= 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.

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 may also want to have a CI job
or two 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.  The early
adopter configuration used for changes for this release is `feature.git3`.

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>.

=== 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>,

== 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>.
