Fix CI permissions for external fork pull requests - #829
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* Serialize Spack buildcache index writers `buildcache update-index` is a read-modify-write of the mirror's single `index.spack` tag: it lists every spec tag, rebuilds the index, and overwrites. Individual spec tags are immutable and safe to push concurrently, but two concurrent index writers are last-writer-wins, and specs banked by the loser can drop out of the index. The workflow-level concurrency groups are keyed on github.ref, so they serialize runs within a branch or PR but not across them. The index is one shared object per mirror, so two different PRs -- or a PR and a push to main -- could refresh it at the same time. Give each mirror's refresh a job-level concurrency group that deliberately omits github.ref, with cancel-in-progress: false so a second writer queues instead of clobbering. In lookahead.yml this requires promoting the refresh step to its own job, since steps cannot carry `concurrency`; it rebuilds a minimal mirror-only Spack environment the same way spack.yml already does. Its previous comment claimed no writer could race it, which held for jobs (nothing else builds against Spack develop) but not for concurrent runs. This narrows but does not close the window: the parent workflow's cancel-in-progress can still cancel a queued refresh. The index remains best-effort, and binaries stay reachable meanwhile through Spack's direct OCI-tag lookup. Extracted from #829, which bundles this with unrelated fork-permission changes. * Update .github/workflows/lookahead.yml Co-authored-by: Hugh Carson <114775781+hughcars@users.noreply.github.com> * Update .github/workflows/spack.yml Co-authored-by: Hugh Carson <114775781+hughcars@users.noreply.github.com> --------- Co-authored-by: Hugh Carson <114775781+hughcars@users.noreply.github.com>
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Summary
Fix external-fork CI failures by separating read-only builds from trusted publication.
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