stats: Stop removing the ed25519 signature if the extra info file is too big

If the signature data was removed, but the keyword was kept, this could
result in an unparseable extra info file.

Fixes bug 30958; bugfix on 0.2.7.2-alpha.

changes/bug30958

@@ -0,0 +1,5 @@
+  o Minor bugfixes (statistics):
+    - Stop removing the ed25519 signature if the extra info file is too big.
+      If the signature data was removed, but the keyword was kept, this could
+      result in an unparseable extra info file. Fixes bug 30958;
+      bugfix on 0.2.7.2-alpha.

src/or/router.c

@@ -3252,11 +3252,13 @@ extrainfo_dump_to_string(char **s_out, extrainfo_t *extrainfo,
   while (strlen(s) > MAX_EXTRAINFO_UPLOAD_SIZE - DIROBJ_MAX_SIG_LEN) {
     /* So long as there are at least two chunks (one for the initial
      * extra-info line and one for the router-signature), we can keep removing
-     * things. */
-    if (smartlist_len(chunks) > 2) {
-      /* We remove the next-to-last element (remember, len-1 is the last
-         element), since we need to keep the router-signature element. */
-      int idx = smartlist_len(chunks) - 2;
+     * things. If emit_ed_sigs is true, we also keep 2 additional chunks at the
+     * end for the ed25519 signature. */
+    const int required_chunks = emit_ed_sigs ? 4 : 2;
+    if (smartlist_len(chunks) > required_chunks) {
+      /* We remove the next-to-last or 4th-last element (remember, len-1 is the
+       * last element), since we need to keep the router-signature elements. */
+      int idx = smartlist_len(chunks) - required_chunks;
       char *e = smartlist_get(chunks, idx);
       smartlist_del_keeporder(chunks, idx);
       log_warn(LD_GENERAL, "We just generated an extra-info descriptor "