Summary:
This fixes a two problems:
* A mistake in `KeyedMutex.runExclusive`.
* Logic about saving a document to s3 when the document is found to match what is already there.
`HostedStorageManager.flushDoc` could get caught in a loop if a document was uploaded to s3 and then, without any change to it, marked as dirty. Low level code would detect there was no change and skip the upload; but then the snapshotId could be unknown, causing an error and retries. This diff fixes that problem by discovering the snapshotId on downloads and tracking it. It also corrects a mutex problem that may have been creating the scenario. A small delay is added to `flushDoc` to mitigate the effect of similar problems in future. Exponential backoff would be good, but `flushDoc` is called in some situations where long delays would negatively impact worker shutdown or user work.
Test Plan: added tests
Reviewers: dsagal
Reviewed By: dsagal
Differential Revision: https://phab.getgrist.com/D2654
Summary:
Deliberate changes:
* save snapshots to s3 prior to migrations.
* label migration snapshots in s3 metadata.
* avoid pruning migration snapshots for a month.
Opportunistic changes:
* Associate document timezone with snapshots, so pruning can respect timezones.
* Associate actionHash/Num with snapshots.
* Record time of last change in snapshots (rather than just s3 upload time, which could be a while later).
This ended up being a biggish change, because there was nowhere ideal to put tags (list of possibilities in diff).
Test Plan: added tests
Reviewers: dsagal
Reviewed By: dsagal
Differential Revision: https://phab.getgrist.com/D2646
