Fix OS-blind apphost probe in MTP bridge on Unix (regression from #16201)

[Azat Mukhametshin (azat-msft)]

Problem

MtpServerConnection.BuildLaunch (added in #16201) probes for a test app's native apphost by unconditionally swapping the source extension to .exe and gating on File.Exists:

string apphost = Path.ChangeExtension(source, ".exe");
return File.Exists(apphost) ? (apphost, ...) : ("dotnet", ...);

The apphost file name is OS-specific: Foo.exe on Windows, but extension-less Foo on Unix. On a normal Linux box the .exe doesn't exist and this harmlessly falls back to dotnet <dll>. But when a test payload is built on Windows and run on Unix (e.g. tests built on a Windows agent, zipped, and executed on a Linux Helix machine), the Windows PE Foo.exe travels alongside the dll — and zip carries no Unix permission bits. File.Exists then finds it and Process.Start fails:

Microsoft.Testing.Platform run failed for '.../Foo.dll':
An error occurred trying to start process '.../Foo.exe' ... Permission denied

(chmod +x would only turn this into Exec format error — it's a Windows binary.)

This surfaced in dotnet/aspnetcore's CI (Windows build agents + Linux Helix queues) on an xunit.v3 (MTP-capable) test assembly, but it affects any repo that builds MTP tests (xunit.v3 / MSTest-on-MTP / TUnit) on Windows and runs them on Linux.

Fix

Make the apphost probe OS-aware, and on Unix require the candidate to actually be executable before launching it (otherwise fall back to dotnet <dll>):

• Windows → Foo.exe; Unix → extension-less Foo.
• New IsUsableApphost: file must exist, and on Unix must carry an execute bit (File.GetUnixFileMode), so a stray non-native sibling (e.g. a Windows PE copied onto Unix) is never selected.

The OS check is #if NETFRAMEWORK-guarded (net462 has no RuntimeInformation and is Windows-only); the Unix-mode check is #if NET-guarded (File.GetUnixFileMode is .NET 7+). On netstandard2.0 it degrades to existence-only, which is still correct because the OS-aware path selection already resolves the bug.

Tests

Adds MtpServerConnectionTests covering: .exe source passes through; dll with a sibling Windows-only .exe on Unix is not selected (falls back to dotnet <dll>); dll with no apphost falls back; and IsUsableApphost execute-bit semantics on Unix.

Verification

Reproduced end-to-end with a source-built runner, A/B-swapping only CrossPlatEngine.dll against a real xunit.v3 app plus a dummy sibling .exe, under the same runtime:

• Before: ... start process '.../MtpRepro.exe' ... Permission denied → Test Run Aborted.
• After: Passed! 1 — the Windows .exe is ignored. A genuine executable Unix apphost is still selected and launched, and a classic (non-MTP) xUnit v2 project is unaffected.

Regression from #16201.

[Copilot]

Pull request overview

This PR fixes a cross-platform regression in the CrossPlatEngine MTP bridge where apphost probing was OS-blind, potentially selecting a Windows *.exe apphost when running on Unix and causing Process.Start failures. The update makes apphost selection OS-aware and adds unit tests around BuildLaunch/apphost usability semantics.

Changes:

• Update MtpServerConnection.BuildLaunch to probe Foo.exe on Windows and extension-less Foo on Unix, and gate selection via IsUsableApphost.
• Add IsUsableApphost (Unix execute-bit check under #if NET) to avoid selecting non-executable apphost candidates on Unix when supported.
• Add new unit tests validating fallback behavior and Unix execute-bit semantics.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/Microsoft.TestPlatform.CrossPlatEngine/Client/MTP/MtpServerConnection.cs	Makes apphost probing OS-aware and introduces IsUsableApphost to avoid launching unusable apphost candidates.
test/Microsoft.TestPlatform.CrossPlatEngine.UnitTests/Client/MTP/MtpServerConnectionTests.cs	Adds unit tests for BuildLaunch and Unix execute-bit behavior (with OS-conditional coverage).

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.

Suppressed comments (1)

src/Microsoft.TestPlatform.CrossPlatEngine/Client/MTP/MtpServerConnection.cs:12

• using System.Runtime.InteropServices; is only needed for the non-NETFRAMEWORK builds (where RuntimeInformation/OSPlatform are referenced). In the net462 target the #if NETFRAMEWORK branch removes all uses, so this using becomes unused and can trigger IDE0005 (this repo commonly wraps such usings with TFM guards; e.g. src/Microsoft.TestPlatform.PlatformAbstractions/net462/System/ProcessHelper.cs). Consider conditionally including the using to avoid build breaks in multi-target builds.

using System.IO;
using System.Net;
using System.Net.Sockets;
using System.Runtime.InteropServices;
using System.Text;