stack build [target] no longer works, builds all targets

[nh2]

There is a regression in stack that stack build myexe no longer builds just the executable myexe, but all executables from the package.

Especially projects with many executables, and projects where linking takes a long time suffer from this (both are the case for IOHK's cardano-sl).

The reason according to @mgsloan:

I think this was the result of switching from using runhaskell Setup.hs install to copy + register - f71a515 .
copy fails if it can't find every executable, and there's no way in released cabal to tell it to only copy particular targets. There was a patch well before the most recent cabal release, but for some reason it is only available on master. So, with future cabal this may be fixable.

[snoyberg]

Let's finally set to rest the theory that it's the switch from install to copy/register (it's not):

stack new foo
cd foo
stack exec --no-ghc-package-path -- runghc Setup.hs configure --user
stack exec --no-ghc-package-path -- runghc Setup.hs build lib:foo
stack exec --no-ghc-package-path -- runghc Setup.hs install

Results in:

Installing library in
/Users/michael/.cabal/lib/x86_64-osx-ghc-8.0.2/foo-0.1.0.0-K7FRl2SdaSPJF1E2Gf1Eav
Installing executable(s) in /Users/michael/.cabal/bin
Warning: The directory /Users/michael/.cabal/bin is not in the system search
path.
dist/build/foo-exe/foo-exe: copyFile: does not exist (No such file or
directory)

[snoyberg]

The problem: Cabal (the library) will fail and die if you call the copy command when some of the executables are unavailable.

Original solution: oh well, user will get an error message, realize he/she has to call build once without arguments, and then can give components.

Current solution: never tell build to only build a subset of components, since that can lead to copy failing.

Proposed solution: before calling Cabal build, Stack should check if all executables are built. If they are, pass in components. Otherwise, give a logInfo that we're building all components once, and then future calls will build individual components.

[chrisdone]

Found the related commit: dc6313f

[snoyberg]

Merged to master, closing.