fix: support runfiles.CurrentRepository with non-hermetic sys.path (#1634)

When using Python 3.10 or earlier, the first `sys.path` entry is the
directory containing the script. This can result in modules being loaded
from the source root rather than the runfiles root, which the runfiles
library previously didn't support.

Fixes #1631

---------

Co-authored-by: Ignas Anikevicius <[email protected]>
Co-authored-by: Richard Levasseur <[email protected]>

Author: 3 people

CHANGELOG.md

@@ -67,6 +67,10 @@ A brief description of the categories of changes:
   cause warnings by default. In order to see the warnings for diagnostic purposes
   set the env var `RULES_PYTHON_REPO_DEBUG_VERBOSITY` to one of `INFO`, `DEBUG` or `TRACE`.
   Fixes [#1818](https://github.com/bazelbuild/rules_python/issues/1818).
+* (runfiles) Make runfiles lookups work for the situation of Bazel 7,
+  Python 3.9 (or earlier, where safepath isn't present), and the Rlocation call
+  in the same directory as the main file.
+  Fixes [#1631](https://github.com/bazelbuild/rules_python/issues/1631).
 
 ### Added
 * (rules) `PYTHONSAFEPATH` is inherited from the calling environment to allow

python/runfiles/runfiles.py

@@ -247,6 +247,20 @@ def CurrentRepository(self, frame: int = 1) -> str:
             raise ValueError("failed to determine caller's file path") from exc
         caller_runfiles_path = os.path.relpath(caller_path, self._python_runfiles_root)
         if caller_runfiles_path.startswith(".." + os.path.sep):
+            # With Python 3.10 and earlier, sys.path contains the directory
+            # of the script, which can result in a module being loaded from
+            # outside the runfiles tree. In this case, assume that the module is
+            # located in the main repository.
+            # With Python 3.11 and higher, the Python launcher sets
+            # PYTHONSAFEPATH, which prevents this behavior.
+            # TODO: This doesn't cover the case of a script being run from an
+            #       external repository, which could be heuristically detected
+            #       by parsing the script's path.
+            if (
+                sys.version_info.minor <= 10
+                and sys.path[0] != self._python_runfiles_root
+            ):
+                return ""
             raise ValueError(
                 "{} does not lie under the runfiles root {}".format(
                     caller_path, self._python_runfiles_root