Plans that use bootstrapping to break cycles

[Ericson2314]

In haskell/happy#175 (no merged) I have

executable happy
  ...
  if flag(bootstrap)
    ...
    -- build-tools: happy
  else
    ...

I would like to uncomment build-tools: happy to indicate that when bootstrapping, Happy does in fact depend on Happy, but then Cabal will complain of the cycle.

There's a lot of nuance on exactly how to make this goal, but I figured I'd start with the problem first, now that I finally have a small example of it.

[gbaz]

Build-tools is deprecated in version 2 format files and removed in version 3 format cabal files. That's not going to be the right approach: https://cabal.readthedocs.io/en/3.6/cabal-package.html?highlight=build-tools#pkg-field-build-tools

[Ericson2314]

@gbaz same issues happens with build-tool-depends. Glad build-tools is going away, though!

[Ericson2314]

I tried with https://github.com/simonmar/happy/pull/212/files using a different name to avoid this and #3502, but then I hit there there is no way to say "use happy-boot for .ly files". It's rather alpha-equivalence-violating, but the dep will only work if it's named happy.

[Mikolaj]

From the little I understand about this, #6459 is related, but it's a serious undertaking.

[Ericson2314]

@Mikolaj Yes, that required for this to work, thanks for pointing out.

[gbaz]

With build-tools-depends it gets introduced as a qualified goal. I think there are two things to do -- one is enable the same package version with different configs under different subgoals. The second is just to enable backtracking instead of hard failure on cycle detection (unless that may already occur!). Its an interesting problem and worthwhile to solve for the next release, if possible.

[grayjay]

The dependency solver should already be able to backtrack in response to a cycle. The findCycles function creates a conflict set from the packages in one of the cycles that is found, which is used for backtracking. However, there is a TODO for adding flag and stanza variables:

cabal/cabal-install-solver/src/Distribution/Solver/Modular/Cycles.hs

Lines 49 to 54 in 8760e3d

	-- | Given the reverse dependency map from a node in the tree, check
	-- if the solution is cyclic. If it is, return the conflict set containing
	-- all decisions that could potentially break the cycle.
	--
	-- TODO: The conflict set should also contain flag and stanza variables.
	findCycles :: QPN -> RevDepMap -> Maybe ConflictSet

I think we would need to fix the TODO and implement #6459 to allow the solver to break the cycle by building the same version of happy with different flags.

@Ericson2314 do you have the solver log (-v3) from trying to add a build-tool-depends dependency on happy so that I can check whether this is the problem?