Can additional data be put into a git commit object?

The traditional way to attach data to a commit are "trailers": special formatted lines at the end of the commit message. The Signed-of-by line added by git commit -s (docu) is such a trailer.

Git has some direct support for these:

• git interpret-trailers (docu) is a helper for adding or parsing these lines
• git log has a format-placeholder %(trailers[:options]) (docu)
• git for-each-ref also has a %(trailers) placeholder (docu)

Trailers have some differences to git notes:

• The trailers are part of the commit message and hence covered by the hash. They are therefore immutable. git notes are add-ons to commits (or other stuff).
• Trailer data is cloned and pushed to/from remote repositories. git notes not by default.
• Trailer data is moved around by git cherry-pick, git rebase and others. git notes ... sometimes.

Yes, you can totally put more fields called “git commit extra headers” in there. They’ll show up with git cat-file -p, but the only way I see to get them in there in the first place with the standard git client is by editing the git source and recompiling.

In commit.h, there is a struct for extra headers, along with functions to add them when committing, and to read them out later.

struct commit_extra_header {
        struct commit_extra_header *next;
        char *key;
        char *value;
        size_t len;
};

int commit_tree_extended(const char *msg, size_t msg_len,
                        const struct object_id *tree,
                        struct commit_list *parents,
                        struct object_id *ret, const char *author,
                        const char *sign_commit,
                        struct commit_extra_header *);

struct commit_extra_header *read_commit_extra_headers(struct commit *, const char **);

commit.c contains a list of ‘standard’ header fields:

static inline int standard_header_field(const char *field, size_t len)
{
        return ((len == 4 && !memcmp(field, "tree", 4)) ||
                (len == 6 && !memcmp(field, "parent", 6)) ||
                (len == 6 && !memcmp(field, "author", 6)) ||
                (len == 9 && !memcmp(field, "committer", 9)) ||
                (len == 8 && !memcmp(field, "encoding", 8)));
}

with everything else showing up as an ‘extra’ header.

Doing a quick grep of the source code, the only current use I found of this is the gpgsig header used to include GPG signatures on commits.

Kiln Harmony, now discontinued, used fields such as kilnhgusername and kilnhgrawdescription here.

git notes will add an arbitrary note to any commit. Usage: git notes add <SHA>, e.g. git notes add HEAD. This will pull up the default text editor and allow you enter a note. You can also specify a message from the CLI in the same manner as you can with a commit, i.e. git notes add -m "my message" HEAD.

If you run a git log after taking a note, you will see:

commit 06ca03a20bb01203e2d6b8996e365f46cb6d59bd
Author: Example User <[email protected]>
Date:   Mon March 28 14:50:15 2016 -0700

    commit message

Notes:
    my message

Notes can be given namespaces as well. This article has the complete documentation.

You can use git hash-object -t commit -w FILE to create your own commit object with your custom git headers from a file, it will print the commit id of the newly created commit to stdout. It's easiest if you start from an existing commit object, add your custom header, create a new commit object and reset your branch to point to the newly created commit object. You can dump a commit object to a file using git cat-file -p COMMIT_ID > FILE.

For example this one-liner adds "myheader foo bar" to the last commit:

git reset "$(git cat-file -p HEAD | sed -e '0,/^$/s/^$/myheader foo bar\n/' | git hash-object -t commit -w --stdin)"