gstreamer/HACKING

THE GOAL
--------
What we are trying to achieve:

satisfy:
  patching of CVS checkout using our patch files placed in our CVS

  passing of
    make
    make distcheck
    non-srcdir build (ie, mkdir build; cd build; ../configure; make)

THE SETUP
---------
There is a "mirror" root CVS module that contains "ffmpeg".
This directory contains a vendor-branch checkout of upstream FFmpeg CVS
of a given day.

On head, the following things have been commited on top of this:
* patches/, which is a directory with a set of patches, and a series file
  listing the order, as generated by quilt
* the result of having all these patches commited (ie, quilt push -a) to the
  ffmpeg tree.

The patched CVS ffmpeg code needs to be commited to CVS so that a checkout 
gives the patched code

The Quilt state .pc hidden directory must NOT be committed to CVS, because 
having CVS subdirs inside it confuses the hell out of quilt and causes it to
start storing diffs against CVS Entries files, and all hell breaks loose

THE WARNING
-----------

***

NEVER EVER commit stuff in gst-libs/ext/ffmpeg UNLESS your quilt stack is
completely applied !
This means, ALWAYS make sure quilt push -a has been run without problems.

What's more, if you want to be on the safe side, make sure that you can
unapply and reapply without problems, by running quilt pop -a then
quilt push -a.

The ONLY exception to this is when you're working on a branch to update
the upstream source you're working with.

***

THE WAY
-------
- If you want to hack on our copy of the FFmpeg code, there are some basic
  rules you need to respect:
  - you need to use quilt.  If you don't use quilt, you can't hack on it.
  - we separate patches based on the functionality they patch, and whether
    or not we want to send stuff upstream.  Make sure you work in the right
    patch.  use "quilt applied" to check which patches are applied.
  - before starting to hack, run cvs diff.  There should be NO diffs, and
    NO files listed with question mark.  If there are, somebody before you
    probably made a mistake.  To manage the state correctly, it is vital that
    none of the files are unknown to CVS.

FIRST TIME:
  - The quilt state is kept in a hidden dir in the gst-libs/ext/ffmpeg dir, 
    but this hidden dir can't be kept in CVS because it confuses patch. Hence
    when you get a clean gst-ffmpeg checkout you have an ffmpeg tree with
    patches applied, but no quilt metadata to modify it with.

  - You need to create the quilt metadata in your checkout:
    1) Unroll the quilt patches. In gst-libs/ext/ffmpeg, run:

       tac patches/series | while read p; do patch -p1 -R < "patches/$p"; done

    2) Now, push all the patches to quilt and it will apply them, but now with
       the appropriate stored metadata:

       quilt push -a

- if you want to add a file to a patchset, you need to:
  - be in the right patchset
  - quilt add (file)
  - cvs add .pc/(patchsetname)/(file)
  - cvs commit .pc/(patchsetname) (to update the state of quilt in cvs)
  - edit the file
  - cvs add the file if it doesn't exist yet
  - quilt refresh
  - quilt push -a (This one is IMPORTANT, otherwise you'll have a huge diff)
  - cvs commit

- if you want to add a patchset, you need to:
  - go over the procedure with thomas to check it's correct
  - decide where in the stack to put it.  ask for help if you don't know.
  - go there in the patch stack (use quilt pop/push)
  - quilt new (patchsetname).patch (don't forget .patch !)
  - quilt add (files)
  - cvs add .pc/(patchsetname) the whole tree
  - cvs commit .pc/(patchsetname)
  - quilt refresh
  - quilt push -a
  - cvs commit
  - cvs diff (to check if any of the files are unknown to CVS; if they are,
    you need to add them to CVS)

THE UPSTREAM
------------
At some points you want to update the upstream snapshot code to a newer date.
This is easy if you follow the steps outlined here, but make sure to follow
them correctly !

- find a good CVS snapshot date for upstream, one that is known to work.
  You're going to save yourself quite a bit of trouble if you verify this
  first !
- check it out to a local directory:
  cvs -z9 -d:pserver:anonymous@mplayerhq.hu:/cvsroot/ffmpeg export -D '2004-04-11 23:00 GMT' ffmpeg
- compile it and test it, make sure it works

- in gst-ffmpeg/gst-libs/ext/ffmpeg:
  - Pre-flight checks:
    - first make sure you don't have local changes, all files are either in
      CVS or in .cvsignore patch, the whole quilt stack is applied, and stuff
      works.
    - do a quilt pop -a and quilt push -a to verify everything is ok.

  - Branch and rollback:
    - tag HEAD with the branch root point:
      cvs tag BRANCH-UPDATE-CVS-2004-04-11-23-00-ROOT
    - branch:
      cvs tag -b BRANCH-UPDATE-CVS-2004-04-11-23-00
    - FIXME: lock cvs HEAD
    - update local copy to branch:
      cvs update -r BRANCH-UPDATE-CVS-2004-04-11-23-00
    - peel off all patches:
      quilt pop -a
    - commit this
      cvs commit
    - check
      cvs diff
      you should only have ? for files that are generated somehow (binaries,
      build files, ...)
      you get warnings about cvs not finding files to diff that are in .pc
      or generated by your patches

  
    - if you want, you can now compare this state of CVS (which should 
      be last upstream CVS combined with your local unapplied quilt state)
    - remember to NOT do cvs update from here on, since you popped your quilt
      state all your added files that are also in CVS are not locally present.

