cabal: Error: Could not find module: "Added_prepossor_module_name" with any suffix

[mdgabriel]

Describe the bug
% cabal v2-sdist
cabal: Error: Could not find module: SciDbAFL with any suffix:
["gc","chs","hsc","x","y","ly","cpphs","hs","lhs","hsig","lhsig"]. If the
module is autogenerated it should be added to 'autogen-modules'.

I have added the preprocessor bnfc to a custom Setup. The file SciDbAFL.cf is not found.

To Reproduce
% cabal v2-sdist

Here is a snip of the custom Setup:

main :: IO ()
main = defaultMainWithHooks simpleUserHooks{hookedPreProcessors =
("cf",ppBnfc):knownSuffixHandlers}

The input file to bnfc is SciDbAFL.cf and the appropriate snip of my scidb-hquery.cabal is

other-modules:
SciDbAFL
AbsSciDbAFL
ErrM
LexSciDbAFL
ParSciDbAFL
PrintSciDbAFL

autogen-modules:
AbsSciDbAFL
ErrM
LexSciDbAFL
ParSciDbAFL
PrintSciDbAFL

Where to find the problem
In the file cabal-install-2.4.1.0/Distribution/Client/CmdSdist.hs, in the the function packageToSdist, the following line of code only uses knownSuffixHandlers:

listPackageSources verbosity (flattenPackageDescription $ packageDescription pkg) knownSuffixHandlers

For example, as a test, if I hard code

listPackageSources verbosity (flattenPackageDescription $ packageDescription pkg) (("cf",undefined):knownSuffixHandlers)

then my hacked cabal v2-sdist works.

Expected behavior
I expect cabal v2-sdist to not only use the the knownSuffixHandlers but also those in hookedPreProcessors.

Work around
I have an (ugly) work around. One just adds SciDbAFL to autogen-modules, which it is of course not, and one adds it also to

extra-source-files:
src/SciDbAFL.cf

As I understand it, this respects what nix wishes to accomplish.

System informataion

• Operating system: Ubuntu 16.04.6 LTS (Xenial Xerus)
• $ cabal --version
  cabal-install version 2.4.1.0
  compiled using version 2.4.1.0 of the Cabal library
• $ ghc --version
  The Glorious Glasgow Haskell Compilation System, version 8.6.5

Additional context
The issue was discovered with a cabal package that I am creating that uses bnfc --haskell via the hookedPreProcessors record.

[phadej]

#5908 is partially related.

IIRC @typedrat and @hvr think that v2-sdist should be completely static, i.e. don't care about build-type: Custom (I cannot find a discussion atm). This is clearly a counterexample.

[Mikolaj]

@mdgabriel: We would love to limit the use of Custom scripts (and surely make cabal dist ignorant of anything of the sort). Since this ticket was raised, did you perchance discover a way forward without them? Or could something by added to cabal to make it possible?