Migrate GenAPI and GenFacades tasks to the multithreaded task model

[ViktorHofer]

Seventh safe subset of #17381: Microsoft.DotNet.GenAPI and Microsoft.DotNet.GenFacades.

One task is annotated with [MSBuildMultiThreadableTask] (ClearAssemblyReferenceVersions); the other three keep IMultiThreadableTask without the attribute, so they continue to route through the TaskHost while their paths still resolve against the project. Routing keys off the attribute alone, so the interface only causes TaskEnvironment to be injected — removing it would revert path resolution to the process current directory while leaving the task exactly as unsafe.

• GenAPITask hands raw LibPath/Assembly values to HostEnvironment, which expands them with Environment.ExpandEnvironmentVariables and probes with Directory.Exists/File.Exists (Microsoft.Cci.Extensions/HostEnvironment.cs:719-740).
• GenPartialFacadeSource and NotSupportedAssemblyGenerator derive from RoslynBuildTask, which subscribes an instance handler to the process-wide AssemblyLoadContext.Resolving event, so with differing RoslynAssembliesPath values one instance can service another's resolution.

AbsolutePath is threaded through SourceGenerator, TypeParser and GenPartialFacadeSourceGenerator. Retyping those helper parameters is what actually resolves the transitive MSBuildTask0005 chain out of GenPartialFacadeSource. The first two were already internal; GenPartialFacadeSourceGenerator is now internal too, so none of this is public API. It was never reachable as API anyway — build task projects set IncludeBuildOutput=false and pack under tools/, so a PackageReference exposes no compile-time assets, and the only caller in the dotnet org is the one in-repo call site.

Two issues found auditing the migration itself

• Canonicalization loss. GenPartialFacadeSourceGenerator deduplicated seeds with Path.GetFullPath, which anchors and canonicalizes — the comment on the line says "Normalizing and Removing Relative Segments". GetAbsolutePath only anchors, so Distinct() no longer collapsed two spellings of the same seed, and the duplicate-name check immediately below would have reported them as "multiple versions of these assemblies". Restored with the GetCanonicalForm() polyfill, which required adding the usual <Compile> link to the GenFacades project. AbsolutePath is a readonly struct implementing IEquatable<AbsolutePath> with a path-aware comparer, so Distinct() is sound on it.
• Empty item specs. TypeParser.GetSourceTrees explicitly skipped empty source files rather than treating them as an error. [Required] on an ITaskItem[] only validates the collection, not each ItemSpec, and GetAbsolutePath throws on an empty path — so absolutizing every item spec turned a skipped entry into a hard failure. They are now filtered before resolution. (ReferenceAssembly and OutputSourcePath are [Required] scalars, which MSBuild rejects when empty, so they need no guard.)

Validation

• dotnet build Arcade.slnx -c Release --no-incremental: 0 errors, 0 warnings. Incremental builds skip up-to-date projects, so the analyzer does not re-run and reports a false "0 warnings"; --no-incremental is required for an authoritative count.
• Positive control: reverting the ClearAssemblyReferenceVersions call site to File.Open(Assembly, ...) produces MSBuildTask0003, confirming the analyzer is live on the newly annotated task and that the clean run is meaningful.
• Neither assembly has an in-repo test suite.

[Copilot]

Copilot review overview

🟡 Changes recommended

The public API change is breaking, and contract-project-relative paths may resolve against the wrong project directory.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Review tier: Lite
Findings: 1

New issues introduced by this change (1)

Severity	Finding
	src/​Microsoft.DotNet.GenFacades/​GenPartialFacadeSourceGenerator.cs — Preserve the public generator API

What changed in this PR

Migrates GenAPI and GenFacades tasks to TaskEnvironment path handling and multithreaded task support.

Changes:

• Threads AbsolutePath through facade generation.
• Restores seed canonicalization and filters empty source items.
• Adds task environment injection and annotates ClearAssemblyReferenceVersions.
• Review identified a breaking public API change and a contract-project path resolution issue.

File	Summary
src/​Microsoft.DotNet.GenFacades/​TypeParser.cs	Parses absolute source paths.
src/​Microsoft.DotNet.GenFacades/​SourceGenerator.cs	Uses absolute compile and output paths.
src/​Microsoft.DotNet.GenFacades/​NotSupportedAssemblyGenerator.cs	Resolves task paths through TaskEnvironment.
src/​Microsoft.DotNet.GenFacades/​Microsoft.DotNet.GenFacades.csproj	Includes the path canonicalization helper.
src/​Microsoft.DotNet.GenFacades/​GenPartialFacadeSourceGenerator.cs	Canonicalizes and deduplicates seeds.
src/​Microsoft.DotNet.GenFacades/​GenPartialFacadeSource.cs	Resolves inputs and outputs and filters empty items.
src/​Microsoft.DotNet.GenFacades/​ClearAssemblyReferenceVersions.cs	Enables multithreaded routing and path resolution.
src/​Microsoft.DotNet.GenAPI/​GenAPITask.cs	Adds environment-aware path handling.

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[akoeplinger]

LGTM but I'd probably change the PR title since the tasks aren't actually migrated

[ViktorHofer]

@akoeplinger sorry, this wasn't yet ready for review. I just drafted the PR.

[Copilot]

Copilot review overview

🔵 Needs a closer look

Address the two unresolved moderate issues involving whitespace-only paths and empty item specifications.

Review tier: Lite
Findings: None

Issues resolved since last review (1)

Severity	Finding
	src/​Microsoft.DotNet.GenFacades/​GenPartialFacadeSourceGenerator.cs — Preserve the public generator API View resolved comment

Previously missed findings (1)

In code that hasn't changed since last review

src/Microsoft.DotNet.GenFacades/NotSupportedAssemblyGenerator.cs:78

• This resolves every item spec before checking it. An empty ITaskItem.ItemSpec is still possible even with [Required] on the array; previously File.Exists("") returned false and the task logged the existing not-found error, whereas GetAbsolutePath("") throws and turns the item into an unhandled task failure. Guard empty specs before this call (preserving the prior error/skip behavior).