Fix data race initializing AttrCursor::_value#557
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AttrCursor::_value and EvalCache::value were lazily initialized without any synchronization, while `nix flake check/show` call into cursors shared between evaluator threads (e.g. the flake outputs cursor used by every Leaf::derivation() call). Two threads could both see an empty RootValue and move-assign it concurrently, double-freeing a root value slot. Since the root value pool uses an intrusive freelist, a double free corrupts the freelist and makes a subsequent allocRootValueSlot() chase a garbage next pointer, crashing with EXC_BAD_ACCESS (e.g. at address 0x28, a Value discriminator word read as a Slot pointer). Wrap both members in Sync so initialization happens exactly once and concurrent callers block until the value is available. The lock is held while the value is computed; lock ordering is strictly child -> parent (and thunk waiting in parallel eval blocks on a condition variable, without work stealing), so this cannot deadlock. Note: AttrCursor::cachedValue has a similar unsynchronized lazy initialization pattern that still needs fixing. Assisted-by: Claude Fable 5 <noreply@anthropic.com>
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ChangesEval-cache synchronization
Estimated code review effort: 3 (Moderate) | ~20 minutes Possibly related PRs
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cole-h
approved these changes
Jul 13, 2026
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Motivation
This field could be lazily initialized by multiple threads, clobbering each other's
RootValue. Since #546 this has been much more likely to lead to a crash. (PreviouslyRootValuewas anstd::shared_ptr, which was not safe either but unlikely to crash.)Context
Summary by CodeRabbit