dir: Look if circuit is closed in connection_dir_is_anonymous()

Before inspecting the p_chan, we must check if the circuit is marked for close
because if it is the case, the channels are nullified from the circuit.

Several valid cases can mark the circuit for close of the directory
connection.

Fixes #31958

Signed-off-by: David Goulet <dgoulet@torproject.org>

changes/ticket31958

@@ -0,0 +1,5 @@
+  o Minor bugfixes (directory):
+    - When checking if a directory connection is anonymous, test if the circuit
+      was marked for close before looking at its channel. This avoids a BUG()
+      stacktrace in case it was previously closed. Fixes bug 31958; bugfix on
+      0.4.2.1-alpha.

src/feature/dircommon/directory.c

@@ -225,7 +225,17 @@ connection_dir_is_anonymous(const dir_connection_t *dir_conn)
     return false;
   }
 
-  /* Get the previous channel to learn if it is a client or relay link. */
+  /* It is possible that the circuit was closed because one of the channel was
+   * closed or a DESTROY cell was received. Either way, this connection can
+   * not continue so return that it is not anonymous since we can not know for
+   * sure if it is. */
+  if (circ->marked_for_close) {
+    return false;
+  }
+
+  /* Get the previous channel to learn if it is a client or relay link. We
+   * BUG() because if the circuit is not mark for close, we ought to have a
+   * p_chan else we have a code flow issue. */
   if (BUG(CONST_TO_OR_CIRCUIT(circ)->p_chan == NULL)) {
     log_info(LD_DIR, "Rejected HSDir request: no p_chan");
     return false;