# Worker Process Launcher

The gateway uses `WorkerProcessLauncher` to validate and start one worker
process for a gateway session. The launcher owns process start semantics only;
pipe handshaking and `WorkerReady` validation remain part of the worker client
startup path.

## Launch Inputs

`WorkerProcessLaunchRequest` carries the per-session bootstrap values:

- `SessionId`,
- `PipeName`,
- `ProtocolVersion`,
- `Nonce`,
- optional `PipeReservation` cleanup handle.

The launcher passes `SessionId`, `PipeName`, and `ProtocolVersion` as command
line arguments:

```text
--session-id <sessionId> --pipe-name <pipeName> --protocol-version <version>
```

The launcher sets the nonce through the `MXGATEWAY_WORKER_NONCE` environment
variable. The nonce is not included in `WorkerProcessCommandLine` so logs and
diagnostics can report the launch command without exposing the secret.

## Validation And Cleanup

Before starting the process, the launcher validates that the configured worker
path exists, has a `.exe` extension, contains a valid Windows Portable
Executable header, and matches the configured `RequiredArchitecture`.

After the process starts, `IWorkerStartupProbe` waits for startup readiness.
The default probe only verifies that the worker did not exit immediately. The
worker client replaces this probe when pipe connection, hello, and
`WorkerReady` handling are implemented.

Startup probing uses a bounded Polly retry policy. The gateway starts the worker
process once, then retries only transient startup-probe failures while the
process remains alive. The policy is configured by
`WorkerOptions.StartupProbeRetryAttempts` and
`WorkerOptions.StartupProbeRetryDelayMilliseconds`; the retry counter is
recorded as `mxgateway.retries.attempted` with `area=worker_startup`.

The launcher also passes
`MXGATEWAY_WORKER_PIPE_CONNECT_ATTEMPT_TIMEOUT_MS` to the worker process from
`WorkerOptions.PipeConnectAttemptTimeoutMilliseconds`. The worker uses that
value as the per-attempt named-pipe connect timeout inside its own bounded
Polly retry loop.

If startup fails or exceeds `WorkerOptions.StartupTimeoutSeconds`, the launcher
kills the worker process tree, disposes the process handle, disposes the
optional pipe reservation, records a worker kill metric, and reports a
`WorkerProcessLaunchException`.

## Verification

Run the focused launcher tests after changing process launch behavior:

```bash
dotnet test src/MxGateway.Tests/MxGateway.Tests.csproj --filter WorkerProcessLauncherTests
```

Run the gateway build because the launcher is part of `MxGateway.Server`:

```bash
dotnet build src/MxGateway.Server/MxGateway.Server.csproj
```

## Related Documentation

- [Gateway Process Detailed Design](./GatewayProcessDesign.md)
- [Worker Frame Protocol](./WorkerFrameProtocol.md)