  - sync with upstream:
    - in a temp dir, redo the export:
      cd ..
      mkdir tmp
      cd tmp
      cvs -z9 -d:pserver:anonymous@mplayerhq.hu:/cvsroot/ffmpeg export -D '2004-04-11 23:00 GMT' ffmpeg
    - rsync it over the old ffmpeg tree
      rsync -arv ffmpeg ..
    - go back and commit this new snapshot
      cd ../ffmpeg
      cvs commit
    - check if any new files got added that you should add to cvs
      cvs diff
      This will list a lot of local files missing, from your quilt state,
      which you shouldn't worry about.  Just inspect all the ?'s and add
      files to cvs that belong to upstream and should be in cvs.
    - if everything's ok and commited, tag the state:
      cvs tag UPSTREAM-CVS-2004-04-11-23-00

  - reapply and fix quilt patches one by one
    - try applying one
      quilt push
    - if that didn't work, inspect the patch and figure out how to fix it:
      - if the patch got applied upstream completely, quilt push will tell
        you the patch looks like a reverse patch.  In that case you can
        remove the patch from your patches file (patches/series), and
        remove the .pc/$(patchname) and patches/$(patchname).patch files from
        cvs.
      - if the patch conflicts somehow, you can force application with
        quilt push -f
        and then resolve all the rejects, and fix the patch completely.
        Then refresh quilt state with 
        quilt refresh
     - when the patch is succesfully removed or reworked, commit current state
       to CVS, then check again if nothing is missing using cvs diff, and
       resolve problems/apply missing files from your quilt state/...

  - after reapplying your complete quilt state, test locally if the complete
    gst-ffmpeg module now works.  Compile and test.  Resolve all problems
    (upstream errors, missing symbols, missing files, ...) until you have
    a working module.  commit again.

  - merge to head:
    - update locally back to head
      cvs update -A
    - FIXME: unlock cvs HEAD
    - merge from your branch
      cvs update -j BRANCH-UPDATE-CVS-2004-04-11-23-00
    - commit
      cvs commit
    - check for diffs
      cvs diff
    - tag merge point
      cvs tag BRANCH-UPDATE-CVS-2004-04-11-23-00-MERGE
    - add upstream date to "THE RECORDS" below

  - get a drink

THE PLUGIN
----------
Some notes on how ffmpeg wrapping inside GStreamer currently works:
* gstffmpeg{dec,enc,demux,mux}.c are wrappers for specific element types from
    their ffmpeg counterpart. If you want to wrap a new type of element in
    wrapper file.
    The ffmpeg element types, define a whole *list* of elements (in 
    GStreamer, each decoder etc. needs to be its own element). 
    We use a set of tricks for that to keep coding simple: codec 
    mapping and dynamic type creation.

* ffmpeg uses CODEC_ID_* enumerations for their codecs. GStreamer uses caps,
    which consists of a mimetype and a defined set of properties. In ffmpeg,
    these properties live in a AVCodecContext struct, which contains anything
    that could configure any codec (which makes it rather messy, but ohwell).
    To convert from one to the other, we use codec mapping, which is done in
    gstffmpegcodecmap.[ch]. This is the most important file in the whole
    ffmpeg wrapping process! It contains functions to go from a codec type
    (video or audio - used as the output format for decoding or the input
    format for encoding), a codec id (to identify each format) or a format id
    (a string identifying a file format - usually the file format extension)
    to a GstCaps, and the other way around.

* to define multiple elements in one source file (which all behave similarly),
    we dynamically create types for each plugin and let all of them operate on
    the same struct (GstFFMpegDec, GstFFMpegEnc, ...). The functions in
    gstffmpeg{dec,enc,demux,mux}.c called gst_ffmpeg*_register() do this.
    The magic is as follows: for each codec or format, ffmpeg has a single
    AVCodec or AV{Input,Output}Format, which are packed together in a list of
    supported codecs/formats. We simply walk through the list, for each of
    those, we check whether gstffmpegcodecmap.c knows about this single one.
    If it does, we get the GstCaps for each pad template that belongs to it,
    and register a type for all of those together. We also leave this inside
    a caching struct, that will later be used by the base_init() function to
    fill in information about this specific codec in the class struct of this
    element (pad templates and codec/format information). Since the actual
    codec information is the only thing that really makes each codec/format
    different (they all behave the same through the ffmpeg API), we don't
    really need to do anything else that is codec-specific, so all other
    functions are rather simple.

* one particular thing that needs mention is how gstffmpeg{mux,demux}.c and
    gstffmpegprotocol.c interoperate. ffmpeg uses URLProtocols for data input
    and output. Now, of course, we want to use the *GStreamer* way of doing
    input and output (filesrc, ...) rather than the ffmpeg way. Therefore, we
    wrap up a GstPad as a URLProtocol and register this with ffmpeg. This is
    what gstffmpegprotocol.c does. The URL is called gstreamer://%p, where %p
    is the address of a GstPad. gstffmpeg{mux,demux}.c then open a file called
    gstreamer://%p, with %p being their source/sink pad, respectively. This
    way, we use GStreamer for data input/output through the ffmpeg API. It's
    rather ugly, but it has worked quite well so far.

* there's lots of things that still need doing. See the TODO file for more
    information.

THE RECORDS
-----------
- list of snapshots used:

  CVS-2004-04-11-23-00
  * other updates people didn't enter :)
  CVS-2006-02-17-04-00

THE REMINDERS
-------------
* the initial ffmpeg checkout was imported using:
  - get CVS ffmpeg
    cvs -z3 -d:pserver:anonymous@mplayerhq.hu:/cvsroot/ffmpeg co -D '2004-03-09 06:00 GMT' ffmpeg