[Python] Export only PyInit_* from macOS extension modules

Each extension statically links protobuf, abseil and cel-cpp. On macOS,
pybind_extension links with only `-undefined dynamic_lookup`, so those
copies are exported, including thousands of weak definitions. dyld
coalesces exported weak definitions across every loaded image, so
importing cel_expr_python alongside another extension that ships protobuf
(e.g. ortools) binds one runtime's code to the other's parse tables and
crashes in AddDescriptors at import time.

Wrap pybind_extension to add `-Wl,-exported_symbol,_PyInit_<name>` on
macOS, matching the isolation Linux already gets from `-Wl,-Bsymbolic`,
and add a macOS test that each extension exports only its init function.

Fixes #115

Author: mmattice

cel_expr_python/BUILD

@@ -1,6 +1,7 @@
-load("@pybind11_bazel//:build_defs.bzl", "pybind_extension", "pybind_library")
+load("@pybind11_bazel//:build_defs.bzl", "pybind_library")
 load("@rules_cc//cc:cc_library.bzl", "cc_library")
 load("@rules_python//python:py_test.bzl", "py_test")
+load(":pybind_extension.bzl", "pybind_extension")
 
 package(default_visibility = ["//visibility:public"])
 
@@ -209,6 +210,37 @@ py_test(
     }),
 )
 
+py_test(
+    name = "exported_symbols_test",
+    srcs = ["exported_symbols_test.py"],
+    data = [
+        ":cel",
+        "//cel_expr_python/ext:ext_bindings",
+        "//cel_expr_python/ext:ext_encoders",
+        "//cel_expr_python/ext:ext_lists",
+        "//cel_expr_python/ext:ext_math",
+        "//cel_expr_python/ext:ext_optional",
+        "//cel_expr_python/ext:ext_proto",
+        "//cel_expr_python/ext:ext_strings",
+    ],
+    deps = [
+        "@com_google_absl_py//absl/testing:absltest",
+        "@com_google_absl_py//absl/testing:parameterized",
+    ] + select({
+        "@platforms//os:windows": [],
+        "//conditions:default": [
+            ":cel",
+            "//cel_expr_python/ext:ext_bindings",
+            "//cel_expr_python/ext:ext_encoders",
+            "//cel_expr_python/ext:ext_lists",
+            "//cel_expr_python/ext:ext_math",
+            "//cel_expr_python/ext:ext_optional",
+            "//cel_expr_python/ext:ext_proto",
+            "//cel_expr_python/ext:ext_strings",
+        ],
+    }),
+)
+
 # Platform definition for macOS x86_64.
 # Used for cross-compiling from arm64 macOS hosts (Apple Silicon) to x86_64.
 # This is passed to Bazel via --platforms in setup.py when building x86_64 wheels.

cel_expr_python/exported_symbols_test.py

@@ -0,0 +1,66 @@
+# Copyright 2026 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Tests that macOS extension modules export only their init function.
+
+Exported copies of the statically linked protobuf and abseil runtimes are
+coalesced by dyld with any other copy loaded in the process (for example
+ortools'), which crashes at import time.
+"""
+
+import subprocess
+import sys
+import types
+
+from absl.testing import absltest
+from absl.testing import parameterized
+from cel_expr_python import cel
+from cel_expr_python.ext import ext_bindings
+from cel_expr_python.ext import ext_encoders
+from cel_expr_python.ext import ext_lists
+from cel_expr_python.ext import ext_math
+from cel_expr_python.ext import ext_optional
+from cel_expr_python.ext import ext_proto
+from cel_expr_python.ext import ext_strings
+
+_MODULES = (
+    cel,
+    ext_bindings,
+    ext_encoders,
+    ext_lists,
+    ext_math,
+    ext_optional,
+    ext_proto,
+    ext_strings,
+)
+
+
+@absltest.skipUnless(sys.platform == "darwin", "macOS-specific linkage")
+class ExportedSymbolsTest(parameterized.TestCase):
+
+  @parameterized.parameters(*_MODULES)
+  def test_exports_only_init_function(self, module: types.ModuleType):
+    output = subprocess.run(
+        ["nm", "-gU", module.__file__],
+        check=True,
+        capture_output=True,
+        text=True,
+    ).stdout
+    exported = {line.split()[-1] for line in output.splitlines() if line}
+    init_symbol = "_PyInit_" + module.__name__.rpartition(".")[2]
+    self.assertEqual(exported, {init_symbol})
+
+
+if __name__ == "__main__":
+  absltest.main()

cel_expr_python/ext/BUILD

@@ -1,4 +1,4 @@
-load("@pybind11_bazel//:build_defs.bzl", "pybind_extension")
+load("//cel_expr_python:pybind_extension.bzl", "pybind_extension")
 
 package(default_visibility = ["//visibility:public"])
 

cel_expr_python/pybind_extension.bzl

@@ -0,0 +1,40 @@
+# Copyright 2026 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""pybind_extension wrapper that keeps statically linked dependencies private."""
+
+load("@pybind11_bazel//:build_defs.bzl", _pybind_extension = "pybind_extension")
+
+def pybind_extension(name, linkopts = [], **kwargs):
+    """Builds a pybind11 extension module that exports only its init function.
+
+    Each extension statically links its own copy of protobuf, abseil and
+    cel-cpp. On macOS, dyld coalesces exported weak definitions across every
+    loaded image, so exporting those copies lets them bind to another
+    extension's protobuf or abseil (e.g. ortools') and crash at import time.
+    Linux is unaffected because pybind_extension links with -Bsymbolic there.
+
+    Args:
+      name: The name of the extension module.
+      linkopts: Additional linker options.
+      **kwargs: Passed through to pybind11_bazel's pybind_extension.
+    """
+    _pybind_extension(
+        name = name,
+        linkopts = linkopts + select({
+            "@platforms//os:osx": ["-Wl,-exported_symbol,_PyInit_" + name],
+            "//conditions:default": [],
+        }),
+        **kwargs
+    )

custom_ext/BUILD

@@ -1,6 +1,6 @@
-load("@pybind11_bazel//:build_defs.bzl", "pybind_extension")
 load("@rules_python//python:py_library.bzl", "py_library")
 load("@rules_python//python:py_test.bzl", "py_test")
+load("//cel_expr_python:pybind_extension.bzl", "pybind_extension")
 
 package(default_visibility = ["//visibility:private"])