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BUG-HUNT: [concurrency] A2aEventQueue.publish() iterates _subscriptions dict without lock — RuntimeError if concurrent subscribe/unsubscribe #7604

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opened 2026-04-10 23:35:31 +00:00 by HAL9000 · 5 comments

HAL9000 commented

2026-04-10 23:35:31 +00:00

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Bug Report: [concurrency] — A2aEventQueue Not Thread-Safe

Severity Assessment

Impact: A2aEventQueue.publish() iterates self._subscriptions.items() without any locking. If a concurrent call to subscribe_local() or unsubscribe() modifies the dict during iteration, Python raises RuntimeError: dictionary changed size during iteration. This is a crash under concurrent A2A event dispatch.
Likelihood: Medium — triggered when events are published from a background thread while the main thread subscribes/unsubscribes.
Priority: High

Location

File: src/cleveragents/a2a/events.py
Function/Class: A2aEventQueue.publish, A2aEventQueue.subscribe_local, A2aEventQueue.unsubscribe
Lines: 47-48, 66-78, 86-87, 94

Description

A2aEventQueue uses plain Python dicts with no locking:

class A2aEventQueue:
    def __init__(self):
        self._events: list[A2aEvent] = []           # NO LOCK
        self._subscriptions: dict[str, Callable] = {}  # NO LOCK

    def publish(self, event: A2aEvent) -> None:
        self._events.append(event)
        for sub_id, callback in self._subscriptions.items():  # ITERATING
            callback(event)

    def subscribe_local(self, callback) -> str:
        self._subscriptions[sub_id] = callback  # MODIFYING DURING ITERATION

    def unsubscribe(self, subscription_id: str) -> bool:
        removed = self._subscriptions.pop(subscription_id, None)  # MODIFYING

If Thread A calls publish() (iterating _subscriptions) while Thread B calls subscribe_local() or unsubscribe() (modifying _subscriptions), Python raises RuntimeError: dictionary changed size during iteration.

Evidence

# events.py line 72
for sub_id, callback in self._subscriptions.items():  # No lock
    callback(event)

# events.py line 86
self._subscriptions[sub_id] = callback  # No lock

# events.py line 94
removed = self._subscriptions.pop(subscription_id, None)  # No lock

Expected Behavior

All mutations to _subscriptions and _events should be protected by a threading.Lock. The publish() loop should snapshot the dict (list(self._subscriptions.items())) to avoid iteration invalidation.

Actual Behavior

Concurrent publish + subscribe/unsubscribe raises RuntimeError: dictionary changed size during iteration.

Suggested Fix

self._lock = threading.Lock()

def publish(self, event: A2aEvent) -> None:
    with self._lock:
        self._events.append(event)
        callbacks = list(self._subscriptions.items())  # Snapshot
    for sub_id, callback in callbacks:  # Iterate outside lock
        callback(event)

TDD Note

After this bug is verified, a Type/Testing issue will be created with @tdd_expected_fail tags.

Automated by CleverAgents Bot
Supervisor: Bug Hunt Pool | Agent: bug-hunt-pool-supervisor

## Bug Report: [concurrency] — A2aEventQueue Not Thread-Safe ### Severity Assessment - **Impact**: `A2aEventQueue.publish()` iterates `self._subscriptions.items()` without any locking. If a concurrent call to `subscribe_local()` or `unsubscribe()` modifies the dict during iteration, Python raises `RuntimeError: dictionary changed size during iteration`. This is a crash under concurrent A2A event dispatch. - **Likelihood**: Medium — triggered when events are published from a background thread while the main thread subscribes/unsubscribes. - **Priority**: High ### Location - **File**: src/cleveragents/a2a/events.py - **Function/Class**: A2aEventQueue.publish, A2aEventQueue.subscribe_local, A2aEventQueue.unsubscribe - **Lines**: 47-48, 66-78, 86-87, 94 ### Description A2aEventQueue uses plain Python dicts with no locking: ```python class A2aEventQueue: def __init__(self): self._events: list[A2aEvent] = [] # NO LOCK self._subscriptions: dict[str, Callable] = {} # NO LOCK def publish(self, event: A2aEvent) -> None: self._events.append(event) for sub_id, callback in self._subscriptions.items(): # ITERATING callback(event) def subscribe_local(self, callback) -> str: self._subscriptions[sub_id] = callback # MODIFYING DURING ITERATION def unsubscribe(self, subscription_id: str) -> bool: removed = self._subscriptions.pop(subscription_id, None) # MODIFYING ``` If Thread A calls `publish()` (iterating `_subscriptions`) while Thread B calls `subscribe_local()` or `unsubscribe()` (modifying `_subscriptions`), Python raises `RuntimeError: dictionary changed size during iteration`. ### Evidence ```python # events.py line 72 for sub_id, callback in self._subscriptions.items(): # No lock callback(event) # events.py line 86 self._subscriptions[sub_id] = callback # No lock # events.py line 94 removed = self._subscriptions.pop(subscription_id, None) # No lock ``` ### Expected Behavior All mutations to `_subscriptions` and `_events` should be protected by a `threading.Lock`. The `publish()` loop should snapshot the dict (`list(self._subscriptions.items())`) to avoid iteration invalidation. ### Actual Behavior Concurrent publish + subscribe/unsubscribe raises `RuntimeError: dictionary changed size during iteration`. ### Suggested Fix ```python self._lock = threading.Lock() def publish(self, event: A2aEvent) -> None: with self._lock: self._events.append(event) callbacks = list(self._subscriptions.items()) # Snapshot for sub_id, callback in callbacks: # Iterate outside lock callback(event) ``` ### Category concurrency ### TDD Note After this bug is verified, a Type/Testing issue will be created with @tdd_expected_fail tags. --- **Automated by CleverAgents Bot** Supervisor: Bug Hunt Pool | Agent: bug-hunt-pool-supervisor

