perf: reduce use of to_list in py_image_layer

[jbedard]

This removes a single .to_list() call, which will actually have minimal impact since the same depset is already flattened elsewhere.

However this also removes the use of some large temp dicts (interpreter_reference_paths) and makes it a bit clearer how the reference_mappings creates the last-write-wins list passed to awk.

Changes are visible to end-users: no

Test plan

• Covered by existing test cases

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: 8d8b49bc-5c9b-48c1-9f98-c6b5c90843c3

📥 Commits

Reviewing files that changed from the base of the PR and between dec1bc1 and 0548a51.

📒 Files selected for processing (1)

• py/private/py_image_layer.bzl

Included review availability: Your plan includes up to 3 reviews per rolling hour; 2 remain after this review.

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📝 Walkthrough

Walkthrough

The image layer implementation now preserves separate symlink mapping functions for source, interpreter, and rule-group files. The tar action applies each mapper to its corresponding files while retaining ordered precedence and directory tree inputs.

Changes

Symlink mapping

Layer / File(s)	Summary
Ordered per-layer mapping and tar integration py/private/py_image_layer.bzl	_py_image_layer_impl creates ordered mappings for source/first-party, interpreter, and rule-group files. _run_tar_action applies each mapping’s files with its corresponding mapper.

Estimated code review effort: 3 (Moderate) | ~15 minutes

Merge Risk: ⚪ Minimal · up to 0548a

This localized refactor removes temporary data structures without changing end-user behavior, and no actionable merge-blocking risk remains beyond normal checks and review.

Suggested reviewers: xangcastle, thesayyn

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the performance cleanup in py_image_layer and matches the main changes.
Description check	✅ Passed	The description accurately covers the removed to_list call, temporary dictionary cleanup, and test coverage.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches 💡 1

📝 Generate docstrings 💡

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch image-less-to_list

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[github-actions]

py_binary startup benchmark

Version	Mean (ms)	Median (ms)	± stddev	vs BCR	vs main	Build (s)
BCR 1.11.7 (baseline)	166.514	158.477	±21.528	—	—	43.62
HEAD main	54.113	54.155	±0.570	-67.5%	—	9.20
This PR	55.222	54.212	±4.528	-66.8%	+2.0%	6.78

Measured with hyperfine --warmup 5 --runs 50 on Linux
Gate: PR vs HEAD main (threshold: 10%). BCR is shown only as a historical baseline.
Build time: cold bazel build //:bench with isolated output base, no disk cache.

sys.path quality

Version	sys.path entries	distinct site-packages roots	duplicate realpaths
BCR 1.11.7 (baseline)	6	1	0
HEAD main	7	2	0
This PR	7	2	0

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.5 (baseline)	10248.265	10276.876	±165.544	—	—	301	13672
HEAD main	8959.331	8939.199	±89.068	-12.6%	—	301	13540
This PR	8899.107	8909.285	±114.342	-13.2%	-0.7%	301	13540

Measured with hyperfine --warmup 1 --runs 10 on Linux
Gate: PR vs HEAD main (threshold: 10%). BCR is shown only as a historical baseline.
Command: cold bazel build --nobuild //workspace/... with isolated output base, no disk cache.