patrol_mcp: don't listen for SIGTERM on Windows

[rsnpettrov]

Closes #3035.

Problem

patrol_mcp 0.1.3 fails to start on Windows. ProcessSignal.sigterm.watch() is not listenable there — it throws SignalException: Failed to listen for SIGTERM, osError: OS Error: The request is not supported, errno = 50. The exception is raised synchronously from _ExitSignal's constructor, before the MCP server can answer an initialize request, so MCP clients (Claude Code, Cursor, etc.) on Windows never see any tools.

Reproduction (from Git Bash or PowerShell on Windows):

dart run patrol_mcp

[INFO][patrol_mcp] Running MCP server on stdio
[INFO][patrol_mcp] Server started
Unhandled exception:
SignalException: Failed to listen for SIGTERM, osError: OS Error: The request is not supported, errno = 50

Fix

Guard the sigterm registration with !Platform.isWindows. The sigterm subscription becomes nullable so _cleanup() works without a LateInitializationError on Windows.

• SIGINT keeps its registration unconditionally — Ctrl-C / Ctrl-Break continue to work on every platform.
• stdio-transport shutdown is unaffected — MCP clients terminate the server by closing stdin, which mcp_dart's stdio transport already handles.
• POSIX behavior is unchanged — kill PID, docker stop, etc. still trigger the SIGTERM path on macOS / Linux.

Windows has no POSIX SIGTERM; process termination there goes through TerminateProcess (uncatchable, like SIGKILL) or the SIGINT path we already listen on. So nothing functional is lost by skipping registration on Windows — the watch() call was already not running there, it was just crashing the process instead.

Verification on Windows

After the patch, an MCP initialize + tools/list exchange now returns all five tools (run, quit, status, screenshot, native-tree):

> {"jsonrpc":"2.0","id":1,"method":"initialize", ...}
< {"jsonrpc":"2.0","id":1,"result":{"protocolVersion":"2024-11-05", ...}}
> {"jsonrpc":"2.0","id":2,"method":"tools/list","params":{}}
< {"jsonrpc":"2.0","id":2,"result":{"tools":[{"name":"run", ...}, {"name":"quit", ...}, {"name":"status", ...}, {"name":"screenshot", ...}, {"name":"native-tree", ...}]}}

Stdin close triggers a clean exit via [DEBUG][mcp_dart.server.stdio] Stdin closed..

Environment

• Windows 11 (10.0.26200.8246)
• Dart SDK 3.11.1 (stable, windows_x64)
• Flutter 3.41.4 (stable)
• patrol_mcp 0.1.3
• Verified in Git Bash; behavior on PowerShell is identical because the root cause is Dart's ProcessSignal.sigterm unavailability, not shell-specific.

Diff

One file, 11 insertions / 3 deletions: packages/patrol_mcp/bin/patrol_mcp.dart. Inline comment in the code explains the guard for future readers.

[gemini-code-assist]

Code Review

This pull request prevents a SignalException on Windows by guarding the ProcessSignal.sigterm subscription, which is unsupported on that platform. The review feedback suggests maintaining the immutability of the _sigtermSubscription field by using a conditional initialization expression and declaring the variable as late final.

[gemini-code-assist]

For better consistency and to adhere to Effective Dart's recommendation of using final for fields that are only assigned once, you can use a conditional expression to initialize _sigtermSubscription. This keeps the initialization logic concise and ensures the field remains immutable after its initial assignment.

    _sigtermSubscription = !Platform.isWindows
        ? ProcessSignal.sigterm.watch().listen(_handleSignal)
        : null;
    _sigintSubscription = ProcessSignal.sigint.watch().listen(_handleSignal);

[pdenert]

Looks good, please add changelog entries to both packages