Documentation/technical/commit-graph.txt - jrn/git - Git at Google

 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 object database 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
 lexicographic 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().

 There are two definitions of generation number:
 1. Corrected committer dates (generation number v2)
 2. Topological levels (generation number v1)

 Define "corrected committer date" of a commit recursively as follows:

  * A commit with no parents (a root commit) has corrected committer date
     equal to its committer date.

  * A commit with at least one parent has corrected committer date equal to
     the maximum of its committer date and one more than the largest corrected
     committer date among its parents.

  * As a special case, a root commit with timestamp zero has corrected commit
     date of 1, to be able to distinguish it from GENERATION_NUMBER_ZERO
     (that is, an uncomputed corrected commit date).

 Define the "topological level" of a commit recursively as follows:

  * A commit with no parents (a root commit) has topological level of one.

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

 Equivalently, the topological level 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.

 The property applies to both versions of generation number, that is both
 corrected committer dates and topological levels.

 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.

 In absence of corrected commit dates (for example, old versions of Git or
 mixed generation graph chains),
 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 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 and 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_V1_MAX = 0x3FFFFFFF for commits whose
 topological levels (generation number v1) 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 topological levels. 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.

 Commit-Graphs Chains
 --------------------

 Typically, repos grow with near-constant velocity (commits per day). Over time,
 the number of commits added by a fetch operation is much smaller than the
 number of commits in the full history. By creating a "chain" of commit-graphs,
 we enable fast writes of new commit data without rewriting the entire commit
 history -- at least, most of the time.

 ## File Layout

 A commit-graph chain uses multiple files, and we use a fixed naming convention
 to organize these files. Each commit-graph file has a name
 `$OBJDIR/info/commit-graphs/graph-{hash}.graph` where `{hash}` is the hex-
 valued hash stored in the footer of that file (which is a hash of the file's
 contents before that hash). For a chain of commit-graph files, a plain-text
 file at `$OBJDIR/info/commit-graphs/commit-graph-chain` contains the
 hashes for the files in order from "lowest" to "highest".

 For example, if the `commit-graph-chain` file contains the lines

 ```
 	{hash0}
 	{hash1}
 	{hash2}
 ```

 then the commit-graph chain looks like the following diagram:

  +-----------------------+
  |  graph-{hash2}.graph  |
  +-----------------------+
 	  |
  +-----------------------+
  |                       |
  |  graph-{hash1}.graph  |
  |                       |
  +-----------------------+
 	  |
  +-----------------------+
  |                       |
  |                       |
  |                       |
  |  graph-{hash0}.graph  |
  |                       |
  |                       |
  |                       |
  +-----------------------+

 Let X0 be the number of commits in `graph-{hash0}.graph`, X1 be the number of
 commits in `graph-{hash1}.graph`, and X2 be the number of commits in
 `graph-{hash2}.graph`. If a commit appears in position i in `graph-{hash2}.graph`,
 then we interpret this as being the commit in position (X0 + X1 + i), and that
 will be used as its "graph position". The commits in `graph-{hash2}.graph` use these
 positions to refer to their parents, which may be in `graph-{hash1}.graph` or
 `graph-{hash0}.graph`. We can navigate to an arbitrary commit in position j by checking
 its containment in the intervals [0, X0), [X0, X0 + X1), [X0 + X1, X0 + X1 +
 X2).

 Each commit-graph file (except the base, `graph-{hash0}.graph`) contains data
 specifying the hashes of all files in the lower layers. In the above example,
 `graph-{hash1}.graph` contains `{hash0}` while `graph-{hash2}.graph` contains
 `{hash0}` and `{hash1}`.

 ## Merging commit-graph files

 If we only added a new commit-graph file on every write, we would run into a
 linear search problem through many commit-graph files.  Instead, we use a merge
 strategy to decide when the stack should collapse some number of levels.

