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

[phst]

🐞 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 "<frozen ntpath>", 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 #3086 (comment) for the initial report.
Locally rolling back a506d77 fixes the issue.
Back-porting the fix to #3579 locally does not fix the issue.