GH-44491: [C++] StatusConstant- cheaply copied const Status

[bkietz]

Rationale for this change

It'd be convenient to construct placeholder error Status cheaply.

What changes are included in this PR?

A constant error Status can now be constructed wrapped in a function like

Status UninitializedResult() {
  static StatusConstant uninitialized_result{StatusCode::UnknownError,
                                             "Uninitialized Result<T>"};
  return uninitialized_result;
}

Copying the constant status is relatively cheap (no heap interaction), so it's suitable for use as a placeholder error status.

Added bool Status::State::is_constant which causes copies to be shallow and skips destruction. Also Status::state_ is a const pointer now; this helps to ensure that it is obvious when we mutate state_ (as in AddContextLine).

Are these changes tested?

Minimal unit test added. The main consideration is probably benchmarks to make sure hot paths don't get much slower.

Are there any user-facing changes?

This API is not currently public; no user-facing changes.

• GitHub Issue: [C++] allocation free error Status #44491

[github-actions]

⚠️ GitHub issue #44491 has been automatically assigned in GitHub to PR creator.

[bkietz]

We could alternatively make StatusConstant a subclass of Status

[bkietz]

@pitrou @felipecrv which benchmark would be convincing as a demonstration this doesn't slow us down? I don't think we have an explicit status or result benchmark

For now (since it seems to me to have the deepest Status bubbling stack),

@ursabot please benchmark command=cpp-micro --suite-filter=grouper

[pitrou]

I don't think we have an explicit status or result benchmark

We do, but for some reason I had stuffed it in type_benchmark.cc

arrow/cpp/src/arrow/type_benchmark.cc

Lines 545 to 555 in 1b40800

	BENCHMARK(ErrorSchemeNoError);
	BENCHMARK(ErrorSchemeBool);
	BENCHMARK(ErrorSchemeStatus);
	BENCHMARK(ErrorSchemeResult);
	BENCHMARK(ErrorSchemeException);
	BENCHMARK(ErrorSchemeNoErrorNoInline);
	BENCHMARK(ErrorSchemeBoolNoInline);
	BENCHMARK(ErrorSchemeStatusNoInline);
	BENCHMARK(ErrorSchemeResultNoInline);
	BENCHMARK(ErrorSchemeExceptionNoInline);

[bkietz]

@ursabot please benchmark command=cpp-micro --suite-filter=type

[ursabot]

Benchmark runs are scheduled for commit f7d3fff. Watch https://buildkite.com/apache-arrow and https://conbench.ursa.dev for updates. A comment will be posted here when the runs are complete.

[conbench-apache-arrow]

Thanks for your patience. Conbench analyzed the 3 benchmarking runs that have been run so far on PR commit f7d3fff.

There were 4 benchmark results indicating a performance regression:

• Pull Request Run on amd64-m5-4xlarge-linux at 2024-10-25 16:10:52Z

  • ErrorSchemeBoolNoInline (C++) with source=cpp-micro, suite=arrow-type-benchmark

• Pull Request Run on amd64-c6a-4xlarge-linux at 2024-10-25 16:06:02Z

  • ErrorSchemeBoolNoInline (C++) with source=cpp-micro, suite=arrow-type-benchmark

• and 2 more (see the report linked below)

The full Conbench report has more details.

[felipecrv]

@pitrou @felipecrv which benchmark would be convincing as a demonstration this doesn't slow us down? I don't think we have an explicit status or result benchmark

Can you check the binary size impact with bloaty?

The performance impact is very tiny but everywhere a Status gets destroyed (all over the place). I think checking binary size is a good proxy to keeping the shape of the code the same.

[bkietz]

@ursabot please benchmark command=cpp-micro --suite-filter=type --benchmark-filter=Error

[ursabot]

Benchmark runs are scheduled for commit 86741d6. Watch https://buildkite.com/apache-arrow and https://conbench.ursa.dev for updates. A comment will be posted here when the runs are complete.

[conbench-apache-arrow]

Thanks for your patience. Conbench analyzed the 3 benchmarking runs that have been run so far on PR commit 86741d6.

There were 2 benchmark results indicating a performance regression:

• Pull Request Run on amd64-c6a-4xlarge-linux at 2024-11-04 18:08:18Z

  • ErrorSchemeBoolNoInline (C++) with source=cpp-micro, suite=arrow-type-benchmark

• Pull Request Run on amd64-m5-4xlarge-linux at 2024-11-04 18:12:34Z

  • ErrorSchemeExceptionNoInline (C++) with source=cpp-micro, suite=arrow-type-benchmark

The full Conbench report has more details.

[bkietz]

Those regressions have got to be flukes; they don't even touch Status

[pitrou]

Yes, the code changes probably trigger random variation in code placement in these benchmarks, that could explain the false regressions.

[pitrou]

@github-actions crossbow submit -g cpp

[pitrou]

Sorry for the delayed review and approval @bkietz . It's a nice improvement.

[pitrou]

(unrelated quick fix for CI compile error)

[github-actions]

Revision: be910af

Submitted crossbow builds: ursacomputing/crossbow @ actions-daf4798abf

Task	Status
example-cpp-minimal-build-static
example-cpp-minimal-build-static-system-dependency
example-cpp-tutorial
test-alpine-linux-cpp
test-build-cpp-fuzz
test-conda-cpp
test-conda-cpp-valgrind
test-cuda-cpp-ubuntu-20.04-cuda-11.2.2
test-cuda-cpp-ubuntu-22.04-cuda-11.7.1
test-debian-12-cpp-amd64
test-debian-12-cpp-i386
test-fedora-39-cpp
test-ubuntu-20.04-cpp
test-ubuntu-20.04-cpp-bundled
test-ubuntu-22.04-cpp
test-ubuntu-22.04-cpp-20
test-ubuntu-22.04-cpp-emscripten
test-ubuntu-22.04-cpp-no-threading
test-ubuntu-24.04-cpp
test-ubuntu-24.04-cpp-gcc-13-bundled
test-ubuntu-24.04-cpp-gcc-14
test-ubuntu-24.04-cpp-minimal-with-formats
test-ubuntu-24.04-cpp-thread-sanitizer

[pitrou]

CI failures are unrelated, I'll merge.

[conbench-apache-arrow]

After merging your PR, Conbench analyzed the 3 benchmarking runs that have been run so far on merge-commit 29e8ea0.

There were no benchmark performance regressions. 🎉

The full Conbench report has more details. It also includes information about 114 possible false positives for unstable benchmarks that are known to sometimes produce them.