[ANNOUNCE] git_fast_filter

[Elijah Newren <newren@gmail.com>]

Just thought I'd make this available, in case there's others with
niche needs that find it useful...


git_fast_filter assists with quickly rewriting the history of a repository
by making it easy to write scripts whose purpose is to serve as safe
filters between fast-export and fast-import.  git_fast_filter comes with
example programs, a basic test-suite, and a double your money back
satisfaction guarantee.  (I love free software.)  You can get it from

  git://gitorious.org/git_fast_filter/mainline.git

In more detail...

=== Purpose ===

git_fast_filter is designed to make it easy to filter or rewrite the
history of a repository.  As such, it fills the same role as
git-filter-branch, and was written primarily to overcome the sometimes
severe speed shortcomings of git-filter-branch.  In particular, using
git_fast_filter can avoid thousands or millions of new process forks, and
can allow you to rewrite the same file only one time instead of 50,000
times.  However, while using git_fast_filter is fairly simple and quick, it
is hard to beat writing a simple git-filter-branch one-liner for efficiency
of human time.  Also, the two tools use very different methods of rewriting
history and do not have exactly overlapping feature sets, so the best tool
for a particular job is going to be very problem dependent.

As human time is often more important than computer time, especially for
one-shot rewrites, git-filter-branch will probably continue to be the more
common tool.  However, git_fast_filter is useful in cases where computer
time of a rewrite matters (particularly larger repositories and more
involved rewrites that need to be run and tested many times on large data
sets).  Also git_fast_filter has a couple features that may come in handy
in special cases (assisting with generating fast-export output from
scratch, interleaving commits from seperate repositories, and bidirectional
collaboration between filtered and unfiltered repositories).

=== Idea ===

The way git_fast_filter works is by providing a simple python library,
git_fast_filter.py.  This library can be used in simple python scripts to
create a filter for the output of git-fast-export.  Thus, the typical
calling convention is of the form:

    git fast-export | filter_script.py | git fast-import

=== Example ===

An example script that renames the 'master' branch to 'other is shown
below (this is similar to the example in the git-fast-export manpage, but
is safe against the string 'refs/heads/master' appearing in some file or
commit message in the repository):

  #!/usr/bin/python

  from git_fast_filter import Commit, FastExportFilter

  def my_commit_callback(commit):
    if commit.branch == "refs/heads/master":
      commit.branch = "refs/heads/other"

  filter = FastExportFilter(commit_callback = my_commit_callback)
  filter.run()

The user can then run this script by:
  $ mkdir target && cd target && git init
  $ (cd /PATH/LEADING/TO/source && git fast-export --all) \
       | /PATH/TO/filter_script.py | git fast-import

(Note: The user can have the script take care of the git init, the cd's,
and the invocations of git fast-export and git fast-import by just passing
directory names to FastExportFilter.run; however, writing out the details
explicitly as in the above example makes it clearer what is going on.)


Elijah