Prefer mmap()ed consensus files over cached_dir_t entries.

Cached_dir_t is a somewhat "legacy" kind of storage when used for
consensus documents, and it appears that there are cases when
changing our settings causes us to stop updating those entries.

This can cause trouble, as @arma found out in #40375, where he
changed his settings around, and consensus diff application got
messed up: consensus diffs were being _requested_ based on the
latest consensus, but were being (incorrectly) applied to a
consensus that was no longer the latest one.

This patch is a minimal fix for backporting purposes: it has Tor do
the same search when applying consensus diffs as we use to request
them.  This should be sufficient for correct behavior.

There's a similar case in GETINFO handling; I've fixed that too.

Fixes #40375; bugfix on 0.3.1.1-alpha.

1 files changed, 13 insertions, 8 deletions

diff --git a/src/feature/dirclient/dirclient.c b/src/feature/dirclient/dirclient.c
index a5dd856729..b3159966d2 100644
--- a/src/feature/dirclient/dirclient.c
+++ b/src/feature/dirclient/dirclient.c
@@ -2263,18 +2263,23 @@ handle_response_fetch_consensus(dir_connection_t *conn,
 
   if (looks_like_a_consensus_diff(body, body_len)) {
     /* First find our previous consensus. Maybe it's in ram, maybe not. */
-    cached_dir_t *cd = dirserv_get_consensus(flavname);
+    cached_dir_t *cd = NULL;
     const char *consensus_body = NULL;
     size_t consensus_body_len;
     tor_mmap_t *mapped_consensus = NULL;
-    if (cd) {
-      consensus_body = cd->dir;
-      consensus_body_len = cd->dir_len;
+
+    /* We prefer the mmap'd version over the cached_dir_t version,
+     * since that matches the logic we used when we picked a consensus
+     * back in dir_consensus_request_set_additional_headers. */
+    mapped_consensus = networkstatus_map_cached_consensus(flavname);
+    if (mapped_consensus) {
+      consensus_body = mapped_consensus->data;
+      consensus_body_len = mapped_consensus->size;
     } else {
-      mapped_consensus = networkstatus_map_cached_consensus(flavname);
-      if (mapped_consensus) {
-        consensus_body = mapped_consensus->data;
-        consensus_body_len = mapped_consensus->size;
+      cd = dirserv_get_consensus(flavname);
+      if (cd) {
+        consensus_body = cd->dir;
+        consensus_body_len = cd->dir_len;
       }
     }
     if (!consensus_body) {