fix: Fix up search path of bootstrapped Python toolchain dylib

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.

Since this needs macOS to be the host platform, restrict this change to
macOS host systems only by checking the host OS name.

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

Author: nicholasjng

CHANGELOG.md

@@ -31,6 +31,10 @@ A brief description of the categories of changes:
   It would work well in cases to reduce merge conflicts.
 
 ### 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",