[oe] Bitbake task dependency stamp handling

[Richard Purdie]

I had some interesting thoughts regarding tasks and their timestamps.
Bitbake's current method of dealing with timestamps walks the task
digraph structure which only knows about a single .bb file. The result
is if you change an earlier task, say do_patch, later tasks like
do_compile would rerun. If you rebuild gtk, the rest of your system
linking to gtk doesn't rebuild though.

If we move away from the digraph which isn't really needed anymore,
taskData has the complete dependency tree available to it and the option
of rebuilding the whole system if you recompile gtk becomes possible.

At face value this sounds really useful. I'd guess that almost every OE
developer would find it insanely annoying in practise though?

Does anyone have any thoughts?

Richard