# Fixes dotnet#13484

### Context
Multithreaded (`/mt`) builds that launch out-of-process task hosts crash
with `ArgumentNullException: Value cannot be null. Parameter name:
path2` in `Path.Combine` when multiple `RequestBuilder` threads
simultaneously resolve the task host executable name.

### Root cause

`GetTaskHostNameFromHostContext` in `NodeProviderOutOfProcTaskHost.cs`
has an unsafe read-modify-write sequence on the static field
`s_msbuildName`:

```csharp
if (string.IsNullOrEmpty(s_msbuildName))            // (1) check
{
    s_msbuildName = Environment.GetEnvironmentVariable("MSBUILD_EXE_NAME"); // (2) write — may be null
    if (!string.IsNullOrEmpty(s_msbuildName))
    {
        return s_msbuildName;
    }
    s_msbuildName = Constants.MSBuildExecutableName;  // (3) write — always non-null
}
return s_msbuildName;                                // (4) read
```

When `MSBUILD_EXE_NAME` is not set, step (2) writes `null` into
`s_msbuildName`. Between steps (2) and (3), another thread can read
`s_msbuildName` at step (4)** and get `null`.

The returned `null` is passed as `toolName` into `Path.Combine(basePath,
toolName)`, which throws `ArgumentNullException` on .NET Framework
(where `Path.Combine` does not accept null arguments).

### Changes Made
Since all computations are deterministic (environment variables +
constants), we use the local-variable snapshot pattern
— read the static into a local, compute if null, write back, return the
local. This is lock-free and safe because reference writes are atomic in
.NET. Applied the same fix to similar patterns in nearby code in
NodeProviderOutOfProcTaskHost.cs and CurrentHost.cs

### Testing
No extra testing. There is no stable local repro.

Co-authored-by: Copilot <[email protected]>

Updated [Microsoft.Build](https://github.com/dotnet/msbuild) from 18.6.3
to 18.7.1.

<details>
<summary>Release notes</summary>

_Sourced from [Microsoft.Build's
releases](https://github.com/dotnet/msbuild/releases)._

