git-filter-repo/README.md

git filter-repo is a versatile tool for rewriting history, which includes capabilities I have not found anywhere else. It roughly falls into the same space of tool as git filter-branch but without the capitulation-inducing poor performance, and with a design that scales usability-wise beyond trivial rewriting cases.

While most users will probably just use filter-repo as a simple command line tool (and likely only use a few of its flags), at its core filter-repo contains a library for creating history rewriting tools. As such, users with specialized needs can leverage it to quickly create entirely new history rewriting tools.

filter-repo is a single-file python script, depending only on the python standard library (and execution of git commands), all of which is designed to make build/installation trivial: just copy it into your $PATH.

Table of Contents

• Background
  • Why filter-repo instead of filter-branch?
  • Example usage, comparing to filter-branch
  • Design rationale behind filter-repo
• Usage
  • The bigger picture
  • Examples

Background

Why filter-repo instead of filter-branch?

filter-branch has a number of problems:

• filter-branch is extremely to unusably slow (multiple orders of magnitude slower than it should be) for non-trivial repositories.

• filter-branch made a number of usability choices that are okay for small repos, but these choices sometimes conflict as more options are combined, and the overall usability often causes difficulties for users trying to work with intermediate or larger repos.

• filter-branch is missing some basic features.

The first two are intrinsic to filter-branch's design at this point and cannot be backward-compatibly fixed.

Example usage, comparing to filter-branch

Let's say that we want to extract a piece of a repository, with the intent on merging just that piece into some other bigger repo. For extraction, we want to:

• extract the history of a single directory, src/. This means that only paths under src/ remain in the repo, and any commits that only touched paths outside this directory will be removed.
• rename all files to have a new leading directory, my-module/ (e.g. so that src/foo.c becomes my-module/src/foo.c)
• rename any tags in the extracted repository to have a 'my-module-' prefix (to avoid any conflicts when we later merge this repo into something else)

Doing this with filter-repo is as simple as the following command:

  git filter-repo --path src/ --to-subdirectory-filter my-module --tag-rename '':'my-module-'

(the single quotes are unnecessary, but make it clearer to a human that we are replacing the empty string as a prefix with my-module-)

By contrast, filter-branch comes with a pile of caveats (more on that below) even once you figure out the necessary invocation(s):

  git filter-branch --tree-filter 'mkdir -p my-module && git ls-files | grep -v ^src/ | xargs git rm -f -q && ls -d * | grep -v my-module | xargs -I files mv files my-module/' --tag-name-filter 'echo "my-module-$(cat)"' --prune-empty -- --all
  git clone file://$(pwd) newcopy
  cd newcopy
  git for-each-ref --format="delete %(refname)" refs/tags/ | grep -v refs/tags/my-module- | git update-ref --stdin
  git gc --prune=now

Some might notice that the above filter-branch invocation will be really slow due to using --tree-filter; you could alternatively use the --index-filter option of filter-branch, changing the above commands to:

  git filter-branch --index-filter 'git ls-files | grep -v ^src/ | xargs git rm -q --cached; git ls-files -s | sed "s-$(printf \\t)-&my-module/-" | git update-index --index-info; git ls-files | grep -v ^my-module/ | xargs git rm -q --cached' --tag-name-filter 'echo "my-module-$(cat)"' --prune-empty -- --all
  git clone file://$(pwd) newcopy
  cd newcopy
  git for-each-ref --format="delete %(refname)" refs/tags/ | grep -v refs/tags/my-module- | git update-ref --stdin
  git gc --prune=now

However, for either filter-branch command there are a pile of caveats. First, some may be wondering why I list five commands here for filter-branch. Despite the use of --all and --tag-name-filter, and filter-branch's manpage claiming that a clone is enough to get rid of old objects, the extra steps to delete the other tags and do another gc are still required to clean out the old objects and avoid mixing new and old history before pushing somewhere. Other caveats:

• Commit messages are not rewritten; so if some of your commit messages refer to prior commits by (abbreviated) sha1, after the rewrite those messages will now refer to commits that are no longer part of the history. It would be better to rewrite those (abbreviated) sha1 references to refer to the new commit ids.
• The --prune-empty flag sometimes misses commits that should be pruned, and it will also prune commits that started empty rather than just ended empty due to filtering. For repositories that intentionally use empty commits for versioning and publishing related purposes, this can be detrimental.
• The commands above are OS-specific. GNU vs. BSD issues for sed, xargs, and other commands often trip up users; I think I failed to get most folks to use --index-filter since the only example in the filter-branch manpage that both uses it and shows how to move everything into a subdirectory is linux-specific, and it is not obvious to the reader that it has a portability issue since it silently misbehaves rather than failing loudly.
• The --index-filter version of the filter-branch command may be two to three times faster than the --tree-filter version, but both filter-branch commands are going to be multiple orders of magnitude slower than filter-repo.

