How to v2-install both library and executable

[elliottslaughter]

Describe the bug
If I install a library followed by its executable, both seem to get rebuilt. This could be avoided if there were an easy way (e.g. --exe --lib) to build both in the first place, or if the command were smart enough to avoid the duplicate build.

To Reproduce
Steps to reproduce the behavior:

$ cabal v2-install --lib pandoc
$ cabal v2-install pandoc

Expected behavior
My original build (with --lib) already built the executable. It would be nice if it didn't have to be rebuilt, or failing that, if there were a way to explicitly request both to be installed.

System information

• Operating system: macOS 10.14
• cabal 3.0.0.0, ghc 8.8.1

Additional context
Sample output:

$ cabal v2-install --lib pandoc
Resolving dependencies...
[ ... ]
 - pandoc-2.9.1.1 (lib) (requires download & build)
 - pandoc-2.9.1.1 (exe:pandoc) (requires download & build)
[ ... ]
Installing   pandoc-2.9.1.1 (exe:pandoc)
Completed    pandoc-2.9.1.1 (exe:pandoc)
$ cabal v2-install pandoc
Resolving dependencies...
Build profile: -w ghc-8.8.1 -O1
In order, the following will be built (use -v for more details):
 - process-1.6.7.0 (lib:process) (requires download & build)
 - hsc2hs-0.68.6 (exe:hsc2hs) (requires build)
 - network-3.1.1.1 (lib:network) (requires build)
 - tls-1.5.3 (lib) (requires build)
 - streaming-commons-0.2.1.2 (lib) (requires build)
 - socks-0.6.1 (lib) (requires build)
 - HTTP-4000.3.14 (lib) (requires build)
 - http-client-0.6.4 (lib) (requires build)
 - connection-0.3.1 (lib:connection) (requires build)
 - http-client-tls-0.3.5.3 (lib) (requires build)
 - pandoc-2.9.1.1 (lib) (requires build)
 - pandoc-2.9.1.1 (exe:pandoc) (requires build)
[ ... ]

It's not quite as bad as a full rebuild, but it still requires extra time if you build the library and executable multiple times.

[phadej]

This is weird. cabal install --lib pandoc should not build an executable. And I don't see a reason in pandoc.cabal why it would do so. (it's not build-type: Custom, e.g.)

@fgaz do you know what may be the cause?

[fgaz]

I can't think of any reason why it should do that. @typedrat?

[fgaz]

I looked a bit at the code and it seems that on the installLibraries branch the component-type filtering is only done after the building and before the installation in the env file:

cabal/cabal-install/Distribution/Client/CmdInstall.hs

Lines 844 to 856 in 0d4ee7b

	entriesForLibraryComponents :: TargetsMap -> [GhcEnvironmentFileEntry]
	entriesForLibraryComponents = Map.foldrWithKey' (\k v -> mappend (go k v)) []
	where
	hasLib :: (ComponentTarget, [TargetSelector]) -> Bool
	hasLib (ComponentTarget (CLibName _) _, _) = True
	hasLib _ = False
	go :: UnitId
	-> [(ComponentTarget, [TargetSelector])]
	-> [GhcEnvironmentFileEntry]
	go unitId targets
	| any hasLib targets = [GhcEnvFilePackageId unitId]
	| otherwise = []

[fgaz]

Which explains why install --lib <package-with-no-libs> succeeds:

> cabal v2-install yabi
Resolving dependencies...
Build profile: -w ghc-8.6.5 -O1
In order, the following will be built (use -v for more details):
 - word8-0.1.3 (lib) (requires download & build)
 - yabi-0.2.0.0 (exe:yabi) (requires download & build)
Downloading  word8-0.1.3
Downloaded   word8-0.1.3
Downloading  yabi-0.2.0.0
Configuring library for word8-0.1.3..
Downloaded   yabi-0.2.0.0
Preprocessing library for word8-0.1.3..
Building library for word8-0.1.3..
[1 of 2] Compiling Data.Char8       ( Data/Char8.hs, dist/build/Data/Char8.o )
[2 of 2] Compiling Data.Word8       ( Data/Word8.hs, dist/build/Data/Word8.o )
Installing library in /home/fgaz/.cabal/store/ghc-8.6.5/incoming/new-13710/home/fgaz/.cabal/store/ghc-8.6.5/word8-0.1.3-a2b361dcca69866e4747c1dcdd9edce50bfcf06017e9a9e3253f16436e28d4e4/lib
Configuring executable 'yabi' for yabi-0.2.0.0..
Preprocessing executable 'yabi' for yabi-0.2.0.0..
Building executable 'yabi' for yabi-0.2.0.0..
[1 of 1] Compiling Main             ( yabi.hs, dist/build/yabi/yabi-tmp/Main.o )
Linking dist/build/yabi/yabi ...
Installing executable yabi in /home/fgaz/.cabal/store/ghc-8.6.5/incoming/new-13710/home/fgaz/.cabal/store/ghc-8.6.5/yabi-0.2.0.0-66c5bafad0f1822c8fed4992e9136d3e02ef01c505ca3ca0a76404c71449ac75/bin
Warning: The directory
/home/fgaz/.cabal/store/ghc-8.6.5/incoming/new-13710/home/fgaz/.cabal/store/ghc-8.6.5/yabi-0.2.0.0-66c5bafad0f1822c8fed4992e9136d3e02ef01c505ca3ca0a76404c71449ac75/bin
is not in the system search path.

(the warning is bogus and has nothing to do with the issue)