fix: Fix up search path of bootstrapped Python toolchain dylib (bazel-contrib#2089)

Previously, `//tests/cc/current_py_cc_libs::python_libs_linking_test`
failed on macOS because the bootstrapped toolchain's dylib had an
incorrect LC_ID_DYLIB field set, pointing to a local directory on the
Python standalone build host machine.

To fix, add a small conditional to the Python repository rule patching
the LC_ID_DYLIB field of the bootstrapped Python's dylib with its fully
qualified file system path. Patching is carried out with macOS's own
`install_name_tool`, which is part of the standard macOS dynamic linking
toolchain.

Qualifies the mentioned test as executable on Mac, now only Windows
linker errors are left to fix.

Author: nicholasjng

CHANGELOG.md

@@ -35,6 +35,10 @@ A brief description of the categories of changes:
     * `3.12 -> 3.12.4`
 
 ### Fixed
+* (rules) Fixes build targets linking against `@rules_python//python/cc:current_py_cc_libs`
+  in host platform builds on macOS, by editing the `LC_ID_DYLIB` field of the hermetic interpreter's
+  `libpython3.x.dylib` using `install_name_tool`, setting it to its absolute path under Bazel's
+  execroot.
 * (rules) Signals are properly received when using {obj}`--bootstrap_impl=script`
   (for non-zip builds).
   ([#2043](https://github.com/bazelbuild/rules_python/issues/2043))

python/private/python_repositories.bzl

@@ -191,6 +191,22 @@ def _python_repository_impl(rctx):
     elif rctx.attr.distutils_content:
         rctx.file(distutils_path, rctx.attr.distutils_content)
 
+    if "darwin" in platform and "osx" == repo_utils.get_platforms_os_name(rctx):
+        # Fix up the Python distribution's LC_ID_DYLIB field.
+        # It points to a build directory local to the GitHub Actions
+        # host machine used in the Python standalone build, which causes
+        # dyld lookup errors. To fix, set the full path to the dylib as
+        # it appears in the Bazel workspace as its LC_ID_DYLIB using
+        # the `install_name_tool` bundled with macOS.
+        dylib = "lib/libpython{}.dylib".format(python_short_version)
+        full_dylib_path = rctx.path(dylib)
+        repo_utils.execute_checked(
+            rctx,
+            op = "python_repository.FixUpDyldIdPath",
+            arguments = [repo_utils.which_checked(rctx, "install_name_tool"), "-id", full_dylib_path, dylib],
+            logger = logger,
+        )
+
     # Make the Python installation read-only. This is to prevent issues due to
     # pycs being generated at runtime:
     # * The pycs are not deterministic (they contain timestamps)

tests/cc/current_py_cc_libs/BUILD.bazel

@@ -20,12 +20,14 @@ current_py_cc_libs_test_suite(name = "current_py_cc_libs_tests")
 cc_test(
     name = "python_libs_linking_test",
     srcs = ["python_libs_linking_test.cc"],
-    # Mac and Windows fail with linking errors, but its not clear why; someone
-    # with more C + Mac/Windows experience will have to figure it out.
+    # Windows fails with linking errors, but its not clear why; someone
+    # with more C + Windows experience will have to figure it out.
     # - rickeylev@
-    target_compatible_with = [
-        "@platforms//os:linux",
-    ],
+    target_compatible_with = select({
+        "@platforms//os:linux": [],
+        "@platforms//os:osx": [],
+        "//conditions:default": ["@platforms//:incompatible"],
+    }),
     deps = [
         "@rules_python//python/cc:current_py_cc_headers",
         "@rules_python//python/cc:current_py_cc_libs",