fix(lint): stream Rust build stdout/stderr + replace deprecated os.system in ci/

[zackees]

Summary

Two related lint-infrastructure fixes surfaced while running bash lint on a fresh worktree, both of which were silently degrading the developer signal.

1. ty (Python type-checker) — 6 sites using deprecated os.system(...)

Python's runtime marks os.system "soft deprecated — use the subprocess module instead." Each site replaced with subprocess.run([...], check=False) using argument-list form (safer against path-with-spaces injection than the prior f-string shell command):

File	What it was invoking
ci/util/create_build_dir.py	attrib -r <path> (Windows readonly clear)
ci/util/map_dump.py	uv run fpvgcc ... (six variants)
ci/util/scrapers/scrape_festival_stick.py	playwright install chromium
ci/wasm_audio_drag_drop_test.py	playwright install chromium
ci/wasm_audio_url_test.py	playwright install chromium
ci/wasm_test.py	playwright install chromium

Result: ty was 6 diagnostics, now 0. python_pipeline was FAILED, now completes.

2. Rust C++ linter — stream cargo output so silent-fails are diagnosable

ci/lint_cpp/rust_binary_cache.py::_run_build used subprocess.run(cmd, cwd=..., check=False) which inherits the parent's stdout/stderr. When the build is invoked from inside a concurrent.futures.ThreadPoolExecutor (which run_all_checkers.py does) and the parent's file descriptors are being indirectly captured by a test runner, logging wrapper, or CI harness, cargo's diagnostics disappear. On a Windows worktree where soldr silently returns exit-code 0 without building the binary, the resulting RuntimeError gave the user zero signal about why the build failed.

Fix:

• Switch to subprocess.Popen with stdout=PIPE / stderr=STDOUT and a line-by-line reader loop that prefixes every cargo line with [cargo] and prints it to sys.stderr with flush=True.
• Log the resolved soldr command + cwd before the build starts so the user sees exactly what will be invoked.
• After the fixed-candidate paths miss in _binary_path, fall back to target.rglob(<name>) so a non-standard cargo output location (e.g. an unpredicted CARGO_TARGET_DIR or a triple our _guess_host_triple heuristic missed) still resolves.
• When the binary genuinely is missing, the error message now shows (a) the checked candidates, (b) what the recursive walk actually found, and (c) points the user at the [cargo] output stream for diagnosis.

Before / after (fresh worktree, from bash lint)

Before:

❌ Unified C++ linter failed
❌ ty failed          (6 deprecation warnings)
❌ python_pipeline FAILED
❌ cpp_linting FAILED

After:

✅ ruff passed
✅ ty passed         (0 diagnostics)
✅ python_pipeline completed
❌ cpp_linting FAILED   ← still fails on this Windows box, but now the
                          soldr silent-fail is visible in the streamed
                          [cargo] output. On healthy Linux/macOS CI
                          runners where soldr works, this stays green.

The remaining cpp_linting failure is a soldr Windows infrastructure issue that FastLED cannot fix — soldr executes with exit 0 but produces no cargo output and no binary. The improved diagnostics + fallback binary walk are what FastLED can contribute to make that failure mode actionable rather than opaque.

Test plan

• bash lint — python_pipeline now completes; ty passes with 0 diagnostics; C++ lint failure is now visibly diagnosable
• All 6 os.system sites verified to use argument-list form (no shell injection risk)
• ci/lint_cpp/rust_binary_cache.py — no other call sites use subprocess.run(cmd) with the same swallowed-output pattern

Co-Authored-By: Claude Opus 4.7 (1M context) noreply@anthropic.com

Generated with Claude Code

Summary by CodeRabbit

• Bug Fixes
  • Improved CI and test tooling so browser setup, build steps, and cleanup commands run more reliably across environments.
  • Build output is now streamed live, making it easier to see progress and diagnose failures during long-running checks.
  • If a required binary is missing, error messages now include clearer details about what was searched and what was found.

[coderabbitai]

Caution

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📥 Commits

Reviewing files that changed from the base of the PR and between 5309dd7 and 0afc0c4.

📒 Files selected for processing (7)

• ci/lint_cpp/rust_binary_cache.py
• ci/util/create_build_dir.py
• ci/util/map_dump.py
• ci/util/scrapers/scrape_festival_stick.py
• ci/wasm_audio_drag_drop_test.py
• ci/wasm_audio_url_test.py
• ci/wasm_test.py

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📝 Walkthrough

Walkthrough

This PR migrates several CI scripts from os.system to subprocess.run/subprocess.Popen for command execution, and enhances rust_binary_cache.py with a recursive target directory search fallback, streamed cargo build output, and expanded error diagnostics for missing lint binaries.

Changes

Rust lint binary caching

Layer / File(s)	Summary
Binary path resolution with recursive fallback ci/lint_cpp/rust_binary_cache.py	_binary_path() performs a recursive target/ search for the binary when deterministic candidates are missing, before falling back to the default candidate.
Streaming build execution ci/lint_cpp/rust_binary_cache.py	_run_build() switches from subprocess.run to Popen, streaming and prefixing merged stdout/stderr cargo output line-by-line, flushing prints, and raising an updated RuntimeError on failure.
Expanded missing-binary diagnostics ci/lint_cpp/rust_binary_cache.py	ensure_rust_lint_binary() error messages now list checked candidate paths and recursive target/ walk results.

CI utility and test subprocess migration

Layer / File(s)	Summary
Windows readonly clearing ci/util/create_build_dir.py	remove_readonly() uses subprocess.run(["attrib", "-r", path], check=False) instead of os.system.
map_dump command execution ci/util/map_dump.py	Commands are built as argument lists and run via subprocess.run(cmd, check=False), replacing shell-string os.system calls; imports updated.
Playwright browser install ci/util/scrapers/scrape_festival_stick.py, ci/wasm_audio_drag_drop_test.py, ci/wasm_audio_url_test.py, ci/wasm_test.py	Chromium installation switches from os.system to subprocess.run(..., check=False), with subprocess imports added.

Estimated code review effort: 2 (Simple) | ~15 minutes

Possibly related PRs

• FastLED/FastLED#3335: Modifies the same rust_binary_cache.py binary locating/building logic (candidate/target path search and Cargo build invocation flow).

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/lint-infra-windows-rust-and-ty

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