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Git Commit Graph Design Notes
=============================

Git walks the commit graph for many reasons, including:

1. Listing and filtering commit history.
2. Computing merge bases.

These operations can become slow as the commit count grows. The merge
base calculation shows up in many user-facing commands, such as 'merge-base'
or 'status' and can take minutes to compute depending on history shape.

There are two main costs here:

1. Decompressing and parsing commits.
2. Walking the entire graph to satisfy topological order constraints.

The commit-graph file is a supplemental data structure that accelerates
commit graph walks. If a user downgrades or disables the 'core.commitGraph'
config setting, then the existing ODB is sufficient. The file is stored
as "commit-graph" either in the .git/objects/info directory or in the info
directory of an alternate.

The commit-graph file stores the commit graph structure along with some
extra metadata to speed up graph walks. By listing commit OIDs in lexi-
cographic order, we can identify an integer position for each commit and
refer to the parents of a commit using those integer positions. We use
binary search to find initial commits and then use the integer positions
for fast lookups during the walk.

A consumer may load the following info for a commit from the graph:

1. The commit OID.
2. The list of parents, along with their integer position.
3. The commit date.
4. The root tree OID.
5. The generation number (see definition below).

Values 1-4 satisfy the requirements of parse_commit_gently().

Define the "generation number" of a commit recursively as follows:

 * A commit with no parents (a root commit) has generation number one.

 * A commit with at least one parent has generation number one more than
   the largest generation number among its parents.

Equivalently, the generation number of a commit A is one more than the
length of a longest path from A to a root commit. The recursive definition
is easier to use for computation and observing the following property:

    If A and B are commits with generation numbers N and M, respectively,
    and N <= M, then A cannot reach B. That is, we know without searching
    that B is not an ancestor of A because it is further from a root commit
    than A.

    Conversely, when checking if A is an ancestor of B, then we only need
    to walk commits until all commits on the walk boundary have generation
    number at most N. If we walk commits using a priority queue seeded by
    generation numbers, then we always expand the boundary commit with highest
    generation number and can easily detect the stopping condition.

This property can be used to significantly reduce the time it takes to
walk commits and determine topological relationships. Without generation
numbers, the general heuristic is the following:

    If A and B are commits with commit time X and Y, respectively, and
    X < Y, then A _probably_ cannot reach B.

This heuristic is currently used whenever the computation is allowed to
violate topological relationships due to clock skew (such as "git log"
with default order), but is not used when the topological order is
required (such as merge base calculations, "git log --graph").

In practice, we expect some commits to be created recently and not stored
in the commit graph. We can treat these commits as having "infinite"
generation number and walk until reaching commits with known generation
number.

We use the macro GENERATION_NUMBER_INFINITY = 0xFFFFFFFF to mark commits not
in the commit-graph file. If a commit-graph file was written by a version
of Git that did not compute generation numbers, then those commits will
have generation number represented by the macro GENERATION_NUMBER_ZERO = 0.

Since the commit-graph file is closed under reachability, we can guarantee
the following weaker condition on all commits:

    If A and B are commits with generation numbers N amd M, respectively,
    and N < M, then A cannot reach B.

Note how the strict inequality differs from the inequality when we have
fully-computed generation numbers. Using strict inequality may result in
walking a few extra commits, but the simplicity in dealing with commits
with generation number *_INFINITY or *_ZERO is valuable.

We use the macro GENERATION_NUMBER_MAX = 0x3FFFFFFF to for commits whose
generation numbers are computed to be at least this value. We limit at
this value since it is the largest value that can be stored in the
commit-graph file using the 30 bits available to generation numbers. This
presents another case where a commit can have generation number equal to
that of a parent.

Design Details
--------------

- The commit-graph file is stored in a file named 'commit-graph' in the
  .git/objects/info directory. This could be stored in the info directory
  of an alternate.

- The core.commitGraph config setting must be on to consume graph files.

- The file format includes parameters for the object ID hash function,
  so a future change of hash algorithm does not require a change in format.

- Commit grafts and replace objects can change the shape of the commit
  history. The latter can also be enabled/disabled on the fly using
  `--no-replace-objects`. This leads to difficultly storing both possible
  interpretations of a commit id, especially when computing generation
  numbers. The commit-graph will not be read or written when
  replace-objects or grafts are present.

- Shallow clones create grafts of commits by dropping their parents. This
  leads the commit-graph to think those commits have generation number 1.
  If and when those commits are made unshallow, those generation numbers
  become invalid. Since shallow clones are intended to restrict the commit
  history to a very small set of commits, the commit-graph feature is less
  helpful for these clones, anyway. The commit-graph will not be read or
  written when shallow commits are present.

Related Links
-------------
[0] https://bugs.chromium.org/p/git/issues/detail?id=8
    Chromium work item for: Serialized Commit Graph

[1] https://public-inbox.org/git/20110713070517.GC18566@sigill.intra.peff.net/
    An abandoned patch that introduced generation numbers.

[2] https://public-inbox.org/git/20170908033403.q7e6dj7benasrjes@sigill.intra.peff.net/
    Discussion about generation numbers on commits and how they interact
    with fsck.

[3] https://public-inbox.org/git/20170908034739.4op3w4f2ma5s65ku@sigill.intra.peff.net/
    More discussion about generation numbers and not storing them inside
    commit objects. A valuable quote:

    "I think we should be moving more in the direction of keeping
     repo-local caches for optimizations. Reachability bitmaps have been
     a big performance win. I think we should be doing the same with our
     properties of commits. Not just generation numbers, but making it
     cheap to access the graph structure without zlib-inflating whole
     commit objects (i.e., packv4 or something like the "metapacks" I
     proposed a few years ago)."

[4] https://public-inbox.org/git/20180108154822.54829-1-git@jeffhostetler.com/T/#u
    A patch to remove the ahead-behind calculation from 'status'.
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