Instructions for updating to a new SOFA version and releasing ERFA
==================================================================

We welcome contributors who would like to help with the steps of updating to a
new version of SOFA! The latter parts of the release process must be done by
a maintainer with permissions, but the first part can (and should) be done as
a pull request by anyone.

Updating SOFA
-------------

These steps can be done by anyone, as they use the Pull Request workflow which
places no contraints on who has permissions to submit code changes.


* Clone your fork of the ERFA repository from github (if you have not already),
  and change to the ERFA directory.

* Create a new branch from the latest ``master`` branch in the ERFA repo.

* If a new version of SOFA exists, run `sofa_deriver.py` from the `erfa-fetch
  repository`_ in its own directory.  That will create a directory called `erfa`
  inside the `erfa-fetch` directory, and   you should copy its contents to the
  `src` directory of `erfa`.  Add any new C files or header files added by SOFA
  to ``src/meson.build``, as appropriate.

* Use ``git diff`` in `erfa` to inspect the changes, ensuring that you are not
  overwriting *intentional* changes between SOFA and ERFA (the README from the
  previous version is generally your friend here.)

* Update the ``SOFA_VERSION`` define in both `meson.build` and `configure.ac`
  to reflect the new SOFA version.

* Update the SOFA version mentioned in `README.rst` to reflect what's now in, as
  well as any relevant changes in the "Differences from SOFA" section or any
  other part of the README that your changes might have affected. This usually
  requires specifying the upcoming version of ERFA - since that isn't definite
  until release time, put ``x.y.z`` as the version, and leave it to the release
  maintainer to fill that in with the correct version (see below).

* Commit the changes and push them up to your fork.

* Create a Pull Request to the main ERFA repository. It is helpful to mention
  in the PR description if the changes in that SOFA version are
  backwards-compatible or not, as that helps the maintainers know how to update
  the version numbers when they do the release (described in more detail in the
  "Version numbering" section below).

* A maintainer will check the Pull Request and merge once it's reviewed.


Releasing ERFA
--------------

These steps should be done by a maintainer, as they require specific Github
permissions.

* Update the version number in the ``project()`` version field of `meson.build`
  as well as ``AC_INIT`` in `configure.ac` to the version number you are about
  to release. Follow the instructions in the `Version numbering`
  "Package version number" section below.

* Update the version info of the shared library in the ``libtool_version``
  variable of `meson.build` and ``ERFA_LIB_VERSION_INFO`` in `configure.ac`.
  Follow the instructions in `Version numbering`
  "Shared library version info" section below.

* Update the `README.rst` to reference the version you filled in above
  (it may be ``x.y.z`` if a maintainer followed the processes described above).
  Ensure the list of changes is accurate for this new version.

* Commit these changes using ``git commit``, with a commit message like
  ``Preparing release v0.0.1``.

* Run ``meson setup builddir``: you need `meson` installed.  If no errors appear,
  this will create a `build.ninja` file in the build directory, as well as a
  `config.h`.
  To avoid possible trouble, check that in the latter, ``PACKAGE_VERSION``
  is set correctly.

* Run ``meson test -C builddir``, which will build the library and run the tests -
  make sure they pass before proceeding. (This is already done by continuous
  integration in Github so it really *should* pass, but better to be safe than
  sorry!)

* Run ``meson dist -C builddir``: this creates the distribution tarball,
  unpackages it and runs the check inside the untarred directory.
  The resulting tarball will be named e.g., `erfa-0.0.1.tar.gz` and
  will be placed in `builddir/meson-dist` (its filename will be printed out as well).

* Tag the current commit with the version number.  A signed tag is preferred if
  you have an a signing key (e.g., do ``git tag -s v0.0.1``).

* Push up your changes and the new tag to github:
  ``git push --tags origin master``. (The command here assumes the git remote
  "origin" points to the "liberfa/erfa" repository.  If not, substitute the
  appropriate name.)

* Go to the "liberfa/erfa" repository for the github page, and click on the
  "releases" banner, "tags", and then the tag name you just pushed up. Click on
  the "..." button and choose "Create release". to change this tag to a release
  (Note that the details of how you find this option might change depending on
  what Github has decided recently, but the goal is to get to the "draft new
  release" page for the existing tag.)

* Put the version number as the title (e.g., ``v0.0.1``) and for the description
  put ``See `README.rst` for release notes.``

* Upload the tarball you created (e.g., `erfa-0.0.1.tar.gz`) by dropping it
  in the area that says "Attach binaries for this release by dropping them
  here or selecting them."

