circ: Don't cannibalize a circuit if the guard state is unusable

Tor preemptiely builds circuits and they can be cannibalized later in their
lifetime. A Guard node can become unusable (from our guard state) but we can
still have circuits using that node opened. It is important to not pick those
circuits for any usage through the cannibalization process.

Fixes #24469

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

1 files changed, 16 insertions, 0 deletions

diff --git a/src/or/entrynodes.c b/src/or/entrynodes.c
index 292a393e51..2b6ff38c9c 100644
--- a/src/or/entrynodes.c
+++ b/src/or/entrynodes.c
@@ -3307,6 +3307,22 @@ entry_guards_update_state(or_state_t *state)
   entry_guards_dirty = 0;
 }
 
+/** Return true iff the circuit's guard can succeed that is can be used. */
+int
+entry_guard_could_succeed(const circuit_guard_state_t *guard_state)
+{
+  if (!guard_state) {
+    return 0;
+  }
+
+  entry_guard_t *guard = entry_guard_handle_get(guard_state->guard);
+  if (!guard || BUG(guard->in_selection == NULL)) {
+    return 0;
+  }
+
+  return 1;
+}
+
 /**
  * Format a single entry guard in the format expected by the controller.
  * Return a newly allocated string.