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Develop VTK with Git

This page documents how to develop VTK through Git. See the README for more information.

Git is an extremely powerful version control tool that supports many different "workflows" for individual development and collaboration. Here we document procedures used by the VTK development community. In the interest of simplicity and brevity we do not provide an explanation of why we use this approach.

Setup

Before you begin, perform initial setup:

  1. Register GitLab Access to create an account and select a user name.

  2. Fork VTK into your user's namespace on GitLab.

  3. Follow the download instructions to create a local clone of the main VTK repository:

    $ git clone https://gitlab.kitware.com/vtk/vtk.git VTK
$ cd VTK

    The main repository will be configured as your origin remote.

  4. Run the developer setup script to prepare your VTK work tree and create Git command aliases used below:

    $ ./Utilities/SetupForDevelopment.sh

    This will prompt for your GitLab user name and configure a remote called gitlab to refer to it.

  5. (Optional but highly recommended.) Register with the VTK project on Kitware's CDash instance to better know how your code performs in regression tests. After registering and signing in, click on "All Dashboards" link in the upper left corner, scroll down and click "Subscribe to this project" on the right of VTK.

Workflow

VTK development uses a branchy workflow based on topic branches. Our collaboration workflow consists of three main steps:

  1. Local Development:

  2. Code Review (requires GitLab Access):

  3. Integrate Changes:

Update

  1. Update your local master branch:

    $ git checkout master
$ git pull

  2. Optionally push master to your fork in GitLab:

    $ git push gitlab master

    to keep it in sync. The git gitlab-push script used to Share a Topic below will also do this.

Create a Topic

All new work must be committed on topic branches. Name topics like you might name functions: concise but precise. A reader should have a general idea of the feature or fix to be developed given just the branch name.

  1. To start a new topic branch:

    $ git fetch origin

  2. For new development, start the topic from origin/master:

    $ git checkout -b my-topic origin/master

    For release branch fixes, start the topic from origin/release, and by convention use a topic name starting in release-:

    $ git checkout -b release-my-topic origin/release

    If backporting a change, you may rebase the branch back onto origin/release:

    $ git checkout -b release-my-topic my-topic
$ git rebase --onto origin/release origin/master

    Alternatively, for more targeted or aggregate backports, use the -x flag when performing git cherry-pick so that a reference to the original commit is added to the commit message:

    $ git checkout -b release-my-topic origin/release
$ git cherry-pick -x $hash_a $hash_b $hash_c
$ git cherry-pick -x $hash_d $hash_e $hash_f

  3. Edit files and create commits (repeat as needed):

    $ edit file1 file2 file3
$ git add file1 file2 file3
$ git commit

    Caveats:

Guidelines for Commit logs

Remember to motivate & summarize. When writing commit logs, make sure that there is enough information there for any developer to read and glean relevant information such as:

  1. Is this change important and why?
  2. If addressing an issue, which issue(s)?
  3. If a new feature, why is it useful and/or necessary?
  4. Are there background references or documentation?

A short description of what the issue being addressed and how will go a long way towards making the log more readable and the software more maintainable.

Style guidelines for commit logs are as follows:

  1. Separate subject from body with a blank line
  2. Limit the subject line to 60 characters
  3. Capitalize the subject line
  4. Use the imperative mood in the subject line e.g. "Refactor foo" or "Fix Issue #12322", instead of "Refactoring foo", or "Fixing issue #12322".
  5. Wrap the body at 80 characters
  6. Use the body to explain what and why and if applicable a brief how.

Share a Topic

When a topic is ready for review and possible inclusion, share it by pushing to a fork of your repository in GitLab. Be sure you have registered and signed in for GitLab Access and created your fork by visiting the main VTK GitLab repository page and using the "Fork" button in the upper right.

  1. Checkout the topic if it is not your current branch:

    $ git checkout my-topic

  2. Check what commits will be pushed to your fork in GitLab:

    $ git prepush

  3. Push commits in your topic branch to your fork in GitLab:

    $ git gitlab-push

    Notes:

    • If you are revising a previously pushed topic and have rewritten the topic history, add -f or --force to overwrite the destination.
    • If the topic adds data see this note.
    • The gitlab-push script also pushes the master branch to your fork in GitLab to keep it in sync with the upstream master.

