improve snapshot generation under load #1380
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Grist uses the SQLite API to make backups. This PR reorganizes the code so that the SQLite connection used by ActiveDoc (the main class representing an open document) is also the one used for backups. Otherwise, if the document is under continuous modification (e.g. because of some automation), attempting to perform the backup may not terminate since SQLite may keep restarting the backup under the hood. With this change, it is now also worth beginning the backup even if the document is busy, so the PR also does that. In testing, when a document is under heavy load, the last step call in the SQLite backup API may be relatively slow, perhaps as all changes that accumulated are flushed also to the backup. But since a since SQLite connection is being used now, there should not be any new SQLite-level busy timeouts.
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Context
Grist uses the SQLite API to make backups. This PR reorganizes the code so that the SQLite connection used by ActiveDoc (the main class representing an open document) is also the one used for backups. Otherwise, if the document is under continuous modification (e.g. because of some automation), attempting to perform the backup may not terminate since SQLite may keep restarting the backup under the hood.
With this change, it is now also worth beginning the backup even if the document is busy, so the PR also does that.
In testing, when a document is under heavy load, the last step call in the SQLite backup API may be relatively slow, perhaps as all changes that accumulated are flushed also to the backup. But since a since SQLite connection is being used now, there should not be any new SQLite-level busy timeouts.
Has this been tested?