Problem using CPATH in modulefiles overwriting system paths

[Flamefire]

TLDR: Stop setting CPATH in module files and set C_INCLUDE_PATH, CPLUS_INCLUDE_PATH, INCLUDE instead

This came up for ROOT and was work-arounded in easybuilders/easybuild-easyblocks#2047

Basically: the C/C++ preprocessor considers the paths in the following order:

• -I
• CPATH
• -isystem
• CPLUS_INCLUDE_PATH
• buildins

Now projects adding paths via -isystem have those ignored if the same folder/file name exists in CPATH which happened for ROOT and LLVM resulting in taking an incompatible, EB installed LLVM over the included LLVM

Talked to a Spack guy about the CPATH issue: They had the same: spack/spack#11555 and solved it by using C_INCLUDE_PATH, CPLUS_INCLUDE_PATH, INCLUDE instead: spack/spack#14749
So I guess we can and should be doing the same. For existing modules we could simply move the contents of CPATH to the other 3 variables after loading a module
Obvious downside: 3 variables with duplicated content instead of 1.

From Bennet Fauber:
https://gcc.gnu.org/onlinedocs/gcc/Environment-Variables.html
Lists the variables for GNU Cs.

Some additional environment variables affect the behavior of the preprocessor.
CPATH
C_INCLUDE_PATH
CPLUS_INCLUDE_PATH
OBJC_INCLUDE_PATH
CPATH specifies a list of directories to be searched as if specified with -I, but after any paths given with -I options on the command line. This environment variable is used regardless of which language is being preprocessed.
The remaining environment variables apply only when preprocessing the particular language indicated. Each specifies a list of directories to be searched as if specified with -isystem, but after any paths given with -isystem options on the command line.

CPLUS_INCLUDE_PATH is particularly useful for --std=c++99 and what have you that are specific to C++. I am blanking on which it was, but one of the geographic libraries was using CFLAGS for that, and it wasn't working well....

[boegel]

@Flamefire Do you think there's a scenario where no longer setting $CPATH could cause trouble?
Incorrect paths being passed to -isystem (like /usr/include), for example?

I'm a bit reluctant to make a change like this in EasyBuild 4.x, it feels like giving up some control (having $CPATH win over -isystem can be seen as a good thing).

Maybe problems that pop up because of this should be dealt with on a per case basis, like we did for ROOT?

@bsteinb Any input on this (since you referenced this issue after hitting a problem at JSC related to this)?

[Flamefire]

Incorrect paths being passed to -isystem (like /usr/include), for example?

having $CPATH win over -isystem can be seen as a good thing

Incorrect things are what we have to deal with anyway. If some software explicitly passes something to the compiler this should win. Us "injecting" unexpected stuff is what I would consider incorrect (which is the reason for this issue anyway)

Maybe problems that pop up because of this should be dealt with on a per case basis, like we did for ROOT?

I'd say this will be hard. Because most often it will "just work". I.e. the application requests some other path (e.g. a local directory) but the system/EB path is taken instead and it happens to work because the API is "compatible enough". But at ABI level it might break in silent ways. So we'll only notice problem when it breaks loudly at build time.

IMO we have some kind of assurance that this change works well: Because Spack already uses it.

[ocaisa]

To maintain backwards compatibility we can just introduce an option to choose one or the other approach, for v4 the default is to use CPATH, for v5 we switch the default (and for maintainers we can recommend to switch already so that we gain plenty of experience with the change)

[SimonPinches]

This is just to add a comment that we are also facing problems associated with the setting of CPATH in modules. The problem arises for Fortran codes and the use of pkg-config. New versions of pkg-config unhelpfully strip out paths they find in CPATH from what they return when run with --cflags. Since compilers like gfortran do not search CPATH for Fortran modules, they are now not found :-(