fix(cc): include toolchain libraries in cc_shared_library (#4166)

`current_py_cc_libs` forwards linker inputs with the Python toolchain
library's owner labels. `cc_shared_library` does not discover those
labels through the toolchain, so it silently omits the static libraries.
Return `CcSharedLibraryHintInfo` with the forwarded owner labels.

Validation: added a `rules_testing` regression with a static library
supplied through `py_cc_toolchain`. It fails before the fix; all three
`current_py_cc_libs` analysis tests pass afterward on Bazel 9.1.1.

news/current-py-cc-libs-shared-library.fixed.md

diff --git a/news/current-py-cc-libs-shared-library.fixed.md b/news/current-py-cc-libs-shared-library.fixed.md
new file mode 100644
index 0000000..9cb1229
--- /dev/null
+++ b/news/current-py-cc-libs-shared-library.fixed.md
@@ -0,0 +1,2 @@
+Include the Python toolchain's static libraries when `current_py_cc_libs` is used
+as a dependency of `cc_shared_library`.

python/private/current_py_cc_libs.bzl

diff --git a/python/private/current_py_cc_libs.bzl b/python/private/current_py_cc_libs.bzl
index ca68346..da4f8d5 100644
--- a/python/private/current_py_cc_libs.bzl
+++ b/python/private/current_py_cc_libs.bzl
@@ -15,15 +15,24 @@
 """Implementation of current_py_cc_libs rule."""
 
 load("@rules_cc//cc/common:cc_info.bzl", "CcInfo")
+load("@rules_cc//cc/common:cc_shared_library_hint_info.bzl", "CcSharedLibraryHintInfo")
 
 def _current_py_cc_libs_impl(ctx):
     py_cc_toolchain = ctx.toolchains["//python/cc:toolchain_type"].py_cc_toolchain
-    return py_cc_toolchain.libs.providers_map.values()
+    providers = py_cc_toolchain.libs.providers_map
+    owners = {
+        linker_input.owner: None
+        for linker_input in providers["CcInfo"].linking_context.linker_inputs.to_list()
+    }
+    return providers.values() + [CcSharedLibraryHintInfo(
+        attributes = [],
+        owners = owners.keys() or [ctx.label],
+    )]
 
 current_py_cc_libs = rule(
     implementation = _current_py_cc_libs_impl,
     toolchains = ["//python/cc:toolchain_type"],
-    provides = [CcInfo],
+    provides = [CcInfo, CcSharedLibraryHintInfo],
     doc = """\
 Provides the currently active Python toolchain's C libraries.
 
@@ -39,5 +48,9 @@
     deps = ["@rules_python//python/cc:current_py_cc_libs"]
 )
 ```
+
+:::{versionchanged} VERSION_NEXT_PATCH
+Static libraries are included when this target is a dependency of `cc_shared_library`.
+:::
 """,
 )

tests/cc/current_py_cc_libs/current_py_cc_libs_tests.bzl

diff --git a/tests/cc/current_py_cc_libs/current_py_cc_libs_tests.bzl b/tests/cc/current_py_cc_libs/current_py_cc_libs_tests.bzl
index 26f9724..756f82a 100644
--- a/tests/cc/current_py_cc_libs/current_py_cc_libs_tests.bzl
+++ b/tests/cc/current_py_cc_libs/current_py_cc_libs_tests.bzl
@@ -14,9 +14,14 @@
 
 """Tests for current_py_cc_libs."""
 
+load("@rules_cc//cc:cc_library.bzl", "cc_library")
+load("@rules_cc//cc:cc_shared_library.bzl", "cc_shared_library")
 load("@rules_cc//cc/common:cc_info.bzl", "CcInfo")
+load("@rules_cc//cc/common:cc_shared_library_info.bzl", "CcSharedLibraryInfo")
 load("@rules_testing//lib:analysis_test.bzl", "analysis_test", "test_suite")
 load("@rules_testing//lib:truth.bzl", "matching")
+load("@rules_testing//lib:util.bzl", "util")
+load("//python/cc:py_cc_toolchain.bzl", "py_cc_toolchain")
 load("//tests/support:cc_info_subject.bzl", "cc_info_subject")
 
 _tests = []
@@ -71,6 +76,51 @@
 
 _tests.append(_test_toolchain_is_registered_by_default)
 
+def _test_shared_library(name):
+    util.helper_target(
+        cc_library,
+        name = name + ".libpython",
+        srcs = ["shared_library.c"],
+    )
+    util.helper_target(
+        py_cc_toolchain,
+        name = name + ".py_cc_toolchain",
+        headers = "//tests/support/cc_toolchains:py_headers",
+        libs = ":" + name + ".libpython",
+        python_version = "3.999",
+    )
+    util.helper_target(
+        native.toolchain,
+        name = name + ".toolchain",
+        toolchain = ":" + name + ".py_cc_toolchain",
+        toolchain_type = "//python/cc:toolchain_type",
+    )
+    util.helper_target(
+        cc_shared_library,
+        name = name + ".shared",
+        deps = ["//python/cc:current_py_cc_libs"],
+    )
+    analysis_test(
+        name = name,
+        impl = _test_shared_library_impl,
+        target = name + ".shared",
+        config_settings = {
+            # This transition replaces the C++ toolchain supplied by RBE.
+            "//command_line_option:extra_toolchains": [
+                str(native.package_relative_label(":" + name + ".toolchain")),
+                str(Label("//tests/support/cc_toolchains:linux_toolchain_definition")),
+                str(Label("//tests/support/cc_toolchains:mac_toolchain_definition")),
+                str(Label("//tests/support/cc_toolchains:windows_toolchain_definition")),
+            ],
+        },
+    )
+
+def _test_shared_library_impl(env, target):
+    libpython = target.label.same_package_label(target.label.name.removesuffix(".shared") + ".libpython")
+    env.expect.that_collection(target[CcSharedLibraryInfo].link_once_static_libs).contains(str(libpython))
+
+_tests.append(_test_shared_library)
+
 def current_py_cc_libs_test_suite(name):
     test_suite(
         name = name,

tests/cc/current_py_cc_libs/shared_library.c

diff --git a/tests/cc/current_py_cc_libs/shared_library.c b/tests/cc/current_py_cc_libs/shared_library.c
new file mode 100644
index 0000000..cea2d11
--- /dev/null
+++ b/tests/cc/current_py_cc_libs/shared_library.c
@@ -0,0 +1 @@
+int python_library_symbol(void) { return 0; }