bisync: rollback listing on error

Before this change, bisync had no mechanism for "retrying" a file again next
time, in the event of an unexpected and possibly temporary error. After this
change, bisync is now essentially able to mark a file as needing to be
rechecked next time. Bisync does this by keeping one prior listing on hand at
all times. In a low-confidence situation, bisync can revert a given file row
back to its state at the end of the last known successful sync, ensuring that
any subsequent changes will be re-noticed on the next run.
This can potentially be helpful for a dynamically changing file system, where
files may be changing quickly while bisync is working with them.

cmd/bisync/queue.go

@@ -153,6 +153,7 @@ func (b *bisyncRun) fastCopy(ctx context.Context, fsrc, fdst fs.Fs, files bilib.
 	if b.opt.Resync {
 		err = sync.CopyDir(ctxCopyLogger, fdst, fsrc, b.opt.CreateEmptySrcDirs)
 	} else {
+		b.testFn()
 		err = sync.Sync(ctxCopyLogger, fdst, fsrc, b.opt.CreateEmptySrcDirs)
 	}
 	fs.Debugf(nil, "logger is: %v", logger)