Context

The FFmpeg project merges all the changes from the Libav project (​https://libav.org) since the origin of the fork (around 2011).

With the exceptions of some commits due to technical/political disagreements or issues, the changes are merged on a more or less regular schedule (daily for years thanks to Michael, but more sparse nowadays).

Reason

The majority of the active developers believe the project needs to keep this policy for various reasons.

The most important one is that we don't want our users to have to choose between two distributors of libraries of the exact same name in order to have a different set of features and bugfixes. By taking the responsibility of unifying the two codebases, we allow users to benefit from the changes from the two teams.

Today, FFmpeg has a much larger user database (we are distributed by every major distribution), so we consider this mission a priority.

A different approach to the merge could have been to pick the changes we are interested in and drop most of the cosmetics and other less important changes. Unfortunately, this makes the following picks much harder, especially since the Libav project is involved in various deep API changes. As a result, we decide to virtually take everything done there.

Any Libav developer is of course welcome anytime to contribute directly to the FFmpeg tree. Of course, we fully understand and are forced to accept that very few Libav developers are interested in doing so, but we still want to recognize their work. This leads us to create merge commits for every single one from Libav. The original commit appears totally unchanged with full authorship in our history (and the conflict are solved in the merge one). That way, not a single thing from Libav will be lost in the future in case some reunification happens, or that project disappears one way or another.

Downsides

Of course, there are many downsides to this approach.

• It causes a non negligible merge commits pollution. We make sure there are not several level of merges entangled (we do a 1:1 merge/commit), but it's still a non-linear history.
• Many duplicated work. For instance, we added libavresample in our tree to keep compatibility with Libav when our libswresample was already covering the exact same purpose. The same thing happened for various elements such as the ProRes support (but differences in features, bugs, licenses, ...). There are many work to do to unify them, and any help is very much welcome.
• So much manpower from both FFmpeg and Libav is lost because of this mess. We know it, and we don't know how to fix it. It takes incredible time to do these merges, so we have even less time to work on things we personally care about. The bad vibes also do not help with keeping our developers motivated.
• There is a growing technical risk factor with the merges due to the codebase differing more and more.

Merge Guidelines

The following gives developer guidelines on how to proceed when merging Libav commits.

Before starting, you can reduce the risk of errors on merge conflicts by using a different merge conflict style:

git config --global merge.conflictstyle diff3

Here is a script to help merging the next commit in the queue:

#!/bin/sh

op="$1"
next_rev=$(git rev-list libav/master --not HEAD --no-merges | tail -n1)

do_merge() {
    if [ -z "$next_rev" ]; then
        printf "Nothing to merge..\n"
        return 0
    fi

    merge_opts=
    [ "$op" = "noop" ] && merge_opts="-s ours"

    printf "Merging $(git log -n 1 --oneline $next_rev)\n"
    git merge --no-commit $merge_opts --no-ff --log $next_rev

    if [ "$op" = "noop" ]; then
        printf "\nThis commit is a no-op." >> .git/MERGE_MSG
        [ -n "$2" ] && printf " See $2." >> .git/MERGE_MSG
        printf "\n" >> .git/MERGE_MSG
    fi

    printf "\nMerged-by: $(git config --get user.name) <$(git config --get user.email)>\n" >> .git/MERGE_MSG
}

case "$op" in
next)
    echo $next_rev
    ;;
show)
    git show $next_rev
    ;;
merge|noop)
    do_merge
    ;;
*)
    echo "Usage: $0 <next|show|merge|noop [REF_HASH]>"
    echo "    next  - Echo hash of the commit to be merged next"
    echo '    show  - git show $next'
    echo '    merge - Prepare to merge the next commit'
    echo '    noop  - Pass up the next commit with an optional "See $REF_HASH" message'
    ;;
esac

The script assumes a remote named libav.

It has two modes: merge, and noop. The noop mode creates a merge with no change to the HEAD. You can pass a hash as extra argument to reference a justification (it is common that we already have the change done in FFmpeg).

TODO/FIXME/UNMERGED

• HEVC DSP and x86 MC SIMD improvements from Libav (see ​http://comments.gmane.org/gmane.comp.video.ffmpeg.devel/204917)