Crate git_repository

Expand description

This crate provides the Repository abstraction which serves as a hub into all the functionality of git.

It’s powerful and won’t sacrifice performance while still increasing convenience compared to using the sub-crates individually. Sometimes it may hide complexity under the assumption that the performance difference doesn’t matter for all but the fewest tools out there, which would be using the underlying crates directly or file an issue.

The prelude and extensions

With use git_repository::prelude::* you should be ready to go as it pulls in various extension traits to make functionality available on objects that may use it.

The method signatures are still complex and may require various arguments for configuration and cache control.

Most extensions to existing objects provide an obj_with_extension.attach(&repo).an_easier_version_of_a_method() for simpler call signatures.

ThreadSafe Mode

By default, the Repository isn’t Sync and thus can’t be used in certain contexts which require the Sync trait.

To help with this, convert it with .to_sync() into a ThreadSafeRepository.

Object-Access Performance

Accessing objects quickly is the bread-and-butter of working with git, right after accessing references. Hence it’s vital to understand which cache levels exist and how to leverage them.

When accessing an object, the first cache that’s queried is a memory-capped LRU object cache, mapping their id to data and kind. On miss, the object is looked up and if ia pack is hit, there is a small fixed-size cache for delta-base objects.

In scenarios where the same objects are accessed multiple times, an object cache can be useful and is to be configured specifically using the object_cache_size(…) method.

Use the cache-efficiency-debug cargo feature to learn how efficient the cache actually is - it’s easy to end up with lowered performance if the cache is not hit in 50% of the time.

Environment variables can also be used for configuration if the application is calling apply_environment() on their Easy* accordingly.

Shortcomings & Limitations

Only a single crate::object or derivatives can be held in memory at a time, per Easy*.
Changes made to the configuration, packs, and alternates aren’t picked up automatically, but the current object store needs a manual refresh.

Design Sketch

Goal is to make the lower-level plumbing available without having to deal with any caches or buffers, and avoid any allocation beyond sizing the buffer to fit the biggest object seen so far.

no implicit object lookups, thus Oid needs to get an Object first to start out with data via object()
Objects with Ref suffix can only exist one at a time unless they are transformed into an owned version of it OR multiple Easy handles are present, each providing another ‘slot’ for an object as long as its retrieved through the respective Easy object.
ObjectRef blocks the current buffer, hence many of its operations that use the buffer are consuming
All methods that access a any field from Easy’s mutable State are fallible, and return easy::Result<_> at least, to avoid panics if the field can’t be referenced due to borrow rules of RefCell.
Anything attached to Access can be detached to lift the object limit or make them Send-able. They can be attached to another Access if needed.
git-repository functions related to Access extensions will always return attached versions of return values, like Oid instead of git_hash::ObjectId, ObjectRef instead of git_odb::data::Object, or Reference instead of git_ref::Reference.
Obtaining mutable is currently a weak spot as these only work with Arc right now and can’t work with Rc<RefCell> due to missing GATs, presumably. All Easy*!Exclusive types are unable to provide a mutable reference to the underlying repository. However, other ways to adjust the Repository of long-running applications are possible. For instance, there could be a flag that indicates a new Repository should be created (for instance, after it was changed) which causes the next server connection to create a new one. This instance is the one to use when spawning new EasyArc instances.
Platform types are used to hold mutable or shared versions of required state for use in dependent objects they create, like iterators. These come with the benefit of allowing for nicely readable call chains. Sometimes these are called Platform for a lack of a more specific term, some are called more specifically like Ancestors.

Terminology

WorkingTree and WorkTree

When reading the documentation of the canonical git-worktree program one gets the impression work tree and working tree are used interchangeably. We use the term work tree only and try to do so consistently as its shorter and assumed to be the same.

Cargo-features

With the optional “unstable” cargo feature

To make using sub-crates easier these are re-exported into the root of this crate. Note that these may change their major version even if this crate doesn’t, hence breaking downstream.

git_repository::

attrs
hash
url
actor
[bstr][bstr]
discover
index
glob
path
credentials
sec
worktree
mailmap
objs
odb
- pack
refs
revision
interrupt
tempfile
lock
traverse
diff
parallel
Progress
progress
interrupt
protocol
- [transport][protocol::transport]
  - [packetline][protocol::transport::packetline]

Feature Flags

Mutually Exclusive Client

Either async-* or blocking-* versions of these toggles may be enabled at a time.

async-network-client — Make git-protocol available along with an async client.
blocking-network-client — Make git-protocol available along with a blocking client.
blocking-http-transport — Stacks with blocking-network-client to provide support for HTTP/S, and implies blocking networking as a whole.

Reducing dependencies

The following toggles can be left disabled to save on dependencies.

local — Provide additional non-networked functionality like git-url and git-diff.
one-stop-shop (enabled by default) — Turns on access to all stable features that are unrelated to networking.

Other

serde1 — Data structures implement serde::Serialize and serde::Deserialize.
max-performance (enabled by default) — Activate other features that maximize performance, like usage of threads, zlib-ng and access to caching in object databases. Note that
local-time-support — Functions dealing with time may include the local timezone offset, not just UTC with the offset being zero.
unstable — Re-export stability tier 2 crates for convenience and make Repository struct fields with types from these crates publicly accessible. Doing so is less stable than the stability tier 1 that git-repository is a member of.
cache-efficiency-debug — Print debugging information about usage of object database caches, useful for tuning cache sizes.

Re-exports

pub use git_actor as actor;

pub use git_attributes as attrs;

pub use git_credentials as credentials;

pub use git_diff as diff;

pub use git_glob as glob;

pub use git_hash as hash;

pub use git_lock as lock;

pub use git_object as objs;

pub use git_object::bstr;

pub use git_odb as odb;

pub use git_protocol as protocol;

pub use git_ref as refs;

pub use git_revision as revision;

pub use git_sec as sec;

pub use git_tempfile as tempfile;

pub use git_traverse as traverse;

pub use git_url as url;

Modules

Feature Flags

Process-global interrupt handling

Run computations in parallel, or not based the parallel feature toggle.

Various prodash types along with various utilities for comfort.

reference

rev_parse

state

Not to be confused with ‘status’.

tag

threading

Type definitions for putting shared ownership and synchronized mutation behind the threading feature toggle.

worktree

Structs

Commit

A decoded commit object with access to its owning repository.

DetachedObject

A detached, self-contained object, without access to its source repository.

Head

The head reference, as created from looking at .git/HEAD, able to represent all of its possible states.

Id

An ObjectId with access to a repository.

Object

A decoded object with a reference to its owning repository.

Permissions

Permissions associated with various resources of a git repository

Reference

A reference that points to an object or reference, with access to its source repository.

Repository

A thread-local handle to interact with a repository from a single thread.

Tag

A decoded tag object with access to its owning repository.

ThreadSafeRepository

An instance with access to everything a git repository entails, best imagined as container implementing Sync + Send for most for system resources required to interact with a git repository which are loaded in once the instance is created.

Tree

A decoded tree object with access to its owning repository.

Url

A URL with support for specialized git related capabilities.

Worktree

A worktree checkout containing the files of the repository in consumable form.

oid

A borrowed reference to a hash identifying objects.