Notify & enable users to stay in the warm shell

Notify & enable users to stay in sbt's shell on the warm JVM by hitting
[ENTER] while sbt is running.

Looks like this; first I run 'sbt about', then I hit [ENTER]:

    $ sbt about
    [info] !!! Executing in batch mode !!! For better performance, hit [ENTER] to remain in the sbt shell

    [info] Loading global plugins from /Users/dnw/.dotfiles/.sbt/0.13/plugins
    [info] Loading project definition from /s/t/project
    [info] Set current project to t (in build file:/s/t/)
    [info] This is sbt 0.13.14-SNAPSHOT
    [info] The current project is {file:/s/t/}t 0.1.0-SNAPSHOT
    [info] The current project is built against Scala 2.12.1
    [info] Available Plugins: sbt.plugins.IvyPlugin, sbt.plugins.JvmPlugin, sbt.plugins.CorePlugin, sbt.plugins.JUnitXmlReportPlugin, sbt.plugins.Giter8TemplatePlugin
    [info] sbt, sbt plugins, and build definitions are using Scala 2.10.6
    >
    >

Fixes #2987

main-command/src/main/scala/sbt/State.scala

@@ -196,7 +196,18 @@ object State {
 
   /** Provides operations and transformations on State. */
   implicit def stateOps(s: State): StateOps = new StateOps {
-    def process(f: (Exec, State) => State): State =
+    def process(f: (Exec, State) => State): State = {
+      def doX(x: Exec, xs: List[Exec]) = {
+        log.debug(s"> $x")
+        f(x, s.copy(remainingCommands = xs, currentCommand = Some(x), history = x :: s.history))
+      }
+      def isInteractive = System.console() != null
+      def hasInput = System.console().reader().ready()
+      s.remainingCommands match {
+        case List()           => if (isInteractive && hasInput) doX(Exec("shell", s.source), Nil) else exit(true)
+        case List(x, xs @ _*) => doX(x, xs)
+      }
+    }
       s.remainingCommands match {
         case List() => exit(true)
         case x :: xs =>

main/src/main/scala/sbt/MainLoop.scala

@@ -72,6 +72,10 @@ object MainLoop {
       val newLogging = state.globalLogging.newAppender(full, out, logBacking)
       // transferLevels(state, newLogging)
       val loggedState = state.copy(globalLogging = newLogging)
+      def isInteractive = System.console() != null
+      def hasShell = state.remainingCommands contains "shell"
+      if (isInteractive && !hasShell)
+        state.log info "!!! Executing in batch mode !!! For better performance, hit [ENTER] to remain in the sbt shell"
       try run(loggedState) finally out.close()
     }
 

notes/0.13.14/stay-in-shell.md

@@ -0,0 +1,7 @@
+### Improvements
+
+- Notifies & enables users to stay in sbt's shell on the warm JVM by hitting \[ENTER\] while sbt is running. [#2987][]/[#2996][] by [@dwijnand][]
+
+[#2987]: https://github.com/sbt/sbt/issues/2987
+[#2996]: https://github.com/sbt/sbt/pull/2996
+[@dwijnand]: https://github.com/dwijnand