= Upcoming breaking changes

The Git project aims to ensure backwards compatibility to the best extent
possible. Minor releases will not break backwards compatibility unless there is
a very strong reason to do so, like for example a security vulnerability.

Regardless of that, due to the age of the Git project, it is only natural to
accumulate a backlog of backwards-incompatible changes that will eventually be
required to keep the project aligned with a changing world. These changes fall
into several categories:

* Changes to long established defaults.
* Concepts that have been replaced with a superior design.
* Concepts, commands, configuration or options that have been lacking in major
  ways and that cannot be fixed and which will thus be removed without any
  replacement.

Explicitly not included in this list are fixes to minor bugs that may cause a
change in user-visible behavior.

The Git project irregularly releases breaking versions that deliberately break
backwards compatibility with older versions. This is done to ensure that Git
remains relevant, safe and maintainable going forward. The release cadence of
breaking versions is typically measured in multiple years. We had the following
major breaking releases in the past:

* Git 1.6.0, released in August 2008.
* Git 2.0, released in May 2014.

We use <major>.<minor> release numbers these days, starting from Git 2.0. For
future releases, our plan is to increment <major> in the release number when we
make the next breaking release. Before Git 2.0, the release numbers were
1.<major>.<minor> with the intention to increment <major> for "usual" breaking
releases, reserving the jump to Git 2.0 for really large backward-compatibility
breaking changes.

The intent of this document is to track upcoming deprecations for future
breaking releases. Furthermore, this document also tracks what will _not_ be
deprecated. This is done such that the outcome of discussions document both
when the discussion favors deprecation, but also when it rejects a deprecation.

Items should have a clear summary of the reasons why we do or do not want to
make the described change that can be easily understood without having to read
the mailing list discussions. If there are alternatives to the changed feature,
those alternatives should be pointed out to our users.

All items should be accompanied by references to relevant mailing list threads
where the deprecation was discussed. These references use message-IDs, which
can visited via

  https://lore.kernel.org/git/$message_id/

to see the message and its surrounding discussion. Such a reference is there to
make it easier for you to find how the project reached consensus on the
described item back then.

This is a living document as the environment surrounding the project changes
over time. If circumstances change, an earlier decision to deprecate or change
something may need to be revisited from time to time. So do not take items on
this list to mean "it is settled, do not waste our time bringing it up again".

== Procedure

Discussing the desire to make breaking changes, declaring that breaking
changes are made at a certain version boundary, and recording these
decisions in this document, are necessary but not sufficient.
Because such changes are expected to be numerous, and the design and
implementation of them are expected to span over time, they have to
be deployable trivially at such a version boundary, prepared over long
time.

The breaking changes MUST be guarded with the a compile-time switch,
WITH_BREAKING_CHANGES, to help this process.  When built with it,
the resulting Git binary together with its documentation would
behave as if these breaking changes slated for the next big version
boundary are already in effect.  We also have a CI job to exercise
the work-in-progress version of Git with these breaking changes.


== Git 3.0

The following subsections document upcoming breaking changes for Git 3.0. There
is no planned release date for this breaking version yet.

Proposed changes and removals only include items which are "ready" to be done.
In other words, this is not supposed to be a wishlist of features that should
be changed to or replaced in case the alternative was implemented already.

=== Changes

* The default hash function for new repositories will be changed from "sha1"
  to "sha256". SHA-1 has been deprecated by NIST in 2011 and is nowadays
  recommended against in FIPS 140-2 and similar certifications. Furthermore,
  there are practical attacks on SHA-1 that weaken its cryptographic properties:
+
  ** The SHAppening (2015). The first demonstration of a practical attack
     against SHA-1 with 2^57 operations.
  ** SHAttered (2017). Generation of two valid PDF files with 2^63 operations.
  ** Birthday-Near-Collision (2019). This attack allows for chosen prefix
     attacks with 2^68 operations.
  ** Shambles (2020). This attack allows for chosen prefix attacks with 2^63
     operations.
+
While we have protections in place against known attacks, it is expected
that more attacks against SHA-1 will be found by future research. Paired
with the ever-growing capability of hardware, it is only a matter of time
before SHA-1 will be considered broken completely. We want to be prepared
and will thus change the default hash algorithm to "sha256" for newly
initialized repositories.
+
An important requirement for this change is that the ecosystem is ready to
support the "sha256" object format. This includes popular Git libraries,
applications and forges.
+
There is no plan to deprecate the "sha1" object format at this point in time.
+
Cf. <2f5de416-04ba-c23d-1e0b-83bb655829a7@zombino.com>,
<20170223155046.e7nxivfwqqoprsqj@LykOS.localdomain>,
<CA+EOSBncr=4a4d8n9xS4FNehyebpmX8JiUwCsXD47EQDE+DiUQ@mail.gmail.com>.