 The diagram below shows such a collapse. As a set of new commits are added, it
 is determined by the merge strategy that the files should collapse to
 `graph-{hash1}`. Thus, the new commits, the commits in `graph-{hash2}` and
 the commits in `graph-{hash1}` should be combined into a new `graph-{hash3}`
 file.

 			    +---------------------+
 			    |                     |
 			    |    (new commits)    |
 			    |                     |
 			    +---------------------+
 			    |                     |
  +-----------------------+  +---------------------+
  |  graph-{hash2}        |->|                     |
  +-----------------------+  +---------------------+
 	  |                 |                     |
  +-----------------------+  +---------------------+
  |                       |  |                     |
  |  graph-{hash1}        |->|                     |
  |                       |  |                     |
  +-----------------------+  +---------------------+
 	  |                  tmp_graphXXX
  +-----------------------+
  |                       |
  |                       |
  |                       |
  |  graph-{hash0}        |
  |                       |
  |                       |
  |                       |
  +-----------------------+

 During this process, the commits to write are combined, sorted and we write the
 contents to a temporary file, all while holding a `commit-graph-chain.lock`
 lock-file.  When the file is flushed, we rename it to `graph-{hash3}`
 according to the computed `{hash3}`. Finally, we write the new chain data to
 `commit-graph-chain.lock`:

 ```
 	{hash3}
 	{hash0}
 ```

 We then close the lock-file.

 ## Merge Strategy

 When writing a set of commits that do not exist in the commit-graph stack of
 height N, we default to creating a new file at level N + 1. We then decide to
 merge with the Nth level if one of two conditions hold:

   1. `--size-multiple=<X>` is specified or X = 2, and the number of commits in
      level N is less than X times the number of commits in level N + 1.

   2. `--max-commits=<C>` is specified with non-zero C and the number of commits
      in level N + 1 is more than C commits.

 This decision cascades down the levels: when we merge a level we create a new
 set of commits that then compares to the next level.

 The first condition bounds the number of levels to be logarithmic in the total
 number of commits.  The second condition bounds the total number of commits in
 a `graph-{hashN}` file and not in the `commit-graph` file, preventing
 significant performance issues when the stack merges and another process only
 partially reads the previous stack.

 The merge strategy values (2 for the size multiple, 64,000 for the maximum
 number of commits) could be extracted into config settings for full
 flexibility.

 ## Handling Mixed Generation Number Chains

 With the introduction of generation number v2 and generation data chunk, the
 following scenario is possible:

 1. "New" Git writes a commit-graph with the corrected commit dates.
 2. "Old" Git writes a split commit-graph on top without corrected commit dates.

 A naive approach of using the newest available generation number from
 each layer would lead to violated expectations: the lower layer would
 use corrected commit dates which are much larger than the topological
 levels of the higher layer. For this reason, Git inspects the topmost
 layer to see if the layer is missing corrected commit dates. In such a case
 Git only uses topological level for generation numbers.

 When writing a new layer in split commit-graph, we write corrected commit
 dates if the topmost layer has corrected commit dates written. This
 guarantees that if a layer has corrected commit dates, all lower layers
 must have corrected commit dates as well.

 When merging layers, we do not consider whether the merged layers had corrected
 commit dates. Instead, the new layer will have corrected commit dates if the
 layer below the new layer has corrected commit dates.

 While writing or merging layers, if the new layer is the only layer, it will
 have corrected commit dates when written by compatible versions of Git. Thus,
 rewriting split commit-graph as a single file (`--split=replace`) creates a
 single layer with corrected commit dates.

 ## Deleting graph-{hash} files

 After a new tip file is written, some `graph-{hash}` files may no longer
 be part of a chain. It is important to remove these files from disk, eventually.
 The main reason to delay removal is that another process could read the
 `commit-graph-chain` file before it is rewritten, but then look for the
 `graph-{hash}` files after they are deleted.

