Fix concurrency issue of Terraform provider cache (Cherry-pick of #21805) #21889
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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
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As an existing bug in an "experimental" backed I don't think this needs to block the 2.24.0 release. I'd like to hold off on merging this until #21879 |
makes sense! |
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(All clear.) |
cburroughs
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cburroughs
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Jan 31, 2025
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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