* The default storage format for references in newly created repositories will
  be changed from "files" to "reftable". The "reftable" format provides
  multiple advantages over the "files" format:
+
  ** It is impossible to store two references that only differ in casing on
     case-insensitive filesystems with the "files" format. This issue is common
     on Windows and macOS platforms. As the "reftable" backend does not use
     filesystem paths to encode reference names this problem goes away.
  ** Similarly, macOS normalizes path names that contain unicode characters,
     which has the consequence that you cannot store two names with unicode
     characters that are encoded differently with the "files" backend. Again,
     this is not an issue with the "reftable" backend.
  ** Deleting references with the "files" backend requires Git to rewrite the
     complete "packed-refs" file. In large repositories with many references
     this file can easily be dozens of megabytes in size, in extreme cases it
     may be gigabytes. The "reftable" backend uses tombstone markers for
     deleted references and thus does not have to rewrite all of its data.
  ** Repository housekeeping with the "files" backend typically performs
     all-into-one repacks of references. This can be quite expensive, and
     consequently housekeeping is a tradeoff between the number of loose
     references that accumulate and slow down operations that read references,
     and compressing those loose references into the "packed-refs" file. The
     "reftable" backend uses geometric compaction after every write, which
     amortizes costs and ensures that the backend is always in a
     well-maintained state.
  ** Operations that write multiple references at once are not atomic with the
     "files" backend. Consequently, Git may see in-between states when it reads
     references while a reference transaction is in the process of being
     committed to disk.
  ** Writing many references at once is slow with the "files" backend because
     every reference is created as a separate file. The "reftable" backend
     significantly outperforms the "files" backend by multiple orders of
     magnitude.
  ** The reftable backend uses a binary format with prefix compression for
     reference names. As a result, the format uses less space compared to the
     "packed-refs" file.
+
Users that get immediate benefit from the "reftable" backend could continue to
opt-in to the "reftable" format manually by setting the "init.defaultRefFormat"
config. But defaults matter, and we think that overall users will have a better
experience with less platform-specific quirks when they use the new backend by
default.
+
A prerequisite for this change is that the ecosystem is ready to support the
"reftable" format. Most importantly, alternative implementations of Git like
JGit, libgit2 and Gitoxide need to support it.

* In new repositories, the default branch name will be `main`. We have been
  warning that the default name will change since 675704c74dd (init:
  provide useful advice about init.defaultBranch, 2020-12-11).  The new name
  matches the default branch name used in new repositories by many of the
  big Git forges.

* Git will require Rust as a mandatory part of the build process. While Git
  already started to adopt Rust in Git 2.49, all parts written in Rust are
  optional for the time being. This includes:
+
  ** The Rust wrapper around libgit.a that is part of "contrib/" and which has
     been introduced in Git 2.49.
  ** Subsystems that have an alternative implementation in Rust to test
     interoperability between our C and Rust codebase.
  ** Newly written features that are not mission critical for a fully functional
     Git client.
+
These changes are meant as test balloons to allow distributors of Git to prepare
for Rust becoming a mandatory part of the build process. There will be multiple
milestones for the introduction of Rust:
+
--
1. Initially, with Git 2.52, support for Rust will be auto-detected by Meson and
   disabled in our Makefile so that the project can sort out the initial
   infrastructure.
2. In Git 2.53, both build systems will default-enable support for Rust.
   Consequently, builds will break by default if Rust is not available on the
   build host. The use of Rust can still be explicitly disabled via build
   flags.
3. In Git 3.0, the build options will be removed and support for Rust is
   mandatory.
--
+
You can explicitly ask both Meson and our Makefile-based system to enable Rust
by saying `meson configure -Drust=enabled` and `make WITH_RUST=YesPlease`,
respectively.
+
The Git project will declare the last version before Git 3.0 to be a long-term
support release. This long-term release will receive important bug fixes for at
least four release cycles and security fixes for six release cycles. The Git
project will hand over maintainership of the long-term release to distributors
in case they need to extend the life of that long-term release even further.
Details of how this long-term release will be handed over to the community will
be discussed once the Git project decides to stop officially supporting it.
+
We will evaluate the impact on downstream distributions before making Rust
mandatory in Git 3.0. If we see that the impact on downstream distributions
would be significant, we may decide to defer this change to a subsequent minor
release. This evaluation will also take into account our own experience with
how painful it is to keep Rust an optional component.

=== Removals

* Support for grafting commits has long been superseded by git-replace(1).
  Grafts are inferior to replacement refs:
+
  ** Grafts are a local-only mechanism and cannot be shared across
     repositories.
  ** Grafts can lead to hard-to-diagnose problems when transferring objects
     between repositories.
+
The grafting mechanism has been marked as outdated since e650d0643b (docs: mark
info/grafts as outdated, 2014-03-05) and will be removed.
+
Cf. <20140304174806.GA11561@sigill.intra.peff.net>.

