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TopGit - A different patch queue manager


DESCRIPTION
-----------

TopGit aims to make handling of large amount of interdependent topic
branches easier. In fact, it is designed especially for the case
when you maintain a queue of third-party patches on top of another
(perhaps Git-controlled) project and want to easily organize, maintain
and submit them - TopGit achieves that by keeping a separate topic
branch for each patch and providing few tools to maintain the branches.


RATIONALE
---------

Why not use something like StGIT or Guilt or rebase -i for maintaining
your patch queue?  The advantage of these tools is their simplicity;
they work with patch _series_ and defer to the reflog facility for
version control of patches (reordering of patches is not
version-controlled at all). But there are several disadvantages -
for one, these tools (especially StGIT) do not actually fit well
with plain Git at all: it is basically impossible to take advantage
of the index efectively when using StGIT. But more importantly,
these tools horribly fail in the face of distributed environment.

TopGit has been designed around three main tenets:

        (i) TopGit is as thin layer on top of Git as possible.
You still maintain your index and commit using Git, TopGit will
only automate few indispensable tasks.

        (ii) TopGit is anxious about _keeping_ your history. It will
never rewrite your history and all metadata is also tracked by Git,
smoothly and non-obnoxiously. It is good to have a _single_ point
when the history is cleaned up, and that is at the point of inclusion
in the upstream project; locally, you can see how your patch has evolved
and easily return to older versions.

        (iii) TopGit is specifically designed to work in distributed
environment. You can have several instances of TopGit-aware repositories
and smoothly keep them all up-to-date and transfer your changes between
them.

As mentioned above, the main intended use-case for TopGit is tracking
third-party patches, where each patch is effectively a single topic
branch.  In order to flexibly accomodate even complex scenarios when
you track many patches where many are independent but some depend
on others, TopGit ignores the ancient Quilt heritage of patch series
and instead allows the patches to freely form graphs (DAGs just like
Git history itself, only "one lever higher"). For now, you have
to manually specify which patches does the current one depend
on, but TopGit might help you with that in the future in a darcs-like
fashion.

A glossary plug: The union (i.e. merge) of patch dependencies is
called a _base_ of the patch (topic branch).

Of course, TopGit is perhaps not the right tool for you:

        (i) TopGit is not complicated, but StGIT et al. are somewhat
simpler, conceptually.  If you just want to make a linear purely-local
patch queue, deferring to StGIT instead might make more sense.

        (ii) When using TopGit, your history can get a little hairy
over time, especially with all the merges rippling through. ;-)


SYNOPSIS
--------

        ## Create and evolve a topic branch
        $ tg create t/gitweb/pathinfo-action
        tg: Automatically marking dependency on master
        tg: Creating t/gitweb/pathinfo-action base from master...
        $ ..hack..
        $ git commit
        $ ..fix a mistake..
        $ git commit

        ## Create another topic branch on top of the former one
        $ tg create t/gitweb/nifty-links
        tg: Automatically marking dependency on t/gitweb/pathinfo-action
        tg: Creating t/gitweb/nifty-links base from t/gitweb/pathinfo-action...
        $ ..hack..
        $ git commit

        ## Create another topic branch on top of master and submit
        ## the resulting patch upstream
        $ tg create t/revlist/author-fixed master
        tg: Creating t/revlist/author-fixed base from master...
        $ ..hack..
        $ git commit
        $ tg patch -m
        tg: Sent t/revlist/author-fixed
        From: pasky@suse.cz
        To: git@vger.kernel.org
        Cc: gitster@pobox.com
        Subject: [PATCH] Fix broken revlist --author when --fixed-string

        ## Create another topic branch depending on two others non-trivially
        $ tg create t/whatever t/revlist/author-fixed t/gitweb/nifty-links
        tg: Creating t/whatever base from t/revlist/author-fixed...
        tg: Merging t/whatever base with t/gitweb/nifty-links...
        Merge failed!
        tg: Please commit merge resolution and call: tg create
        tg: It is also safe to abort this operation using `git reset --hard`
        tg: but please remember you are on the base branch now;
        tg: you will want to switch to a different branch.
        $ ..resolve..
        $ git commit
        $ tg create
        tg: Resuming t/whatever setup...
        $ ..hack..
        $ git commit

