How to find all commits having a particular parent?

Question

How may I find all commits in a repository that have a specific parent?

For example, if I have a commit A, I would like to find all other commits that share the parent with A. What would be the most effective, i.e. performant yet correct way to do this in LibGit2Sharp?

Answer 1

That's a tricky question ;-)

The Git object allows to retrieve the parents of a commit. However, there's no easy way to find the children of a commit.

The following code would however partially solve this. The idea is to perform a git log from all the references of the repository (heads, tags, ...) and, along the way, select every commit which bear a parent with the requested SHA.

As the walk is being done from the most recent commits down the ancestor path, it may take some time, if you're searching for the children of a very early commit in a repository with a very large history and many branches.

[Fact]
public void CanRetrieveChildrenOfASpecificCommit()
{
    TemporaryCloneOfTestRepo path = BuildTemporaryCloneOfTestRepo(StandardTestRepoPath);
    using (var repo = new Repository(path.RepositoryPath))
    {
        const string parentSha = "5b5b025afb0b4c913b4c338a42934a3863bf3644";

        var filter = new Filter
                         {
                             /* Revwalk from all the refs (git log --all) ... */
                             Since = repo.Refs, 

                             /* ... and stop when the parent is reached */
                             Until = parentSha
                         };

        var commits = repo.Commits.QueryBy(filter);

        var children = from c in commits
                    from p in c.Parents
                    let pId = p.Id
                    where pId.Sha == parentSha
                    select c;

        var expectedChildren = new[] { "c47800c7266a2be04c571c04d5a6614691ea99bd", 
                                        "4a202b346bb0fb0db7eff3cffeb3c70babbd2045" };

        Assert.Equal(expectedChildren, children.Select(c => c.Id.Sha));
    }
}

Limits:

• This will not retrieve commits that have been rewritten (through amend or rebase, for instance) as LibGit2Sharp doesn't expose a way to access the reflog (yet)
• Unreachable (dangling) commits won't either be retrieved with this proposal.

Test repository:

The content of the repository being queried is show below

$ git log --all --graph
* commit 4c062a6361ae6959e06292c1fa5e2822d9c96345
| Author: gor <gorbach.alexey@gmail.com>
| Date:   Thu Apr 14 18:44:16 2011 +0300
|
|     directory was added
|
*   commit be3563ae3f795b2b4353bcce3a527ad0a4f7f644
|\  Merge: 9fd738e c47800c
| | Author: Scott Chacon <schacon@gmail.com>
| | Date:   Tue May 25 11:58:27 2010 -0700
| |
| |     Merge branch 'br2'
| |
| | * commit e90810b8df3e80c413d903f631643c716887138d
| | | Author: Vicent Marti <tanoku@gmail.com>
| | | Date:   Thu Aug 5 18:42:20 2010 +0200
| | |
| | |     Test commit 2
| | |
| | * commit 6dcf9bf7541ee10456529833502442f385010c3d
| |   Author: Vicent Marti <tanoku@gmail.com>
| |   Date:   Thu Aug 5 18:41:33 2010 +0200
| |
| |       Test commit 1
| |
| | *   commit a4a7dce85cf63874e984719f4fdd239f5145052f
| | |\  Merge: c47800c 9fd738e
| |/ /  Author: Scott Chacon <schacon@gmail.com>
| | /   Date:   Tue May 25 12:00:23 2010 -0700
| |/
|/|         Merge branch 'master' into br2
| |
* | commit 9fd738e8f7967c078dceed8190330fc8648ee56a
| | Author: Scott Chacon <schacon@gmail.com>
| | Date:   Mon May 24 10:19:19 2010 -0700
| |
| |     a fourth commit
| |
* | commit 4a202b346bb0fb0db7eff3cffeb3c70babbd2045
| | Author: Scott Chacon <schacon@gmail.com>
| | Date:   Mon May 24 10:19:04 2010 -0700
| |
| |     a third commit
| |
| * commit c47800c7266a2be04c571c04d5a6614691ea99bd
|/  Author: Scott Chacon <schacon@gmail.com>
|   Date:   Tue May 25 11:58:14 2010 -0700
|
|       branch commit one
|
* commit 5b5b025afb0b4c913b4c338a42934a3863bf3644
| Author: Scott Chacon <schacon@gmail.com>
| Date:   Tue May 11 13:38:42 2010 -0700
|
|     another commit
|
* commit 8496071c1b46c854b31185ea97743be6a8774479
  Author: Scott Chacon <schacon@gmail.com>
  Date:   Sat May 8 16:13:06 2010 -0700

      testing

* commit 41bc8c69075bbdb46c5c6f0566cc8cc5b46e8bd9
| Author: Scott Chacon <schacon@gmail.com>
| Date:   Tue May 11 13:40:41 2010 -0700
|
|     packed commit two
|
* commit 5001298e0c09ad9c34e4249bc5801c75e9754fa5
  Author: Scott Chacon <schacon@gmail.com>
  Date:   Tue May 11 13:40:23 2010 -0700

      packed commit one