During the search, the region between `upper_bound` and `target_pos`
should contain no data items. The search lowers `upper_bound` and raises
`lower_bound` until they both point to the last item before `target_pos`.
The `lower_bound` is increased to the position of the last item returned
by a search (`high_pos`) when that item is lower than `target_pos`.
This avoids some loop iterations compared to a strict binary search
algorithm, which would increase `lower_bound` only as far as `probe_pos`.
When the search runs over live extent items, occasionally a new extent
will appear between `upper_bound` and `target_pos`. When this happens,
`lower_bound` is bumped up to the position of one of the new items, but
that position is in the "unoccupied" space between `upper_bound` and
`target_pos`, where no items are supposed to exist, so `seek_backward`
throws an exception.
To cut down on the noise, only increase `lower_bound` as far as
`upper_bound`. This avoids the exception without increasing the number
of loop iterations for normal cases.
In the exceptional cases, extra loop iterations are needed to skip over
the new items. This raises the worst-case number of loop iterations
by one.
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
This allows detailed but selective debugging when using the library,
particularly when something goes wrong.
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
The debug log is only revealed when something goes wrong, but it is
created and discarded every time `seek_backward` is called, and it
is quite CPU-intensive.
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
This template turns a forward search primitive (e.g. lower_bound, FIEMAP,
TREE_SEARCH_V2) into a backward search primitive.
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