 To allow holding old split commit-graphs for a while after they are unreferenced,
 we update the modified times of the files when they become unreferenced. Then,
 we scan the `$OBJDIR/info/commit-graphs/` directory for `graph-{hash}`
 files whose modified times are older than a given expiry window. This window
 defaults to zero, but can be changed using command-line arguments or a config
 setting.

 ## Chains across multiple object directories

 In a repo with alternates, we look for the `commit-graph-chain` file starting
 in the local object directory and then in each alternate. The first file that
 exists defines our chain. As we look for the `graph-{hash}` files for
 each `{hash}` in the chain file, we follow the same pattern for the host
 directories.

 This allows commit-graphs to be split across multiple forks in a fork network.
 The typical case is a large "base" repo with many smaller forks.

 As the base repo advances, it will likely update and merge its commit-graph
 chain more frequently than the forks. If a fork updates their commit-graph after
 the base repo, then it should "reparent" the commit-graph chain onto the new
 chain in the base repo. When reading each `graph-{hash}` file, we track
 the object directory containing it. During a write of a new commit-graph file,
 we check for any changes in the source object directory and read the
 `commit-graph-chain` file for that source and create a new file based on those
 files. During this "reparent" operation, we necessarily need to collapse all
 levels in the fork, as all of the files are invalid against the new base file.

 It is crucial to be careful when cleaning up "unreferenced" `graph-{hash}.graph`
 files in this scenario. It falls to the user to define the proper settings for
 their custom environment:

  1. When merging levels in the base repo, the unreferenced files may still be
     referenced by chains from fork repos.

  2. The expiry time should be set to a length of time such that every fork has
     time to recompute their commit-graph chain to "reparent" onto the new base
     file(s).

  3. If the commit-graph chain is updated in the base, the fork will not have
     access to the new chain until its chain is updated to reference those files.
     (This may change in the future [5].)

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

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

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

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

 [5] https://lore.kernel.org/git/f27db281-abad-5043-6d71-cbb083b1c877@gmail.com/
     A discussion of a "two-dimensional graph position" that can allow reading
     multiple commit-graph chains at the same time.
	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 object database 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
	lexicographic 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().

	There are two definitions of generation number:
	1. Corrected committer dates (generation number v2)
	2. Topological levels (generation number v1)

	Define "corrected committer date" of a commit recursively as follows:

	* A commit with no parents (a root commit) has corrected committer date
	equal to its committer date.

	* A commit with at least one parent has corrected committer date equal to
	the maximum of its committer date and one more than the largest corrected
	committer date among its parents.

	* As a special case, a root commit with timestamp zero has corrected commit
	date of 1, to be able to distinguish it from GENERATION_NUMBER_ZERO
	(that is, an uncomputed corrected commit date).

	Define the "topological level" of a commit recursively as follows:

	* A commit with no parents (a root commit) has topological level of one.

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

	Equivalently, the topological level 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.

	The property applies to both versions of generation number, that is both
	corrected committer dates and topological levels.

	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.

	In absence of corrected commit dates (for example, old versions of Git or
	mixed generation graph chains),
	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 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 and 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_V1_MAX = 0x3FFFFFFF for commits whose
	topological levels (generation number v1) 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 topological levels. 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.

	Commit-Graphs Chains
	--------------------

	Typically, repos grow with near-constant velocity (commits per day). Over time,
	the number of commits added by a fetch operation is much smaller than the
	number of commits in the full history. By creating a "chain" of commit-graphs,
	we enable fast writes of new commit data without rewriting the entire commit
	history -- at least, most of the time.

	## File Layout

	A commit-graph chain uses multiple files, and we use a fixed naming convention
	to organize these files. Each commit-graph file has a name
	`$OBJDIR/info/commit-graphs/graph-{hash}.graph` where `{hash}` is the hex-
	valued hash stored in the footer of that file (which is a hash of the file's
	contents before that hash). For a chain of commit-graph files, a plain-text
	file at `$OBJDIR/info/commit-graphs/commit-graph-chain` contains the
	hashes for the files in order from "lowest" to "highest".

