fix(bzlmod): keep the lockfile platform independent when resolving python

Before this PR the lockfile would become platform dependent when the
`requirements` file would have env markers. This was not caught because
we do not have MODULE.bazel.lock checked into the `rules_python`
repository because the CI is running against many versions and the lock
file is different, therefor we would not be able to run with
`bazel build --lockfile_mode=error`.

With this change we use the label to `BUILD.bazel` which is living next
to the `python` symlink and since the `BUILD.bazel` is the same on all
platforms, the lockfile will remain the same.

Work towards bazel-contrib#1105, bazel-contrib#1868.

Author: aignas

CHANGELOG.md

@@ -33,6 +33,11 @@ A brief description of the categories of changes:
 ### Removed
 * Nothing yet
 
+### Fixed
+* (bzlmod) get the path to the host python interpreter without calling
+  `mctx.path` on Labels that have differing contents on different OSes.
+* (bzlmod) correctly watch sources when using `pypi_repo_utils`.
+
 ## [0.35.0] - 2024-08-15
 
 [0.35.0]: https://github.com/bazelbuild/rules_python/releases/tag/0.35.0

examples/bzlmod/MODULE.bazel.lock

@@ -51,7 +51,20 @@ def _resolve_python_interpreter(mrctx, *, python_interpreter = None, python_inte
     python_interpreter = _get_python_interpreter_attr(mrctx, python_interpreter = python_interpreter)
 
     if python_interpreter_target != None:
-        python_interpreter = mrctx.path(python_interpreter_target)
+        # The following line would make the MODULE.bazel.lock platform
+        # independent, because the lock file will then contain a hash of the
+        # file so that the lock file can be recalculated, hence the best way is
+        # to add this directory to PATH.
+        #
+        # hence we add the root BUILD.bazel file and get the directory of that
+        # and construct the path differently. At the end of the day we don't
+        # want the hash of the interpreter to end up in the lock file.
+        if hasattr(python_interpreter_target, "same_package_label"):
+            root_build_bazel = python_interpreter_target.same_package_label("BUILD.bazel")
+        else:
+            root_build_bazel = python_interpreter_target.relative(":BUILD.bazel")
+
+        python_interpreter = mrctx.path(root_build_bazel).dirname.get_child(python_interpreter_target.name)
 
         os = repo_utils.get_platforms_os_name(mrctx)
 
@@ -110,7 +123,7 @@ def _execute_checked(mrctx, *, srcs, **kwargs):
         # This will ensure that we will re-evaluate the bzlmod extension or
         # refetch the repository_rule when the srcs change. This should work on
         # Bazel versions without `mrctx.watch` as well.
-        repo_utils.watch(mrctx.path(src))
+        repo_utils.watch(mrctx, mrctx.path(src))
 
     env = kwargs.pop("environment", {})
     pythonpath = env.get("PYTHONPATH", "")