introduce EasyBlock.post_init method to correctly define builddir variable when build-in-installdir mode is enabled in easyconfig or easyblock

[boegel]

fixes #3895

problem to investigate

[branfosj]

This looks correct, but is not solving the issue I am seeing with OpenFOAM.

eb OpenFOAM-8-foss-2020b.eb -Tr --sanity-check-only

with this PR is not hitting the right section of code.

[branfosj]

So, setting buildininstalldir = True in the OpenFOAM easyconfig means this works correctly. However, the self.build_in_installdir = True in the OpenFOAM easyblock acts after this.

[smoors]

a possible solution would then be to replace self.build_in_installdir = True with an EasyBlock method self.build_in_installdir():

build_in_installdir(self):
    if self.builddir != self.installdir:
        self.log.info("Changing build dir to %s", self.installdir)
        self.builddir = self.installdir

[boegel]

a possible solution would then be to replace self.build_in_installdir = True with an EasyBlock method self.build_in_installdir()

@smoors That's possible, but the downside of this approach is that easyblocks need to follow suite in order to pick up this bugfix...

I think a better option is to introduce a post_init method which is always run after initializing the EasyBlock instance (like I did in 9623629), before any steps are run; not making that part of a step makes sure it's always run.

That way, existing easyblocks that set build_in_installdir to True in the constructor don't need to be changed.
There are only two exceptions in the central easybuild-easyblocks repository: the custom easyblocks for PETSc and SLEPc (which are among the oldest easyblocks we have).
I'll open a trivial PR to also make those two set build_in_installdir in the easyblock constructor.

[branfosj]

lgtm

[branfosj]

Going in, thanks @boegel!