command propagates RUNFILES_MANIFEST_FILE to child binaries, breaking targets with their own runfiles trees

[manuelnaranjo]

When a command wraps a binary that performs its own runfiles discovery (e.g. a py_binary from aspect_rules_py), the child binary fails to locate files in its own runfiles tree because it inherits RUNFILES_MANIFEST_FILE from the parent multirun/command process.

I created a repro repository here

The outputs are:

$ bazel run :command-that-fails
foo.__main__ loaded
RUNFILES_MANIFEST_FILE: /home/mnaranjo/.cache/bazel/_bazel_mnaranjo/738fc0b40850cb253097925d8e75c2d7/execroot/_main/bazel-out/k8-fastbuild/bin/command-that-fails.bash.runfiles_manifest
Traceback (most recent call last):
  File "/usr/local/git_tree/multirun-aspect_rules_py-repro/foo/__main__.py", line 7, in <module>
    from bar import baz
ModuleNotFoundError: No module named 'bar'

$ bazel run :command-that-works
foo.__main__ loaded
RUNFILES_MANIFEST_FILE: 
bar.baz loaded

Not sure if this is worth to fix in rules_multirun or if it's actually something to fix in aspect_rules_py or even the hermetic_launcher but I think it's worth to share with others specially given that the workaround is quite simple:

  command(
      name = "my-command",
      command = ":my-tool",
      environment = {
          "RUNFILES_MANIFEST_FILE": "",  # actual workaround
      },
  )

Note: clearing RUNFILES_DIR as well is not viable — the rules_shell wrapper that command uses internally relies on it to locate the child binary.