* ps/odb-for-each-object:
  odb: drop unused `for_each_{loose,packed}_object()` functions
  reachable: convert to use `odb_for_each_object()`
  builtin/pack-objects: use `packfile_store_for_each_object()`
  odb: introduce mtime fields for object info requests
  treewide: drop uses of `for_each_{loose,packed}_object()`
  treewide: enumerate promisor objects via `odb_for_each_object()`
  builtin/fsck: refactor to use `odb_for_each_object()`
  odb: introduce `odb_for_each_object()`
  packfile: introduce function to iterate through objects
  packfile: extract function to iterate through objects of a store
  object-file: introduce function to iterate through objects
  object-file: extract function to read object info from path
  odb: fix flags parameter to be unsigned
  odb: rename `FOR_EACH_OBJECT_*` flags

* ps/object-info-bits-cleanup:
  odb: convert `odb_has_object()` flags into an enum
  odb: convert object info flags into an enum
  odb: drop gaps in object info flag values
  builtin/fsck: fix flags passed to `odb_has_object()`
  builtin/backfill: fix flags passed to `odb_has_object()`

Subsequent commits will expand the `struct odb_source` to become a
generic interface for accessing an object database source. As part of
these refactorings we'll add a set of function pointers that will
significantly expand the structure overall.

Prepare for this by splitting out the `struct odb_source` into a
separate header. This keeps the high-level object database interface
detached from the low-level object database sources.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Introduce a new "files" object database source. This source encapsulates
access to both loose object files and the packfile store, similar to how
the "files" backend for refs encapsulates access to loose refs and the
packed-refs file.

Note that for now the "files" source is still a direct member of a
`struct odb_source`. This architecture will be reversed in the next
commit so that the files source contains a `struct odb_source`.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

The "files" backend is implemented as a pointer in the `struct
odb_source`. This contradicts our typical pattern for pluggable backends
like we use it for example in the ref store or for object database
streams, where we typically embed the generic base structure in the
specialized implementation. This pattern has a couple of small benefits:

  - We avoid an extra allocation.

  - We hide implementation details in the generic structure.

  - We can easily downcast from a generic backend to the specialized
    structure and vice versa because the offsets are known at compile
    time.

  - It becomes trivial to identify locations where we depend on backend
    specific logic because the cast needs to be explicit.

Refactor our "files" object database source to do the same and embed the
`struct odb_source` in the `struct odb_source_files`.

There are still a bunch of sites in our code base where we do have to
access internals of the "files" backend. The intent is that those will
go away over time, but this will certainly take a while. Meanwhile,
provide a `odb_source_files_downcast()` function that can convert a
generic source into a "files" source.

As we only have a single source the downcast succeeds unconditionally
for now. Eventually though the intent is to make the cast `BUG()` in
case the caller requests to downcast a non-"files" backend to a "files"
backend.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

The primary object database source may be initialized with a relative
path. When the process changes its current working directory we thus
have to update this path and have it point to the same path, but
relative to the new working directory.

This logic is handled in the object database layer. It consists of three
steps:

  1. We undo any potential temporary object directory, which are used
     for transactions. This is done so that we don't end up modifying
     the temporary object database source that got applied for the
     transaction.

  2. We then iterate through the non-transactional sources and reparent
     their respective paths.

  3. We reapply the temporary object directory, but update its path.

All of this logic is heavily tied to how the object database source
handles paths in the first place. It's an internal implementation
detail, and as sources may not even use an on-disk path at all it is not
a mechanism that applies to all potential sources.

Refactor the code so that the logic to reparent the sources is hosted by
the "files" source and the temporary object directory subsystems,
respectively. This logic is easier to reason about, but it also ensures
that this logic is handled at the correct level.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

When a caller holds a `struct odb_source`, they have no way of telling
what type the source is. This doesn't really cause any problems in the
current status quo as we only have a single type anyway, "files". But
going forward we expect to add more types, and if so it will become
necessary to tell the sources apart.

Introduce a new enum to cover this use case and assert that the given
source actually matches the target source when performing the downcast.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Introduce a new callback function in `struct odb_source` to make the
function pluggable.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Introduce a new callback function in `struct odb_source` to make the
function pluggable.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Introduce a new callback function in `struct odb_source` to make the
function pluggable.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Introduce a new callback function in `struct odb_source` to make the
function pluggable.

Note that this function is a bit less straight-forward to convert
compared to the other functions. The reason here is that the logic to
read an object is:

  1. We try to read the object. If it exists we return it.

  2. If the object does not exist we reprepare the object database
     source.

  3. We then try reading the object info a second time in case the
     reprepare caused it to appear.

The second read is only supposed to happen for the packfile store
though, as reading loose objects is not impacted by repreparing the
object database.

Ideally, we'd just move this whole logic into the ODB source. But that's
not easily possible because we try to avoid the reprepare unless really
required, which is after we have found out that no other ODB source
contains the object, either. So the logic spans across multiple ODB
sources, and consequently we cannot move it into an individual source.

Instead, introduce a new flag `OBJECT_INFO_SECOND_READ` that tells the
backend that we already tried to look up the object once, and that this
time around the ODB source should try to find any new objects that may
have surfaced due to an on-disk change.

With this flag, the "files" backend can trivially skip trying to re-read
the object as a loose object. Furthermore, as we know that we only try
the second read via the packfile store, we can skip repreparing loose
objects and only reprepare the packfile store.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Introduce a new callback function in `struct odb_source` to make the
function pluggable.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Introduce a new callback function in `struct odb_source` to make the
function pluggable.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Introduce a new callback function in `struct odb_source` to make the
function pluggable.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Introduce a new callback function in `struct odb_source` to make the
function pluggable.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Introduce a new callback function in `struct odb_source` to make the
function pluggable.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Introduce a new callback function in `struct odb_source` to make the
function pluggable.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Introduce a new callback function in `struct odb_source` to make the
function pluggable.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Introduce a new callback function in `struct odb_source` to make the
function pluggable.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>