        ## Update a single topic branch and propagate the changes to
        ## a different one
        $ git checkout t/gitweb/nifty-links
        $ ..hack..
        $ git commit
        $ git checkout t/whatever
        $ tg info
        Topic Branch: t/whatever (1 commit)
        Subject: [PATCH] Whatever patch
        Base: 3f47ebc1
        Depends: t/revlist/author-fixed t/gitweb/nifty-links
        Needs update from:
                t/gitweb/nifty-links (1 commit)
        $ tg update
        tg: Updating base with t/gitweb/nifty-links changes...
        Merge failed!
        tg: Please commit merge resolution and call `tg update` again.
        tg: It is also safe to abort this operation using `git reset --hard`,
        tg: but please remember you are on the base branch now;
        tg: you will want to switch to a different branch.
        $ ..resolve..
        $ git commit
        $ tg update
        tg: Updating t/whatever against new base...
        Merge failed!
        tg: Please resolve the merge and commit. No need to do anything else.
        tg: You can abort this operation using `git reset --hard` now
        tg: and retry this merge later using `tg update`.
        $ ..resolve..
        $ git commit

        ## Update a single topic branch and propagate the changes
        ## further through the dependency chain
        $ git checkout t/gitweb/pathinfo-action
        $ ..hack..
        $ git commit
        $ git checkout t/whatever
        $ tg info
        Topic Branch: t/whatever (1/2 commits)
        Subject: [PATCH] Whatever patch
        Base: 0ab2c9b3
        Depends: t/revlist/author-fixed t/gitweb/nifty-links
        Needs update from:
                t/gitweb/pathinfo-action (<= t/gitweb/nifty-links) (1 commit)
        $ tg update
        tg: Recursing to t/gitweb/nifty-links...
        [t/gitweb/nifty-links] tg: Updating base with t/gitweb/pathinfo-action changes...
        Merge failed!
        [t/gitweb/nifty-links] tg: Please commit merge resolution and call `tg update` again.
        [t/gitweb/nifty-links] tg: It is also safe to abort this operation using `git reset --hard`,
        [t/gitweb/nifty-links] tg: but please remember you are on the base branch now;
        [t/gitweb/nifty-links] tg: you will want to switch to a different branch.
        [t/gitweb/nifty-links] tg: You are in a subshell. If you abort the merge,
        [t/gitweb/nifty-links] tg: use `exit` to abort the recursive update altogether.
        [t/gitweb/nifty-links] $ ..resolve..
        [t/gitweb/nifty-links] $ git commit
        [t/gitweb/nifty-links] $ tg update
        [t/gitweb/nifty-links] tg: Updating t/gitweb/nifty-links against new base...
        Merge failed!
        [t/gitweb/nifty-links] tg: Please resolve the merge and commit.
        [t/gitweb/nifty-links] tg: You can abort this operation using `git reset --hard`.
        [t/gitweb/nifty-links] tg: You are in a subshell. After you either commit or abort
        [t/gitweb/nifty-links] tg: your merge, use `exit` to proceed with the recursive update.
        [t/gitweb/nifty-links] $ ..resolve..
        [t/gitweb/nifty-links] $ git commit
        [t/gitweb/nifty-links] $ exit
        tg: Updating base with t/gitweb/nifty-links changes...
        tg: Updating t/whatever against new base...


USAGE
-----

The 'tg' tool of TopGit has several subcommands:

tg help
~~~~~~~
        Our sophisticated integrated help facility. Doesn't do
        a whole lot for now.

tg create
~~~~~~~~~
        Create a new TopGit-controlled topic branch of a given name
        (required argument) and switch to it. If no dependencies
        are specified (by extra arguments passed after the first one),
        the current branch is assumed to be the only dependency.

        After `tg create`, you should insert the patch description
        to the '.topmsg' file, which will already contain some
        pre-filled bits. You can set topgit.to, topgit.cc and topgit.bcc
        configuration variables in order to have `tg create`
        add these headers with given default values to '.topmsg'.

