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158 lines
6.5 KiB
ReStructuredText
158 lines
6.5 KiB
ReStructuredText
waf build system overview
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=========================
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mpv's new build system is based on waf and it should completly replace the
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custom ./configure + Makefile based system inherited from MPlayer.
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Goals and the choice of waf
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===========================
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The new system comes with some goals, which can be summed up as: be as good as
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the old one at what it did well (customizability) and fix some of it's major
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shortcomings:
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1) The build system must be uniform in how it handles any single feature check.
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Repetition and boilerplate have to be avoided.
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When adding a new feature using the old configure, one had to add a fair
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amount of code to the shell script to do option parsing, detection of the
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feature and declaration of variables for the Makefile to pickup. The worst
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part is this pieces are spread apart in the configure and copy pasted for
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any single case. That brings us to..
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2) --enable-feature has to be overridden by the user and helps them understand that
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they have libraries missing and should install them for the feature to be
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enabled.
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3) Must be customizable, hackable, pleasant to the developer eyes and to work
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with in general.
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4) Must have separate configuration and build steps.
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Goal 2 comes as a given on pretty much any build system, since autotools made
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this behaviour very popular among users (and rightly so).
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Goal 1+3 were somewhat harder to accomplish as it looks like all of the build
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systems we evaluated (waf included!) had problems with them. For reference we
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had proof of concept build systems with waf, CMake and autotools.
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What puts waf apart from CMake and autotools, is that projects using it use
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Python to program their build system. Also while the Waf Book shows really
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simple API usages, you can write your own build system on top of waf that is
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tailored to the project's specific needs.
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mpv's custom configure step on top of waf
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=========================================
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To some extents mpv has a custom build system written on top of waf. This
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document will not go over the standard waf behaviour as that is documented in
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the `Waf book <http://docs.waf.googlecode.com/git/book_17/single.html>`_.
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All of the configuration process is handled with a declarative approach. Lists
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of dictionaries define the checks, and some custom Python code traverses these
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lists and depending on the check definition it calls into the actual waf API.
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A simple example using pkg-config would be::
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{
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'name': '--vdpau',
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'desc': 'VDPAU acceleration',
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'deps': [ 'x11' ],
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'func': check_pkg_config('vdpau', '>= 0.2'),
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}
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This defines a feature called ``vdpau`` which can be enabled or disabled by
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the users with configure flags (that's the meaning of ``--``). This feature
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depends on another feature whose name is ``x11``, and the autodetection check
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consists of running ``pkg-config`` and looking for ``vdpau`` with version
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``>= 0.2``. If the check succeds a ``#define HAVE_VDPAU 1`` will be added to
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``config.h``, if not ``#define HAVE_VDPAU 0`` will be added.
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The defines names are automatically prepended with ``HAVE_``, capitalized and
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special characters are replaced with underscores. This happens in
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``waftools/inflectors.py``.
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Mandatory fields:
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-----------------
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``name``: indicates the unique identifier used by the custom dependency code
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to refer to a feature. If the unique identifier is prepended with ``--``
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the build system will also generate options for ``./waf configure`` so that
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the feature can be enabled and disabled.
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``desc``: this is the textual representation of the feature used in the
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interactions with the users.
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``func``: function that will perform the check. These functions are defined in
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``waftools/checks``. The reusable checks are all functions that return
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functions. The return functions will then be applied using waf's configuration
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context.
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The source code for the reusable checks is a bit convoluted, but it should be
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easy to pick up their usage from the ``wscript``. Their signature mirrors
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the semantics of some of the shell functions used in mplayer.
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If someone expresses some interest, I will extend this document with official
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documentation for each check function.
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Optional fields
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---------------
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``deps``: list of dependencies of this feature. It is a list of names of
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other features as defined in the ``name`` field (minus the eventual leading
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``--``). All of the dependencies must be satisfied. If they are not the check
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will be skipped without even running ``func``.
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``deps_any``: like deps but it is satisfied even if only one of the dependencies
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is satisfied. You can think of ``deps`` as a 'and' condition and ``deps_any``
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as a 'or' condition.
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``deps_neg``: like deps but it is satisfied when none of the dependencies is
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satisfied.
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``req``: defaults to False. If set to True makes this feature a hard
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dependency of mpv (configuration will fail if autodetection fails). If set to
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True you must also provide ``fmsg``.
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``fmsg``: string with the failure message in case a required dependency is not
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satisfied.
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``os_specific_checks``: this takes a dictionary that has ``os-`` dependencies
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as keys (such as ``os-win32``), and by values has another dictionary that is
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merged on top of the current feature definition only for that specific OS.
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For example::
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{
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'name': '--pthreads',
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'desc': 'POSIX threads',
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'func': check_pthreads,
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'os_specific_checks': {
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'os-win32': {
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'func': check_pthreads_w32_static.
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}
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}
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}
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will override the value of ``func`` with ``check_pthreads_w32_static`` only
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if the target OS of the build is Windows.
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``groups``: groups a dependency with another one. This can be used to disabled
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all the grouped dependencies with one ``--disable-``. At the moment this is
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only used for OpenGL backends, where you want to disable them when
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``--disable-gl`` is passed to the configure.
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mpv's custom build step on top of waf
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=====================================
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Build step is pretty much vanilla waf. The only difference being that the list
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of source files can contain both strings or tuples. If a tuple is found,
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the second element in the tuple will the used to match the features detected
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in the configure step (the ``name`` field described above). If this feature
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was not enabled during configure, the source file will not be compiled in.
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All of the custom Python for this is inside the function ``filtered_sources``
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contained in the file ``waftools/dependencies.py``.
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Also ``dependencies_use`` and ``dependencies_includes`` collect cflags and
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ldflags that were generated from the features checks in the configure step.
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