libstore: only acquire big write lock when needed, hold read lock#15967
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libstore: only acquire big write lock when needed, hold read lock#15967lisanna-dettwyler wants to merge 1 commit into
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Check if a schema migration is actually needed before acquiring exclusive on the big nix store lock, and always hold a shared lock throughout the duration of the LocalStore. Checking if it's needed prevents the build hook from trying to acquire exclusive while the parent is holding shared. Signed-off-by: Lisanna Dettwyler <lisanna.dettwyler@gmail.com>
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Check if a schema migration is actually needed before acquiring exclusive on the big nix store lock, and always hold a shared lock throughout the duration of the LocalStore. Checking if it's needed prevents the build hook from trying to acquire exclusive while the parent is holding shared.
Motivation
We need to hold the shared lock throughout the lifetime of the LocalStore to prevent other schema upgrades from happening while we're using the store.
Context
Prior attempt to fix this logic: #15941
There are no schema migration tests that I know of, so this logic should be thoroughly examined before merging, although keeping in mind that the current state of affairs is already broken and can result in a hang.
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