Design rationale behind filter-repo (why create a new tool?)

None of the existing repository filtering tools do what I want. They're all good in their own way, but come up short for my needs. No tool provided any of the first eight traits below I wanted, and all failed to provide at least one of the last four traits as well:

1. [Starting report] Provide user an analysis of their repo to help them get started on what to prune or rename, instead of expecting them to guess or find other tools to figure it out. (Triggered, e.g. by running the first time with a special flag, such as --analyze.)

2. [Keep vs. remove] Instead of just providing a way for users to easily remove selected paths, also provide flags for users to only keep certain paths. Sure, users could workaround this by specifying to remove all paths other than the ones they want to keep, but the need to specify all paths that ever existed in any version of the repository could sometimes be quite painful. For filter-branch, using pipelines like git ls-files | grep -v ... | xargs -r git rm might be a reasonable workaround but can get unwieldy and isn't as straightforward for users; plus those commands are often operating-system specific (can you spot the GNUism in the snippet I provided?).

3. [Renaming] It should be easy to rename paths. For example, in addition to allowing one to treat some subdirectory as the root of the repository, also provide options for users to make the root of the repository just become a subdirectory. And more generally allow files and directories to be easily renamed. Provide sanity checks if renaming causes multiple files to exist at the same path. (And add special handling so that if a commit merely copied oldname->newname without modification, then filtering oldname->newname doesn't trigger the sanity check and die on that commit.)

4. [More intelligent safety] Writing copies of the original refs to a special namespace within the repo does not provide a user-friendly recovery mechanism. Many would struggle to recover using that. Almost everyone I've ever seen do a repository filtering operation has done so with a fresh clone, because wiping out the clone in case of error is a vastly easier recovery mechanism. Strongly encourage that workflow by detecting and bailing if we're not in a fresh clone, unless the user overrides with --force.

5. [Auto shrink] Automatically remove old cruft and repack the repository for the user after filtering (unless overridden); this simplifies things for the user, helps avoid mixing old and new history together, and avoids problems where the multi-step process for shrinking the repo documented in the manpage doesn't actually work in some cases. (I'm looking at you, filter-branch.)

6. [Clean separation] Avoid confusing users (and prevent accidental re-pushing of old stuff) due to mixing old repo and rewritten repo together. (This is particularly a problem with filter-branch when using the --tag-name-filter option, and sometimes also an issue when only filtering a subset of branches.)

7. [Versatility] Provide the user the ability to extend the tool or even write new tools that leverage existing capabilities, and provide this extensibility in a way that (a) avoids the need to fork separate processes (which would destroy performance), (b) avoids making the user specify OS-dependent shell commands (which would prevent users from sharing commands with each other), (c) takes advantage of rich data structures (because hashes, dicts, lists, and arrays are prohibitively difficult in shell) and (d) provides reasonable string manipulation capabilities (which are sorely lacking in shell).

8. [Old commit references] Provide a way for users to use old commit IDs with the new repository (in particular via mapping from old to new hashes with refs/replace/ references).

9. [Commit message consistency] If commit messages refer to other commits by ID (e.g. "this reverts commit 01234567890abcdef", "In commit 0013deadbeef9a..."), those commit messages should be rewritten to refer to the new commit IDs.

10. [Become-empty pruning] Commits which become empty due to filtering should be pruned. If the parent of a commit is pruned, the first non-pruned ancestor needs to become the new parent. If no non-pruned ancestor exists and the commit was not a merge, then it becomes a new root commit. If no non-pruned ancestor exists and the commit was a merge, then the merge will have one less parent (and thus make it likely to become a non-merge commit which would itself be pruned if it had no file changes of its own). One special thing to note here is that we prune commits which become empty, NOT commits which start empty. Some projects intentionally create empty commits for versioning or publishing reasons, and these should not be removed. (As a special case, commits which started empty but whose parent was pruned away will also be considered to have "become empty".)

