fix: Dynamic code execution stays fast with system .NET 6 installed

[hatayama]

Summary

• Prevent the shared Roslyn worker from probing system-wide .NET runtimes when Unity launches it.
• Keep the worker runtime aligned with the Unity bundled runtime so .NET 8 references are not executed on system .NET 6.
• Add an EditMode regression test for the worker runtime environment setting.

Reproduction

• Confirmed on main with Unity 6000.4.3f1 and system Microsoft.NETCore.App 6.0.36 installed.
• The shared worker logs execute-dynamic-code shared Roslyn worker failed to operate correctly and falls back to one-shot compiler execution.

Verification

• uloop compile --force-recompile false --wait-for-domain-reload true completed with 0 errors and 0 warnings.
• SharedRoslynCompilerWorkerHostTests.ConfigureWorkerDotnetRuntimeEnvironment_WhenCalled_ShouldDisableMultilevelLookup passed.
• On this PR branch with Unity 6000.4.3f1 and system .NET 6.0.36, execute-dynamic-code returned 6000.4.3f1 successfully.
• uloop get-logs --search-text "shared Roslyn worker" --max-count 50 --include-stack-trace true returned TotalCount: 0.
• uloop get-logs --log-type Error --max-count 50 --include-stack-trace false returned TotalCount: 0.

Fixes #985

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: d93f0f0f-7f91-481d-9b3a-5f0cab0eb522

📥 Commits

Reviewing files that changed from the base of the PR and between 53f8e1e and 8c4adf8.

📒 Files selected for processing (1)

• Packages/src/Editor/Compilation/SharedRoslynCompilerWorkerHost.cs

📝 Walkthrough

Walkthrough

The changes add functionality to disable .NET multilevel runtime lookup in the shared Roslyn compiler worker host, preventing system-wide .NET installations from interfering with Unity's bundled runtime. A new test verifies this environment variable configuration.

Changes

Cohort / File(s)	Summary
Worker Configuration Packages/src/Editor/Compilation/SharedRoslynCompilerWorkerHost.cs	Added ConfigureWorkerDotnetRuntimeEnvironment method with two internal constants (DotnetMultilevelLookupEnvironmentVariableName, DotnetMultilevelLookupDisabledValue) to set DOTNET_MULTILEVEL_LOOKUP to "0" during worker process startup, preventing runtime version mismatch.
Test Fixture Assets/Tests/Editor/DynamicCodeToolTests/SharedRoslynCompilerWorkerHostTests.cs	Added test method ConfigureWorkerDotnetRuntimeEnvironment_WhenCalled_ShouldDisableMultilevelLookup to verify environment variable configuration updates ProcessStartInfo.EnvironmentVariables correctly.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately describes the main fix: preventing the shared Roslyn worker from probing system runtimes to maintain compatibility with Unity's bundled .NET 8.
Linked Issues check	✅ Passed	The code implements the suggested fix from #985 by setting DOTNET_MULTILEVEL_LOOKUP=0, with a regression test added, directly addressing the worker runtime mismatch issue.
Out of Scope Changes check	✅ Passed	All changes are scoped to the shared Roslyn compiler worker runtime configuration and its corresponding test; no unrelated or extraneous modifications are present.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch codex/fix-985-shared-roslyn-worker-runtime

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[coderabbitai]

Caution

Some comments are outside the diff and can’t be posted inline due to platform limitations.

⚠️ Outside diff range comments (1)

Packages/src/Editor/Compilation/SharedRoslynCompilerWorkerHost.cs (1)

542-551: ⚠️ Potential issue | 🟡 Minor

Add ConfigureWorkerDotnetRuntimeEnvironment to the worker-assembly build process at line 551.

CompileWorkerAssembly spawns the Roslyn compiler via ProcessStartInfo (lines 542–551) without calling ConfigureWorkerDotnetRuntimeEnvironment, whereas CreateWorkerStartInfo (lines 491–508) correctly applies this guard when spawning the compiled worker. The compiler DLL's runtimeconfig.json is subject to the same multilevel-lookup probing that this PR addresses, and without DOTNET_MULTILEVEL_LOOKUP=0, the process can fail with FileLoadException if a system .NET 6 runtime is discovered before the bundled .NET 8 runtime, blocking the worker fast path on the first compile after shutdown.

