libstore: skip optimisation when GC removes a link concurrently - #15904
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xokdvium merged 1 commit intoMay 26, 2026
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A concurrent garbage collection can remove an entry from the links directory between the moment optimisePath_ checks for it and the moment it creates the hard link. This produced "cannot create hard link: No such file or directory" and failed the whole build. Treat the vanished link as a benign race and skip optimising the path, since a later pass will dedup it. Guard both the lstat (now maybeLstat) and the create_hard_link call, mirroring the existing too_many_links and file_exists handling. Relates to NixOS#7273 Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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…oved-links libstore: skip optimisation when GC removes a link concurrently
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Motivation
nix store optimiseandauto-optimise-storecan abort a build with:https://github.com/cachix/devenv/actions/runs/26294238442/job/77408282471#step:6:2534
when a garbage collection runs at the same time.
The race
In
optimisePath_, when a file with matching contents is already indexed, the flow is:/nix/store/.links/<hash>exists,lstatit,create_hard_link(linkPath, tempLink)and thenrename(tempLink, path).The links-directory entry can have a link count of 1 (the original store path that created it was already collected, but
removeUnusedLinkshas not run yet). A concurrent GC'sremoveUnusedLinksthen deletes that entry in the window between the existence check andcreate_hard_link. The source is gone, socreate_hard_linkfails withENOENT, and the exception propagates and fails the whole operation.This is the
No such file or directoryvariant of #7273. The variant reported there isFile exists, which was addressed by switching tomakeTempPathin #14676; theENOENTwindow described above is still unhandled.Fix
Store optimisation is best effort, so a link disappearing underneath us is benign: skip optimising this path and let a later pass dedup it. This mirrors how the existing
too_many_linksandfile_existscases already shrug and return.Two windows are guarded:
lstat(linkPath)becomesmaybeLstat(linkPath), returning early if the entry vanished before the inode comparison.create_hard_linkcatch now returns onstd::errc::no_such_file_or_directory.Context
This is a race condition. There is no hook to deterministically remove
linkPathmid call, so a non flaky regression test is not feasible, consistent with #7273 which also has none.