extern networkstatus_t *current_ns_consensus;
 extern networkstatus_t *current_md_consensus;
 #endif /* defined(TOR_UNIT_TESTS) */