Re: [PATCH v3 8/9] odb: introduce object filters to `odb_for_each_object()`

[Toon Claes <toon@iotcl.com>]

Patrick Steinhardt <ps@pks.im> writes:

> The function `for_each_bitmapped_object()` can be used to iterate
> through all objects covered by a bitmap. The benefit of this function is
> that it allows the caller to efficiently handle some object filters. For
> example, this can be used to filter out objects of a specific type with
> some simple bitmap operations. But callers are currently required to
> manually wire up the use of bitmaps though, and to do so they have to
> reach into internals of a given object database source.
>
> Introduce a new `struct odb_for_each_object_options::filter` field so
> that the interface becomes generic. When set, then a backend may
> optionally use the filter to skip some objects that it would have
> otherwise yielded.
>
> Note that the respective backends are free to ignore this field if they
> cannot meaningfully optimize for a given filter, and consequently
> callers need to verify whether they actually want the returned objects.
> While annoying, we cannot easily lift this restriction anyway as the
> object filter infrastructure supports some filters that cannot be
> answered by the object database alone.
>
> An alternative might be to limit the filters to only those that _can_ be
> answered by backends. But ultimately, the filters that can be answered
> efficiently by the "packed" backend are completely disjunct from those
> that can be answered by the "loose" backend, and consequently the set of
> filters supported by all backends would be empty. Furthermore, it would
> require us to make assumptions about capabilities of future backends,
> which may be able to efficiently handle more filters than current ones.
> So in the end, this alternative would only limit us artificially.
>
> Implement the logic for the "packed" source. Note that we use the new
> function `prepare_source_bitmap_git()` to open the bitmap: as the

Also here: `prepare_bitmap_git_for_source()`

> backend operates on a single object source, we must only use bitmaps
> that belong to that specific source. Otherwise we might yield objects
> that are not part of the source at all, and with multiple sources we
> would enumerate the same bitmap once per source.


-- 
Cheers,
Toon