Enable `buildFromSdist` by default - to ensure release-worthiness

[srid]

https://community.flake.parts/haskell-flake/settings#buildfromsdist

@roberth made a good point here:

haskell-flake has a role in CI to ensure release-worthiness. Building from sdist is an important part of that.

Currently, enabling buildFromSdist break things (see #253). But we should a) add tests for those (one exists for jailbreak, we need another one for check), b) find the root cause and fix the breakage, and c) re-enable it as the default.

[srid]

Release-worthiness is not the only benefit; building from sdist also avoids unnecessary rebuilds, which otherwise has to be hacked around with source filtering!

So we must definitely do this.

[roberth]

avoids unnecessary rebuilds

Only if content addressed derivations were to be used, which are not a production ready feature yet.

You could emulate it with IFD though, building and unpacking the sdist, readDir the output, and then reassemble it with a custom source filter or filesets. This will be slow, because whereas cabal2nix is almost instance, sdist can take a while, especially if it's generic, Setup.hs-based instead of the quicker cabal-install one which doesn't work for everyone (horizon-haskell? static?).

[srid]

Right, sadly, I notice that rebuilds will happen regardless of the use of buildFromSdist. Change ./subdir/README.md - then ./subdir/subdir.cabal Haskell package will rebuild, despite the README.md not being used in the sdist. What's the normal way to fix this?

[roberth]

I get by with this:

• Make sure all packages have their own subdir
• If you can get away with it, don't put "unnecessary" files in them
• Maybe accept that some files trigger a rebuild. In my experience, rebuilding for something README.md is not great, but not terrible.
• If needed, use a source filter, such as filesets

README.md is an interesting one btw. It should be part of the sdist, so that Hackage can show it, but it's probably not needed for the build.

I wouldn't be surprised if haskell.nix has a custom reimplementation of cabal's default sdist behavior in pure Nix.

Something that might be a bit helpful to get the source filter right, is to compare the filtered sources to the sdists in the checks output. For a special case like README.md where you want it in the sdist, it would help to have e.g. two filesets

• buildSrc: exactly what we pass to the derivation
• extraSdist: extra files needed for the sdist. When generating the sdist for checking, we pass toSource { fileset = unions [ buildSrc extraSdist ]; }.

It's a bit convoluted, but this approach seems to at least guide the user somewhat to get it right and have no surprises at release time. Using ca-derivations would be simpler, but doesn't actually solve the extraSdist problem.

[srid]

Turned out to be useful:

srid/haskell-template#129

Though, not all projects need it; for eg., https://github.com/nammayatri/nammayatri does not, as there are no plans to put it on Hackage.