🛠️ Proposed change

             ProcessStartInfo startInfo = new ProcessStartInfo
             {
                 FileName = externalCompilerPaths.DotnetHostPath,
                 Arguments = $"{QuoteCommandLineArgument(externalCompilerPaths.CompilerDllPath)} @{QuoteCommandLineArgument(workerCompileResponseFilePath)}",
                 WorkingDirectory = Path.GetDirectoryName(workerSourcePath),
                 UseShellExecute = false,
                 RedirectStandardOutput = true,
                 RedirectStandardError = true,
                 CreateNoWindow = true
             };
+            ConfigureWorkerDotnetRuntimeEnvironment(startInfo);

🤖 Prompt for AI Agents

Verify each finding against the current code and only fix it if needed.

In `@Packages/src/Editor/Compilation/SharedRoslynCompilerWorkerHost.cs` around
lines 542 - 551, CompileWorkerAssembly creates a ProcessStartInfo (startInfo) to
invoke the Roslyn compiler but does not apply the same DOTNET_MULTILEVEL_LOOKUP
guard used in CreateWorkerStartInfo; to fix, call
ConfigureWorkerDotnetRuntimeEnvironment(startInfo) after the ProcessStartInfo is
constructed (the same place CreateWorkerStartInfo applies it) so the compiler
process has the multilevel lookup disabled and won’t pick up a system .NET
runtime before the bundled one.

🤖 Prompt for all review comments with AI agents

Verify each finding against the current code and only fix it if needed.

Outside diff comments:
In `@Packages/src/Editor/Compilation/SharedRoslynCompilerWorkerHost.cs`:
- Around line 542-551: CompileWorkerAssembly creates a ProcessStartInfo
(startInfo) to invoke the Roslyn compiler but does not apply the same
DOTNET_MULTILEVEL_LOOKUP guard used in CreateWorkerStartInfo; to fix, call
ConfigureWorkerDotnetRuntimeEnvironment(startInfo) after the ProcessStartInfo is
constructed (the same place CreateWorkerStartInfo applies it) so the compiler
process has the multilevel lookup disabled and won’t pick up a system .NET
runtime before the bundled one.

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: c36206ad-7bc1-45ed-9c3d-1a64ff155c94

📥 Commits

Reviewing files that changed from the base of the PR and between 694cb47 and 53f8e1e.

⛔ Files ignored due to path filters (1)

• Assets/Tests/Editor/DynamicCodeToolTests/SharedRoslynCompilerWorkerHostTests.cs.meta is excluded by none and included by none

📒 Files selected for processing (2)

• Assets/Tests/Editor/DynamicCodeToolTests/SharedRoslynCompilerWorkerHostTests.cs
• Packages/src/Editor/Compilation/SharedRoslynCompilerWorkerHost.cs

[cubic-dev-ai]

No issues found across 3 files

[hatayama]

Addressed the CodeRabbit review comment in 8c4adf82.

The Roslyn compiler process used by CompileWorkerAssembly now also calls ConfigureWorkerDotnetRuntimeEnvironment(startInfo), so both the worker build process and the shared worker runtime disable .NET multilevel lookup before launching through Unity's bundled dotnet host.

Verification performed after the change:

• uloop compile --force-recompile false --wait-for-domain-reload true completed with 0 errors. Unity 6000.4 reported existing obsolete API warnings unrelated to this file.
• SharedRoslynCompilerWorkerHostTests.ConfigureWorkerDotnetRuntimeEnvironment_WhenCalled_ShouldDisableMultilevelLookup passed.
• execute-dynamic-code returned 6000.4.3f1 successfully.
• uloop get-logs --search-text "shared Roslyn worker" --max-count 50 --include-stack-trace true returned TotalCount: 0.
• uloop get-logs --log-type Error --max-count 50 --include-stack-trace false returned TotalCount: 0.