        The main task of `tg create` is to set up the topic branch
        base from the dependencies. This may fail due to merge conflicts.
        In that case, after you commit the conflicts resolution,
        you should call `tg create` again (without any arguments);
        it will detect that you are on a topic branch base ref and
        resume the topic branch creation operation.

tg delete
~~~~~~~~~
        Remove a TopGit-controlled topic branch of given name
        (required argument). Normally, this command will remove
        only empty branch (base == head); use '-f' to remove
        non-empty branch.

        Currently, this command will _NOT_ remove the branch from
        the dependency list in other branches. You need to take
        care of this _manually_. This is even more complicated
        in combination with '-f', in that case you need to manually
        unmerge the removed branch's changes from the branches
        depending on it.

        TODO: '-a' to delete all empty branches, depfix, revert

tg info
~~~~~~~
        Show a summary information about the current or specified
        topic branch.

tg patch
~~~~~~~~
        Generate a patch from the current or specified topic branch.
        This means that the diff between the topic branch base and
        head (latest commit) is shown, appended to the description
        found in the .topmsg file.

        The patch is by default simply dumped to stdout. In the future,
        tg patch will be able to automatically send the patches by mail
        or save them to files. (TODO)

        TODO: tg patch -i to base at index instead of branch,
                -w for working tree

tg remote
~~~~~~~~~
        Register given remote as TopGit-controlled. This will create
        the namespace for the remote branch bases and teach 'git fetch'
        and 'git push' to operate on them.

        It takes a mandatory remote name argument, and optional
        '--populate' switch - use that for your origin-style remote,
        it will seed the local topic branch system based on the
        remote topic branches. '--populate' will also make 'tg remote'
        automatically fetch the remote and 'tg update' to look at
        branches of this remote for updates by default.

tg summary
~~~~~~~~~~
        Show overview of all TopGit-tracked topic branches and their
        up-to-date status ('0' marks that it introduces no own changes,
        'D' marks that it is out-of-date wrt. its dependencies,
        '!' marks that it has missing dependencies (even recursively),
        'B' marks that it is out-of-date wrt. its base).

        TODO: Speed up by an order of magnitude
        TODO: Graph view

tg export
~~~~~~~~~
        Export a tidied-up history of the current topic branch
        and its dependencies, suitable for feeding upstream.
        Each topic branch corresponds to a single commit or patch
        in the cleaned up history (corresponding basically exactly
        to `tg patch` output for the topic branch).

        The command has two posible outputs now - either a Git branch
        with the collapsed history, or a quilt series in new directory.

        In case of producing collapsed history in new branch,
        You can use this collapsed structure either for providing
        a pull source for upstream, or further linearization e.g.
        for creation of a quilt series using git log:

                git log --pretty=email -p --topo-order origin..exported

        To better understand the function of `tg export`,
        consider this dependency structure of topic branches:

        origin/master - t/foo/blue - t/foo/red - master
                     `- t/bar/good <,----------'
                     `- t/baz      ------------'

        (Where each of the branches may have hefty history.) Then

        master$ tg export for-linus

        will create this commit structure on branch for-linus:

        origin/master - t/foo/blue -. merge - t/foo/red -.. merge - master
                     `- t/bar/good <,-------------------'/
                     `- t/baz      ---------------------'

        In case of the quilt mode,

        master$ tg export --quilt for-linus

        would create this directory for-linus:

        for-linus/t/foo/blue.diff
        for-linus/t/foo/red.diff
        for-linus/t/bar/good.diff
        for-linus/t/baz.diff
        for-linus/series:
                t/foo/blue.diff -p1
                t/bar/good.diff -p1
                t/foo/red.diff -p1
                t/baz.diff -p1

        The command works on the current topic branch
        and can be called either without a parameter
        (in that case, '--collapse' is assumed)
        and with one mandatory argument: the name of the branch
        where the exported result shall be stored.
        The branch will be silently overwritten if it exists already!
        Use git reflog to recover in case of mistake.

        Alternatively, call it with the '--quilt' parameter
        and an argument specifying the directory
        where the quilt series should be saved.

