compile_pip_requirements on Windows (GitHub workers) fails; bug is probably in the runfiles library

# 🐞 bug report

### Affected Rule

The issue is caused by the rule: compile\_pip\_requirements (but probably an issue in the runfiles library)

### Is this a regression?

Yes, the previous version in which this bug was not present was: 1.7.0

### Description

On the GitHub workers on MS-Windows, compile\_pip\_requirements rules fail with an error such as

```
File "C:\Users\RUNNER~1\AppData\Local\Temp\Bazel.runfiles_flx90ick\runfiles\rules_python+\python\runfiles\runfiles.py", line 306, in Rlocation
source_repo = self.CurrentRepository(frame=2)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "C:\Users\RUNNER~1\AppData\Local\Temp\Bazel.runfiles_flx90ick\runfiles\rules_python+\python\runfiles\runfiles.py", line 384, in CurrentRepository
caller_runfiles_path = os.path.relpath(caller_path, self._python_runfiles_root)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "", line 777, in relpath
ValueError: path is on mount 'C:', start on mount 'D:'
```

## 🔬 Minimal Reproduction

https://github.com/phst/rules_python-bug-runfiles-windows

See e.g. the action logs at https://github.com/phst/rules_python-bug-runfiles-windows/actions/runs/22157518229

## 🔥 Exception or Error

==================== Test output for //:requirements.test:

Traceback (most recent call last):

  File "C:\Users\RUNNER~1\AppData\Local\Temp\Bazel.runfiles_flx90ick\runfiles\_main\_requirements.test_stage2_bootstrap.py", line 499, in 

    main()

  File "C:\Users\RUNNER~1\AppData\Local\Temp\Bazel.runfiles_flx90ick\runfiles\_main\_requirements.test_stage2_bootstrap.py", line 493, in main

    _run_py_path(main_filename, args=sys.argv[1:])

  File "C:\Users\RUNNER~1\AppData\Local\Temp\Bazel.runfiles_flx90ick\runfiles\_main\_requirements.test_stage2_bootstrap.py", line 287, in _run_py_path

    runpy.run_path(main_filename, run_name="__main__")

  File "", line 291, in run_path

  File "", line 98, in _run_module_code

  File "", line 88, in _run_code

  File "C:\Users\RUNNER~1\AppData\Local\Temp\Bazel.runfiles_flx90ick\runfiles\rules_python+\python\private\pypi\dependency_resolver\dependency_resolver.py", line 270, in 

    main()

  File "C:\Users\RUNNER~1\AppData\Local\Temp\Bazel.runfiles_flx90ick\runfiles\rules_python++config+pypi__click\click\core.py", line 1157, in __call__

    return self.main(*args, **kwargs)

           ^^^^^^^^^^^^^^^^^^^^^^^^^^

  File "C:\Users\RUNNER~1\AppData\Local\Temp\Bazel.runfiles_flx90ick\runfiles\rules_python++config+pypi__click\click\core.py", line 1078, in main

    rv = self.invoke(ctx)

         ^^^^^^^^^^^^^^^^

  File "C:\Users\RUNNER~1\AppData\Local\Temp\Bazel.runfiles_flx90ick\runfiles\rules_python++config+pypi__click\click\core.py", line 1434, in invoke

    return ctx.invoke(self.callback, **ctx.params)

           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

  File "C:\Users\RUNNER~1\AppData\Local\Temp\Bazel.runfiles_flx90ick\runfiles\rules_python++config+pypi__click\click\core.py", line 783, in invoke

    return __callback(*args, **kwargs)

           ^^^^^^^^^^^^^^^^^^^^^^^^^^^

  File "C:\Users\RUNNER~1\AppData\Local\Temp\Bazel.runfiles_flx90ick\runfiles\rules_python+\python\private\pypi\dependency_resolver\dependency_resolver.py", line 111, in main

    resolved_srcs = [_locate(bazel_runfiles, src) for src in srcs]

                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

  File "C:\Users\RUNNER~1\AppData\Local\Temp\Bazel.runfiles_flx90ick\runfiles\rules_python+\python\private\pypi\dependency_resolver\dependency_resolver.py", line 111, in 

    resolved_srcs = [_locate(bazel_runfiles, src) for src in srcs]

                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^

  File "C:\Users\RUNNER~1\AppData\Local\Temp\Bazel.runfiles_flx90ick\runfiles\rules_python+\python\private\pypi\dependency_resolver\dependency_resolver.py", line 82, in _locate

    return bazel_runfiles.Rlocation(file)

           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

  File "C:\Users\RUNNER~1\AppData\Local\Temp\Bazel.runfiles_flx90ick\runfiles\rules_python+\python\runfiles\runfiles.py", line 306, in Rlocation

    source_repo = self.CurrentRepository(frame=2)

                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

  File "C:\Users\RUNNER~1\AppData\Local\Temp\Bazel.runfiles_flx90ick\runfiles\rules_python+\python\runfiles\runfiles.py", line 384, in CurrentRepository

    caller_runfiles_path = os.path.relpath(caller_path, self._python_runfiles_root)

                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

  File "", line 777, in relpath

ValueError: path is on mount 'C:', start on mount 'D:'

================================================================================

## 🌍 Your Environment

**Operating System:**

  

Windows

  

**Output of `bazel version`:**

  

Bazelisk version: 1.28.1

INFO: Invocation ID: 0c29aa33-c4d6-490c-ae50-a131af7e76a6

Build label: 9.0.0

Build target: @@//src/main/java/com/google/devtools/build/lib/bazel:BazelServer

Build time: Tue Jan 20 18:20:43 2026 (1768933243)

Build timestamp: 1768933243

Build timestamp as int: 1768933243


  

**Rules_python version:**

  

1.8.4

  

**Anything else relevant?**

See https://github.com/bazel-contrib/rules_python/pull/3086#issuecomment-3765803662 for the initial report.
Locally rolling back https://github.com/bazel-contrib/rules_python/commit/a506d774dd60eedf6fce94dcccd2c233553b06d4 fixes the issue.
Back-porting the fix to https://github.com/bazel-contrib/rules_python/issues/3579 locally does not fix the issue.

Start with python/runfiles/runfiles.py, especially CurrentRepository and Rlocation, then inspect private/pypi/dependency_resolver/dependency_resolver.py and reproduce the failure from the linked Windows example. Done means compile_pip_requirements succeeds on GitHub Windows workers when the runfiles root and caller are on different drives, with coverage for that case.

Written by the indexing model from the issue text.