Git the Hard Way

If you have no git experience, we recommend you to read the Collaborative Development with Git-Trac chapter instead. The git-trac simplifies the interaction with our git and trac servers.

If you want to contribute using git only, you are at the right place. This chapter will tell you how to do so, assuming some basic familiarity with git. In particular, you should have read Sage Development Process first.

We assume that you have a copy of the Sage git repository, for example by running:

[user@localhost ~]$ git clone git://github.com/sagemath/sage.git
[user@localhost ~]$ cd sage
[user@localhost sage]$ make

The Trac Server

The Sage trac server also holds a copy of the Sage repository, it is served via the ssh and git protocols. To add it as a remote repository to your local git repository, use these commands:

[user@localhost sage]$ git remote add trac git://trac.sagemath.org/sage.git -t master
[user@localhost sage]$ git remote set-url --push trac git@trac.sagemath.org:sage.git
[user@localhost sage]$ git remote -v
origin      git://github.com/sagemath/sage.git (fetch)
origin      git://github.com/sagemath/sage.git (push)
trac        git://trac.sagemath.org/sage.git (fetch)
trac        git@trac.sagemath.org:sage.git (push)

Instead of trac you can use any local name you want, of course. It is perfectly fine to have multiple remote repositories for git, think of them as bookmarks. You can then use git pull to get changes and git push to upload your local changes using:

[user@localhost sage]$ git <push|pull> trac [ARGS]

Note

In the command above we set up the remote to only track the master branch on the trac server (the -t master option). This avoids clutter by not automatically downloading all branches ever created. But it also means that you will not fetch everything that is on trac by default, and you need to explicitly tell git which branch you want to get from trac. See the Checking Out Tickets section for examples.

We set up the remote here to perform read-only operations (fetch) using the git protocol and write operations (push) using the ssh protocol (specified by the git@ part). To use the ssh protocol you need to have a trac account and to set up your ssh public key as described in Manually Linking your Public Key to your Trac Account. Authentication is necessary if you want to upload anything to ensure that it really is from you.

If you want to use ssh only, use these commands:

[user@localhost sage]$ git remote add trac git@trac.sagemath.org:sage.git -t master
[user@localhost sage]$ git remote -v
origin      git://github.com/sagemath/sage.git (fetch)
origin      git://github.com/sagemath/sage.git (push)
trac        git@trac.sagemath.org:sage.git (fetch)
trac        git@trac.sagemath.org:sage.git (push)

Checking Out Tickets

Trac tickets that are finished or in the process of being worked on can have a git branch attached to them. This is the “Branch:” field in the ticket description. The branch name is generally of the form u/user/description, where user is the name of the user who made the branch and description is some free-form short description (and can include further slashes).

If you want to work with the changes in that remote branch, you must make a local copy. In particular, git has no concept of directly working with the remote branch, the remotes are only bookmarks for things that you can get from/to the remote server. Hence, the first thing you should do is to get everything from the trac server’s branch into your local repository. This is achieved by:

[user@localhost sage]$ git fetch trac u/user/description
remote: Counting objects: 62, done.
remote: Compressing objects: 100% (48/48), done.
remote: Total 48 (delta 42), reused 0 (delta 0)
Unpacking objects: 100% (48/48), done.
From trac.sagemath.org:sage
* [new branch]      u/user/description -> FETCH_HEAD

The u/user/description branch is now temporarily (until you fetch something else) stored in your local git database under the alias FETCH_HEAD. In the second step, we make it available as a new local branch and switch to it. Your local branch can have a different name, for example:

[user@localhost sage]$ git checkout -b my_branch FETCH_HEAD
Switched to a new branch 'my_branch'

creates a new branch in your local git repository named my_branch and modifies your local Sage filesystem tree to the state of the files in that ticket. You can now edit files and commit changes to your local branch.

Pushing Your Changes to a Ticket

To add your local branch to a trac ticket, you should first decide on a name on the Sage trac repository.

For read/write permissions on git branches, see Note on Branch Names

In order to avoid name clashes, you can use u/your_username/a_description_of_your_branch (the description can contain slashes, but no spaces). Then:

  • Fill the Branch field of the trac ticket with that name.

  • Push your branch to trac with either:

    [user@localhost sage]$ git push --set-upstream trac HEAD:u/user/description
    

    if you started the branch yourself and do not follow any other branch, or use:

    [user@localhost sage]$ git push trac HEAD:u/user/description
    

    if your branch already has an upstream branch.

Here, HEAD means that you are pushing the most recent commit (and, by extension, all of its parent commits) of the current local branch to the remote branch.

The Branch field on the trac ticket can appear in red/green. See The Ticket Fields to learn what it means.

