Add remote cancellation support

This commit makes it possible for a remote client to cancel a running
task initiated by a different client by typing `cancel` into the shell.
It can be useful if the remote client has run something blocking like
console.

The console task can't safely be interrupted, so instead we write some
newlines filed by ctrl+d to exit the console.

internal/util-logging/src/main/scala/sbt/internal/util/ConsoleAppender.scala

@@ -658,6 +658,7 @@ private[sbt] object ProgressState {
             }
           } else {
             pe.command.toSeq.flatMap { cmd =>
+              val tail = if (isWatch) Nil else "enter 'cancel' to stop evaluation" :: Nil
               s"sbt server is running '$cmd'" :: tail
             }
           }

main/src/main/scala/sbt/internal/CommandExchange.scala

@@ -350,6 +350,15 @@ private[sbt] final class CommandExchange {
     commandQueue.add(exit)
     ()
   }
+  private[this] def cancel(e: Exec): Unit = {
+    if (e.commandLine.startsWith("console")) {
+      val terminal = Terminal.get
+      terminal.write(13, 13, 13, 4)
+      terminal.printStream.println("\nconsole session killed by remote sbt client")
+    } else {
+      Util.ignoreResult(NetworkChannel.cancel(e.execId, e.execId.getOrElse("0")))
+    }
+  }
 
   private[this] class MaintenanceThread
       extends Thread("sbt-command-exchange-maintenance")

main/src/main/scala/sbt/internal/server/NetworkChannel.scala

@@ -698,6 +698,40 @@ object NetworkChannel {
   case object SingleLine extends ChannelState
   case object InHeader extends ChannelState
   case object InBody extends ChannelState
+  private[sbt] def cancel(
+      execID: Option[String],
+      id: String
+  ): Either[String, String] = {
+
+    Option(EvaluateTask.currentlyRunningEngine.get) match {
+      case Some((state, runningEngine)) =>
+        val runningExecId = state.currentExecId.getOrElse("")
+
+        def checkId(): Boolean = {
+          if (runningExecId.startsWith("\u2668")) {
+            (
+              Try { id.toLong }.toOption,
+              Try { runningExecId.substring(1).toLong }.toOption
+            ) match {
+              case (Some(id), Some(eid)) => id == eid
+              case _                     => false
+            }
+          } else runningExecId == id
+        }
+
+        // direct comparison on strings and
+        // remove hotspring unicode added character for numbers
+        if (checkId) {
+          runningEngine.cancelAndShutdown()
+          Right(runningExecId)
+        } else {
+          Left("Task ID not matched")
+        }
+
+      case None =>
+        Left("No tasks under execution")
+    }
+  }
 
   private[sbt] val disconnect: Command =
     Command.arb { s =>