Add buildifier composite action and a test workflow

.github/workflows/test-ci-buildifier.yaml

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+# A workflow to test buildifier composite action
+
+name: Test CI Buildifier Action
+# Run on pull_request
+on:
+  pull_request:
+    paths:
+    - ci_buildifier/**
+    - tests/ci_buildifier/**
+    - .github/workflows/test-ci-buildifier.yaml
+    branches:
+      - main
+defaults:
+  run:
+    shell: bash
+jobs:
+  run-buildifier:
+    runs-on: "linux-x86-n2-16"
+    container: "us-central1-docker.pkg.dev/tensorflow-sigs/tensorflow/ml-build:latest"
+    timeout-minutes: 60
+    if: |
+      github.event.sender.type == 'User' ||
+      contains(github.event.pull_request.body, 'RUN_BUILDIFIER')
+    steps:
+      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
+        with:
+          persist-credentials: false
+      - name: "Run buildifier action"
+        uses: ./ci_buildifier/

ci_buildifier/README.md

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+# CI Buildifier
+
+This composite action helps you maintain consistent formatting for your Bazel
+`BUILD` and `.bzl` files. It automatically runs `buildifier` on any modified
+files in your pull requests, ensuring code quality and adherence to style
+conventions. If buildifier detects any formatting violations, your workflow
+will fail, preventing unformatted code from being merged.
+
+## Resolving Formatting Failures
+If a workflow run fails due to formatting violations, you're expected to
+fix the issues locally. Simply run `buildifier` on the problematic
+files, e.g., using
+`buildifier -v <files>`,
+and then commit the formatted code to your pull request.
+
+## Requirements
+This action expects `buildifier` to be available in your workflow's runtime
+environment. If `buildifier` is not pre-installed in your chosen runner image
+or container, you will need to install it before this action runs.

ci_buildifier/action.yaml

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+# Copyright 2025 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.
+name: "Run buildifier"
+description: 'Action to run buildifier on changed build files'
+
+runs:
+  using: "composite"
+  steps:
+  - name: "Checking out repository"
+    uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
+    with:
+      fetch-depth: 0 # Fetch full history for accurate diffing
+  - name: Run buildifier check
+    id: buildifier-check
+    shell: bash
+    env:
+      TARGET_BRANCH: ${{ github.base_ref }}
+    run: |
+      FILE_PATTERNS="BUILD */BUILD *.bzl"
+
+      # Add this line to resolve the dubious git ownership error
+      git config --global --add safe.directory "$GITHUB_WORKSPACE"
+
+      # Compare PR head against the base branch to find changed files.
+      GIT_DIFF_CMD="git diff -z --name-only --diff-filter=d origin/$TARGET_BRANCH HEAD -- $FILE_PATTERNS"
+
+      $GIT_DIFF_CMD | xargs -0 buildifier --mode=check -v

test/ci_buildifier/BUILD

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+# This is a correctly formatted BUILD file.
+
+package(default_visibility = ["//visibility:public"])
+
+licenses(["notice"])  # Apache 2.0
+
+cc_library(
+    name = "my_library",
+    srcs = [
+        "my_library.cc",
+    ],
+    hdrs = [
+        "my_library.h",
+    ],
+    deps = [
+        "//third_party/some_lib",
+        "//util:common_utils",
+    ],
+)
+
+cc_binary(
+    name = "my_binary",
+    srcs = [
+        "main.cc",
+    ],
+    deps = [
+        ":my_library",
+        "//framework:base",
+    ],
+)
+
+py_library(
+    name = "py_utils",
+    srcs = ["py_utils.py"],
+    deps = [
+        "//other_python:dependencies",
+    ],
+)

test/ci_buildifier/subfolder/subfolder/BUILD

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+# This BUILD file contained intentional formatting errors
+# and is then formatted by running the buildifier locally.
+
+package(default_visibility = ["//visibility:public"])
+
+licenses(["notice"])  # Apache 2.0
+
+cc_library(
+    name = "my_library",
+    srcs = [
+        "my_library.cc",
+    ],
+    hdrs = [
+        "my_library.h",
+    ],
+    deps = [
+        "//third_party/some_lib",
+        "//util:common_utils",
+    ],
+)
+
+cc_binary(
+    name = "my_binary",
+    srcs = [
+        "main.cc",
+    ],
+    deps = [
+        ":my_library",
+        "//framework:base",
+    ],
+)

test/ci_buildifier/subfolder/test.bzl

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+"""
+This is a correctly formatted .bzl file for testing.
+It defines a simple custom rule.
+"""
+
+load("@bazel_skylib//lib:attrs.bzl", "attrs")
+load("@bazel_skylib//lib:collections.bzl", "dedup_list")
+load("//some/dependency:defs.bzl", "dependency_macro")
+
+def _my_custom_rule_impl(ctx):
+    """Implementation for my_custom_rule."""
+    input_file = ctx.files.src[0]
+    output_file = ctx.actions.declare_file(ctx.attr.name + ".out")
+
+    ctx.actions.run(
+        inputs = [input_file],
+        outputs = [output_file],
+        executable = "/usr/bin/some_tool",
+        arguments = [
+            "--input",
+            input_file.path,
+            "--output",
+            output_file.path,
+            "--verbose",
+        ],
+        mnemonic = "MyCustomRule",
+    )
+
+    dependency_macro(
+        name = ctx.attr.name + "_dep",
+        data = ctx.attr.data,
+    )
+
+    return [DefaultInfo(files = depset([output_file]))]
+
+my_custom_rule = rule(
+    implementation = _my_custom_rule_impl,
+    attrs = {
+        "data": attrs.list(
+            default = [],
+            allow_files = True,
+            doc = "Additional data files.",
+        ),
+        "src": attrs.label(
+            allow_single_file = True,
+            mandatory = True,
+            doc = "The primary source file.",
+        ),
+    },
+    doc = "A custom rule that processes a source file.",
+)
+
+def another_macro(name, value, some_list = []):
+    """Another example macro."""
+    print("Running another_macro with value: %s" % value)
+    dedup_list(some_list)