Plan of Attack for Parallel Builds

[Frank Murphy]

I've been looking around and trying to develop a plan for parallelizing builds
in cabal-install. Here's my idea so far:

- Parallelize executeInstallPlan. When given a target load average as a flag it
 will determine whether it should spawn a worker (if below the target load
 average) or wait. If waiting, it will listen to all worker status channels
 and print out their current build status and the load average. Once a worker
 exits, it will again check the load average and spawn a new thread if
 necessary.
- Rewrite install.*Package and their callees to use the CHP (Communicating
 Haskell Process) monad where possible. Use channels to communicate build
 status back to the main thread.
- It might be necessary to parse the output of external builds in some way so
 that meaningful status can be communicated back to the user.
- Add a default parallel build log path template. Allow the user to specify one
 on the command line to override the default. All output of parallel package
 builds will be logged in the background silently instead of displayed to the
 user.
- On single-threaded (sequential) builds, revert to the old output style. On
 multi-threaded builds, display the current status of all running builds, load
 averages and nothing else. Possible output:

Resolving dependencies...
Building derive-2.3.0.2...                                            [17 of 58]
Building regex-base-0.93.1...                                           [1 of 4]
Building dyre-0.8.6...                                                  [5 of 7]
Configuring xdg-basedir-0.2...                                     [in progress]

                                                  Dependencies Built:  [0 of 9]
                                                        Load Average: [3.4/4.0]
                                                                Running 4 Jobs.

A possible error message might look like:

derive-2.3.0.2 failed during the building phase.
Log stored in /home/frank/cabal/logs/build/derive-2.3.0.2.log

What does everyone think?

Thanks,
Frank