Resolve bug18812 by logging fallback key changes at info level

changes/bug18812

@@ -0,0 +1,4 @@
+  o Minor bugfixes (bootstrap):
+    - When a fallback changes its fingerprint from the hard-coded
+      fingerprint, log a less severe, more explanatory log message.
+      Fixes bug 18812; bugfix on 0.2.8.1-alpha. Patch by teor.

src/or/connection_or.c

@@ -31,6 +31,7 @@
 #include "geoip.h"
 #include "main.h"
 #include "link_handshake.h"
+#include "microdesc.h"
 #include "networkstatus.h"
 #include "nodelist.h"
 #include "reasons.h"
@@ -1706,6 +1707,9 @@ connection_or_check_valid_tls_handshake(or_connection_t *conn,
  *  - log a protocol warning whenever the fingerprints don't match;
  * On clients:
  *  - if a relay's fingerprint doesn't match, log a warning;
+ *  - if we don't have updated relay fingerprints from a recent consensus, and
+ *    a fallback directory mirror's hard-coded fingerprint has changed, log an
+ *    info explaining that we will try another fallback.
  *
  * If we're testing reachability, remember what we learned.
  *
@@ -1740,19 +1744,43 @@ connection_or_client_learned_peer_id(or_connection_t *conn,
     base16_encode(seen, sizeof(seen), (const char*)peer_id, DIGEST_LEN);
     base16_encode(expected, sizeof(expected), conn->identity_digest,
                   DIGEST_LEN);
+    const int using_hardcoded_fingerprints =
+      !networkstatus_get_reasonably_live_consensus(time(NULL),
+                                                   usable_consensus_flavor());
+    const int is_fallback_fingerprint = router_digest_is_fallback_dir(
+                                                   conn->identity_digest);
+    const int is_authority_fingerprint = router_digest_is_trusted_dir(
+                                                                      conn->identity_digest);
     int severity;
+    const char *extra_log = "";
 
     if (server_mode(options)) {
       severity = LOG_PROTOCOL_WARN;
     } else {
-      /* a relay has changed its fingerprint from the one in the consensus */
-      severity = LOG_WARN;
+      if (using_hardcoded_fingerprints) {
+        /* We need to do the checks in this order, because the list of
+         * fallbacks includes the list of authorities */
+        if (is_authority_fingerprint) {
+          severity = LOG_WARN;
+        } else if (is_fallback_fingerprint) {
+          /* we expect a small number of fallbacks to change from their
+           * hard-coded fingerprints over the life of a release */
+          severity = LOG_INFO;
+          extra_log = " Tor will try a different fallback.";
+        } else {
+          /* it's a bridge, it's either a misconfiguration, or unexpected */
+          severity = LOG_WARN;
+        }
+      } else {
+        /* a relay has changed its fingerprint from the one in the consensus */
+        severity = LOG_WARN;
+      }
     }
 
     log_fn(severity, LD_HANDSHAKE,
            "Tried connecting to router at %s:%d, but identity key was not "
-           "as expected: wanted %s but got %s.",
-           conn->base_.address, conn->base_.port, expected, seen);
+           "as expected: wanted %s but got %s.%s",
+           conn->base_.address, conn->base_.port, expected, seen, extra_log);
     entry_guard_register_connect_status(conn->identity_digest, 0, 1,
                                         time(NULL));
     control_event_or_conn_status(conn, OR_CONN_EVENT_FAILED,