[tbot] U-Boot Building

[Harald Seiler]

Hello!

As mentioned in a previous mail, the current implementation for
building U-Boot is quite rigid and unwieldy in some cases
(I have included the relevant quotation in the end of this mail).

This means, the relevant testcases need quite some overhaul.  Before
hacking some solution, that works for me, I thought it might make
sense to collect the use-cases you all have so the future implementation
supports them all.

To start off, I will list my personal ones.  Please extend this list
as necessary:

* Completely fresh build:
	(Example: CI, nightly tests)
	clone/fetch; clean; checkout rev; patch; make
* Rebuild from existing tree, without using existing artifacts:
	(Example: during a bisect)
	clean; make
* Rebuild from existing dirty tree:
	(Example: during development, debugging)
	make

In my opinion, tbot should behave like this: In any case, if no
repo exists yet, it should clone upstream and try building with
that (as in the first case). If a repo exists already it will use
the specified behaviour.

What are your opinions?

One open question: Do you want/need to apply patches during a bisect?

The triggering issue:
>    Just a few days ago, I was writing a testcase to bisect U-Boot.  The current
> design for building U-Boot in tbot is one that is strictly configured (and does not
> really use composition).  The issue arose that the `uboot.build` testcase internally
> checks out the U-Boot repository.  During a git-bisect, this will make troubles because
> we do not want to checkout the current master but build on the revision the bisect
> put us on.  How to solve this?  Well, you can't.  You have to change the `uboot.build`
> testcase.  Luckily you can do this without actually touching tbot code, but the solution
> is still less than ideal:
> 
>         class BuildInfo(getattr(tbot.selectable.UBootMachine, "build")):
>             def checkout(self, clean):
>                 return super().checkout(False)
> 
>         builddir = uboot.build(
>             bh,
>             BuildInfo,
>             clean=True,
>         )
> 
> This code overwrites the checkout method in the U-Boot build-info (which is a composable
> part of this config!) and tells tbot to always do a dirty checkout, even if we supply
> clean=True to `uboot.build`.  This is arguably not good programming and definitely not
> pythonic in any way.  If the U-Boot build was more composable instead of this config-approach,
> this issue would never arise.
-- 
Harald

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