Fixes sbt#1345 - do not buffer logging for native-image output

src/main/scala/com/typesafe/sbt/packager/graalvmnativeimage/GraalVMNativeImagePlugin.scala

@@ -1,7 +1,6 @@
 package com.typesafe.sbt.packager.graalvmnativeimage
 
-import java.io.ByteArrayInputStream
-
+import java.io.{ByteArrayInputStream, PrintStream, PrintWriter}
 import sbt._
 import sbt.Keys.{mainClass, name, _}
 import com.typesafe.sbt.packager.{MappingsHelper, Stager}
@@ -73,7 +72,7 @@ object GraalVMNativeImagePlugin extends AutoPlugin {
             className,
             classpathJars.map(_._1),
             extraOptions,
-            streams.log
+            UnbufferedProcessLogger(streams.log)
           )
 
         case Some(image) =>
@@ -152,7 +151,7 @@ object GraalVMNativeImagePlugin extends AutoPlugin {
       s"-H:Name=$binaryName"
     ) ++ extraOptions ++ Seq(className)
 
-    sys.process.Process(command) ! streams.log match {
+    sys.process.Process(command) ! UnbufferedProcessLogger(streams.log) match {
       case 0 => targetDirectory / binaryName
       case x => sys.error(s"Failed to run $command, exit status: " + x)
     }
@@ -218,3 +217,11 @@ object GraalVMNativeImagePlugin extends AutoPlugin {
     Stager.stage(GraalVMBaseImage)(streams, stageDir, mappings)
   }
 }
+
+private case class UnbufferedProcessLogger(logger: Logger) extends ProcessLogger {
+  override def out(s: => String): Unit = logger.out(s)
+
+  override def err(s: => String): Unit = logger.err(s)
+
+  override def buffer[T](f: => T): T = f
+}