Before this change, if there were changes to sync, bisync listed each path
twice: once before the sync and once after. The second listing caused quite
a lot of problems, in addition to making each run much slower and more
expensive. A serious side-effect was that file changes could slip through
undetected, if they happened to occur while a sync was running (between the
first and second listing snapshots.)
After this change, the second listing is eliminated by getting the underlying
sync operation to report back a list of what it changed. Not only is this more
efficient, but also much more robust to concurrent modifications. It should no
longer be necessary to avoid make changes while it's running -- bisync will
simply learn about those changes next time and handle them on the next run.
Additionally, this also makes --check-sync usable again.
For further discussion, see:
https://forum.rclone.org/t/bisync-bugs-and-feature-requests/37636#:~:text=5.%20Final%20listings%20should%20be%20created%20from%20initial%20snapshot%20%2B%20deltas%2C%20not%20full%20re%2Dscans%2C%20to%20avoid%20errors%20if%20files%20changed%20during%20sync
Before this change, bisync handled copies and deletes in separate operations.
After this change, they are combined in one sync operation, which is faster
and also allows bisync to support --track-renames and --backup-dir.
Bisync uses a --files-from filter containing only the paths bisync has
determined need to be synced. Just like in sync (but in both directions),
if a path is present on the dst but not the src, it's interpreted as a delete
rather than a copy.
This commit documents my learnings after having encountered a failure
I reported in the rclone forum[0].
I may be a fool for having failed to understand the previous
documentation, but I am likely not the only fool to get snared by it.
This commit therefore adds details to clarify what the user must do in
order to allow `--check-access` to succeed.
While at it, I've also added some basic documentation for `--check-filename`.
[0]: https://forum.rclone.org/t/bisync-check-file-check-failed/35682
