Prevent checkpoints during snapshots #477
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Use a mutex that is held during snapshots and only try locking it during checkpoints. If the database is in high write pressure and taking a snapshot takes long it is possible for the automatic checkpointing to go from PASSIVE to RESTART. When a RESTART checkpoint is issued SQLite will block new read transactions and wait for the existing ones to finish. However, during a snapshot Litestream keeps a persistent read transaction open until it finishes which will in turn create a deadlock situation for the checkpointer as the RESTART checkpoint will start blocking writers as long as the snapshot is being written out. This failure condition doesn't break everything persistently but it will create an unfortunate persistent write lock for the application until the snapshot finishes.
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This actually fixes #237 as we hit the same issue. |
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Use a mutex that is held during snapshots and only try locking it during checkpoints.
If the database is in high write pressure and taking a snapshot takes long it is possible for the automatic checkpointing to go from PASSIVE to RESTART.
When a RESTART checkpoint is issued SQLite will block new read transactions and wait for the existing ones to finish. However, during a snapshot Litestream keeps a persistent read transaction open until it finishes which will in turn create a deadlock situation for the checkpointer as the RESTART checkpoint will start blocking writers as long as the snapshot is being written out.
This failure condition doesn't break everything persistently but it will create an unfortunate persistent write lock for the application until the snapshot finishes.