Contributing changes

If you have a patch to fix a problem in Xapian, or to add a new feature, please send it to us for inclusion. Any major changes should be discussed on the xapian-devel mailing list first.

The rest of this section gives information on how to get your changes adopted quickly into Xapian. We have noted some things we aim for in our code and our changes, as well as how to get changes to us and some other details.

License grant

We ask everyone contributing changes to Xapian to dual-license under the GPL (which Xapian currently uses) and the MIT/X license (which we would like to move to in future). The simplest way to do this is to drop an email to the xapian-devel mailing list stating that you own the copyright on your changes and are happy to dual-license accordingly.

Some things that we look for

Beyond looking for changes that improve Xapian, code that works and so forth, there are a number of things that we aim for when accepting changes. This then is a list of good practices when contributing changes.

Code that compiles cleanly and looks like existing code

We like Xapian to compile without any warnings. In “maintainer mode”, which will be how you’re building Xapian if you’re working from a git clone, all warnings will actually become errors. You should fix the problems rather than change the compilation settings to ignore these warnings.

Please configure your editor to:

  • indent each block of code 4 columns from the containing block
  • display the tab character by advancing to the next column that is a multiple of 8
  • “tab fill” indents: all indents should start with as many tab characters as possible followed by as few spaces as possible
  • use Unix line endings (so each line ends with just LF, rather than CR+LF)

We don’t currently have a formal coding standards document, so you should try to follow the style of the existing code. In particular, it’s a good idea to pay close attention to code alignment and where we have spaces.

We have a small tool that can help spot common formatting problems. It’s run on all pull requests, so it’s a good idea to run it on your changes. From a clone of the Xapian source tree, the following will tell you if there are any problems in the changes you’ve made since you branched from master:

git diff master..HEAD | xapian-maintainer-tools/xapian-check-patch

Updated documentation

If you add a new feature, please ensure that you’ve documented it. Don’t worry too much about the language you use, or if English isn’t your first language. Others can help get the documentation into shape, but having a first draft from the person who wrote the feature is usually the best way to get started.

  • API classes, methods, functions, and types should be documented by documentation comments alongside the declaration in include/xapian/*.h.

    These are collated by doxygen – see doxygen’s documentation for details of the supported syntax. We’ve decided to prefer to use @ rather than \ to introduce doxygen commands (the choice is essentially arbitrary, but \ introduces C/C++ escape sequences so @ is likely to make for easier to read mark up for C/C++ coders).

  • The documentation comments don’t give users a good overview, so we also need documentation which gives a good overview of how to achieve particular tasks.

    If there’s relevant documentation already in the user guide, then you should update that. For completely new features, you should create either a “how to” or an “advanced feature” document in the user manual, so that people can get started without having to start with the API documentation.

  • Internal classes, etc should also be documented by documentation comments where they are declared.

Automated tests

If you’re fixing a bug, you should first write a regression test. The test will fail on the existing code, then when you fix the bug it will pass. In the future, the test will make sure no one accidentally re-introduces the same bug.

If you’re adding a new feature, you’ll want to write tests that it behaves correctly. Thinking about the tests you need to write can often help you plan how to implement the feature; it can also help when thinking about what API any new classes or methods should expose. A good set of tests will both ensure that the feature isn’t broken to start with and detect if later changes stop it working as intended.

And of course you should check that the existing tests all continue to pass. It’s good practice to get into the habit of running tests locally as part of your development process, and certainly before you share changes with others, such as by opening a Pull Request or sending us a patch.

If you don’t know how to write tests using the Xapian test rig, then please ask. It’s reasonably simple once you’ve done it once. There is a brief introduction to the Xapian test system in docs/tests.html, as well as some helpful information in the Testing Xapian section of this guide.


If you’re adding a new testcase to demonstrate an existing bug, and not checking a fix in at the same time, mark the testcase as a known failure (by calling XFAIL("explanatory message") at the start of your testcase (if necessary this can be conditional on backend or other factors - the backend case has explicit support via XFAIL_FOR_BACKEND("backend", "message")).

