Support executable-only packages and non-Haskell libraries

[mietek]

Add uniform support for dependencies which may be Haskell executable-only packages or non-Haskell libraries.

Relevant issues:

• cabal-install doesn't consider build-tools as dependencies haskell/cabal#220
• build-depends on packages that provide no library haskell/cabal#779

Examples:

• language-javascript and haskell-src-exts need alex and happy
• type-eq needs cpphs
• pcre-light needs libpcre

[begriffs]

There are a couple more ones you'll want.

• A modern libgmp. Heroku has an old one (version 3 I think) installed by default
• Symlink libncurses to the name libtinfo since some packages are looking for it by that name

[mietek]

libgmp and libtinfo are already handled, as they are prerequisites for GHC and Cabal.

Do you mean some packages specifically require libgmp.so.10?

[begriffs]

Oh, nevermind I didn't review the code carefully enough to notice those libraries were already handled.

[mietek]

Work in progress report:

We can now easily determine what are the exact executable-only packages and non-Haskell libraries needed to install any Cabal package.

$ time (
>     lib_constraints=$( cabal_list_planned_lib_constraints /tmp/haskell-on-heroku-examples/hello-yesod-platform ) &&
>     echo "${lib_constraints}" | cabal_get_all_package_files 1 /tmp/lib_package_files &&
>     echo "${lib_constraints}" | cabal_dump_all_package_build_tools /tmp/lib_package_files | sed 's/^/build-tools: /' &&
>     echo "${lib_constraints}" | cabal_dump_all_package_extra_libs /tmp/lib_package_files | sed 's/^/extra-libs: /'
> )
build-tools: hsc2hs
build-tools: alex
build-tools: happy
extra-libs: z

real  0m11.015s
user  0m15.550s
sys   0m2.530s

This work would benefit from cabal get being able to output just the package description. (haskell/cabal#1954)

[mietek]

A proper implementation of my ideas for solving this issue ought to be done as an improvement to Cabal. My first step towards this (PR haskell/cabal#1977) has been stuck in the review pipeline for a while now.

I am leaving this for later, as there is now another way to achieve the same results.

[mietek]

Pre- and post-build hooks are now included, hashed and tracked as dependencies. This work turned out to be a great start towards complete automation.

It is now possible to automatically deploy extra apps with a single Halcyon invocation. For example, here’s how to deploy Idris in 0m51.807s:

$ deploy --sandbox-extra-apps=alex-3.1.3 idris

Support for non-Haskell libraries is now tracked as #33.

[mietek]

Executable-only packages needed as build-tools can be declared in .halcyon/sandbox-extra-apps.

Non-Haskell libraries can be declared as native OS packages in .halcyon/sandbox-extra-os-packages for build-time dependencies, and in .halcyon/extra-os-packages for run-time dependencies.