Our new binary object map code avoids needing to be intimately involved
with file handling by simply writing data to an object implement Write.
This makes it very easy to test by writing to a Cursor wrapping a Vec
for tests, and thus decouples it from intimate knowledge about how we
handle files.
However, we will actually want to write our data to an actual file,
since that's the most practical way to persist data. Implement a
wrapper around the hashfile code that implements the Write trait so that
we can write our object map into a file.
Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Our current loose object format has a few problems. First, it is not
efficient: the list of object IDs is not sorted and even if it were,
there would not be an efficient way to look up objects in both
algorithms.
Second, we need to store mappings for things which are not technically
loose objects but are not packed objects, either, and so cannot be
stored in a pack index. These kinds of things include shallows, their
parents, and their trees, as well as submodules. Yet we also need to
implement a sensible way to store the kind of object so that we can
prune unneeded entries. For instance, if the user has updated the
shallows, we can remove the old values.
For these reasons, introduce a new binary object map format. The
careful reader will notice that it resembles very closely the pack index
v3 format. Add an in-memory object map as well, and allow writing to a
batched map, which can then be written later as one of the binary object
maps. Include several tests for round tripping and data lookup across
algorithms.
Note that the use of this code elsewhere in Git will involve some C code
and some C-compatible code in Rust that will be introduced in a future
commit. Thus, for example, we ignore the fact that if there is no
current batch and the caller asks for data to be written, this code does
nothing, mostly because this code also does not involve itself with
opening or manipulating files. The C code that we will add later will
implement this functionality at a higher level and take care of this,
since the code which is necessary for writing to the object store is
deeply involved with our C abstractions and it would require extensive
work (which would not be especially valuable at this point) to port
those to Rust.
Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
We'd like to be able to write some Rust code that can work with object
IDs. Add a structure here that's identical to struct object_id in C,
for easy use in sharing across the FFI boundary. We will use this
structure in several places in hot paths, such as index-pack or
pack-objects when converting between algorithms, so prioritize efficient
interchange over a more idiomatic Rust approach.
Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Implement a trivial test balloon for our Rust build infrastructure by
reimplementing the "varint.c" subsystem in Rust. This subsystem is
chosen because it is trivial to convert and because it doesn't have any
dependencies to other components of Git.
If support for Rust is enabled, we stop compiling "varint.c" and instead
compile and use "src/varint.rs".
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Add the infrastructure into Meson to build an internal Rust library.
Building the Rust parts of Git are for now entirely optional, as they
are mostly intended as a test balloon for both Git developers, but also
for distributors of Git. So for now, they may contain:
- New features that are not mission critical to Git and that users can
easily live without.
- Alternative implementations of small subsystems.
If these test balloons are successful, we will eventually make Rust a
mandatory dependency for our build process in Git 3.0.
The availability of a Rust toolchain will be auto-detected by Meson at
setup time. This behaviour can be tweaked via the `-Drust=` feature
toggle.
Next to the linkable Rust library, also wire up tests that can be
executed via `meson test`. This allows us to use the native unit testing
capabilities of Rust.
Note that the Rust edition is currently set to 2018. This edition is
supported by Rust 1.49, which is the target for the upcoming gcc-rs
backend. For now we don't use any features of Rust that would require a
newer version, so settling on this old version makes sense so that
gcc-rs may become an alternative backend for compiling Git. If we _do_
want to introduce features that were added in more recent editions of
Rust though we should reevaluate that choice.
Inspired-by: Ezekiel Newren <ezekielnewren@gmail.com>
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
