Fix concurrency issue of Terraform provider cache (Cherry-pick of #21805) #21887
Conversation
This file contains 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
The Terraform provider cache is not concurrency safe. The worst concurrency bug with the Terraform provider cache is a race condition when nonatomic moves result in another process capturing incorrect hashes in the lockfile which gets pulled into the Pants cache and then poisons it. After [lots of debugging and help from the community](https://chat.pantsbuild.org/t/26730983/i-m-finding-terraform-init-may-not-cache-stably-from-2-23-it#84153d7a-cedd-4600-838f-bd6ef99f6cc9), we've determined that the best way forward is to isolate each module into its own Terraform cache. This prevents concurrent access to the Terraform cache. One possible solution would have been to not cache during cache-unsafe operations. This is harder than it seems, because envvars can override user cache settings in Terraform RC files but they cannot _unset_ it. fixes pantsbuild#21804
cburroughs
added
the
release-notes:not-required
cburroughs
approved these changes
Jan 31, 2025
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
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Outdated suggestions cannot be applied.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.
The Terraform provider cache is not concurrency safe.
The worst concurrency bug with the Terraform provider cache is a race condition when nonatomic moves result in another process capturing incorrect hashes in the lockfile which gets pulled into the Pants cache and then poisons it.
After lots of debugging and help from the community, we've determined that the best way forward is to isolate each module into its own Terraform cache. This prevents concurrent access to the Terraform cache.
One possible solution would have been to not cache during cache-unsafe operations. This is harder than it seems, because envvars can override user cache settings in Terraform RC files but they cannot unset it.
fixes #21804