    The output will include a link to the topic branch in your fork in GitLab and a link to a page for creating a Merge Request.

Create a Merge Request

(If you already created a merge request for a given topic and have reached this step after revising it, skip to the next step.)

Visit your fork in GitLab, browse to the "Merge Requests" link on the left, and use the "New Merge Request" button in the upper right to reach the URL printed at the end of the previous step. It should be of the form:

https://gitlab.kitware.com/<username>/vtk/merge_requests/new

Follow these steps:

  1. In the "Source branch" box select the <username>/vtk repository and the my-topic branch.

  2. In the "Target branch" box select the vtk/vtk repository and the master branch. It should be the default.

    If your change is a fix for the release branch, you should still select the master branch as the target because the change needs to end up there too.

    For other release branches (e.g., release-6.3), merge requests should go directly to the branch (they are not tied with master in our workflow).

  3. Use the "Compare branches" button to proceed to the next page and fill out the merge request creation form.

  4. In the "Title" field provide a one-line summary of the entire topic. This will become the title of the Merge Request.

    Example Merge Request Title:

    Wrapping: Add Java 1.x support

  5. In the "Description" field provide a high-level description of the change the topic makes and any relevant information about how to try it.

    • Use @username syntax to draw attention of specific developers. This syntax may be used anywhere outside literal text and code blocks. Or, wait until the next step and add comments to draw attention of developers.
    • If your change is a fix for the release branch, indicate this so that a maintainer knows it should be merged to release.
    • Optionally use a fenced code block with type message to specify text to be included in the generated merge commit message when the topic is merged.

    Example Merge Request Description:

    This branch requires Java 1.x which is not generally available yet.
Get Java 1.x from ... in order to try these changes.

```message
Add support for Java 1.x to the wrapping infrastructure.
```

Cc: @user1 @user2

  6. The "Assign to", "Milestone", and "Labels" fields may be left blank.

  7. Use the "Submit merge request" button to create the merge request and visit its page.

Guidelines for Merge Requests

Remember to motivate & summarize. When creating a merge request, consider the reviewers and future perusers of the software. Provide enough information to motivate the merge request such as:

  1. Is this merge request important and why?
  2. If addressing an issue, which issue(s)?
  3. If a new feature, why is it useful and/or necessary?
  4. Are there background references or documentation?

Also provide a summary statement expressing what you did and if there is a choice in implementation or design pattern, the rationale for choosing a certain path. Notable software or data features should be mentioned as well.

A well written merge request will motivate your reviewers, and bring them up to speed faster. Future software developers will be able to understand the reasons why something was done, and possibly avoid chasing down dead ends, Although it may take you a little more time to write a good merge request, you’ll likely see payback in faster reviews and better understood and maintainable software.

Review a Merge Request

Add comments mentioning specific developers using @username syntax to draw their attention and have the topic reviewed. After typing @ and some text, GitLab will offer completions for developers whose real names or user names match.

Comments use GitLab Flavored Markdown for formatting. See GitLab documentation on Special GitLab References to add links to things like merge requests and commits in other repositories.

Human Reviews

Reviewers may add comments providing feedback or to acknowledge their approval. Lines of specific forms will be extracted during merging and included as trailing lines of the generated merge commit message.

A commit message consists of up to three parts which must be specified in the following order: the leading line, then middle lines, then trailing lines. Each part is optional, but they must be specified in this order.

Leading Line

The leading line of a comment may optionally be exactly one of the following votes followed by nothing but whitespace before the end of the line:

  • -1 or 👎 (:-1:) means "The change is not ready for integration."
  • +1 or 👍 (:+1:) means "I like the change but defer to others."
  • +2 means "The change is ready for integration."
  • +3 means "I have tested the change and verified it works."

Middle Lines

The middle lines of a comment may be free-form GitLab Flavored Markdown.

Trailing Lines

Zero or more trailing lines in the last section of a comment may each contain exactly one of the following votes followed by nothing but whitespace before the end of the line:

  • Rejected-by: me means "The change is not ready for integration."
  • Acked-by: me means "I like the change but defer to others."
  • Reviewed-by: me means "The change is ready for integration."
  • Tested-by: me means "I have tested the change and verified it works."

