CMakeMake easyblock: Suggestion to set -DFETCHCONTENT_FULLY_DISCONNECTED=ON by default

[ThomasHoffmann77]

Probably, please open an issue about it so we have it on record.

Originally posted by @akesandgren in easybuilders/easybuild-easyconfigs#22155 (comment)

[boegel]

Can you share some more context/info in this issue itself? What would this imply, why should we do it, etc.?

[ThomasHoffmann77]

Can you share some more context/info in this issue itself? What would this imply, why should we do it, etc.?

Many cmake-base software packages download third-party packages at build time by fetch_content(). Such sources are not tracked by default. E.g. libcifpp downloads catch2 and other for which also a builddepenceny could be used.
Setting FETCHCONTENT_FULLY_DISCONNECTED=ON by default would help to enforce using such packeges by (build)dependencies. Some existing easyconfigs might fail with this default setting. However, dependencies could be adapted accordingly.

Therefore offline-builds could be improved with this new default parameter.

[Flamefire]

We can download required sources and use FETCHCONTENT_SOURCE_DIR_FOO to point CMake to it: https://cmake.org/cmake/help/latest/module/FetchContent.html#variable:FETCHCONTENT_SOURCE_DIR_%3CuppercaseName%3E

So this shouldn't be causing too much trouble

[Micket]

just some bike shedding:

offline = ...

to determine this (which is used by cargo* easyblocks already)