lz4 fails to cross-compile due to gen_manual rule

[riastradh]

Describe the bug
lz4 uses $(CXX) to compile gen_manual, and then executes it during the build.

In a cross-build, say using a cross-compiler on an x86 system to build an lz4 that runs on aarch64, $(CXX) is a cross-compiler, so using it here yields an aarch64 executable for gen_manual, which can't be executed on x86.

Expected behavior
lz4 uses a native compiler for gen_manual when cross-compiling everything else.

For example, perhaps lz4 could sprout a parameter CXX_FOR_BUILD, which defaults to $(CXX), and which is used to compile gen_manual before executing it.

To Reproduce
Cross-compile lz4.

Additional context
Many other software packages that do something similar expose CC_FOR_BUILD or CXX_FOR_BUILD parameters, so there is plenty of precedent for this approach.

[Cyan4973]

A number of things to unpack here.

• gen_manual is not part of the standard lz4 package. If one runs the command make, this will not generate (nor run) gen_manual.
• gen_manual is part of an extended list of binaries that can be created from the lz4 repository. This extended list also includes for example test programs, such as tests/fuzzer . The command make all can be used to generate this extended list. It's primarily useful for CI tests.
• If your goal is to generate a binary package for a cross-compiled target, I'm not sure to understand why it would need the extended list of binaries, as opposed to the "core" useful package, featuring only the libraries and the CLI.
• The goal of gen_manual is just to generate a manual in html format from the source code. It's a repository-level tool. There should not be any reason to run this binary on client side, the html manual is supposed to be already updated.

So, I suspect the first question is, we should talk about the need to generate gen_manual,
and if there is no such need,
why is it generated, can or should it be excluded ?

[riastradh]

I came here from pkgsrc, which follows the convention that all is the standard make target for building everything important in a source tree. The pkgsrc packaging for lz4 doesn't do anything to override this.

I didn't investigate whether what gen_manual produces is relevant to the resulting package -- I'm just going through and fixing cross-compilation bugs in every package I stumble upon, and this is a common bug with a common fix, which is what I submitted in #1241.

Is there a better target to use instead? Should it use the default target?

[riastradh]

Looks like there's no trouble with leaving the target empty in pkgsrc so it will pick whatever is the default from make, which avoids the cross-compilation issue. Thanks!

[Cyan4973]

Yeah, the regular default make is the one intended to build lz4 packages.

make all is essentially used in CI, to build everything in the repository, and ensure all targets, including secondary ones, still work fine after every modification.

Maybe it's a mis-usage of the all target. I should have a deeper look at that, understand what's the established convention and see if it's respected.