Getting Changes

A common task during development is to synchronize your local copy of the branch with the branch on trac. In particular, assume you downloaded somebody else’s branch made some suggestions for improvements on the trac ticket. Now the original author incorporated your suggestions into his branch, and you want to get the added changesets to complete your review. Assuming that you originally got your local branch as in Checking Out Tickets, you can just issue:

[user@localhost sage]$ git pull trac u/user/description
From trac.sagemath.org:sage
 * branch            u/user/description -> FETCH_HEAD
Updating 8237337..07152d8
Fast-forward
 src/sage/tests/cmdline.py      | 3 ++-
 1 file changed, 2 insertions(+), 1 deletions(-)

where now user is the other developer’s trac username and description is some description that he chose. This command will download the changes from the originally-used remote branch and merge them into your local branch. If you haven’t published your local commits yet then you can also rebase them via:

[user@localhost sage]$ git pull -r trac u/user/description
From trac.sagemath.org:sage
 * branch            u/user/description -> FETCH_HEAD
First, rewinding head to replay your work on top of it...
Applying: my local commit

See Merging and Rebasing section for an in-depth explanation of merge vs. rebase.

So far, we assumed that there are no conflicts. It is unavoidable in distributed development that, sometimes, the same location in a source source file is changed by more than one person. Reconciling these conflicting edits is explained in the Conflict Resolution section.

Updating Master

The master branch can be updated just like any other branch. However, your local copy of the master branch should stay identical to the trac master branch.

If you accidentally added commits to your local copy of master, you must delete them before updating the branch.

One way to ensure that you are notified of potential problems is to use git pull --ff-only, which will raise an error if a non-trivial merge would be required:

[user@localhost sage]$ git checkout master
[user@localhost sage]$ git pull --ff-only trac master

If this pull fails, then something is wrong with the local copy of the master branch. To switch to the correct Sage master branch, use:

[user@localhost sage]$ git checkout master
[user@localhost sage]$ git reset --hard trac/master

Merging and Rebasing

Sometimes, a new version of Sage is released while you work on a git branch.

Let us assume you started my_branch at commit B. After a while, your branch has advanced to commit Z, but you updated master (see Updating Master) and now your git history looks like this (see The History):

      X---Y---Z my_branch
     /
A---B---C---D master

How should you deal with such changes? In principle, there are two ways:

  • Rebase: The first solution is to replay commits X,Y,Z atop of the new master. This is called rebase, and it rewrites your current branch:

    git checkout my_branch
    git rebase -i master
    

    In terms of the commit graph, this results in:

                  X'--Y'--Z' my_branch
                 /
    A---B---C---D master
    

    Note that this operation rewrites the history of my_branch (see Rewriting History). This can lead to problems if somebody began to write code atop of your commits X,Y,Z. It is safe otherwise.

    Alternatively, you can rebase my_branch while updating master at the same time (see Getting Changes):

    git checkout my_branch
    git pull -r master
    
  • Merging your branch with master will create a new commit above the two of them:

    git checkout my_branch
    git merge master
    

    The result is the following commit graph:

          X---Y---Z---W my_branch
         /           /
    A---B---C-------D master
    
    • Pros: you did not rewrite history (see Rewriting History).The additional commit is then easily pushed to the git repository and distributed to your collaborators.
    • Cons: it introduced an extra merge commit that would not be there had you used rebase.

    Alternatively, you can merge my_branch while updating master at the same time (see Getting Changes):

    git checkout my_branch
    git pull master
    

In case of doubt use merge rather than rebase. There is less risk involved, and rebase in this case is only useful for branches with a very long history.

Finally, do nothing unless necessary: it is perfectly fine for your branch to be behind master. You can always merge/rebase if/when your branch’s name appears in red on its trac page (see The Ticket Fields), or when you will really need a feature that is only available in the current master.

Merge Tools

Simple conflicts can be easily solved with git only (see Conflict Resolution)

For more complicated ones, a range of specialized programs are available. Because the conflict marker includes the hash of the most recent common parent, you can use a three-way diff:

[alice@laptop]$ git mergetool

This message is displayed because 'merge.tool' is not configured.
See 'git mergetool --tool-help' or 'git help config' for more details.
'git mergetool' will now attempt to use one of the following tools:
meld opendiff kdiff3 [...] merge araxis bc3 codecompare emerge vimdiff
Merging:
fibonacci.py

Normal merge conflict for 'fibonacci.py':
  {local}: modified file
  {remote}: modified file
Hit return to start merge resolution tool (meld):

If you don’t have a favourite merge tool we suggest you try meld (cross-platform). The result looks like the following screenshot.

_images/meld-screenshot.png

The middle file is the most recent common parent; on the right is Bob’s version and on the left is Alice’s conflicting version. Clicking on the arrow moves the marked change to the file in the adjacent pane.