Hackage roundtrip tests are failing for package `io-classes` due to dependency splitting

[fgaz]

jasagredo

This cabal file just broke the hackage roundtrip tests I think https://hackage.haskell.org/package/io-classes-1.6.0.0/io-classes.cabal

12345678910111213              +Dependency
                (PackageName "io-classes")
                (OrLaterVersion (mkVersion [0]))
                mainLibSet,
              Dependency
                (PackageName "io-classes")
                (OrLaterVersion (mkVersion [0]))
                (NonEmptySet.fromNonEmpty
                  (NE.fromList
                    [
                      -LMainLibName,
                      LSubLibName (UnqualComponentName "si-timers")]))],
            buildTools = ...},

Seems like parse . toUTF8BS . showGenericPackageDescription . parse changes the build-depends: io-classes:{io-classes, si-timers} to build-depends: io-classes, io-classes:si-timers
^ To reproduce: cabal run hackage-tests -- roundtrip io-classes

Originally posted by @mpickering in https://github.com/haskell/cabal/issues/10277#issuecomment-2312199548

[mpickering]

The error is introduced by this function

257 -- See Note [Dependencies on sublibraries] in Distribution.PackageDescription.Parsec
258 --                                                                              
259 preProcessInternalDeps :: CabalSpecVersion -> GenericPackageDescription -> GenericPackageDescription
260 preProcessInternalDeps specVer gpd                                                    
261   | specVer >= CabalSpecV3_4 = gpd                                              
262   | otherwise = transformAllBuildInfos transformBI transformSBI gpd             
263   where                                                                         
264     transformBI :: BuildInfo -> BuildInfo                                                       
265     transformBI =                                                               
266       over L.targetBuildDepends (concatMap transformD)                          
267         . over L.mixins (map transformM)                                        
268                                                                                 
269     transformSBI :: SetupBuildInfo -> SetupBuildInfo                            
270     transformSBI = over L.setupDepends (concatMap transformD)                   
271                                                                                 
272     transformD :: Dependency -> [Dependency]                                    
273     transformD (Dependency pn vr ln)                                            
274       | pn == thisPn =                                                          
275           if LMainLibName `NES.member` ln                                       
276             then Dependency thisPn vr mainLibSet : sublibs                      
277             else sublibs                                                        
278       where                                                                     
279         sublibs =                                                               
280           [ Dependency (unqualComponentNameToPackageName uqn) vr mainLibSet       
281           | LSubLibName uqn <- NES.toList ln                                    
282           ]                                                                     
283     transformD d = [d]                                                          
284                                                                                 
285     transformM :: Mixin -> Mixin                                                
286     transformM (Mixin pn (LSubLibName uqn) inc)                                 
287       | pn == thisPn =                                                          
288           mkMixin (unqualComponentNameToPackageName uqn) LMainLibName inc       
289     transformM m = m                                                            
290                                                                                 
291     thisPn :: PackageName                                                       
292     thisPn = pkgName (package (packageDescription gpd)) 

[mpickering]

Setting the cabal-version to 3.4 resolves the roundtripping error (see first clause in preProcessInternalDeps).

[mpickering]

Linked to https://github.com/haskell/cabal/issues/6083

[mpickering]

Minimised reproducer:

cabal-version:       3.0
name:                io-classes
version:             1.6.0.0

library

library io-classes-mtl
    build-depends:  io-classes:{io-classes,si-timers}

[mpickering]

It seems a bit baffling to me that preProcessInternalDeps is called only before pretty printing. I think this code can probably be deleted. It seems that is used to be called in more places.

[coot]

Do you want me to publish a revision which is using cabal-version: 3.4? Or maybe you don't want me to do that before you have a general fix?

[mpickering]

@coot At this point it doesn't matter what you do as the cabal file which exposes the bug is in the index tarball and will never be deleted. (Not a problem, it was a real bug which was exposed by the package)