Improve MainThreadCheckerLibsCopy performance (#2599)

apple/internal/partials/main_thread_checker_dylibs.bzl

@@ -56,10 +56,7 @@ def _run_main_thread_checker(
     apple_support.run(
         actions = actions,
         apple_fragment = platform_prerequisites.apple_fragment,
-        arguments = [
-            binary_artifact.path,
-            main_thread_checker_dylib.path,
-        ],
+        arguments = [main_thread_checker_dylib.path],
         executable = main_thread_checker_tool,
         execution_requirements = {"no-sandbox": "1"},
         inputs = [binary_artifact] + dylibs,

tools/main_thread_checker_tool/BUILD

@@ -1,10 +1,8 @@
 licenses(["notice"])
 
-py_binary(
+sh_binary(
     name = "main_thread_checker_tool",
-    srcs = ["main_thread_checker_tool.py"],
-    python_version = "PY3",
-    srcs_version = "PY3",
+    srcs = ["main_thread_checker_tool.sh"],
     visibility = [
         "//apple/internal:__pkg__",
     ],

tools/main_thread_checker_tool/main_thread_checker_tool.sh

@@ -0,0 +1,36 @@
+# Copyright 2023 The Bazel Authors. All rights reserved.
+#
+# 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.
+
+# A tool that gets the libMainThreadChecker.dylib from the `DEVELOPER_DIR` and
+# copies it to the requested output path for bundling.
+
+set -euo pipefail
+
+output_path=$1
+developer_dir="${DEVELOPER_DIR:-}"
+
+if [[ -z "${developer_dir}" ]]; then
+  echo "DEVELOPER_DIR is not set, unable to find libMainThreadChecker.dylib"
+  exit 1
+fi
+
+# Xcode 15+ example: /Applications/Xcode.15.0.0.15A240d.app/Contents/Developer/usr/lib/libMainThreadChecker.dylib
+lib_path="${developer_dir}/usr/lib/libMainThreadChecker.dylib"
+
+if [[ ! -f "${lib_path}" ]]; then
+  echo "libMainThreadChecker.dylib not found at: ${lib_path}"
+  exit 1
+fi
+
+cp "${lib_path}" "${output_path}"