Running a test on a Windows executor from a Linux host is not possible

[tjgq]

It's currently not possible to run a *_test rule on a remote Windows executor when Bazel is running on a Linux host.

• Every test rule has a dependency on @bazel_tools//tools/test:test_wrapper and @bazel_tools//tools/test:xml_writer, which point to bundled precompiled tw.exe and xml.exe files if Bazel was built for Windows, and are missing otherwise (causing the test rule to fail during loading).
• In addition, some test rules (including sh_test) have a dependency on @bazel_tools//tools/launcher:launcher which points to a bundled precompiled launcher.exe if Bazel was built for Windows, and to the C++ source code otherwise. This makes a C++ cross-compiler required for a build that is otherwise not dependent on C++. In addition, C++ cross-compilation from Linux to Windows is currently broken (see #19208).

Note that it's possible to run a genrule on Windows from a Linux host, showing that this is a testing-specific issue rather than a more general one with cross-platform Windows builds.

A minimal repro is available at https://github.com/jtattermusch/win-from-linux-rbe-repro (thanks @jtattermusch!). You must use a Bazel built at or after 2f0948b.

We should consider publishing the tw.exe, xml.exe and launcher.exe binaries to an external repository and fetch it on demand when a test is configured for Windows. This would avoid unnecessarily increasing the Bazel binary size, introducing useless dependencies on other platforms, or requiring a C++ cross compiler only for running tests.

[jtattermusch]

Since tw.exe and xml.exe (and launcher.exe) binaries are easily obtained (they are bundled with bazel.exe on windows), it would be good to document a workaround for providing these .exe binaries myself and convincing bazel linux binary to use them in a build.

If lucky, such workaround (if it exists) would allow me to run sh_test and 'native_test` rules on windows executor from linux host or it would possibly reveal whether other non-yet-known issues exist with the cross-platform execution.

[tjgq]

A Bazel with (unconditionally) embedded {tw,xml,launcher}.exe binaries can be built from https://github.com/tjgq/bazel/tree/win-rbe-fixes (for investigation purposes only; not planning to submit as-is).

[meisterT]

cc @meteorcloudy

[meteorcloudy]

Yeah, I agree we should move stuff out of @bazel_tools as much as possible, especially platform specific dependencies, hopefully that is easier in the Bzlmod world.

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