ci: redirect uv cache to node-local TMPDIR self-hosted runners

[sbryngelson]

Summary

• On GitHub Actions self-hosted runners, the default uv cache (~/.cache/uv) often lives on a shared NFS $HOME (e.g. OLCF /ccs/home), where uv's file-lock implementation hits os error 524 when concurrent runners on different nodes contend for the same .lock file.
• Redirects UV_CACHE_DIR to node-local ${TMPDIR:-/tmp}/uv-cache-${USER} when running in CI, leaving non-CI users' normal reusable cache unaffected.

Test plan

• Verify CI passes on self-hosted runners (OLCF/Frontier or similar NFS-home systems)
• Confirm local builds are unaffected (no GITHUB_ACTIONS env var set)

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[coderabbitai]

📝 Walkthrough

Walkthrough

The script toolchain/bootstrap/python.sh has been modified to configure the uv package manager's cache behavior for CI environments. When USE_UV=1, the script now exports a UV_CACHE_DIR environment variable pointing to a temporary directory on GitHub Actions self-hosted runners. The cache directory is scoped to the TMPDIR and keyed by the USER variable to isolate cache usage per runner instance. In non-CI environments, the default cache behavior remains unchanged. This configuration change consists of 8 new lines of code.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Description check	⚠️ Warning	The description explains the problem, solution, and test plan clearly, but does not follow the repository's template structure with required sections like 'Type of change', 'Testing', and the checklist.	Restructure the description to follow the template: add 'Type of change' section, formal 'Testing' section with how changes were tested, and complete the 'Checklist' items.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately and specifically describes the main change: redirecting the uv cache to a node-local TMPDIR on GitHub Actions self-hosted runners.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
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.

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[coderabbitai]

Actionable comments posted: 1

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: ab0daafb-b0b3-426a-a1d0-60f40c201c53

📥 Commits

Reviewing files that changed from the base of the PR and between 00aa603 and bee5332.

📒 Files selected for processing (1)

• toolchain/bootstrap/python.sh

[coderabbitai]

⚠️ Potential issue | 🟠 Major

Harden CI cache-root fallback to avoid silently reverting to shared ~/.cache/uv.

At Line 188, if TMPDIR is set but unusable, the condition fails and UV_CACHE_DIR is left unset, so CI can still hit the shared-home lock path this PR is fixing.

Suggested patch

-        if [ "${GITHUB_ACTIONS:-}" = "true" ] && [ -w "${TMPDIR:-/tmp}" ]; then
-            export UV_CACHE_DIR="${TMPDIR:-/tmp}/uv-cache-${USER:-$(id -un)}"
-        fi
+        if [ "${GITHUB_ACTIONS:-}" = "true" ]; then
+            uv_cache_root="${TMPDIR:-}"
+            if [ -z "$uv_cache_root" ] || [ ! -d "$uv_cache_root" ] || [ ! -w "$uv_cache_root" ]; then
+                uv_cache_root="/tmp"
+            fi
+            if [ -d "$uv_cache_root" ] && [ -w "$uv_cache_root" ]; then
+                export UV_CACHE_DIR="${uv_cache_root}/uv-cache-${USER:-$(id -un)}"
+            fi
+        fi

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 64.76%. Comparing base (00aa603) to head (bee5332).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1385   +/-   ##
=======================================
  Coverage   64.76%   64.76%           
=======================================
  Files          71       71           
  Lines       18713    18713           
  Branches     1549     1549           
=======================================
  Hits        12119    12119           
  Misses       5638     5638           
  Partials      956      956           

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