GIT

   1 =========================== WHY USE GIT+TOPGIT? ==========================
   2
   3 Three major concerns were on our mind when setting up this project.
   4
   5   o First we needed to structure the project in such a way that it would be
   6     easy to rebase all of our changes on the latest official ZFS release
   7     from Sun.  We absolutely need to be able to benefit from the upstream
   8     improvements and not get locked in to an old version of the code base.
   9
  10   o Secondly, we wanted to be able to easily manage our changes in terms
  11     of a patch stack or graph.  This allows us to easily isolate specific
  12     changes and push them upstream for inclusion.  It also allows us to
  13     easily update or drop specific changes based on what occurs upstream.
  14
  15   o Thirdly we needed our DVCS to be integrated with the management of this
  16     patch stack or graph.  We have tried other methods in the past such as
  17     SVN+Quilt but have found managing the patch stack becomes cumbersome.
  18     By using Git+TopGit to more tightly integrate our patches in to the repo
  19     we expect several benefits.  One of the most important will be the
  20     ability to easily work on the patch's with a distributed development
  21     team, additionally the repo can track patch history, and we can utilize
  22     Git to merge patches and resolve conflicts.
  23
  24 TopGit is designed to specifically address these concerns by providing
  25 tools to simplify the handling of large numbers of interdependent topic
  26 branches.  When using a TopGit aware repo every topic branch represents
  27 a 'patch' and that branch references its dependent branches.  The union
  28 of all these branches is your final source base.
  29
  30 ========================= SETTING UP GIT+TOPGIT ==========================
  31
  32 First off you need to install a Git package on your system.  For my
  33 purposes I have been working on a RHEL5 system with git version 1.5.4.5
  34 installed and it has been working well.  You will also need to go get
  35 the latest version of TopGit which likely is not packaged nicely so you
  36 will need to build it from source.  You can use Git to clone TopGit
  37 from the official site here and your all set:
  38
  39         > git clone git://repo.or.cz/w/topgit.git
  40         > make
  41         > make install    # Default installs to $(HOME)
  42
  43 ========================== TOPGIT AND ZFS ================================
  44
  45 One you have Git and TopGit installed you will want to clone a copy of
  46 the Linux ZFS repo.  While this project does not yet have a public home
  47 it hopefully will some day.  In the meanwhile if you have VPN access to
  48 LLNL you can clone the latest official repo here.  Cloning a TopGit
  49 controlled repo is very similar to cloning a normal Git repo, but you
  50 need to remember to use 'tg remote' to populate all topic branches.
  51
  52         > git clone http://eris.llnl.gov/git/zfs.git zfs
  53         > cd zfs
  54         > tg remote --populate origin
  55
  56 Now that you have the Linux ZFS repo the first thing you will probably
  57 want to do is have a look at all the topic branches.  TopGit provides
  58 a summary command which shows all the branches and a brief summary for
  59 each branch obtained from the .topmsg files.
  60
  61         > tg summary
  62  0      feature-branch                  [PATCH] feature-branch
  63         feature-commit-cb               [PATCH] feature commit cb
  64         feature-zap-cursor-to-key       [PATCH] feature zap cursor to key
  65         ...
  66
  67 By convention all TopGit branches are usually prefixed with 't/', however
  68 I have chosen not to do this for simplicity.  A different convention I have
  69 adopted is to tag the top most TopGit branch as 'top' for easy reference.
  70 This provides a consistent label to be used when you need to reference the
  71 branch which contains the union of all topic branches.
  72
  73 One thing you may also notice about the 'tg summary' command is it does
  74 not show the branches in dependent order.  This is done because TopGit allows
  75 each branch to express multiple dependencies as a DAC.  Initially this seemed
  76 like an added complication which I planned to avoid by just implementing a
  77 stack using the graph.   However, this ended up being problematic because
  78 with a stack when a change was made to a branch near the base, it was a
  79 very expensive operation to merge the change up to the top of the stack.
  80 By defining the dependencies as a graph it is possible to keep the depth
  81 much shallower thus minimizing the merging.  It has also proved insightful
  82 as to each patches actual dependencies.
  83
  84 To see the dependencies you will need to use the --graphviz option and pipe
  85 the result to dot for display.  The following command works fairly well for
  86 me.  Longer term it would be nice to update this option to use a preferred
  87 config options stored in the repo.