Each me reference may instead be an @username reference or a full Real Name <user@domain> reference to credit someone else for performing the review. References to me and @username will automatically be transformed into a real name and email address according to the user's GitLab account profile.

Fetching Changes

One may fetch the changes associated with a merge request by using the git fetch command line shown at the top of the Merge Request page. It is of the form:

$ git fetch https://gitlab.kitware.com/$username/vtk.git $branch

This updates the local FETCH_HEAD to refer to the branch.

There are a few options for checking out the changes in a work tree:

  • One may checkout the branch:

    $ git checkout FETCH_HEAD -b $branch

    or checkout the commit without creating a local branch:

    $ git checkout FETCH_HEAD

  • Or, one may cherry-pick the commits to minimize rebuild time:

    $ git cherry-pick ..FETCH_HEAD

Robot Reviews

The "Kitware Robot" automatically performs basic checks on the commits and adds a comment acknowledging or rejecting the topic. This will be repeated automatically whenever the topic is pushed to your fork again. A re-check may be explicitly requested by adding a comment with a single trailing line:

Do: check

A topic cannot be merged until the automatic review succeeds.

Testing

VTK has a buildbot instance watching for merge requests to test. A developer must issue a command to buildbot to enable builds:

Do: test

The buildbot user (@buildbot) will respond with a comment linking to the CDash results when it schedules builds.

The Do: test command accepts the following arguments:

  • --stop clear the list of commands for the merge request
  • --superbuild build the superbuilds related to the project
  • --clear clear previous commands before adding this command
  • --regex-include <arg> or -i <arg> only build on builders matching <arg> (a Python regular expression)
  • --regex-exclude <arg> or -e <arg> excludes builds on builders matching <arg> (a Python regular expression)

Multiple Do: test commands may be given in separate comments. A new Do: test command must be explicitly issued for each branch update for which testing is desired. Buildbot may skip tests for older branch updates that have not started before a test for a new update is requested.

Builder names always follow this pattern:

    project-host-os-libtype-buildtype+feature1+feature2
  • project: always vtk for vtk
  • host: the buildbot host
  • os: one of windows, osx, or linux
  • libtype: shared or static
  • buildtype: release or debug
  • feature: alphabetical list of features enabled for the build

For a list of all builders, see:

Revise a Topic

If a topic is approved during GitLab review, skip to the next step. Otherwise, revise the topic and push it back to GitLab for another review as follows:

  1. Checkout the topic if it is not your current branch:

    $ git checkout my-topic

  2. To revise the 3rd commit back on the topic:

    $ git rebase -i HEAD~3

    (Substitute the correct number of commits back, as low as 1.) Follow Git's interactive instructions.

  3. Return to the above step to share the revised topic.

Merge a Topic

After a topic has been reviewed and approved in a GitLab Merge Request, authorized developers may add a comment with a single trailing line:

Do: merge

to ask that the change be merged into the upstream repository. By convention, do not request a merge if any -1 or Rejected-by: review comments have not been resolved and superseded by at least +1 or Acked-by: review comments from the same user.

Merge Success

If the merge succeeds the topic will appear in the upstream repository master branch and the Merge Request will be closed automatically.

Merge Failure

If the merge fails (likely due to a conflict), a comment will be added describing the failure. In the case of a conflict, fetch the latest upstream history and rebase on it:

$ git fetch origin
$ git rebase origin/master

(If you are fixing a bug in the latest release then substitute origin/release for origin/master.)

Return to the above step to share the revised topic.

Delete a Topic

After a topic has been merged upstream the Merge Request will be closed. Now you may delete your copies of the branch.

  1. In the GitLab Merge Request page a "Remove Source Branch" button will appear. Use it to delete the my-topic branch from your fork in GitLab.

  2. In your work tree checkout and update the master branch:

    $ git checkout master
$ git pull

  3. Delete the local topic branch:

    $ git branch -d my-topic

    The branch -d command works only when the topic branch has been correctly merged. Use -D instead of -d to force the deletion of an unmerged topic branch (warning - you could lose commits).