Custom Build-type with --package-db and --global encounters runtime error

[cygnus]

To reproduce:

• Run cabal init, change Build-type to Custom (Setup.hs already provided by cabal init)
• Run ghc-pkg init db
• Run cabal configure --package-db=./db --global
• Output:

  Resolving dependencies...
  cabal: internal error: unexpected package db stack
  

In cabal-install-0.8.2, this appears to be caused by the code on lines 112-114 of Distribution.Client.Configure:

usePackageDB = if UserPackageDB `elem` packageDBs
                 then packageDBs
                 else packageDBs ++ [UserPackageDB]

in combination with the code on lines 297-306 of Distribution.Client.SetupWrapper:

ghcPackageDbOptions :: PackageDBStack -> [String]
ghcPackageDbOptions dbstack = case dbstack of
  (GlobalPackageDB:UserPackageDB:dbs) -> concatMap specific dbs
  (GlobalPackageDB:dbs)               -> "-no-user-package-conf"
                                       : concatMap specific dbs
  _                                   -> ierror
  where
    specific (SpecificPackageDB db) = [ "-package-conf", db ]
    specific _ = ierror
    ierror     = error "internal error: unexpected package db stack"

Essentially, the order of the package databases in the stack is not as expected since the list already contains a SpecificPackageDB when the UserPackageDB is appended.

(Note: this applies to GHC 6.12.3 and Cabal 1.8.0.6 on my system, but I could not select those versions in the ticket properties.)

[hvr]

Alas, this still applies to latest (unreleased) Cabal-1.13.3 / cabal-install-0.13.3 / GHC-7.3 versions

Also, cabal-dev calls cabal with that --package-db/--global combination, causing this "unexpected package db stack" error...

[duncan]

Thanks for the report, and pinpointing the location of the problem.

Wed Mar 28 23:05:56 BST 2012  Duncan Coutts <duncan@community.haskell.org>
  * Fix ticket #731