Fix PBS encodings error by making pyvenv.cfg home absolute at runtime

[xangcastle]

PBS encodings fix

Python 3.11/3.12 PBS reimplemented prefix discovery in getpath.py. Its resolvedpath() fails on multi-hop relative symlinks that traverse components across runfiles boundaries. When resolution fails Python falls back to the /install compile-time prefix.

The launcher workaround (exporting PYTHONHOME) doesn't help when nvoked through a venv symlink with -I, because getpath.py reads the venv's pyvenv.cfg first and ignores PYTHONHOME when the relative home = be resolved.

Fix: rewrite pyvenv.cfg at runtime with an absolute home= path so getpath.py discovers the stdlib without following symlinks.

Dep group fallback fix:
Packages with a single available dep_group or single cfg previously required an explicit dep_group attribute, even though there was no ambiguity. Add //conditions:default fallbacks in uv_hub/repository.bzl and uv_project/repository.bzl so these packages resolve automatically.

Changes are visible to end-users: yes/no

• Searched for relevant documentation and updated as needed: yes/no
• Breaking change (forces users to change their own code or config): yes/no
• Suggested release notes appear below: no

Test plan

• Covered by existing test cases
• New test cases added
• Manual testing; please provide instructions so we can reproduce:

[aspect-workflows]

✨ Aspect Workflows Tasks

📅 Tue Jun 2 13:31:56 UTC 2026

❌ 3 failed tasks

• ❌ buildifier · ⏱ 32.9s · 🐙 GitHub Actions · ☑️ Check
  💬 failed in diff · Format failed (2 files need format)
• ❌ test (test-e2e-bazel-8) · ⏱ 33.4s · 🐙 GitHub Actions · ☑️ Check
  💬 failed in test · Bazel test failed
• ❌ test (test-e2e-bazel-9) · ⏱ 35.3s · 🐙 GitHub Actions · ☑️ Check
  💬 failed in test · Bazel test failed

✅ 4 successful tasks

• ✅ gazelle · ⏱ 23.2s · 🐙 GitHub Actions · ☑️ Check
  💬 Gazelle complete (clean)
• ✅ test (test-examples-uv_pip_compile-bazel-8) · ⏱ 20.8s · 🐙 GitHub Actions · ☑️ Check
  💬 Bazel test complete (1/1 passed · 1 cached)
• ✅ test (test-root-bazel-8) · ⏱ 1m 53s · 🐙 GitHub Actions · ☑️ Check
  💬 Bazel test complete (185/185 passed)
• ✅ test (test-root-bazel-9) · ⏱ 2m 23s · 🐙 GitHub Actions · ☑️ Check
  💬 Bazel test complete (184/184 passed)

🛠️ Fix

❌ buildifier (buildifier)

aspect buildifier --severity=info -- \
  e2e/snapshots/pypi_single.defs.bzl \
  uv/private/uv_hub/snapshots/pypi.defs.bzl

# without Aspect CLI
bazel run --run_in_cwd @buildifier_prebuilt//buildifier -- \
  e2e/snapshots/pypi_single.defs.bzl \
  uv/private/uv_hub/snapshots/pypi.defs.bzl

_{Install aspect: docs.aspect.build/cli/install}

🔁 Reproduce

❌ buildifier (buildifier)

aspect buildifier --severity=fail --base-ref=origin/main

❌ test (test-e2e-bazel-8 · test-e2e-bazel-9)

aspect test -- //...

# without Aspect CLI
bazel test -- //...

_{Install aspect: docs.aspect.build/cli/install}

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[github-actions]

py_binary startup benchmark

Version	Mean (ms)	Median (ms)	± stddev	vs BCR	vs main	Build (s)
BCR 1.11.5 (baseline)	328.836	327.025	±6.724	—	—	39.59
HEAD main	135.083	134.957	±0.848	-58.9%	—	11.76
This PR	147.132	146.281	±5.537	-55.3%	+8.9%	10.47

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 1.11.5 (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.