11. [Become-degenerate pruning] Pruning of commits which become empty can potentially cause topology changes, and there are lots of special cases. Normally, merge commits are not removed since they are needed to preserve the graph topology, but the pruning of parents and other ancestors can ultimately result in the loss of one or more parents. A simple case was already noted above: if a merge commit loses enough parents to become a non-merge commit and it has no file changes, then it too can be pruned. Merge commits can also have a topology that becomes degenerate: it could end up with the merge_base serving as both parents (if all intervening commits from the original repo were pruned), or it could end up with one parent which is an ancestor of its other parent. In such cases, if the merge has no file changes of its own, then the merge commit can also be pruned. However, much as we do with empty pruning we do not prune merge commits that started degenerate (which indicates it may have been intentional, such as with --no-ff merges) but only merge commits that become degenerate and have no file changes of their own.

12. [Speed] Filtering should be reasonably fast

Usage

The bigger picture

Using filter-repo is relatively simple, but rewriting history is part of a larger discussion in terms of collaboration. When you rewrite history, the old and new histories are no longer compatible; if you push this history somewhere for others to view, it will look as though you've done a rebase of all branches and tags. Make sure you are familiar with the "Recovering from upstream rebase" section of git-rebase(1) (and in particular, "The hard case") before proceeding, in addition to this section.

Steps to use filter-repo as part of the bigger picture of doing a history rewrite are roughly as follows:

1. Create a clone of your repository (if you created special refs outside of refs/heads/ or refs/tags/, make sure to fetch those too). Note that --bare and --mirror clones are supported too, if you prefer.

2. (Optional) Run git filter-repo --analyze. This will create a directory of reports mentioning renames that have occurred in your repo and also listing sizes of objects aggregated by path/directory/extension/blob-id; this information may be useful in choosing how to filter your repo. It can also be useful to re-run --analyze after filtering to verify the changes look correct.

3. Run filter-repo with your desired filtering options. Many examples are given below. For more complex cases, note that doing the filtering in multiple steps (by running multiple filter-repo invocations in a sequence) is supported. If anything goes wrong here, simply delete your clone and restart.

4. Push your new repository to its new home (note that refs/remotes/origin/* will have been moved to refs/heads/* as the first part of filter-repo, so you can just deal with normal branches instead of remote tracking branches). While you can force push this to the same URL you cloned from, there are good reasons to consider pushing to a different location instead:

   1. People who cloned from the original repo will have old history. When they fetch the new history you force pushed up, unless they do a git reset --hard @{u} on their branches or rebase their local work, git will think they have hundreds or thousands of commits with very similar commit messages as what exist upstream (but which include files you wanted excised from history), and allow the user to merge the two histories, resulting in what looks like two copies of each commit. If they then push this history back up, then everyone now has history with two copies of each commit and the bad files have returned. You're more likely to succeed in forcing people to get rid of the old history if they have to clone a new URL.

   2. Rewriting history will rewrite tags; those who have already downloaded tags will not get the updated tags by default (see the "On Re-tagging" section of the git-tag(1) manpage). Every user trying to use an existing clone will have to forcibly delete all tags and re-fetch them; it may be easier for them to just re-clone, which they are more likely to do with a new clone URL.

   3. Rewriting history may delete some refs (e.g. branches that only had files that you wanted excised from history); unless you run git push with the --mirror or --prune options, those refs will continue to exist on the server. If folks then merge these branches into others, then people have started mixing old and new history. If users had already cloned these branches, removing them from the server isn't enough; you need all users to delete any local branches based on these refs and run fetch with the --prune option as well. Simply re-cloning from a new URL is easier.

   4. The server may not allow you to force push over some refs. For example, code review systems may have special ref namespaces (e.g. refs/changes/, refs/pull/, refs/merge-requests/) that they have locked down.

5. (Optional) Some additional considerations

   1. filter-repo by default creates replace refs (see git-replace(1)) for each rewritten commit ID, allowing you to use old (unabbreviated) commit hashes to refer to the newly rewritten commits. If you want to use these replace refs, push them to the relevant clone URL and tell users to adjust their fetch refspec (e.g. git config --add remote.origin.fetch +refs/replace/*:refs/replace/*) Sadly, some existing git servers (e.g. Gerrit, GitHub) do not yet understand replace refs, and thus one can't use old commit hashes within their UI; this may change in the future. But replace refs at least help users locally within the git CLI.

   2. If you have a central repo, you may want to prevent people from pushing old commit IDs, in order to avoid mixing old and new history. Every repository manager does this differently, some provide specialized commands, others require you to write hooks.

Examples

git clone URL Run git filter-repo -h; more detailed docs will be added soon...