Fix start_dir for components of Bundle easyblock

[Flamefire]

(created using eb --new-pr)

I tried using a patch for a bunde component but it failed because the start_dir is wrong. It will always use build_dir (or a path relative to it if set in start_dir EC parameter) because at the point of guess_startdir there are no sources in self.src hence it can't be relative to the unpacked source.

The comment and code don't add up too:

# location of first unpacked source is used to determine where to apply patch(es)
comp.src[-1]['finalpath'] = comp.cfg['start_dir']

Why does it set the last sources final path to the start_dir?

So I'd say the only correct way would be to set comp.src correctly and call guess_start_dir afterwards. Touching a sources finalpath can never be right ever, can it?

Edit: Further investigation shows that finalpath isn't set correctly for multiple sources. What happens for bundles of multiple components with a single source:

• Start with an empty builddir
• Extract source of 1st component -> Only folder -> finalpath set to that folder
• Extract source of 2nd component -> 2 folders -> Parent folder (builddir) is used as finalpath
• When setting component2.src = src2 and then calling guess_start_dir it will find the source and use its finalpath as the start_dir, hence wrongly using the builddir
• If start_dir is manually set in the easyconfig it will still find the correct start_dir
• However apply_patches uses the finalpath of the source at an index given in the patch spec, defaulting to 0
• Hence it will try to apply the patch in the build dir instead of the component dir

So this requires either easybuilders/easybuild-framework#4922 or comp.src[0]['finalpath'] = comp.start_dir after the comp.guess_start_dir so that patches work correctly.

[Flamefire]

The actual issue here is

Turns out the Bundle easyblock has multiple issues and quite a few easyconfigs & easyblocks already rely on the broken behavior.

E.g. Clang-AOMP does a configure-step in the Bundle-level using the directories that would be the correct finalpath for src_dir, e.g. src_dir = 'omp-1.2.3/llvm'.
With this fix this results in starting at omp-1.2.3 where the subfolder omp-1.2.3 does not exist (it tries <builddir>/omp-1.2.3/omp-1.2.3/llvm)

The confusing behavior of the code contradicting the comment is fixed in #3778, the rest needs further discussion.