Patch HPC for better compatibility with large projects

[peterbecich]

This is a patch to HPC authored by @purefn . I've received permission from him to attempt to upstream this patch.

Paraphrasing @purefn ,
HPC currently requires all inputs as command line arguments.

Usage: hpc report [OPTION] .. <TIX_FILE> [<MODULE> [<MODULE> ..]]
Output textual report about program coverage
Options:
    --per-module                  show module level detail
    --decl-list                   show unused decls
    --exclude=[PACKAGE:][MODULE]  exclude MODULE and/or PACKAGE
    --include=[PACKAGE:][MODULE]  include MODULE and/or PACKAGE
    --srcdir=DIR                  path to source directory of .hs files
                                  multi-use of srcdir possible
    --hpcdir=DIR                  append sub-directory that contains .mix files
                                  default .hpc [rarely used]
    --reset-hpcdirs               empty the list of hpcdir's
                                  [rarely used]
    --xml-output                  show output in XML
    --verbosity=[0-2]             verbosity level, 0-2
                                  default 1

With large projects this can result in an argument list too long error: NixOS/nix#5593
This patch to HPC adds the arguments srcdirs-from=<FILE>, hpcdirs-from=<FILE>, and includes-from=<FILE>.
Then, haskell.nix passes these inputs as files rather than as direct command line arguments.

@purefn notes this improvement to HPC ought to be upstreamed to HPC directly: https://gitlab.haskell.org/ghc/packages/hpc
However, would this result in the change being available to old GHC versions such as 8.10.7?

---

Here are two tests of this patch on public projects:

Pandoc
peterbecich/pandoc@93bc858
This HTML report can be generated with: nix build -f default.nix projectCoverageReport
No other arguments are needed because they are here: https://github.com/peterbecich/pandoc/blob/93bc85844c539a81b3a31eb8e76f7efb0d5aac63/default.nix#L13-L20

more tests TODO...

I do not have a test which exhibits the error without the patch. This is because projects sufficiently large to cause unpatched HPC to fail also hit other instances of the argument list too long error, unrelated to HPC: #1297
So, the best I can do is to show that the patch is not a regression.

[michaelpj]

I think this needs to be upstreamed. I would guess that it would be pretty safe to make hpc reinstallable, so even if this patch is only available in recent versions of hpc, it'll probably be usable by people on older versions of GHC.

[michaelpj]

We already have way too many GHC patches, if we can possibly avoid more that would be great...

[peterbecich]

Thanks, I will try to upstream the change. I see now it applies to hpc-bin (https://gitlab.haskell.org/ghc/ghc/-/tree/master/utils/hpc); not hpc (https://gitlab.haskell.org/ghc/packages/hpc)

Because this change applies to the GHC source tree, not the HPC source tree, does that mean it will only take effect on future GHCs, unless explicitly back-ported?

[peterbecich]

Here is the upstream change: https://gitlab.haskell.org/ghc/ghc/-/merge_requests/8194 I should probably provide a test to show it works without haskell.nix before asking for review.
If/when it is merged, I will open a new PR to make the corresponding changes to haskell.nix. Thanks again

[michaelpj]

That looks super, thanks.

Like I said, I think we might be able to get away with just:

• A new release of HPC on Hackage
• Letting HPC be re-installed so that users can just pick up the new Hackage version rather than taking the one bundled with their GHC.

[BinderDavid]

That looks super, thanks.

Like I said, I think we might be able to get away with just:

* A new release of HPC on Hackage

* Letting HPC be re-installed so that users can just pick up the new Hackage version rather than taking the one bundled with their GHC.

@michaelpj Just to let you know that HPC has a new maintainership team now, and we are working on this. I just created a ticket for a reinstallable HPC here: https://gitlab.haskell.org/hpc/hpc-bin/-/issues/13 If you have any requirements on the HPC binary, feel free to chime in with feature requests :)

[michaelpj]

Great! We may well have such in future, should I tag you in that case?

[BinderDavid]

Great! We may well have such in future, should I tag you in that case?

Yes, feel free to tag me :)