* The git-pack-redundant(1) command can be used to remove redundant pack files.
  The subcommand is unusably slow and the reason why nobody reports it as a
  performance bug is suspected to be the absence of users. We have nominated
  the command for removal and have started to emit a user-visible warning in
  c3b58472be (pack-redundant: gauge the usage before proposing its removal,
  2020-08-25) whenever the command is executed.
+
So far there was a single complaint about somebody still using the command, but
that complaint did not cause us to reverse course. On the contrary, we have
doubled down on the deprecation and starting with 4406522b76 (pack-redundant:
escalate deprecation warning to an error, 2023-03-23), the command dies unless
the user passes the `--i-still-use-this` option.
+
There have not been any subsequent complaints, so this command will finally be
removed.
+
Cf. <xmqq1rjuz6n3.fsf_-_@gitster.c.googlers.com>,
    <CAKvOHKAFXQwt4D8yUCCkf_TQL79mYaJ=KAKhtpDNTvHJFuX1NA@mail.gmail.com>,
    <20230323204047.GA9290@coredump.intra.peff.net>,

* Support for storing shorthands for remote URLs in "$GIT_COMMON_DIR/branches/"
  and "$GIT_COMMON_DIR/remotes/" has been long superseded by storing remotes in
  the repository configuration.
+
The mechanism has originally been introduced in f170e4b39d ([PATCH] fetch/pull:
short-hand notation for remote repositories., 2005-07-16) and was superseded by
6687f8fea2 ([PATCH] Use .git/remote/origin, not .git/branches/origin.,
2005-08-20), where we switched from ".git/branches/" to ".git/remotes/". That
commit already mentions an upcoming deprecation of the ".git/branches/"
directory, and starting with a1d4aa7424 (Add repository-layout document.,
2005-09-01) we have also marked this layout as deprecated. Eventually we also
started to migrate away from ".git/remotes/" in favor of config-based remotes,
and we have marked the directory as legacy in 3d3d282146 (Documentation:
Grammar correction, wording fixes and cleanup, 2011-08-23)
+
As our documentation mentions, these directories are unlikely to be used in
modern repositories and most users aren't even aware of these mechanisms. They
have been deprecated for almost 20 years and 14 years respectively, and we are
not aware of any active users that have complained about this deprecation.
Furthermore, the ".git/branches/" directory is nowadays misleadingly named and
may cause confusion as "branches" are almost exclusively used in the context of
references.
+
These features will be removed.

* Support for "--stdin" option in the "name-rev" command was
  deprecated (and hidden from the documentation) in the Git 2.40
  timeframe, in preference to its synonym "--annotate-stdin".  Git 3.0
  removes the support for "--stdin" altogether.

* The git-whatchanged(1) command has outlived its usefulness more than
  10 years ago, and takes more keystrokes to type than its rough
  equivalent `git log --raw`.  We have nominated the command for
  removal, have changed the command to refuse to work unless the
  `--i-still-use-this` option is given, and asked the users to report
  when they do so.
+
The command will be removed.

* Support for `core.commentString=auto` has been deprecated and will
  be removed in Git 3.0.
+
cf. <xmqqa59i45wc.fsf@gitster.g>

* Support for `core.preferSymlinkRefs=true` has been deprecated and will be
  removed in Git 3.0. Writing symbolic refs as symbolic links will be phased
  out in favor of using plain files using the textual representation of
  symbolic refs.
+
Symbolic references were initially always stored as a symbolic link. This was
changed in 9b143c6e15 (Teach update-ref about a symbolic ref stored in a
textfile., 2005-09-25), where a new textual symref format was introduced to
store those symbolic refs in a plain file. In 9f0bb90d16
(core.prefersymlinkrefs: use symlinks for .git/HEAD, 2006-05-02), the Git
project switched the default to use the textual symrefs in favor of symbolic
links.
+
The migration away from symbolic links has happened almost 20 years ago by now,
and there is no known reason why one should prefer them nowadays. Furthermore,
symbolic links are not supported on some platforms.
+
Note that only the writing side for such symbolic links is deprecated. Reading
such symbolic links is still supported for now.

== Superseded features that will not be deprecated

Some features have gained newer replacements that aim to improve the design in
certain ways. The fact that there is a replacement does not automatically mean
that the old way of doing things will eventually be removed. This section tracks
those features with newer alternatives.

* The features git-checkout(1) offers are covered by the pair of commands
  git-restore(1) and git-switch(1). Because the use of git-checkout(1) is still
  widespread, and it is not expected that this will change anytime soon, all
  three commands will stay.
+
This decision may get revisited in case we ever figure out that there are
almost no users of any of the commands anymore.
+
Cf. <xmqqttjazwwa.fsf@gitster.g>,
<xmqqleeubork.fsf@gitster.g>,
<112b6568912a6de6672bf5592c3a718e@manjaro.org>.