        Usage: tg export ([--collapse] BRANCH | --quilt DIR)

        TODO: Make stripping of non-essential headers configurable
        TODO: Make stripping of [PATCH] and other prefixes configurable
        TODO: --mbox option for other mode of operation
        TODO: -n option to prevent exporting of empty patches
        TODO: -a option to export all branches
        TODO: Allow branches to be exported to be passed as arguments, default
              to the current branch if none are specified
        TODO: For quilt exporting, use a temporary branch and remove it when
              done - this would allow producing conflict-less series

tg update
~~~~~~~~~
        Update the current topic branch wrt. changes in the branches
        it depends on and remote branches.
        This is performed in two phases - first,
        changes within the dependencies are merged to the base,
        then the base is merged into the topic branch.
        The output will guide you in case of conflicts.

        In case your dependencies are not up-to-date, tg update
        will first recurse into them and update these.

        TODO: tg update -a for updating all topic branches

TODO: Some infrastructure for sharing topic branches between
        repositories easily
TODO: tg depend for adding/removing dependencies smoothly
TODO: tg rename


IMPLEMENTATION
--------------

TopGit stores all the topic branches in the regular refs/heads/
namespace, (we recommend to mark them with the 't/' prefix).
Except that, TopGit also maintains a set of auxiliary refs in
refs/top-*. Currently, only refs/top-bases/ is used, containing
the current _base_ of the given topic branch - this is basically
a merge of all the branches the topic branch depends on; it is
updated during `tg update` and then merged to the topic branch,
and it is the base of a patch generated from the topic branch by
`tg patch`.

All the metadata is tracked within the source tree and history
of the topic branch itself, in .top* files; these files are kept
isolated within the topic branches during TopGit-controlled merges
and are of course omitted during `tg patch`. The state of these
files in base commits is undefined; look at them only in the topic
branches themselves.  Currently, two files are defined:

        .topmsg: Contains the description of the topic branch
in a mail-like format, plus the author information,
whatever Cc headers you choose or the post-three-dashes message.
When mailing out your patch, basically only few extra headers
mail headers are inserted and the patch itself is appended.
Thus, as your patches evolve, you can record nuances like whether
the paricular patch should have To-list/Cc-maintainer or vice
versa and similar nuances, if your project is into that.
From is prefilled from your current GIT_AUTHOR_IDENT, other headers
can be prefilled from various optional topgit.* config options.

        .topdeps: Contains the one-per-line list of branches
your patch depends on, pre-seeded with `tg create`. (Continuously
updated) merge of these branches will be the "base" of your topic
branch.

TopGit also automagically installs a bunch of custom commit-related
hooks that will verify if you are committing the .top* files in sane
state. It will add the hooks to separate files within the hooks/
subdirectory and merely insert calls of them to the appropriate hooks
and make them executable (but make sure the original hooks code
is not called if the hook was not executable beforehand).

Another automagically installed piece is .git/info/attributes specifier
for an 'ours' merge strategy for the files .topmsg and .topdeps, and
the (intuitive) 'ours' merge strategy definition in .git/config.


REMOTE HANDLING
---------------

There are three issues with accessing topic branches in remote repositories:

        (i) Fetching/pushing accurate picture of the remote topic branch setup
        (ii) Referring to remote topic branches from your local repository
        (iii) Developing some of the remote topic branches locally

(ii) and (iii) are fairly interconnected problems, while (i) is largely
independent. The issue is to accurately reflect the current state of the
quickly changing topic branches set - this can be easily done
with the current facilities like 'git remote prune' and 'git push --mirror' -
and to properly upload also the bases of the topic branches.
For this, we need to modify the fetch/push refspecs to also include
the refs/top-bases/ ref namespace; we shall provide a special 'tg remote'
command to set up an existing remote for TopGit usage.

About (ii) and (iii), there are two somewhat contradicting design
considerations:

        (a) Hacking on multiple independent TopGit remotes in a single
        repository
        (b) Having a self-contained topic system in local refs space

To us, (a) does not appear to be very convincing, while (b) is quite desirable
for 'git-log topic' etc. working, 'git push' automatically creating
self-contained topic system in the remote repository, and increased conceptual
simplicity.

Thus, we choose to instantiate all the topic branches of given remote locally;
this is performed by 'tg remote --populate'.
'tg update' will also check if a branch can be updated from its corresponding
remote branch. The logic is somewhat involved if we should DTRT.
First, we update the base, handling the remote branch as if it was the first
dependency; thus, conflict resolutions made in the remote branch will be
carried over to our local base automagically. Then, the base is merged into
remote branch and the result is merged to local branch - again, to carry over
remote conflict resolutions. In the future, this order might be adjustable
per-update in case local changes are diverging more than the remote ones.