Arch Linux

Well, it looks like https://github.com/xbmc/xbmc/blob/17.4-Krypton/project/cmake/modules/FindFFMPEG.cmake tries to detect the system ffmpeg via pkg-config and only downloads an internal copy if it fails or if you use ENABLE_INTERNAL_FFMPEG. But it does need ffmpeg as a makedepends, which it doesn't seem to have...

There have been issues where system ffmpeg could not playback some formats supported by kodi, and all was working fine with internal ffmpeg. Since then we always build with internal ffmpeg to avoid this sort of issues.

That sounds quite weird, maybe the system ffmpeg package should be fixed!

But be that as it may, all sources used in a PKGBUILD should still be added to source=() and downloaded as part of makepkg itself then symlinked into the build directory as needed, rather than allowing the build system to download completely arbitrary and unauthenticated source code archives.
The fact that this allows offline builds is simply icing on the cake.

Also if you really want it to build with an internal ffmpeg copy, you should make this explicit in the build configuration, rather than relying on the fact that the *current* recursive makedepends graph doesn't include ffmpeg. This also helps other people realize when something is explicitly bundled rather than accidentally.

can we try to make system ffmpeg work? And figure out which format actually breaks and check why thats the case (compare internal and external ffmpeg).
I can provide debugging help if wanted

> all sources used in a PKGBUILD should still be added to source=() and downloaded as part of makepkg itself

this sounds like the second best option, assuming there's a way to make kodi get along system ffmpeg (or the other way around). definitely more satisfactory than the current setup.

I noticed that there are other components that are downloaded upon building.

% grep 'download step' log
[ 2%] Performing download step (download, verify and extract) for 'crossguid'
[ 2%] Performing download step (download, verify and extract) for 'dvdcss'
[ 14%] Performing download step (download, verify and extract) for 'dvdread'
[ 15%] Performing download step (download, verify and extract) for 'dvdnav'

Is there a reason to want to use the system-provided versions of there corresponding packages as well? I will say that the change to including ffmpeg and ditching the internal version lead to breakage when ffmpeg was updated to 1:3.4-1 ( FS#56065 ) and also, the change pulls down all the ffmpeg deps which are sizable compared to having ffmpeg built into kodi and not pulling these down.