	For example, if the `commit-graph-chain` file contains the lines

	```
	{hash0}
	{hash1}
	{hash2}
	```

	then the commit-graph chain looks like the following diagram:

	+-----------------------+
	\| graph-{hash2}.graph \|
	+-----------------------+
	\|
	+-----------------------+
	\| \|
	\| graph-{hash1}.graph \|
	\| \|
	+-----------------------+
	\|
	+-----------------------+
	\| \|
	\| \|
	\| \|
	\| graph-{hash0}.graph \|
	\| \|
	\| \|
	\| \|
	+-----------------------+

	Let X0 be the number of commits in `graph-{hash0}.graph`, X1 be the number of
	commits in `graph-{hash1}.graph`, and X2 be the number of commits in
	`graph-{hash2}.graph`. If a commit appears in position i in `graph-{hash2}.graph`,
	then we interpret this as being the commit in position (X0 + X1 + i), and that
	will be used as its "graph position". The commits in `graph-{hash2}.graph` use these
	positions to refer to their parents, which may be in `graph-{hash1}.graph` or
	`graph-{hash0}.graph`. We can navigate to an arbitrary commit in position j by checking
	its containment in the intervals [0, X0), [X0, X0 + X1), [X0 + X1, X0 + X1 +
	X2).

	Each commit-graph file (except the base, `graph-{hash0}.graph`) contains data
	specifying the hashes of all files in the lower layers. In the above example,
	`graph-{hash1}.graph` contains `{hash0}` while `graph-{hash2}.graph` contains
	`{hash0}` and `{hash1}`.

	## Merging commit-graph files

	If we only added a new commit-graph file on every write, we would run into a
	linear search problem through many commit-graph files. Instead, we use a merge
	strategy to decide when the stack should collapse some number of levels.

	The diagram below shows such a collapse. As a set of new commits are added, it
	is determined by the merge strategy that the files should collapse to
	`graph-{hash1}`. Thus, the new commits, the commits in `graph-{hash2}` and
	the commits in `graph-{hash1}` should be combined into a new `graph-{hash3}`
	file.

	+---------------------+
	\| \|
	\| (new commits) \|
	\| \|
	+---------------------+
	\| \|
	+-----------------------+ +---------------------+
	\| graph-{hash2} \|->\| \|
	+-----------------------+ +---------------------+
	\| \| \|
	+-----------------------+ +---------------------+
	\| \| \| \|
	\| graph-{hash1} \|->\| \|
	\| \| \| \|
	+-----------------------+ +---------------------+
	\| tmp_graphXXX
	+-----------------------+
	\| \|
	\| \|
	\| \|
	\| graph-{hash0} \|
	\| \|
	\| \|
	\| \|
	+-----------------------+

	During this process, the commits to write are combined, sorted and we write the
	contents to a temporary file, all while holding a `commit-graph-chain.lock`
	lock-file. When the file is flushed, we rename it to `graph-{hash3}`
	according to the computed `{hash3}`. Finally, we write the new chain data to
	`commit-graph-chain.lock`:

	```
	{hash3}
	{hash0}
	```

	We then close the lock-file.

	## Merge Strategy

	When writing a set of commits that do not exist in the commit-graph stack of
	height N, we default to creating a new file at level N + 1. We then decide to
	merge with the Nth level if one of two conditions hold:

	1. `--size-multiple=<X>` is specified or X = 2, and the number of commits in
	level N is less than X times the number of commits in level N + 1.

	2. `--max-commits=<C>` is specified with non-zero C and the number of commits
	in level N + 1 is more than C commits.

	This decision cascades down the levels: when we merge a level we create a new
	set of commits that then compares to the next level.

	The first condition bounds the number of levels to be logarithmic in the total
	number of commits. The second condition bounds the total number of commits in
	a `graph-{hashN}` file and not in the `commit-graph` file, preventing
	significant performance issues when the stack merges and another process only
	partially reads the previous stack.