## 18.7.1

## What's Changed
* Fix TraceEngine file contention deadlock in multithreaded mode by
@​JanProvaznik in dotnet/msbuild#13446
* Remove duplicate test cases in MultithreadableTaskAnalyzer by
@​Youssef1313 in dotnet/msbuild#13483
* Ensure ThreadSafeTaskAnalyzer.Tests is considered as a unit test
project by @​Youssef1313 in dotnet/msbuild#13481
* Fix MSBuildTask0002 analyzer warnings in already-migrated tasks by
@​JanProvaznik in dotnet/msbuild#13466
* Fix race conditions in task host path resolution by @​AR-May in
dotnet/msbuild#13485
* Migrate ToolTask and Al task to TaskEnvironment API by @​OvesN in
dotnet/msbuild#13423
* Bump main to 18.7, add vs18.6 to merge flow by @​MichalPavlik in
dotnet/msbuild#13472
* Avoid allocations in GetHashCode implementations by @​DustinCampbell
in dotnet/msbuild#13475
* Add PATs rotation to agentic workflow(s) by @​JanKrivanek in
dotnet/msbuild#13496
* Fix ASP.NET WebSite projects to copy netstandard.dll facade when
required by @​JanProvaznik in
dotnet/msbuild#13058
* Migrate AspNetCompiler to TaskEnvironment API by @​OvesN in
dotnet/msbuild#13424
* Add review workflow by @​JanKrivanek in
dotnet/msbuild#13503
* Strengthen reviewer skill: add step-back analysis dimensions by
@​JanProvaznik in dotnet/msbuild#13504
* Add 'Request Speedometer Perf Run' to VS experimental insertion build
policies by @​Copilot in dotnet/msbuild#13505
* Remove duplicate @ prefix from issueAuthor in GitOps by @​akoeplinger
in dotnet/msbuild#13492
* Improve review aw by @​JanKrivanek in
dotnet/msbuild#13510
* Migrates unit tests to use RoslynCodeTaskFactory to enable running
tests under .NET Core by @​jankratochvilcz in
dotnet/msbuild#13500
* Fix cross-AppDomain TaskItem modifier cache regression by
@​DustinCampbell in dotnet/msbuild#13493
* Discourage review agent from approving PRs by @​JanKrivanek in
dotnet/msbuild#13512
* Stop trying to deploy ValueTuple by @​rainersigwald in
dotnet/msbuild#13507
* Ad-hoc re-sign bootstrap dotnet on macOS to prevent SIGKILL by
@​jankratochvilcz in dotnet/msbuild#13513
* RoslynCodeTaskFactory: Log MSB3753 when task class does not implement
ITask by @​jankratochvilcz in
dotnet/msbuild#13517
* Update gh-aw (upon mcp policy changes) by @​JanKrivanek in
dotnet/msbuild#13526
* Eliminate XmlChildNodes allocations in GetXmlNodeInnerContents by
@​nareshjo in dotnet/msbuild#13509
* Fix telemetry allocation regression: per-engine collector ownership by
@​JanProvaznik in dotnet/msbuild#13516
* Migrate to xunit.v3 by @​Youssef1313 in
dotnet/msbuild#13482
* Fix stray brace in HandleBuildCancel trace string causing MSB1025 by
@​Copilot in dotnet/msbuild#13535
* Bumping to 10.0.4 runtime packages by @​MichalPavlik in
dotnet/msbuild#13533
* Remove early return in GetCanonicalForm, always call System.IO.Path by
@​OvesN in dotnet/msbuild#13532
* Do not overwrite GetCopyToOutputDirectoryItemsDependsOn, just add new…
by @​snechaev in dotnet/msbuild#13474
* Migrate GetReferenceAssemblyPaths task to TaskEnvironment API by
@​OvesN in dotnet/msbuild#13495
* Stabilize ToolTaskThatTimeoutAndRetry test by @​rainersigwald in
dotnet/msbuild#13489
* [automated] Merge branch 'vs18.6' => 'main' by @​github-actions[bot]
in dotnet/msbuild#13506
* Add extra test assertions around tests by @​Youssef1313 in
dotnet/msbuild#13536
* Add static eval for repo skills/agents via skill-validator by
@​JanKrivanek in dotnet/msbuild#13537
* Migrate SGen task to Task environment API by @​OvesN in
dotnet/msbuild#13457
* Fix TerminalLogger assert failure for metaproj files and cached
project eval ID by @​OvesN in
dotnet/msbuild#13480
* Filter out approving review from pr-reviewer agent by @​JanKrivanek in
dotnet/msbuild#13553
* Use a unique task name per invocation to tabilize
RoslynCodeTaskFactory_ReuseCompilation test by @​huulinhnguyen-dev in
dotnet/msbuild#13551
* Brief doc on feedback/logging/data systems by @​rainersigwald in
dotnet/msbuild#13554
* Localized file check-in by OneLocBuild Task: Build definition ID 9434:
Build ID 13881982 by @​dotnet-bot in
dotnet/msbuild#13437
* Stage 3: Forward BuildProjectFile* callbacks from OOP TaskHost to
worker node by @​JanProvaznik in
dotnet/msbuild#13350
* Enable TaskHost Callbacks by default by @​JanProvaznik in
dotnet/msbuild#13579
* Remove unactionable info from reviewer agent by @​JanKrivanek in
dotnet/msbuild#13578
* Enlighten RequiresFramework35SP1Assembly task for multithreaded mode
by @​jankratochvilcz in dotnet/msbuild#13575
* Make SdkResolver-provided environment variables take precedence over
ambient environment by @​Copilot in
dotnet/msbuild#12655
* Add dotnet/skills marketplace and enable plugins by @​Evangelink in
dotnet/msbuild#13582
* The skills/agents check filters-in only touched files by @​JanKrivanek
in dotnet/msbuild#13586
* Fix skill-validation workflow failing when agents directory is deleted
by @​JeremyKuhne in dotnet/msbuild#13592
 ... (truncated)

Commits viewable in [compare
view](dotnet/msbuild@v18.6.3...v18.7.1).
</details>

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>