Fix potential deadlock in SyncedEnforcer with Redis Watcher

[Copilot]

Problem

A potential deadlock existed in SyncedEnforcer when using Redis watcher (or similar watchers) for policy synchronization between multiple nodes. The deadlock occurred due to circular lock acquisition between the enforcer's write lock and the watcher's mutex.

Deadlock Scenario

Consider two threads running concurrently:

Thread A (watcher subscribe thread):

1. Acquires watcher mutex in subscribe() method
2. Receives a policy update notification and calls the callback (load_policy())
3. Attempts to acquire the enforcer's write lock (_wl)
4. Blocks waiting for the lock

Thread B (policy modification thread):

1. Calls add_policy() which acquires the enforcer's write lock (_wl)
2. The underlying enforcer calls watcher.update() while holding the lock
3. watcher.update() attempts to acquire the watcher's mutex
4. Blocks waiting for the mutex

Result: Circular dependency → Deadlock

• Thread A holds watcher mutex, waiting for enforcer write lock
• Thread B holds enforcer write lock, waiting for watcher mutex

Solution

This PR fixes the deadlock by ensuring watcher notifications happen after releasing the enforcer's locks, eliminating the circular wait condition.

Key Changes

1. Added _notify_watcher() helper method: Calls watcher notification methods outside of any lock context, preventing lock ordering issues.

2. Modified all policy modification methods: Updated 26 methods (including add_policy, remove_policy, add_policies, add_grouping_policy, etc.) to:

   • Temporarily disable auto_notify_watcher before acquiring the lock
   • Perform the operation with the lock held
   • Release the lock
   • Notify the watcher outside the lock context

3. Enhanced set_watcher() method: Now properly sets the watcher's update callback to use self.load_policy, ensuring the callback acquires locks correctly when invoked by the watcher.

4. Added enable_auto_notify_watcher() method: Exposed this method in SyncedEnforcer for consistency with the base Enforcer class.

Example Pattern Applied

def add_policy(self, *params):
    # Temporarily disable auto_notify to prevent deadlock
    old_auto_notify = self._e.auto_notify_watcher
    self._e.auto_notify_watcher = False
    try:
        with self._wl:
            result = self._e.add_policy(*params)
    finally:
        self._e.auto_notify_watcher = old_auto_notify
    
    # Notify watcher outside of lock to prevent deadlock
    if result and old_auto_notify:
        self._notify_watcher("update_for_add_policy", "p", "p", list(params))
    
    return result

Testing

Added comprehensive tests in test_synced_enforcer_deadlock.py:

• Concurrent operations test: Simulates the deadlock scenario with actual threading to verify it's resolved
• Notification verification test: Ensures watchers are still properly notified for all operations
• Disable notification test: Verifies auto_notify_watcher can be disabled

All 304 tests pass, including both the new tests and all existing tests, confirming no regressions.

Impact

• ✅ Eliminates deadlock risk when using SyncedEnforcer with Redis watcher or similar implementations
• ✅ 100% backward compatible - no breaking changes to public API
• ✅ No functional changes - watcher notifications still work correctly, just happen outside lock context
• ✅ Minimal overhead - only adds necessary safety checks

This fix makes SyncedEnforcer safe to use with Redis watcher in production multi-threaded environments where policy updates and synchronization happen concurrently.

Fixes #XXX

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Issue: Potential Deadlock in SyncedEnforcer with Redis Watcher</issue_title>
<issue_description>## Problem Description
I've identified a potential deadlock scenario in the SyncedEnforcer implementation when using Redis watcher for policy consistence between multiple nodes. The deadlock occurs due to conflicting lock acquisition orders between different components.

Details

SyncedEnforcer

class SyncedEnforcer:
    def __init__(self, model=None, adapter=None):
        self._e = Enforcer(model, adapter)
        self._rwlock = RWLockWrite()
        self._rl = self._rwlock.gen_rlock()
        self._wl = self._rwlock.gen_wlock()
        self._auto_loading = AtomicBool(False)
        self._auto_loading_thread = None

    def load_policy(self):
        with self._wl:
            return self._e.load_policy()
    
    def add_policy(self, *params):        
        with self._wl:
            return self._e.add_policy(*params)

class InternalEnforcer(CoreEnforcer):
    def _add_policy(self, sec, ptype, rule):
        """adds a rule to the current policy."""
        if self.model.has_policy(sec, ptype, rule):
            return False

        if self.adapter and self.auto_save:
            if self.adapter.add_policy(sec, ptype, rule) is False:
                return False

            if self.watcher and self.auto_notify_watcher:
                if callable(getattr(self.watcher, "update_for_add_policy", None)):
                    self.watcher.update_for_add_policy(sec, ptype, rule)
                else:
                    self.watcher.update()

        rule_added = self.model.add_policy(sec, ptype, rule)

Redis Watcher(https://github.com/officialpycasbin/redis-watcher/tree/master)

Watcher Update Method:

def update(self):
    def func():
        with self.mutex:  # Acquires watcher mutex
            msg = MSG("Update", self.options.local_ID, "", "", "")
            return self.pub_client.publish(self.options.channel, msg.marshal_binary())

    return self.log_record(func)

Redis Watcher Subscribe Method:

def subscribe(self):
    time.sleep(self.sleep)
    try:
        if not self.sub_client:
            rds = self._get_redis_conn()
            self.sub_client = rds.client().pubsub()
        self.sub_client.subscribe(self.options.channel)
        print(f"Waiting for casbin updates... in the worker: {self.options.local_ID}")
        if self.execute_update:
            self.update()
        try:
            for item in self.sub_client.listen():
                if not self.subscribe_event.is_set():
                    self.subscribe_event.set()
                if item is not None and item["type"] == "message":
                    try:
                        with self.mutex:  # Acquires watcher mutex for callback
                            self.callback(str(item))  # Typically calls load_policy()
                    except Exception as listen_exc:
                        print(
                            "Casbin Redis watcher failed sending update to teh callback function "
                            " process due to: {}".format(str(listen_exc))
                        )
                        if self.sub_client:
                            self.sub_client.close()
                        break
        except Exception as sub_exc:
            print("Casbin Redis watcher failed to get message from redis due to {}".format(str(sub_exc)))
            if self.sub_client:
                self.sub_client.close()
    except Exception as redis_exc:
        print("Casbin Redis watcher failed to subscribe due to: {}".format(str(redis_exc)))
    finally:
        if self.sub_client:
            self.sub_client.close()

Deadlock Scenario

Thread A (Subscribe thread):

• Acquires mutex in subscribe() method (line: with self.mutex:)
• Calls callback (typically load_policy())
• Attempts to acquire _wl (write lock) in load_policy() (line: with self._wl:)

Thread B (Policy modification thread):

• Acquires _wl (write lock) in add_policy() (line: with self._wl:)
• Calls _e.add_policy() which triggers watcher's update() method
• Attempts to acquire mutex in update() (line: with self.mutex:)

Circular Dependency:
Thread A holds mutex, waiting for _wl
Thread B holds _wl, waiting for mutex
</issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes #408

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