	The merge strategy values (2 for the size multiple, 64,000 for the maximum
	number of commits) could be extracted into config settings for full
	flexibility.

	## Handling Mixed Generation Number Chains

	With the introduction of generation number v2 and generation data chunk, the
	following scenario is possible:

	1. "New" Git writes a commit-graph with the corrected commit dates.
	2. "Old" Git writes a split commit-graph on top without corrected commit dates.

	A naive approach of using the newest available generation number from
	each layer would lead to violated expectations: the lower layer would
	use corrected commit dates which are much larger than the topological
	levels of the higher layer. For this reason, Git inspects the topmost
	layer to see if the layer is missing corrected commit dates. In such a case
	Git only uses topological level for generation numbers.

	When writing a new layer in split commit-graph, we write corrected commit
	dates if the topmost layer has corrected commit dates written. This
	guarantees that if a layer has corrected commit dates, all lower layers
	must have corrected commit dates as well.

	When merging layers, we do not consider whether the merged layers had corrected
	commit dates. Instead, the new layer will have corrected commit dates if the
	layer below the new layer has corrected commit dates.

	While writing or merging layers, if the new layer is the only layer, it will
	have corrected commit dates when written by compatible versions of Git. Thus,
	rewriting split commit-graph as a single file (`--split=replace`) creates a
	single layer with corrected commit dates.

	## Deleting graph-{hash} files

	After a new tip file is written, some `graph-{hash}` files may no longer
	be part of a chain. It is important to remove these files from disk, eventually.
	The main reason to delay removal is that another process could read the
	`commit-graph-chain` file before it is rewritten, but then look for the
	`graph-{hash}` files after they are deleted.

	To allow holding old split commit-graphs for a while after they are unreferenced,
	we update the modified times of the files when they become unreferenced. Then,
	we scan the `$OBJDIR/info/commit-graphs/` directory for `graph-{hash}`
	files whose modified times are older than a given expiry window. This window
	defaults to zero, but can be changed using command-line arguments or a config
	setting.

	## Chains across multiple object directories

	In a repo with alternates, we look for the `commit-graph-chain` file starting
	in the local object directory and then in each alternate. The first file that
	exists defines our chain. As we look for the `graph-{hash}` files for
	each `{hash}` in the chain file, we follow the same pattern for the host
	directories.

	This allows commit-graphs to be split across multiple forks in a fork network.
	The typical case is a large "base" repo with many smaller forks.

	As the base repo advances, it will likely update and merge its commit-graph
	chain more frequently than the forks. If a fork updates their commit-graph after
	the base repo, then it should "reparent" the commit-graph chain onto the new
	chain in the base repo. When reading each `graph-{hash}` file, we track
	the object directory containing it. During a write of a new commit-graph file,
	we check for any changes in the source object directory and read the
	`commit-graph-chain` file for that source and create a new file based on those
	files. During this "reparent" operation, we necessarily need to collapse all
	levels in the fork, as all of the files are invalid against the new base file.

	It is crucial to be careful when cleaning up "unreferenced" `graph-{hash}.graph`
	files in this scenario. It falls to the user to define the proper settings for
	their custom environment:

	1. When merging levels in the base repo, the unreferenced files may still be
	referenced by chains from fork repos.

	2. The expiry time should be set to a length of time such that every fork has
	time to recompute their commit-graph chain to "reparent" onto the new base
	file(s).

	3. If the commit-graph chain is updated in the base, the fork will not have
	access to the new chain until its chain is updated to reference those files.
	(This may change in the future [5].)

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

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

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

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

	[5] https://lore.kernel.org/git/f27db281-abad-5043-6d71-cbb083b1c877@gmail.com/
	A discussion of a "two-dimensional graph position" that can allow reading
	multiple commit-graph chains at the same time.