* Click the "Publish release" button.

* Update the release listing on Github Pages to include this release:
  Do ``git checkout gh-pages``, add a new ``<li>...</li>`` entry for the
  release in `index.html`, do ``git commit``, and then
  ``git push origin gh-pages``.  Or just edit it directly on the Github web
  interface if that's more your thing.

Version numbering
=================

ERFA needs to provide two different version numbers.  You need to update both.
The first is the
**package version number** or **version number** proper. ERFA uses
`semantic versioning <http://semver.org/>`_ to create this number.
For more on this choice, see
`liberfa/erfa#6 <https://github.com/liberfa/erfa/issues/6>`_.

The second number is `shared library version info`. When a program has been
linked with the ERFA shared library, the dynamic linker checks the version
info of the library requested by the program with those of the libraries
present if the system. This version info is important to binary distributions
(such as Linux distributions). ERFA uses `libtool versioning <http://www.gnu.org/software/libtool/manual/html_node/Libtool-versioning.html>`_.


Package version number
----------------------

Semantic versioning dictates how to change the version number according to
changes to the API of the library. In the case of ERFA the API is:

  * The public C macros defined in erfam.h
  * The names, return types, number of arguments and types of the functions in erfa.h

To update the package version, the release manager has to check the relevant
information about the release, such as:

  * upstream SOFA documentation in http://www.iausofa.org/current_changes.html
  * relevant bug reports in the github project page

If the version is given in the form MAJOR.MINOR.PATCH, then

  * if there is a backwards incompatible change (function removed, types of
    arguments altered, macros renamed...) then increase MAJOR by one and set
    the others to zero.
  * else if there is backwards compatible change (new function added or
    new macro added) then do not change MAJOR, increase MINOR by one and
    set PATCH to zero.
  * else
        you are either fixing a bug or making other improvements. Increase
        patch by one and do not change the others.

Change the version number in `README.rst` and the `project()` version argument.

Shared library version info
---------------------------

For the shared  library version info, we are only interested in a subset of
the API, the **interfaces of the shared library**. As the C macros are
interpolated away at compile time, the interfaces in the ERFA
shared library are:

  * The names, return types, number of arguments and types of the functions

Again, the release manager has to review the relevant information:

  * upstream SOFA documentation in http://www.iausofa.org/current_changes.html
  * relevant bug reports in the github project page

The shared library version info is stored in three numbers called *current*,
*revision* and *age*. These numbers appear in the variable `libtool_version`
in the mentioned order.

If the version is given in the form CURRENT,REVISION,AGE then

  * if there is a backwards incompatible change (function removed, types of
    arguments altered...) then increase CURRENT by one and set
    the others to zero (c,r,a -> c+1,0,0).
  * else if there is backwards compatible change (new function added)
    then increase both CURRENT and AGE by one, set REVISON to zero
    (c,r,a -> c+1,0,a+1).
  * else if the library code has been modified at all
    then increase REVISION by one (c,r,a -> c,r+1,a)
  * else
       do not change the version info (c,r,a -> c,r,a)

Change the version info in `libtool_version`

Examples
---------
We start with ERFA version 1.0.0 and library version info 0,0,0

* SOFA makes a new release. A function is added and two functions change their
  arguments. This is a backawars incompatible change, so the new package will
  have version 2.0.0 and the shared library version info will be 1,0,0

* We forgot to add README.rst to the release. We make a new one. The change
  is a bugfix (no API changes), the new release will be 2.0.1. The shared
  library version is not modified (no changes in the library source code).

* SOFA makes a new release. They just add a new function. The new package
  version will be 2.1.0. The shared library info will be 2,0,1 (both current
  and age are incremented).

* SOFA makes a new relase fixing some bugs in the code without changing the
  API. New package version is 2.1.1. The shared library version is 2,1,1

* A contributor finds a bug in ERFA. The fix doesn't change the API. New
  package version is 2.1.2. The shared library version is 2,2,1

* SOFA makes a new release incorporating the bug fix and adding new functions.
  The new package version is 2.2.0. The shared library version is 3,0,2

* SOFA makes a new release removing functions. This is a backawars
  incompatible change, so the new package will
  have version 3.0.0 and the shared library version info will be 4,0,0

.. _erfa-fetch repository: https://github.com/liberfa/erfa-fetch
