Before this change if using --fast-list on a directory with more than
a few thousand directories in it DirTree.CheckParents became very slow
taking up to 24 hours for a directory with 1,000,000 directories in
it.
This is because it becomes an O(N²) operation as DirTree.Find has to
search each directory in a linear fashion as it is stored as a slice.
This patch fixes the problem by scanning the DirTree for directories
before starting the CheckParents process so it never has to call
DirTree.Find.
After the fix calling DirTree.CheckParents on a directory with
1,000,000 directories in it will take about 1 second.
Anything which calls DirTree.Find can potentially have bad performance
so in the future we should redesign the DirTree to use a different
underlying datastructure or have an index.
https://forum.rclone.org/t/almost-24-hours-cpu-compute-time-during-sync-between-two-large-s3-buckets/39375/
In commit
da404dc0f2 sync,copy: Fix --fast-list --create-empty-src-dirs and --exclude
The fix caused DirTree.AddDir to be called with the root directory.
This in turn caused a spurious directory entry in the DirTree which
caused tests with the -fast-list flag to fail with directory not found
errors.
This is possible now that we no longer support go1.12 and brings
rclone into line with standard practices in the Go world.
This also removes errors.New and errors.Errorf from lib/errors and
prefers the stdlib errors package over lib/errors.
