Empty package: cabal-hash.txt: Does not exist

[23Skidoo]

Got this while trying to build ircbrowse with a recent snapshot of cabal-install HEAD:

$ cabal new-build
[...]
cabal: Failed to build snap-app-0.7.0 (which is required by exe:ircbrowse from
ircbrowse-0.1.0.0). The failure occurred during the final install step. The
exception was:
/home/refold/.cabal/store/ghc-8.0.1/snap-app-0.7.0-e7ebb350f8c439e859941200457b8734ec0e234e736ed19e834da281aa9c8440/cabal-hash.txt:
openBinaryFile: does not exist (No such file or directory)

The /home/refold/.cabal/store/ghc-8.0.1/snap-app-0.7.0-e7ebb350f8c439e859941200457b8734ec0e234e736ed19e834da281aa9c8440/ directory doesn't seem to exist.

[23Skidoo]

snap-app 0.7 is an empty package, and we don't seem to handle that case correctly either:

$ cabal new-build
Resolving dependencies...
In order, the following will be built (use -v for more details):
 - snap-app-0.7.0 (lib) (first run)
Configuring component lib from snap-app-0.7.0
Preprocessing library snap-app-0.7.0...

[23Skidoo]

@hvr showed me an example where a (conditionally) empty library stanza is actually useful: http://hackage.haskell.org/package/nats-1.1.1/nats.cabal

tl;dr: when you want reexported-modules, but want to support old cabal-install versions.

[ezyang]

Since hashing is only done for packages from a repository, it will be difficult to construct a test case (all our test cases rely on inplace builds); so from a tooling perspective, this is blocking on #4016.

[hvr]

@23Skidoo here's another canonical one btw: http://hackage.haskell.org/package/fail-4.9.0.0/fail.cabal

[ezyang]

#4016 is solved so we should be able to make a test case for this.

[hvr]

Just in case there was any doubt, this is a regression relative to old-build (which handles the weirdo-case snap-app-0.7.0 just fine);

simple repro-case:

cabal-version:       1.12
name:                bug
version:             0
build-type:          Simple
extra-source-files:  ChangeLog.md

library
  build-depends:       snap-app == 0.7.0
  default-language:    Haskell2010

cabal install --dep will succeed, while cabal new-build will run into the cabal-hash.txt-not-found error

[hasufell]

I have a similar problem, but on cabal new-install test:spec. I have no idea how to mitigate this.

[phadej]

@hasufell, installing test-suites is something which shouldn't work anyway, should it? (I agree that error should be about that, and not about something internal).

[phadej]

With cabal-install-3.0 it's better, as it doesn't make store broken:

% cabal build --dep
Resolving dependencies...
Build profile: -w ghc-8.6.5 -O1
In order, the following will be built (use -v for more details):
 - snap-app-0.7.0 (lib:snap-app) (requires download & build)
Downloading  snap-app-0.7.0
Downloaded   snap-app-0.7.0
Starting     snap-app-0.7.0 (all, legacy fallback)
Building     snap-app-0.7.0 (all, legacy fallback)
Installing   snap-app-0.7.0 (all, legacy fallback)
cabal: Failed to build snap-app-0.7.0. The failure occurred during the final
install step. The exception was:
/cabal/store/ghc-8.6.5/incoming/new-99767/cabal/store/ghc-8.6.5/snap-app-0.7.0-69381acbcfc33e2f42e1fee72b15c9d7d1d268b1e258f52f29e71a36232fc0bb/cabal-hash.txt:
openBinaryFile: does not exist (No such file or directory)