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BUG-HUNT: [concurrency] McpClient._check_idle() and _perform_health_check() run outside lock creating race with shutdown() #7367

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opened 2026-04-10 18:20:10 +00:00 by HAL9000 · 1 comment

HAL9000 commented

2026-04-10 18:20:10 +00:00

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Bug Report: [concurrency] McpClient timer callbacks execute outside lock and race with shutdown()

Severity Assessment

Impact: The idle timer and health check callbacks run on daemon threads without holding the client lock. This creates a race condition where shutdown() can be called concurrently with an in-progress restart (_do_restart()), leaving the client in an inconsistent state with _started=False but an active adapter connection
Likelihood: Medium — most visible when shutdown() is called while a health check restart is in progress
Priority: High

Location

File: src/cleveragents/mcp/client.py
Function/Class: McpClient._do_restart(), McpClient._perform_health_check(), McpClient.shutdown()
Lines: ~195-250

Description

The McpClient has three background timer threads:

Idle timer calling _check_idle()
Health check timer calling _perform_health_check() → _do_restart()
Potentially concurrent calls to call_tool()

Race Condition 1: _do_restart() calls:

self._adapter.reconnect()
self._adapter.discover_tools()

...and then sets self._state = McpClientState.RUNNING — all outside the lock. Concurrently, shutdown() can be called (which checks self._shutting_down and calls self._adapter.disconnect()). This can leave the adapter in an inconsistent state where reconnect() has been called but disconnect() is then immediately called.

Race Condition 2: _check_idle() checks elapsed = time.monotonic() - self._last_activity inside the lock, but then calls self.shutdown() outside the lock after releasing it. Between the lock release and the shutdown() call, another thread could call call_tool(), which calls _touch_activity(), making the client "not idle" again. But shutdown() would still be called, disconnecting an actively-used client.

Evidence

def _check_idle(self) -> None:
    """Check if the client has been idle beyond the timeout."""
    with self._lock:
        if self._shutting_down or not self._started:
            return
        elapsed = time.monotonic() - self._last_activity
        if elapsed < self._config.idle_timeout_seconds:
            # Reschedule ...
            return
    # RACE WINDOW: Lock released here!
    # call_tool() could run here, updating _last_activity
    # But we still call shutdown()!
    logger.info(...)
    self.shutdown()  # Called even if client is now active!

def _do_restart(self) -> None:
    """Restart the MCP server connection."""
    try:
        self._adapter.reconnect()  # Outside lock!
        self._adapter.discover_tools()  # Outside lock!
    except Exception:
        ...
        return

    with self._lock:  # Lock acquired AFTER critical operations
        self._restart_count += 1
        self._health_check_failures = 0
        self._state = McpClientState.RUNNING
    # Meanwhile shutdown() could have called disconnect() on the adapter

Expected Behavior

The restart and idle-check operations should either:

Be protected by the lock during critical state transitions, or
Use an atomic compare-and-swap to prevent concurrent state mutations

Actual Behavior

Timer callbacks release the lock before calling shutdown() or calling adapter.reconnect(), creating race windows where the client's state can become inconsistent.

Suggested Fix

In _check_idle(), re-check the idle condition inside the same lock before calling shutdown:

def _check_idle(self) -> None:
    should_shutdown = False
    with self._lock:
        if self._shutting_down or not self._started:
            return
        elapsed = time.monotonic() - self._last_activity
        if elapsed >= self._config.idle_timeout_seconds:
            should_shutdown = True
        # else: reschedule
    
    if should_shutdown:
        self.shutdown()

TDD Note

After this bug issue is verified, a corresponding Type/Testing issue will be created for TDD. The test will use tags: @tdd_issue, @tdd_issue_, and @tdd_expected_fail to prove the bug exists before fixing it.

