[florentianayuwono]

Summary

Three L0 tests in StepHostL0.cs (DetermineNodeRuntimeVersionInAlpineContainerAsync, DetermineNode20RuntimeVersionInAlpineContainerAsync, DetermineNode24RuntimeVersionInAlpineContainerAsync) fail deterministically whenever the Test assembly is built natively on linux-arm64. This surfaced in #233's CI run once the dead L0-test matrix condition there was fixed and dotnet test actually ran on the linux-arm64 leg for the first time.

Root cause

StepHost.CheckPlatformForAlpineContainer gates Alpine Node support with:

if (!Constants.Runner.PlatformArchitecture.Equals(Constants.Architecture.X64))
{
    throw new NotSupportedException(...);
}

Constants.Runner.PlatformArchitecture is not detected at runtime — it's baked in at compile time via #if X86/#if ARM/#if ARM64/else directives in Constants.cs, driven by the X64/ARM/ARM64 preprocessor symbols that Directory.Build.props sets from the PackageRuntime MSBuild property.

./dev.sh test auto-detects the host architecture (via uname -m) and sets PackageRuntime accordingly when no explicit target runtime is passed. On a linux-arm64 self-hosted runner, that means the Test assembly is built with PackageRuntime=linux-arm64, the ARM64 symbol is defined, and PlatformArchitecture compiles to Arm64 — so CheckPlatformForAlpineContainer always throws, regardless of what the test is actually trying to verify. This is 100% reproducible, not flaky, and is unrelated to the actual Alpine-container Node-version-detection logic the tests exercise.

This was previously invisible because the build.yml L0-test-trigger condition was dead (see #233), so the test step never actually executed on the linux-arm64 matrix leg.

Fix

Gate the three Alpine-container tests with #if X64, matching the production code's own intentional X64-only restriction for Alpine support. They now only compile/run as part of an X64-targeted Test assembly build, which is the only configuration where the behavior they exercise is actually reachable.

Testing

• ./dev.sh test — full local suite builds and runs cleanly (one unrelated pre-existing local-environment failure in ProcessExtensionL0.SuccessReadProcessEnv, caused by a missing cached node20 external in this sandbox, not by this change).
• dotnet test --filter "FullyQualifiedName~StepHostL0|FullyQualifiedName~JobExtensionL0" — 41/41 passed.

[florentianayuwono]

Consolidating this fix directly into #233 so CI goes green in one PR (this fix needed #233's build.yml matrix-condition change to actually exercise dotnet test on linux-arm64 anyway). Closing in favor of #233.