Recover better from empty/invalid storagedir files

If we can't read a file because of an FS issue, we say "we can't
read that" and move on.  But if we can't read it because it's empty,
because it has no labels, or because its labels are misformatted, we
should remove it.

Fixes bug 24099; bugfix on 0.3.1.1-alpha.

changes/bug24099

@@ -0,0 +1,4 @@
+  o Minor bugfixes (directory cache):
+    - Recover better from empty or corrupt files in the consensus cache
+      directory. Fixes bug 24099; bugfix on 0.3.1.1-alpha.
+

src/or/conscache.c

@@ -539,9 +539,16 @@ consensus_cache_rescan(consensus_cache_t *cache)
     map = storage_dir_map_labeled(cache->dir, fname,
                                   &labels, &body, &bodylen);
     if (! map) {
-      /* Can't load this; continue */
-      log_warn(LD_FS, "Unable to map file %s from consensus cache: %s",
-               escaped(fname), strerror(errno));
+      if (errno == ERANGE || errno == EINVAL) {
+        log_warn(LD_FS, "Found %s file %s in consensus cache; removing it.",
+                 errno == ERANGE ? "empty" : "misformatted",
+                 escaped(fname));
+        storage_dir_remove_file(cache->dir, fname);
+      } else {
+        /* Can't load this; continue */
+        log_warn(LD_FS, "Unable to map file %s from consensus cache: %s",
+                 escaped(fname), strerror(errno));
+      }
       continue;
     }
     consensus_cache_entry_t *ent =