How to reduce the depth of an existing git clone?

Question

I have a clone. I want to reduce the history on it, without cloning from scratch with a reduced depth. Worked example:

$ git clone git@github.com:apache/spa         


        

Answer 1

Edit, Feb 2017: this answer is now outdated / wrong. Git can make a shallow clone shallower, at least internally. Git 2.11 also has --deepen to increase the depth of a clone, and it looks as though there are eventual plans to allow negative values (though right now they are rejected). It's not clear how well this works in the real world, and your best bet is still to clone the clone, as in jthill's answer.

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You can only deepen a repository. This is primarily because Git is built around adding new stuff. The way shallow clones work is that your (receiving) Git gets the sender (another Git) to stop sending "new stuff" upon reaching the shallow-clone-depth argument, and coordinates with the sender so as to understand why they have stopped at that point even though more history is obviously required. They then write the IDs of "truncated" commits into a special file, .git/shallow, that both marks the repository as shallow, and notes which commits are truncated.

Note that during this process, your Git is still adding new stuff. (Also, when it has finished cloning and exits, Git forgets what the depth was, and over time it becomes impossible even to figure out what it was. All Git can tell is that this is a shallow clone, because the .git/shallow file containing commit IDs still exists.)

The rest of Git continues to be built around this "add new stuff" concept, so you can deepen the clone, but not increase its shallowness. (There's no good, agreed-upon verb for this: the opposite of deepening a pit is filling it in, but fill has the wrong connotation. Diminish might work; I think I'll use that.)

In theory, git gc, which is the only part of Git that ever actually throws anything out,¹ could perhaps diminish a repository, even converting a full clone into a shallow one, but no one has written code to do that. There are some tricky bits, e.g., do you discard tags? Shallow clones start out sans tags for implementation reasons, so converting a repository to shallow, or diminishing an existing shallow repository, might call for discarding at least some tags. Certainly any tag pointing to a commit wiped out by the diminish action would have to go.

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Meanwhile, the --depth argument to git-pack-objects (passed through from git repack) means something else entirely: it's the maximum length of a delta chain, when Git uses its modified xdelta compression on Git objects stored in each pack-file. This has nothing to do with the depth of particular parts of the commit DAG (as computed from each branch head).

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¹Well, git repack winds up throwing things out as a side effect, depending on which flags are used, but it's invoked this way by git gc. This is also true of git prune. For these two commands to really do their job properly, they need git reflog expire run first. The "normal user" end of the clean-things-up sequence is git gc; it deals with all of this. So we can say that git gc is how you discard accumulated "new stuff" that turned out to be unwanted after all.