[oe] A new bitbake extension: .bbappend files

[Richard Purdie]

Whilst our layers mechanism, is great it does have a drawback which has
bugged me for a while. If you have a recipe like pointercal which has
machine specific information in it and you have your new machine code in
a layer, how do you add a pointercal file for your machine?

Answer is you copy the whole pointercal recipe and files into your
layer, then add the single file for your machine. To me this is ugly,
ugly, ugly. We hate code duplication and as soon as you create two
copies of the same information, we've failed.

So how could we do this better? Somehow we need to say that a given
directory X has some information which should be merged with the
original recipe. I've thought through several different ways of doing
this and the best solution I found was "bbappend".

The idea is that if bitbake finds any X.bbappend files, when it loads
X.bb, it will also include these files after it parses the base .bb file
(but before finalise and the anonymous methods run). This means that
the .bbappend file can poke around and do whatever it might want to the
recipe to customise it.

I went ahead and tried it out as its quite simple to code this in
bitbake. I liked the result enough I've already merged this into Poky:

http://git.pokylinux.org/cgit.cgi/poky/commit/?id=63e6ba85677b8aa9f4cf9942a1fccbb8a8c72660

I'm proposing to push it to bitbake master if there are no serious
objections.

As an example use case, for the pointercal case above in another later
you could add a pointercal_0.0.bbappend file which contained something
like:

FILESPATH := "${FILESPATH}:${@os.path.dirname(bb.data.getVar('FILE', d, True))}"

which would then cause the directory containing the bbappend file to be
searched for pointercal files.

There are of course many other uses this could be put to for creating
customised layers, its totally generic.

For the specific case of paths, I have wondered if there would be a way
to leverage help from bitbake in creating a sane set of search paths but
I'm still thinking about that. This extension is good enough in its own
right in my opinion to be worthwhile.

Cheers,

Richard