This will mean that this testcase failing will be reported as “XFAIL” which won’t cause the test run to fail. If such a testcase in fact passes, that gets reported as “XPASS” and will cause the test run to fail. A testcase should not be flagged as “XFAIL” for a long time, but it can be useful to be able to add such testcases during development. It also allows a patch series which fixes a bug to first demonstrate the bug via a new testcase marked as “XFAIL”, then fix the bug and remove the “XFAIL” – this makes it clear that the regression test actually failed before the fix.

Note that failures which are due to valgrind errors or leaked fds are not affected by this macro – such errors are inherently not suitable for “XFAIL” as they go away when the testsuite is run without valgrind or on a platform where our fd leak detector code isn’t supported.

Updated attributions

If necessary, modify the copyright statement at the top of any files you’ve altered. If there is no copyright statement, you may add one (there are a couple of’s and similar that don’t have copyright statements; anything that small doesn’t really need one anyway, so it’s a judgement call). If you’ve added files which you’ve written from scratch, they should include the GPL boilerplate with your name only.

If you’re not in there already, add yourself to the xapian-core/AUTHORS file. If you forget, as well as if we have used patches from you, or received helpful reports or advice, we will add you to this file, unless you specifically request us not to. If you see we have forgotten to do this, please draw it to our attention so that we can address the omission.

Update trac

If there’s a related trac ticket or other reference for a bug or feature, update it (if the issue is completely addressed by the changes you’ve made, then close it). It’s also good to mention it in the commit message – it’s a great help to future developers trying to work out why a change was made.

If you’ve fixed a bug, it’s helpful to update the release notes for the most recent release with a copy of the patch. If the commit from git applies cleanly, you can just link to it. If it fails to apply, please attach an adjusted patch which does. If there are conflicts in test cases which aren’t easy to resolve, it is fine to just drop those changes from the patch if we can still be confident that the issue is actually fixed by the patch.

Consider backporting bug fixes

If there’s an active release branch, please check if the bug is present in that branch, and if the fix is appropriate to backport - if the fix breaks ABI compatibility or is very invasive, you may need to fix it in a different way for the release branch, or decide not to backport the fix.

Submit your patch

There are two ways of working, depending on whether you want to use Github or not. In both cases, review and acceptance of the changes will generally go more easily if you’ve included tests, updated documentation and so on as discussed earlier.

Pull requests via Github will have the tests run automatically on a variety of platforms. This means that you should run the tests before creating a pull request (since it’s not worth reviewing something where the tests are failing – you may have to make significant changes to get the tests to pass, so reviewing too early could be a waste of everyone’s time).

Of course, if you have difficulty getting the tests to pass on your local machine, or if locally they do pass but fail on the automated systems connected to Github but you can’t figure out why, then get in touch and someone should be able to help.

Attach a patch directly to the trac ticket

We find patches in unified diff format easiest to work with. git diff produces the right output for a single commit (or git format-patch for a series of commits).

If you’re working from a tarball, you can unpack a second clean copy of the files and compare the two versions with diff -pruN (-p reports the function name for each chunk, -r acts recursively, -u does a unified diff, and -N shows new files in the diff). Alternatively ptardiff (which comes with perl, at least on Debian and Ubuntu) can diff against the original tarball, unpacking it on the fly.

Someone from the community will then be able to review the patch and decide if it needs further work before integrating. If so, they’ll leave comments on the trac ticket (trac will generally email you if you’re marked as the owner, or you can explicitly add yourself to the “cc” list for a ticket).

Open a Pull Request on github

Github pull requests provide a web-based interface for review and discussion of changes before they are accepted into Xapian. Github’s documentation explains how you can go about opening them.

If your patch is a sub-project in a larger piece of work, then it’s important not to assume the patch is fine as it stands and to immediately start the next sub-project. Instead you should concentrate on completing the sub-project before moving on. Since you’ll almost always have to wait at least a little time to get feedback on any changes, you may want to put the code and tests up while still working on documentation.

You should add further changes to pull requests by creating additional commits locally, typically by using git commit --fixup, and then pushing the branch up to Github. Only once everything’s been approved should you squash your commits together to keep the history clean.


Once you’ve opened a pull request, you shouldn’t have to close it until it’s merged (in which case we’ll generally close it for you). Even if you need to redo some work, you can either add fixup commits or (with agreement from whoever is reviewing the PR) unwind your work and create completely new commits, force pushing to replace the previous commits in the pull request.

It makes it much harder to review if you close a pull request in the middle of a review only to open another with similar code.