test: MCP PID lock tests

Part of #734

tests/test_mcp_pid_lock.py

@@ -0,0 +1,75 @@
+"""Tests for MCP PID file lock (#734)."""
+
+import os
+import sys
+import tempfile
+from pathlib import Path
+
+sys.path.insert(0, str(Path(__file__).parent.parent))
+
+# Override MCP_DIR for testing
+import tools.mcp_pid_lock as lock_mod
+_test_dir = Path(tempfile.mkdtemp())
+lock_mod._MCP_DIR = _test_dir
+
+
+def test_acquire_and_release():
+    """Lock can be acquired and released."""
+    pid = lock_mod.acquire_lock("test_server")
+    assert pid == os.getpid()
+    assert lock_mod.is_locked("test_server")
+    lock_mod.release_lock("test_server")
+    assert not lock_mod.is_locked("test_server")
+
+
+def test_concurrent_lock_blocked():
+    """Second acquire returns None when server running."""
+    lock_mod.acquire_lock("test_concurrent")
+    result = lock_mod.acquire_lock("test_concurrent")
+    assert result is None
+    lock_mod.release_lock("test_concurrent")
+
+
+def test_stale_lock_cleaned():
+    """Stale PID files are cleaned up."""
+    # Write a fake stale PID
+    pid_file = _test_dir / "stale.pid"
+    pid_file.write_text("99999999")
+    assert not lock_mod.is_locked("stale")
+    assert not pid_file.exists()
+
+
+def test_list_locks():
+    """list_locks returns only active locks."""
+    lock_mod.acquire_lock("list_test")
+    locks = lock_mod.list_locks()
+    assert "list_test" in locks
+    assert locks["list_test"] == os.getpid()
+    lock_mod.release_lock("list_test")
+
+
+def test_cleanup_stale():
+    """cleanup_stale_locks removes dead PID files."""
+    (_test_dir / "dead1.pid").write_text("99999998")
+    (_test_dir / "dead2.pid").write_text("99999999")
+    count = lock_mod.cleanup_stale_locks()
+    assert count >= 2
+
+
+def test_force_release():
+    """force_release kills process and removes lock."""
+    lock_mod.acquire_lock("force_test")
+    assert lock_mod.is_locked("force_test")
+    lock_mod.force_release("force_test")
+    assert not lock_mod.is_locked("force_test")
+
+
+if __name__ == "__main__":
+    tests = [test_acquire_and_release, test_concurrent_lock_blocked,
+             test_stale_lock_cleaned, test_list_locks, test_cleanup_stale,
+             test_force_release]
+    for t in tests:
+        print(f"Running {t.__name__}...")
+        t()
+        print("  PASS")
+    print("\nAll tests passed.")

tools/mcp_pid_lock.py

@@ -0,0 +1,158 @@
+"""
+MCP PID File Lock — Prevent concurrent MCP server instances.
+
+Uses PID files at ~/.hermes/mcp/{name}.pid to ensure only one instance
+of each MCP server runs at a time. Prevents zombie accumulation (#714).
+
+Usage:
+    from tools.mcp_pid_lock import acquire_lock, release_lock, is_locked
+    
+    lock = acquire_lock("morrowind")
+    if lock:
+        try:
+            # run server
+            pass
+        finally:
+            release_lock("morrowind")
+"""
+
+import fcntl
+import os
+import signal
+import time
+from pathlib import Path
+from typing import Optional
+
+_MCP_DIR = Path(os.getenv("HERMES_HOME", str(Path.home() / ".hermes"))) / "mcp"
+
+
+def _pid_file(name: str) -> Path:
+    """Get the PID file path for an MCP server."""
+    _MCP_DIR.mkdir(parents=True, exist_ok=True)
+    return _MCP_DIR / f"{name}.pid"
+
+
+def _is_process_alive(pid: int) -> bool:
+    """Check if a process is running."""
+    try:
+        os.kill(pid, 0)  # Signal 0 = check if alive
+        return True
+    except ProcessLookupError:
+        return False
+    except PermissionError:
+        return True  # Exists but we can't signal it
+
+
+def _read_pid_file(name: str) -> Optional[int]:
+    """Read PID from file, returns None if invalid."""
+    path = _pid_file(name)
+    if not path.exists():
+        return None
+    try:
+        content = path.read_text().strip()
+        return int(content) if content else None
+    except (ValueError, OSError):
+        return None
+
+
+def _write_pid_file(name: str, pid: int):
+    """Write PID to file."""
+    path = _pid_file(name)
+    path.write_text(str(pid))
+
+
+def _remove_pid_file(name: str):
+    """Remove PID file."""
+    path = _pid_file(name)
+    try:
+        path.unlink()
+    except FileNotFoundError:
+        pass
+
+
+def is_locked(name: str) -> bool:
+    """Check if an MCP server is already running."""
+    pid = _read_pid_file(name)
+    if pid is None:
+        return False
+    if _is_process_alive(pid):
+        return True
+    # Stale PID file
+    _remove_pid_file(name)
+    return False
+
+
+def acquire_lock(name: str) -> Optional[int]:
+    """
+    Acquire a PID lock for an MCP server.
+    
+    Returns the PID if lock acquired, None if server already running.
+    """
+    # Check existing lock
+    existing_pid = _read_pid_file(name)
+    if existing_pid is not None:
+        if _is_process_alive(existing_pid):
+            return None  # Server already running
+        # Stale lock — clean up
+        _remove_pid_file(name)
+    
+    # Write our PID
+    pid = os.getpid()
+    _write_pid_file(name, pid)
+    return pid
+
+
+def release_lock(name: str):
+    """Release the PID lock."""
+    # Only remove if it's our PID
+    existing_pid = _read_pid_file(name)
+    if existing_pid == os.getpid():
+        _remove_pid_file(name)
+
+
+def force_release(name: str):
+    """Force release a lock (for cleanup scripts)."""
+    pid = _read_pid_file(name)
+    if pid and _is_process_alive(pid):
+        try:
+            os.kill(pid, signal.SIGTERM)
+            time.sleep(0.5)
+            if _is_process_alive(pid):
+                os.kill(pid, signal.SIGKILL)
+        except (ProcessLookupError, PermissionError):
+            pass
+    _remove_pid_file(name)
+
+
+def list_locks() -> dict:
+    """List all active MCP locks."""
+    locks = {}
+    if not _MCP_DIR.exists():
+        return locks
+    
+    for pid_file in _MCP_DIR.glob("*.pid"):
+        name = pid_file.stem
+        pid = _read_pid_file(name)
+        if pid and _is_process_alive(pid):
+            locks[name] = pid
+        else:
+            # Clean up stale
+            _remove_pid_file(name)
+    
+    return locks
+
+
+def cleanup_stale_locks() -> int:
+    """Remove all stale PID files. Returns count cleaned."""
+    cleaned = 0
+    if not _MCP_DIR.exists():
+        return 0
+    
+    for pid_file in _MCP_DIR.glob("*.pid"):
+        name = pid_file.stem
+        pid = _read_pid_file(name)
+        if pid is None or not _is_process_alive(pid):
+            _remove_pid_file(name)
+            cleaned += 1
+    
+    return cleaned