Add buildifier as a BUILD file formatter

src/python/pants/backend/build_files/fmt/buildifier/BUILD

@@ -0,0 +1,6 @@
+# Copyright 2022 Pants project contributors (see CONTRIBUTORS.md).
+# Licensed under the Apache License, Version 2.0 (see LICENSE).
+
+python_sources()
+
+python_tests(name="tests")

src/python/pants/backend/build_files/fmt/buildifier/register.py

@@ -0,0 +1,10 @@
+# Copyright 2022 Pants project contributors (see CONTRIBUTORS.md).
+# Licensed under the Apache License, Version 2.0 (see LICENSE).
+
+from pants.backend.build_files.fmt.buildifier import rules as buildifier_rules
+
+
+def rules():
+    return [
+        *buildifier_rules.rules(),
+    ]

src/python/pants/backend/build_files/fmt/buildifier/rules.py

@@ -0,0 +1,48 @@
+# Copyright 2022 Pants project contributors (see CONTRIBUTORS.md).
+# Licensed under the Apache License, Version 2.0 (see LICENSE).
+
+from pants.backend.build_files.fmt.buildifier.subsystem import Buildifier
+from pants.core.goals.fmt import FmtResult, _FmtBuildFilesRequest
+from pants.core.util_rules.external_tool import DownloadedExternalTool, ExternalToolRequest
+from pants.engine.internals.native_engine import Digest, MergeDigests, Snapshot
+from pants.engine.internals.selectors import Get
+from pants.engine.platform import Platform
+from pants.engine.process import Process, ProcessResult
+from pants.engine.rules import collect_rules, rule
+from pants.engine.unions import UnionRule
+from pants.util.logging import LogLevel
+from pants.util.strutil import pluralize
+
+
+class BuildifierRequest(_FmtBuildFilesRequest):
+    name = "buildifier"
+
+
+@rule(desc="Format with Buildifier", level=LogLevel.DEBUG)
+async def buildfier_fmt(request: BuildifierRequest, buildifier: Buildifier) -> FmtResult:
+    buildifier_tool = await Get(
+        DownloadedExternalTool, ExternalToolRequest, buildifier.get_request(Platform.current)
+    )
+    input_digest = await Get(
+        Digest,
+        MergeDigests((request.snapshot.digest, buildifier_tool.digest)),
+    )
+    result = await Get(
+        ProcessResult,
+        Process(
+            argv=[buildifier_tool.exe, "-type=build", *request.snapshot.files],
+            input_digest=input_digest,
+            output_files=request.snapshot.files,
+            description=f"Run buildifier on {pluralize(len(request.snapshot.files), 'file')}.",
+            level=LogLevel.DEBUG,
+        ),
+    )
+    output_snapshot = await Get(Snapshot, Digest, result.output_digest)
+    return FmtResult.create(request, result, output_snapshot)
+
+
+def rules():
+    return [
+        *collect_rules(),
+        UnionRule(_FmtBuildFilesRequest, BuildifierRequest),
+    ]

