Add RFC 017: Custom test host launcher

[Evangelink]

Summary

Adds RFC 017 — Custom test host launcher (docs/RFCs/017-TestHost-Launcher.md).

The RFC proposes a public, experimental MTP extension point — ITestHostLauncher — that lets an extension control how the out-of-process test host is launched, instead of the platform always calling Process.Start. The platform keeps owning everything around the launch (argument/environment preparation, the controller↔host IPC pipe, PID tracking, ITestHostProcessLifetimeHandler callbacks, and exit-code reconciliation) and delegates only the single "create and start the test host" step.

The hook is deliberately agnostic of the launch mechanism ("we don't know if it's a process"): the launcher does not have to start a local OS process. It returns an ITestHostHandle (an IDisposable) that exposes only lifecycle (WaitForExitAsync(CancellationToken), ExitCode, HasExited, Terminate) plus an optional free-form string? Identifier used purely for diagnostics. This covers packaged-app activation, containers, and remote launches where no local PID exists.

The motivating scenario is testing packaged Windows apps — both UWP and packaged WinUI (which ship as MSIX and share the same deploy + AUMID-activate mechanism, the reason VSTest has a single UwpTestHostRuntimeProvider). This is the blocker behind #2784. The RFC also includes worked examples for debugger attach, elevation, container, and remote.

What changed since the first draft

This reworks the original process-centric draft to be generic:

• ITestHostProcessLauncher → ITestHostLauncher; ITestHostProcessHandle → ITestHostHandle.
• The mandatory int ProcessId is replaced by an optional free-form string? Identifier (diagnostics only); the redundant Exited event is dropped in favor of WaitForExitAsync; Kill() → Terminate().
• ITestHostHandle extends IDisposable, and WaitForExitAsync takes a CancellationToken (both supported by the runtime and the repo's netstandard2.0 polyfill).
• File renamed 017-TestHost-Process-Launcher.md → 017-TestHost-Launcher.md.
• Motivation reframed around packaged Windows apps (UWP + WinUI) rather than WinUI alone.
• Documents the platform integration detail that makes PID-less launchers work (premature-exit gated on HasExited only).

Implementation

The hook is implemented, with a reference consumer (Microsoft.Testing.Extensions.PackagedApp) and unit + acceptance tests, in #9454.

Related: #2784, #9454

[Copilot]

Pull request overview

Adds a new design document (RFC 017) proposing an experimental Microsoft.Testing.Platform extension point (ITestHostProcessLauncher) to let extensions customize how the out-of-process test host is launched, while keeping platform-owned responsibilities (args/env prep, IPC, lifetime handling, exit reconciliation) unchanged.

Changes:

• Introduces RFC 017 documentation describing the proposed ITestHostProcessLauncher API surface and registration shape.
• Documents intended platform integration points (controller host selection, launch-site delegation, singleton enforcement).
• Provides worked examples (WinUI/MSIX activation, debugger attach, elevation, container, remote) plus alternatives and open questions.

Show a summary per file

File	Description
docs/RFCs/017-TestHost-Process-Launcher.md	New RFC describing an experimental extension point for customizing out-of-process test host process launching.

Copilot's findings

• Files reviewed: 1/1 changed files
• Comments generated: 4

[Copilot]

Copilot's findings

• Files reviewed: 1/1 changed files
• Comments generated: 5

[Copilot]

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 4
• Review effort level: Low

[Copilot]

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 2
• Review effort level: Low

[Evangelink]

Note

🤖 Automated review by GitHub Copilot. Posted via a maintainer's GitHub token, so it appears under their account — the account owner did not write or approve this content personally. Generated by the Expert Code Review workflow. To request a follow-up action, reply by tagging @copilot directly.

Documentation-only PR — only docs/RFCs/017-TestHost-Launcher.md changed (new file, 406 lines). Most of the 22 review dimensions (Threading, Performance, Security, Cross-TFM, IPC wire, Test Isolation, Analyzer Quality, etc.) are N/A. Dimensions evaluated: 17 — Documentation Accuracy and 21 — Scope & PR Discipline.

---

Scope & PR Discipline — ✅

• RFC references the blocking issue (#2784) and the implementation PR (#9454).
• Goals / Non-goals / Alternatives / Open Questions sections are all present and well-structured.
• The design rationale (why ITestHostProcessLauncher → ITestHostLauncher, why not ITestHostExecutionOrchestrator, why not expose IProcessHandler) is thorough.

---

Documentation Accuracy — 4 findings

#	Severity	Finding
1	🟡 MODERATE	WaitForExitAsync() has no CancellationToken parameter — platform cannot propagate overall-run cancellation cooperatively (inline comment on line 160)
2	🟡 MODERATE	ITestHostHandle does not extend IDisposable — OS resources held by adapters (process handles, sockets) have no deterministic cleanup path via the platform (inline comment on line 163)
3	🟡 MODERATE	ExitCode behavior is undefined when read before HasExited — implementers need a contract to code against (inline comment on line 158)
4	⚪ NIT	Quote / PasteArguments helpers in Examples 1 and 5 are called but never defined or referenced, leaving a potential argument-injection pitfall for readers (inline comment on line 340)

✅ 20/22 dimensions clean (2 N/A, 2 have findings above).

• WaitForExitAsync — add CancellationToken overload or document that callers must drive cancellation through Terminate()
• ITestHostHandle — add IDisposable (or IAsyncDisposable) or add this to the Open Questions for #9454 to decide
• ExitCode — document "must not be read before HasExited is true"
• Example 5 — define or reference Quote, note it is a placeholder