CachingFilteringSourceAccessor: Make the isAllowed() cache thread-safe - #16341
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The cache in CachingFilteringSourceAccessor was a plain std::map mutated on every cache miss. With parallel evaluation, multiple eval threads can call isAllowed() on the same accessor concurrently (e.g. via builtins.readDir on a Git input fetched with exportIgnore), racing find() against emplace() and corrupting the tree. This caused segfaults like DETERMINATE-NIX-8W (a SEGV_MAPERR at offset 0x10, i.e. reading _M_left of a null red-black tree node). Use boost::concurrent_flat_map instead, following the same pattern as CachingSourceAccessor. Assisted-by: Claude Fable 5 <noreply@anthropic.com>
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| struct CachingFilteringSourceAccessor : FilteringSourceAccessor | ||
| { | ||
| std::map<CanonPath, bool> cache; | ||
| const ref<boost::concurrent_flat_map<CanonPath, bool>> cache; |
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| const ref<boost::concurrent_flat_map<CanonPath, bool>> cache; | |
| std::unique_ptr<boost::concurrent_flat_map<CanonPath, bool>> cache; |
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@edolstra brought up that it should be fine to share the cache actually. Though this doesn't really happen in the current code base.
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Motivation
The cache in CachingFilteringSourceAccessor was a plain std::map mutated on every cache miss. With parallel evaluation, multiple eval threads can call isAllowed() on the same accessor concurrently (e.g. via builtins.readDir on a Git input fetched with exportIgnore), racing find() against emplace() and corrupting the tree. This caused segfaults like DETERMINATE-NIX-8W (a SEGV_MAPERR at offset 0x10, i.e. reading _M_left of a null red-black tree node).
Use boost::concurrent_flat_map instead, following the same pattern as CachingSourceAccessor.
Assisted-by: Claude Fable 5 noreply@anthropic.com
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