feat(py): use the sysmon coverage core on Python 3.12+ by xangcastle · Pull Request #1534 · aspect-build/rules_py

xangcastle · 2026-09-08T19:38:04Z

What

start_coverage() now drives coveragepy through a per-process rcfile instead of constructor kwargs, which lets it:

select core = sysmon (the sys.monitoring / PEP 669 backend) on Python 3.12+, falling back to coveragepy's default core on older interpreters;
give every test process its own data_file under TEST_TMPDIR, so shards or chdir tests running concurrently out of one directory no longer race on the default .coverage database.

write_lcov() additionally tolerates coverage.exceptions.NoSource alongside NoDataError, writing an empty LCOV rather than failing an otherwise passing test.

Why

The sysmon core is markedly cheaper than the C tracer under bazel coverage. In a large monorepo consuming these rules the same change (carried as a local patch against the pre-launcher_env template) cut wall-clock of instrumented py_pytest_test targets by roughly a quarter to a third relative to the default core, across tests ranging from a few seconds to half a minute.

coveragepy only exposes the core choice through configuration ([run] core), hence the rcfile.

Notes for review

The NoSource tolerance mirrors the downstream patch. It is debatable upstream since it can hide a genuinely broken manifest; happy to drop it into a separate PR if preferred.
e2e/cases/coverage-drivers runs on 3.10, 3.11 and 3.12 and should cover both branches of the version gate. Not run locally yet; relying on CI.
sysmon needs coveragepy >= 7.4.0; the e2e lock pins 7.15.2.

coderabbitai · 2026-09-08T19:38:14Z

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aspect-workflows · 2026-09-08T19:38:31Z

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github-actions · 2026-09-08T19:44:45Z

py_binary startup benchmark

Version	Mean (ms)	Median (ms)	± stddev	vs BCR	vs main	Build (s)
BCR 1.11.7 (baseline)	160.804	160.453	±1.741	—	—	1.67
HEAD main	57.238	57.286	±0.672	-64.3%	—	1.85
This PR	57.512	57.427	±2.045	-64.2%	+0.2%	1.64

Measured with hyperfine --warmup 5 --runs 50 --shell=none on Linux
Gate: PR vs HEAD main median (threshold: 10%, and must exceed the 2×SE noise floor, here 1.1%). BCR is shown only as a historical baseline.
Build time: cold bazel build //:bench with isolated output base, no disk cache; external repos prefetched so network is excluded.

sys.path quality

Version	sys.path entries	distinct site-packages roots
BCR 1.11.7 (baseline)	6	1
HEAD main	7	2
This PR	7	2

sys.path quality measured by bench_syspath inside the assembled venv. Duplicate realpaths indicate symlink redundancy; many distinct site-packages roots suggest an inefficient venv layout.

Bazel analysis benchmark

Version	Mean (ms)	Median (ms)	± stddev	vs BCR	vs main	Targets	Actions
BCR 2.0.0-alpha.6 (baseline)	1362.763	1236.274	±255.101	—	—	304	13746
HEAD main	1346.330	1257.699	±257.527	+1.7%	—	304	13546
This PR	1432.548	1335.636	±267.412	+8.0%	+6.2%	304	13546

Measured with hyperfine --warmup 1 --runs 10 on Linux
Gate: PR vs HEAD main median (threshold: 10%, and must exceed the 2×SE noise floor, here 18.7%). BCR is shown only as a historical baseline.
Command: warm-server bazel build --nobuild //workspace/..., analysis cache discarded each run via a fresh --action_env value; no disk cache.

py_image_layer benchmark

Scenario	Version	Mean (s)	Median (s)	± stddev	Actions	vs BCR	vs main
Analysis	BCR 2.0.0-alpha.6 (baseline)	0.69	0.66	±0.12	747	—	—
Analysis	HEAD main	0.69	0.67	±0.15	725	+1.3%	—
Analysis	This PR	0.69	0.67	±0.12	725	+1.9%	+0.6%
1p Source Change	BCR 2.0.0-alpha.6 (baseline)	0.69	0.67	±0.06	4	—	—
1p Source Change	HEAD main	0.70	0.68	±0.10	5	+2.2%	—
1p Source Change	This PR	0.72	0.70	±0.11	5	+4.9%	+2.7%
3p Source Change	BCR 2.0.0-alpha.6 (baseline)	0.80	0.79	±0.05	5	—	—
3p Source Change	HEAD main	0.84	0.84	±0.05	6	+6.9%	—
3p Source Change	This PR	0.84	0.85	±0.06	6	+7.1%	+0.2%

Measured with hyperfine on Linux, building //workspace:image_layers (10 binaries, ~30-wheel dep pool, grouped first-party/pip/interpreter tier) with isolated output base, no disk cache.
Scenarios: analysis = warm-server bazel build --nobuild, re-analyzed each run via a fresh --action_env value; incrementals run against a built state with warm analysis: source = append to the last package's lib.py, wheel = rewrite click post_install_patches content.
Actions: for Analysis, the total action count behind the image target from aquery deps(...); for incrementals, actions re-executed for the mutation, from a single instrumented run's BEP build metrics (deterministic; per-mnemonic breakdown in the *-actions.json artifacts). Informational only, not gated.
Gate: PR vs HEAD main median per scenario (threshold: 10%, and must exceed the 2×SE noise floor). BCR is shown only as a historical baseline.

feat(py): use the sysmon coverage core on Python 3.12+

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feat(py): use the sysmon coverage core on Python 3.12+ - #1534

feat(py): use the sysmon coverage core on Python 3.12+#1534
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