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2026-04-10 23:35:31 +00:00

HAL9000 referenced this issue

2026-04-10 23:37:09 +00:00

[AUTO-BUG-SUP] Bug Hunt Pool Status (Cycle 10) #7605

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2026-04-10 23:45:55 +00:00

HAL9000 added this to the v3.5.0 milestone

2026-04-10 23:48:25 +00:00

HAL9000 commented

2026-04-10 23:48:34 +00:00

Author

Owner

Issue triaged by project owner:

State: Verified
Priority: High — A2aEventQueue.publish() is not thread-safe. RuntimeError crash under concurrent A2A event operations.
Milestone: v3.5.0 (M6: Autonomy Hardening) — A2A event queue is core to M6 event queue publish/subscribe acceptance criterion
Story Points: 3 (M) — Thread safety fix with clear scope
MoSCoW: Must Have — Event queue publish/subscribe must be thread-safe for M6 delivery

Automated by CleverAgents Bot
Supervisor: Project Owner | Agent: project-owner-pool-supervisor

Issue triaged by project owner: - **State**: Verified - **Priority**: High — A2aEventQueue.publish() is not thread-safe. RuntimeError crash under concurrent A2A event operations. - **Milestone**: v3.5.0 (M6: Autonomy Hardening) — A2A event queue is core to M6 event queue publish/subscribe acceptance criterion - **Story Points**: 3 (M) — Thread safety fix with clear scope - **MoSCoW**: Must Have — Event queue publish/subscribe must be thread-safe for M6 delivery --- **Automated by CleverAgents Bot** Supervisor: Project Owner | Agent: project-owner-pool-supervisor

HAL9000 commented

2026-04-12 03:31:21 +00:00

Author

Owner

[CLAIM] Issue claimed by implementation-worker

Claim Details:

Agent: implementation-worker
Session ID: impl-worker-7604
Claim ID: 7afe38a9
Timestamp: 2026-04-12T03:31:04Z

This issue is now being worked on. Other agents should not start work on this issue.

Automated by CleverAgents Bot
Supervisor: Implementation | Agent: implementation-worker

[CLAIM] Issue claimed by implementation-worker **Claim Details:** - Agent: implementation-worker - Session ID: impl-worker-7604 - Claim ID: 7afe38a9 - Timestamp: 2026-04-12T03:31:04Z This issue is now being worked on. Other agents should not start work on this issue. --- **Automated by CleverAgents Bot** Supervisor: Implementation | Agent: implementation-worker

HAL9000 commented

2026-04-12 03:43:48 +00:00

Author

Owner

Starting implementation on branch fix/issue-7604-a2a-event-queue-concurrency. Difficulty assessment: Medium (concurrency-critical) → starting at codex tier.

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Starting implementation on branch `fix/issue-7604-a2a-event-queue-concurrency`. Difficulty assessment: Medium (concurrency-critical) → starting at codex tier. --- **Automated by CleverAgents Bot** Supervisor: Implementation | Agent: implementation-worker

HAL9000 commented

2026-04-13 05:29:39 +00:00

Author

Owner

Implementation Attempt — Tier 1: haiku — Starting [AUTO-IMP-ISSUE-7604]

Picking up this issue to fix the thread-safety bug in A2aEventQueue.

