fix: keep Unity responsive while building the shared Roslyn worker DLL

[hatayama]

Summary

• The rare first-time shared Roslyn worker DLL build no longer blocks the Unity main thread with synchronous WaitForExit.
• Existing timeout (30s) + Kill + stream drain behavior is unchanged.

User Impact

• Before: When the compile gate was free, EnsureWorkerReady → EnsureWorkerAssemblyBuilt could run on the calling thread before the first await, so a missing worker DLL could freeze the Editor for up to the build timeout.
• After: That build runs on the thread pool via Task.Run; the Editor stays responsive. Timeout/kill remain as the safety net.

Design (plan A)

• Offload boundary: entire CompileWorkerAssembly (including drain) via CompileWorkerAssemblyOffMainThreadAsync.
• No Unity APIs inside the build path (Process + File IO only).
• Build remains outside the session state lock.
• Rejected plan B: WaitForExitAsync is not available on Unity’s .NET Standard 2.1 profile; rewriting Exited+TCS would add moving parts without benefit.

Call-site threading (condition 3)

• TryCompileAsync → RunSerializedCompileAsync → TryCompileWithRetriesAsync / TryCompileOnceAsync.
• When the async compile gate is acquired without yielding, work before the first await of EnsureWorkerReadyAsync (generation check, path sync) can still run on the caller thread — including the Unity main thread.
• The long WaitForExit(timeout) body is the part that must leave that thread; that is what Task.Run covers.

Verification

• uloop compile → Success
• CompileWorkerAssemblyAsync_WhenTestHookIsInstalled_ShouldReturnHookResult → Passed

Test plan

• Confirm test-hook async path still returns hooked messages
• Optional: delete worker DLL under temp RoslynWorker-* and run execute-dynamic-code; Editor should stay responsive during rebuild

Made with Cursor

[hatayama]

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Review details

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: b9376ac4-4047-4aa4-8fca-c249b4b00e20

📥 Commits

Reviewing files that changed from the base of the PR and between d59a625 and 409a911.

📒 Files selected for processing (4)

• Assets/Tests/Editor/DynamicCodeToolTests/SharedRoslynCompilerWorkerHostTests.cs
• Packages/src/Editor/FirstPartyTools/ExecuteDynamicCode/DynamicCompilation/SharedRoslynCompilerWorkerAssemblyBuilder.cs
• Packages/src/Editor/FirstPartyTools/ExecuteDynamicCode/DynamicCompilation/SharedRoslynCompilerWorkerHost.cs
• Packages/src/Editor/FirstPartyTools/ExecuteDynamicCode/DynamicCompilation/SharedRoslynCompilerWorkerSession.cs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/roslyn-worker-assembly-wait-async

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