CachingFilteringSourceAccessor: Make the isAllowed() cache thread-safe - #16341
Open
edolstra wants to merge 1 commit into
Open
CachingFilteringSourceAccessor: Make the isAllowed() cache thread-safe#16341edolstra wants to merge 1 commit into
edolstra wants to merge 1 commit into
Conversation
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>
github-actions
Bot
added
the
fetching
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
1 participant
Add this suggestion to a batch that can be applied as a single commit.This suggestion is invalid because no changes were made to the code.Suggestions cannot be applied while the pull request is closed.Suggestions cannot be applied while viewing a subset of changes.Only one suggestion per line can be applied in a batch.Add this suggestion to a batch that can be applied as a single commit.Applying suggestions on deleted lines is not supported.You must change the existing code in this line in order to create a valid suggestion.Outdated suggestions cannot be applied.This suggestion has been applied or marked resolved.Suggestions cannot be applied from pending reviews.Suggestions cannot be applied on multi-line comments.Suggestions cannot be applied while the pull request is queued to merge.Suggestion cannot be applied right now. Please check back later.
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
Context
Add 👍 to pull requests you find important.
The Nix maintainer team uses a GitHub project board to schedule and track reviews.