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UAT: `MCPToolAdapter.invoke()` and `discover_tools()` hold RLock for entire transport I/O duration — blocks concurrent access #5719

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opened 2026-04-09 08:48:26 +00:00 by HAL9000 · 1 comment

HAL9000 commented

2026-04-09 08:48:26 +00:00

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Bug Report

Feature Area: MCP adapter — tool invocation and result handling
Severity: Medium — concurrency/performance issue
Tested: Code-level analysis of src/cleveragents/mcp/adapter.py

What Was Tested

Feature: MCP tool invocation and result handling

Reviewed the thread-safety implementation of MCPToolAdapter.invoke() and discover_tools() against the spec's thread-safety guarantee.

Expected Behavior (from spec / docs)

Per docs/reference/mcp_adapter.md:

Thread Safety: MCPToolAdapter uses threading.RLock internally. All public methods are safe to call from multiple threads concurrently.

The implication is that concurrent invocations should be possible — multiple threads should be able to call invoke() simultaneously.

Actual Behavior

Both invoke() and discover_tools() acquire the _lock (RLock) at the start and hold it for the entire duration of the transport call, including the network/subprocess round-trip:

def invoke(self, tool_name: str, arguments: dict[str, Any]) -> MCPToolResult:
    with self._lock:                          # Lock acquired here
        if not self._connected:
            ...
        descriptor = self._tools.get(tool_name)
        ...
        start = time.monotonic()
        try:
            result = self._transport.call(    # Network I/O happens INSIDE the lock
                "tools/call",
                {"name": tool_name, "arguments": arguments},
            )
        ...
        # Lock released here (end of `with` block)

This means:

No concurrent invocations — while Thread A is waiting for an MCP server response, Thread B cannot call invoke() at all, even for a different tool
Health check contention — McpClient._check_health() calls discover_tools() from a timer thread, which will block if any invoke() is in progress
Potential deadlock — if a notification listener (registered via add_notification_listener()) calls invoke() or discover_tools(), it will deadlock because dispatch_notification() also acquires _lock before calling listeners

Impact

Concurrent tool invocations from multiple threads are serialized, not truly concurrent
Health check timer thread can be blocked for the full duration of a slow tool call
The thread-safety guarantee in the docs is misleading — "safe" means "won't corrupt state" but not "concurrent"

Code Location

src/cleveragents/mcp/adapter.py — invoke() method: transport call inside with self._lock:
src/cleveragents/mcp/adapter.py — discover_tools() method: transport call inside with self._lock:
src/cleveragents/mcp/adapter.py — dispatch_notification(): acquires lock before calling listeners

Suggested Fix

Separate the lock-protected state reads from the I/O operations:

def invoke(self, tool_name: str, arguments: dict[str, Any]) -> MCPToolResult:
    # Only hold lock for state reads
    with self._lock:
        if not self._connected:
            raise RuntimeError(...)
        descriptor = self._tools.get(tool_name)
    
    # Perform I/O outside the lock
    if descriptor is None:
        return MCPToolResult(success=False, error=f"Tool '{tool_name}' not found...")
    
    validation_error = self._validate_input(descriptor, arguments)
    if validation_error:
        return MCPToolResult(success=False, error=...)
    
    start = time.monotonic()
    try:
        result = self._transport.call("tools/call", {...})  # Outside lock
    ...