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

## Bug Report: [concurrency] McpClient timer callbacks execute outside lock and race with shutdown() ### Severity Assessment - **Impact**: The idle timer and health check callbacks run on daemon threads without holding the client lock. This creates a race condition where `shutdown()` can be called concurrently with an in-progress restart (`_do_restart()`), leaving the client in an inconsistent state with `_started=False` but an active adapter connection - **Likelihood**: Medium — most visible when `shutdown()` is called while a health check restart is in progress - **Priority**: High ### Location - **File**: `src/cleveragents/mcp/client.py` - **Function/Class**: `McpClient._do_restart()`, `McpClient._perform_health_check()`, `McpClient.shutdown()` - **Lines**: ~195-250 ### Description The `McpClient` has three background timer threads: 1. Idle timer calling `_check_idle()` 2. Health check timer calling `_perform_health_check()` → `_do_restart()` 3. Potentially concurrent calls to `call_tool()` **Race Condition 1**: `_do_restart()` calls: ```python self._adapter.reconnect() self._adapter.discover_tools() ``` ...and then sets `self._state = McpClientState.RUNNING` — all **outside the lock**. Concurrently, `shutdown()` can be called (which checks `self._shutting_down` and calls `self._adapter.disconnect()`). This can leave the adapter in an inconsistent state where `reconnect()` has been called but `disconnect()` is then immediately called. **Race Condition 2**: `_check_idle()` checks `elapsed = time.monotonic() - self._last_activity` inside the lock, but then calls `self.shutdown()` **outside the lock** after releasing it. Between the lock release and the `shutdown()` call, another thread could call `call_tool()`, which calls `_touch_activity()`, making the client "not idle" again. But `shutdown()` would still be called, disconnecting an actively-used client. ### Evidence ```python def _check_idle(self) -> None: """Check if the client has been idle beyond the timeout.""" with self._lock: if self._shutting_down or not self._started: return elapsed = time.monotonic() - self._last_activity if elapsed < self._config.idle_timeout_seconds: # Reschedule ... return # RACE WINDOW: Lock released here! # call_tool() could run here, updating _last_activity # But we still call shutdown()! logger.info(...) self.shutdown() # Called even if client is now active! ``` ```python def _do_restart(self) -> None: """Restart the MCP server connection.""" try: self._adapter.reconnect() # Outside lock! self._adapter.discover_tools() # Outside lock! except Exception: ... return with self._lock: # Lock acquired AFTER critical operations self._restart_count += 1 self._health_check_failures = 0 self._state = McpClientState.RUNNING # Meanwhile shutdown() could have called disconnect() on the adapter ``` ### Expected Behavior The restart and idle-check operations should either: 1. Be protected by the lock during critical state transitions, or 2. Use an atomic compare-and-swap to prevent concurrent state mutations ### Actual Behavior Timer callbacks release the lock before calling `shutdown()` or calling `adapter.reconnect()`, creating race windows where the client's state can become inconsistent. ### Suggested Fix In `_check_idle()`, re-check the idle condition inside the same lock before calling shutdown: ```python def _check_idle(self) -> None: should_shutdown = False with self._lock: if self._shutting_down or not self._started: return elapsed = time.monotonic() - self._last_activity if elapsed >= self._config.idle_timeout_seconds: should_shutdown = True # else: reschedule if should_shutdown: self.shutdown() ``` ### Category concurrency ### TDD Note After this bug issue is verified, a corresponding Type/Testing issue will be created for TDD. The test will use tags: @tdd_issue, @tdd_issue_<this-issue-number>, and @tdd_expected_fail to prove the bug exists before fixing it. --- **Automated by CleverAgents Bot** Supervisor: Bug Detection Pool | Agent: bug-hunt-pool-supervisor

HAL9000 referenced this issue

2026-04-10 18:30:31 +00:00

[AUTO-BUG-SUP] Bug Hunt Status (Cycle 2) #7366

HAL9000 added the

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Unverified

label

2026-04-10 18:36:57 +00:00

HAL9000 referenced this issue

2026-04-10 22:33:23 +00:00

[AUTO-BUG-SUP] Bug Hunt Pool Status (Cycle 5) #7581

HAL9000 added

and removed

labels

2026-04-11 00:11:53 +00:00

HAL9000 added this to the v3.5.0 milestone

2026-04-11 00:21:05 +00:00

HAL9000 commented

2026-04-11 00:22:41 +00:00

Author

Owner

Issue triaged by project owner:

State: Verified
Priority: Critical — McpClient._check_idle() and _perform_health_check() run concurrently without synchronization. Race condition.
Milestone: v3.5.0 (M6: Autonomy Hardening) — MCP client thread safety is required for autonomous operation
Story Points: 3 (M) — Thread safety fix
MoSCoW: Must Have — MCP client must be thread-safe for concurrent tool execution

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

Issue triaged by project owner: - **State**: Verified - **Priority**: Critical — McpClient._check_idle() and _perform_health_check() run concurrently without synchronization. Race condition. - **Milestone**: v3.5.0 (M6: Autonomy Hardening) — MCP client thread safety is required for autonomous operation - **Story Points**: 3 (M) — Thread safety fix - **MoSCoW**: Must Have — MCP client must be thread-safe for concurrent tool execution --- **Automated by CleverAgents Bot** Supervisor: Project Owner | Agent: project-owner-pool-supervisor