  88
  89         > tg summary --graphviz | dot -Txlib -Nfontsize=8
  90
  91 ========================= UPDATING A TOPIC BRANCH ========================
  92
  93 Updating a topic branch in TopGit is a pretty straight forward but there
  94 are a few rules you need to be aware of.  The basic process involves
  95 checking out the relevant topic branch where the changes need to be made,
  96 making the changes, committing the changes to the branch and then merging
  97 those changes in to dependent branches.  TopGit provides some tools to make
  98 this pretty easy, although it may be a little sluggish depending on how many
  99 dependent branches are impacted by the change.  Here is an example:
 100
 101         > git checkout modify-topic-branch  # Checkout the proper branch
 102         > ...update branch...               # Update the branch
 103         > git commit -a                     # Commit your changes
 104         > git checkout top                  # Checkout the top branch
 105         > tg update                         # Recursively merge in new branch
 106
 107 Assuming you change does not introduce any conflicts your done.  All branches
 108 were dependent on your change will have had the changed merged in.  If your
 109 change introduced a conflict you will need to resolve the conflict and then
 110 continue on with the update.
 111
 112 ========================== ADDING A TOPIC BRANCH =========================
 113
 114 Adding a topic branch in TopGit can be pretty straight forward.  If your
 115 adding a non-conflicting patch in parallel with other patches of the same
 116 type, then things are pretty easy and TopGit does all the work.
 117
 118         > git co existing-topic-branch      # Checkout the branch to add after
 119         > tg create new-topic-branch        # Create a new topic branch
 120         > ...update .topmsg...              # Update the branch message
 121         > ...create patch...                # Update with your changes
 122         > git commit -a                     # Commit your changes
 123         > git co dependent-topic-branch     # Checkout dependent branch
 124         > tg depend add new-topic-branch    # Update dependencies
 125         > git checkout top                  # Checkout the top branch
 126         > tg update                         # Recursively merge in new branch
 127
 128 If you need to add your patch in series with another change things are
 129 a little more complicated.  In this case TopGit does not yet support removing
 130 dependencies so you will need to do it by hand, as follows.
 131
 132         > git co existing-topic-branch      # Checkout the branch to add after
 133         > tg create new-topic-branch        # Create a new topic branch
 134         > ...update .topmsg...              # Update the branch message
 135         > ...create patch...                # Update with your changes
 136         > git commit -a                     # Commit your changes
 137         > git co dependent-topic-branch     # Checkout dependent branch
 138         > ...update .topdeps...             # Manually update dependencies
 139         > git commit -a                     # Commit your changes
 140         > tg update                         # TopGit update
 141         > git checkout top                  # Checkout the top branch
 142         > tg update                         # Recursively merge in new branch
 143
 144 Once your done, I find it is a good idea view the repo using the
 145 'tg summary --graphviz' command and verify the updated dependency graph.
 146
 147 ========================= REMOVING A TOPIC BRANCH ========================
 148
 149 Removing a topic branch in TopGit is also currently not very easy.  To remove
 150 a dependent branch the basic process is to commit a patch which reverts all
 151 changes on the branch.  Then that reversion must be merged in to all dependent
 152 branches, the dependencies manually updated and finally the branch removed.
 153 If the branch is not empty you will not be able to remove it.
 154
 155         > git co delete-topic-branch        # Checkout the branch to delete
 156         > tg patch | patch -R -p1           # Revert all branch changes
 157         > git commit -a                     # Commit your changes
 158         > git checkout top                  # Checkout the top branch
 159         > tg update                         # Recursively merge revert
 160         > git co dependent-topic-branch     # Checkout dependent branch
 161         > ...update .topdeps...             # Manually update dependencies
 162         > git commit -a                     # Commit your changes
 163         > tg delete delete-topic-branch     # Delete empty topic branch
 164
 165 Once your done, I find it is a good idea view the repo using the
 166 'tg summary --graphviz' command and verify the updated dependency graph.
 167
 168 ============================ TOPGIT TODO =================================
 169
 170 TopGit is still a young package which seems to be under active development
 171 by its author.  It provides the minimum set of commands needed but there
 172 are clearly areas which simply have not yet been implemented.  My short
 173 list of features includes:
 174
 175   o 'tg summary --deps', option to display a text version of the topic
 176     branch dependency DAC.
 177
 178   o 'tg depend list', list all topic branch dependencies.
 179
 180   o 'tg depend delete', cleanly remove a topic branch dependency.
 181
 182   o 'tg create', cleanly insert a topic branch in the middle
 183     of the graph and properly take care updating all dependencies.
 184
 185   o 'tg delete', cleanly delete a topic branch in the middle
 186     of the graph and properly take care updating all dependencies.