Setup per-branch target directories in sbt

branch-targets.sbt

// Helper snippet to enable per-git-branch target directories
//
// This will move the target directory for a sub-module away
// from <project-root>/<sub-module-name>/target
// to   <project-root>/per-branch-targets/<branch-name>/<sub-module-name>
//
// Why?: shorter wait times if you switch branches often enough
//       (Example: Akka's project file take 20+ seconds to compile and 
//                 a single line change in AkkaBuild.scala will always 
//                 trigger a complete rebuild.)
//
// Instructions:
//   * put this file into every sub-project directory including `project`
//   * don't forget to restart sbt after each branch change
//
// Caveat: you will need to rebuild your project once from scratch for every new branch 
//         (which can be seen as a feature)
//
// Bonus points:
//  * if you can be adapt this to run on a btrfs which auto-initializes a new branch by 
//    copying data over from the "most related" branch
//  * for adding some cleanup logic for pruning old/non-existant branch directories, 
//    for big projects this will quickly use lots of space (one akka-http Build ~= 100MB)
//  * for making this an auto plugin, so you don't need to put it into every submodule manually

target := {
  def gitBranchName(): String = "git rev-parse --abbrev-ref HEAD".!!.trim
  def safeDirName(name: String): String = name.replaceAll("/", "_")
  target.value.getParentFile.getParentFile / // TODO: replace by proper detection of the parent project root
    "per-branch-targets" / safeDirName(gitBranchName()) / normalizedName.value
}

crossPaths := true

Awesome! What's the motivation behind moving it up in the tree?

Alternative: target.value.getParentFile / target.value.getName + "-" + safeDirName(gitBranchName())

Stumbled upon the idea here the other day while thinking about sbt/sbt-boilerplate#16.

I guess the reason was to keep everything belonging to one branch together. This simplifies maintenance somewhat.

I see. Makes wiping the target dir easier.

One more thing: Is there a way to add this to akka's defaultSettings from an external *.sbt file?

I added

target := {
  try { target.value / "git rev-parse --abbrev-ref HEAD".!!.trim }
  catch { case t: Throwable => println(t); target.value }
}

to akka's defaultSettings and it works like a charm.