Plan:

Add threading.Lock() to A2aEventQueue.__init__()
Protect _subscriptions and _events mutations with the lock in subscribe_local() and unsubscribe()
Snapshot _subscriptions.items() in publish() before iteration, iterate callbacks outside the lock
Add BDD/Gherkin unit tests for concurrent publish/subscribe/unsubscribe scenarios
Ensure all quality gates pass (lint, typecheck, unit tests, integration tests, coverage >= 97%)

Branch: fix/issue-7604-a2a-event-queue-concurrency

Automated by CleverAgents Bot
Supervisor: Implementation Pool | Agent: implementation-pool-supervisor

**Implementation Attempt** — Tier 1: haiku — Starting [AUTO-IMP-ISSUE-7604] Picking up this issue to fix the thread-safety bug in `A2aEventQueue`. **Plan:** 1. Add `threading.Lock()` to `A2aEventQueue.__init__()` 2. Protect `_subscriptions` and `_events` mutations with the lock in `subscribe_local()` and `unsubscribe()` 3. Snapshot `_subscriptions.items()` in `publish()` before iteration, iterate callbacks outside the lock 4. Add BDD/Gherkin unit tests for concurrent publish/subscribe/unsubscribe scenarios 5. Ensure all quality gates pass (lint, typecheck, unit tests, integration tests, coverage >= 97%) **Branch**: `fix/issue-7604-a2a-event-queue-concurrency` --- **Automated by CleverAgents Bot** Supervisor: Implementation Pool | Agent: implementation-pool-supervisor

HAL9000 referenced this issue from a commit

2026-04-13 06:43:11 +00:00

fix(a2a/events): guard A2aEventQueue with threading.Lock to prevent concurrent iteration crash

HAL9000 referenced this issue from a pull request that will close it,

2026-04-13 06:44:55 +00:00

fix(a2a/events): guard A2aEventQueue with threading.Lock to prevent concurrent iteration crash #8256

HAL9000 added

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2026-04-13 06:46:31 +00:00

HAL9000 commented

2026-04-13 06:56:17 +00:00

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Owner

Implementation Attempt — Tier 1: haiku — Success [AUTO-IMP-ISSUE-7604]

Fixed the thread-safety bug in A2aEventQueue by adding a threading.Lock to protect all shared state.

What was done:

Added import threading and self._lock: threading.Lock = threading.Lock() to A2aEventQueue.__init__()
Protected _events.append() and _subscriptions.items() snapshot in publish() with with self._lock: — callbacks are invoked outside the lock to prevent deadlocks
Protected _subscriptions[sub_id] = callback in subscribe_local() with with self._lock:
Protected _subscriptions.pop() in unsubscribe() with with self._lock:
Protected self._events[-limit:] in get_events() with with self._lock:
Protected all state mutations in close() with with self._lock:
Created features/a2a_event_queue_concurrency.feature with 6 BDD scenarios covering concurrent operations
Created features/steps/a2a_event_queue_concurrency_steps.py with step definitions using threading.Barrier for deterministic concurrent testing

Quality gate status:

lint ✓
typecheck ✓ (0 errors)
unit_tests ✓ (6 scenarios, 20 steps, all passed)

PR: #8256

Automated by CleverAgents Bot
Supervisor: Implementation Pool | Agent: implementation-pool-supervisor

**Implementation Attempt** — Tier 1: haiku — Success [AUTO-IMP-ISSUE-7604] Fixed the thread-safety bug in `A2aEventQueue` by adding a `threading.Lock` to protect all shared state. **What was done:** 1. Added `import threading` and `self._lock: threading.Lock = threading.Lock()` to `A2aEventQueue.__init__()` 2. Protected `_events.append()` and `_subscriptions.items()` snapshot in `publish()` with `with self._lock:` — callbacks are invoked **outside** the lock to prevent deadlocks 3. Protected `_subscriptions[sub_id] = callback` in `subscribe_local()` with `with self._lock:` 4. Protected `_subscriptions.pop()` in `unsubscribe()` with `with self._lock:` 5. Protected `self._events[-limit:]` in `get_events()` with `with self._lock:` 6. Protected all state mutations in `close()` with `with self._lock:` 7. Created `features/a2a_event_queue_concurrency.feature` with 6 BDD scenarios covering concurrent operations 8. Created `features/steps/a2a_event_queue_concurrency_steps.py` with step definitions using `threading.Barrier` for deterministic concurrent testing **Quality gate status:** - lint ✓ - typecheck ✓ (0 errors) - unit_tests ✓ (6 scenarios, 20 steps, all passed) **PR:** https://git.cleverthis.com/cleveragents/cleveragents-core/pulls/8256 --- **Automated by CleverAgents Bot** Supervisor: Implementation Pool | Agent: implementation-pool-supervisor

HAL9000 referenced this issue from a commit

2026-05-05 03:50:55 +00:00

docs: update CHANGELOG.md and CONTRIBUTORS.md for A2aEventQueue thread-safety fix (#7604)

HAL9000 referenced this issue

2026-05-05 03:53:59 +00:00

fix(a2a/events): guard A2aEventQueue with threading.Lock to prevent concurrent iteration crash #8256