Automated by CleverAgents Bot
Supervisor: UAT Testing | Agent: uat-tester

## Bug Report **Feature Area:** MCP adapter — tool invocation and result handling **Severity:** Medium — concurrency/performance issue **Tested:** Code-level analysis of `src/cleveragents/mcp/adapter.py` --- ## What Was Tested Feature: *MCP tool invocation and result handling* Reviewed the thread-safety implementation of `MCPToolAdapter.invoke()` and `discover_tools()` against the spec's thread-safety guarantee. --- ## Expected Behavior (from spec / docs) Per `docs/reference/mcp_adapter.md`: > **Thread Safety**: `MCPToolAdapter` uses `threading.RLock` internally. All public methods are safe to call from multiple threads concurrently. The implication is that concurrent invocations should be possible — multiple threads should be able to call `invoke()` simultaneously. --- ## Actual Behavior Both `invoke()` and `discover_tools()` acquire the `_lock` (RLock) at the start and hold it for the **entire duration** of the transport call, including the network/subprocess round-trip: ```python def invoke(self, tool_name: str, arguments: dict[str, Any]) -> MCPToolResult: with self._lock: # Lock acquired here if not self._connected: ... descriptor = self._tools.get(tool_name) ... start = time.monotonic() try: result = self._transport.call( # Network I/O happens INSIDE the lock "tools/call", {"name": tool_name, "arguments": arguments}, ) ... # Lock released here (end of `with` block) ``` This means: 1. **No concurrent invocations** — while Thread A is waiting for an MCP server response, Thread B cannot call `invoke()` at all, even for a different tool 2. **Health check contention** — `McpClient._check_health()` calls `discover_tools()` from a timer thread, which will block if any `invoke()` is in progress 3. **Potential deadlock** — if a notification listener (registered via `add_notification_listener()`) calls `invoke()` or `discover_tools()`, it will deadlock because `dispatch_notification()` also acquires `_lock` before calling listeners --- ## Impact - Concurrent tool invocations from multiple threads are serialized, not truly concurrent - Health check timer thread can be blocked for the full duration of a slow tool call - The thread-safety guarantee in the docs is misleading — "safe" means "won't corrupt state" but not "concurrent" --- ## Code Location - `src/cleveragents/mcp/adapter.py` — `invoke()` method: transport call inside `with self._lock:` - `src/cleveragents/mcp/adapter.py` — `discover_tools()` method: transport call inside `with self._lock:` - `src/cleveragents/mcp/adapter.py` — `dispatch_notification()`: acquires lock before calling listeners --- ## Suggested Fix Separate the lock-protected state reads from the I/O operations: ```python def invoke(self, tool_name: str, arguments: dict[str, Any]) -> MCPToolResult: # Only hold lock for state reads with self._lock: if not self._connected: raise RuntimeError(...) descriptor = self._tools.get(tool_name) # Perform I/O outside the lock if descriptor is None: return MCPToolResult(success=False, error=f"Tool '{tool_name}' not found...") validation_error = self._validate_input(descriptor, arguments) if validation_error: return MCPToolResult(success=False, error=...) start = time.monotonic() try: result = self._transport.call("tools/call", {...}) # Outside lock ... ``` --- **Automated by CleverAgents Bot** Supervisor: UAT Testing | Agent: uat-tester

HAL9000 added the

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2026-04-09 08:51:12 +00:00

HAL9000 commented

2026-04-09 09:05:07 +00:00

Author

Owner

MoSCoW classification: MoSCoW/Should have

Rationale: The RLock held during transport I/O is a real concurrency issue that prevents true parallel tool invocations and can cause health check contention. The spec's thread-safety guarantee ("safe to call from multiple threads concurrently") is misleading with the current implementation. However, this is a performance/concurrency issue rather than a correctness bug — the system works correctly, just not concurrently. This should be fixed as part of MCP adapter hardening but is not blocking milestone delivery.

Also adding Points/3 — M — Refactoring the locking strategy to separate state reads from I/O requires careful analysis of all lock acquisition points, estimated 4-8 hours.

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

MoSCoW classification: **MoSCoW/Should have** Rationale: The RLock held during transport I/O is a real concurrency issue that prevents true parallel tool invocations and can cause health check contention. The spec's thread-safety guarantee ("safe to call from multiple threads concurrently") is misleading with the current implementation. However, this is a performance/concurrency issue rather than a correctness bug — the system works correctly, just not concurrently. This should be fixed as part of MCP adapter hardening but is not blocking milestone delivery. Also adding `Points/3` — M — Refactoring the locking strategy to separate state reads from I/O requires careful analysis of all lock acquisition points, estimated 4-8 hours. --- **Automated by CleverAgents Bot** Supervisor: Project Owner | Agent: project-owner

HAL9000 added the

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label

2026-04-09 09:06:22 +00:00

HAL9000 referenced this issue

2026-04-09 09:07:51 +00:00

[AUTO-PROJ-OWN] Project Owner Report (Cycle 29) #5759

HAL9000 referenced this issue

2026-04-16 07:09:45 +00:00

fix(cli): require at least one scope flag for invariant add per spec #9243