src/python/pants/backend/build_files/fmt/buildifier/rules_integration_test.py

@@ -0,0 +1,97 @@
+# Copyright 2022 Pants project contributors (see CONTRIBUTORS.md).
+# Licensed under the Apache License, Version 2.0 (see LICENSE).
+
+from __future__ import annotations
+
+from textwrap import dedent
+
+import pytest
+
+from pants.backend.build_files.fmt.buildifier.rules import BuildifierRequest
+from pants.backend.build_files.fmt.buildifier.rules import rules as buildifier_rules
+from pants.backend.codegen.protobuf.target_types import rules as target_types_rules
+from pants.core.goals.fmt import FmtResult
+from pants.core.util_rules import external_tool
+from pants.engine.fs import CreateDigest, FileContent, PathGlobs
+from pants.engine.internals.native_engine import Digest, Snapshot
+from pants.testutil.rule_runner import QueryRule, RuleRunner
+
+
+class Materials:
+    def __init__(self, **kwargs):
+        pass
+
+
+@pytest.fixture
+def rule_runner() -> RuleRunner:
+    return RuleRunner(
+        rules=[
+            *buildifier_rules(),
+            *external_tool.rules(),
+            *target_types_rules(),
+            QueryRule(FmtResult, [BuildifierRequest]),
+        ],
+        # NB: Objects are easier to test with
+        objects={"materials": Materials},
+    )
+
+
+GOOD_FILE = dedent(
+    """\
+    materials(
+        drywall = 40,
+        status = "paid",
+        studs = 200,
+    )
+    """
+)
+
+BAD_FILE = dedent(
+    """\
+    materials(status='paid', studs=200, drywall=40)
+    """
+)
+
+
+def run_buildifier(rule_runner: RuleRunner) -> FmtResult:
+    rule_runner.set_options(
+        ["--backend-packages=pants.backend.build_files.fmt.buildifier"],
+        env_inherit={"PATH", "PYENV_ROOT"},
+    )
+    snapshot = rule_runner.request(Snapshot, [PathGlobs(["**/BUILD"])])
+    fmt_result = rule_runner.request(FmtResult, [BuildifierRequest(snapshot)])
+    return fmt_result
+
+
+def get_snapshot(rule_runner: RuleRunner, source_files: dict[str, str]) -> Snapshot:
+    files = [FileContent(path, content.encode()) for path, content in source_files.items()]
+    digest = rule_runner.request(Digest, [CreateDigest(files)])
+    return rule_runner.request(Snapshot, [digest])
+
+
+def test_passing(rule_runner: RuleRunner) -> None:
+    rule_runner.write_files({"BUILD": GOOD_FILE})
+    fmt_result = run_buildifier(rule_runner)
+    assert fmt_result.output == get_snapshot(rule_runner, {"BUILD": GOOD_FILE})
+    assert fmt_result.did_change is False
+
+
+def test_failing(rule_runner: RuleRunner) -> None:
+    rule_runner.write_files({"BUILD": BAD_FILE})
+    fmt_result = run_buildifier(rule_runner)
+    assert fmt_result.output == get_snapshot(rule_runner, {"BUILD": GOOD_FILE})
+    assert fmt_result.did_change is True
+
+
+def test_multiple_files(rule_runner: RuleRunner) -> None:
+    rule_runner.write_files(
+        {
+            "good/BUILD": GOOD_FILE,
+            "bad/BUILD": BAD_FILE,
+        }
+    )
+    fmt_result = run_buildifier(rule_runner)
+    assert fmt_result.output == get_snapshot(
+        rule_runner, {"good/BUILD": GOOD_FILE, "bad/BUILD": GOOD_FILE}
+    )
+    assert fmt_result.did_change is True

src/python/pants/backend/build_files/fmt/buildifier/subsystem.py

@@ -0,0 +1,46 @@
+# Copyright 2022 Pants project contributors (see CONTRIBUTORS.md).
+# Licensed under the Apache License, Version 2.0 (see LICENSE).
+
+from pants.core.util_rules.external_tool import TemplatedExternalTool
+from pants.option.option_types import ArgsListOption, SkipOption
+from pants.util.strutil import softwrap
+
+
+class Buildifier(TemplatedExternalTool):
+    options_scope = "buildifier"
+    name = "Buildifier"
+    help = softwrap(
+        """
+        Buildifier is a tool for formatting BUILD files with a standard convention.
+
+        Pants supports running Buildifier on your Pants BUILD files for several reasons:
+          - You might like the style that buildifier uses.
+          - You might be incrementally adopting Pants from Bazel, and are already using buildifier.
+
+        Please note that there are differences from Bazel's BUILD files (which are Starlark) and
+        Pants' BUILD files (which are Python), so buildifier may issue a syntax error.
+        In practice, these errors should be rare. See https://bazel.build/rules/language#differences_with_python.
+        """
+    )
+
+    default_version = "5.1.0"
+    default_known_versions = [
+        "5.1.0|macos_x86_64|c9378d9f4293fc38ec54a08fbc74e7a9d28914dae6891334401e59f38f6e65dc|7125968",
+        "5.1.0|macos_arm64 |745feb5ea96cb6ff39a76b2821c57591fd70b528325562486d47b5d08900e2e4|7334498",
+        "5.1.0|linux_x86_64|52bf6b102cb4f88464e197caac06d69793fa2b05f5ad50a7e7bf6fbd656648a3|7226100",
+        "5.1.0|linux_arm64 |917d599dbb040e63ae7a7e1adb710d2057811902fdc9e35cce925ebfd966eeb8|7171938",
+    ]
+    default_url_template = (
+        "https://github.com/bazelbuild/buildtools/releases/download/{version}/buildifier-{platform}"
+    )
+    default_url_platform_mapping = {
+        "macos_arm64": "darwin-arm64",
+        "macos_x86_64": "darwin-amd64",
+        "linux_arm64": "linux-arm64",
+        "linux_x86_64": "linux-amd64",
+    }
+
+    skip = SkipOption("fmt")
+    args = ArgsListOption(example="-lint=fix")
+
+    # NB: buildifier doesn't (yet) support config files https://github.com/bazelbuild/buildtools/issues/479

src/python/pants/bin/BUILD

@@ -13,6 +13,7 @@ python_sources(
 target(
     name="plugins",
     dependencies=[
+        "src/python/pants/backend/build_files/fmt/buildifier",
         "src/python/pants/backend/awslambda/python",
         "src/python/pants/backend/codegen/protobuf/lint/buf",
         "src/python/pants/backend/codegen/protobuf/python",