Before this change, Transfer.Done closed the account of a completed transfer but
never released it, because it assigned nil to only a local copy of the pointer.
As a result, every completed transfer continued to reference its account.
An Account holds the transfer context and the source reader, and the stats keep
completed transfers around up to `MaxCompletedTransfers` per group, with groups
discarded only at --max-stats-groups. On a long-running `rclone rcd`, this adds up.
It was noticeable when running bisync repeatedly via the rc, where the transfer
context carries `b.WriteResults` (bisync's `LoggerFn`). A `*bisyncRun` holds
Path1 and Path2 listings, which can be quite large, and are not supposed to be
retained between runs. (Naturally they aren't, when running bisync on the
command line -- which is probably why we didn't notice this issue sooner.)
This change fixes the issue by releasing `tr.acc` in Done, instead of the local
copy. `tr.Snapshot` reads the byte counts off the account, so the progress is
recorded on the transfer first. That read happens before taking `tr.mu`, because
`acc.progress()` locks `acc.values.mu`, `checkReadBefore` holds that lock while
calling `StatsInfo.GetBytes`, and `StatsInfo` locks back into `Transfer` in
`Transferred` and `_removeTransfer`.
nielash
71a0932126