Add warning for slow hostname lookups on OS X

I spent a lot of time debugging why it took 5 seconds to run tests each
time. It turns out that if the hostname is not set explicitly on os x,
then getaddrinfo takes 5 seconds to try (and fail) to resolve the dns
entry for the localhostname. This is easily fixed by setting the
hostname, but it is not at all easy to figure out that a slow hostname
lookup is the reason why tests are slow to start.

I don't know if this is a common issue on other platforms, so only issue
the warning on OS X.

main/src/main/scala/sbt/internal/Load.scala

@@ -1039,7 +1039,9 @@ private[sbt] object Load {
         // Grab all the settings we already loaded from sbt files
         def settings(files: Seq[File]): Seq[Setting[_]] = {
           if (files.nonEmpty)
-            log.info(s"${files.map(_.getName).mkString(s"Loading settings for project ${p.id} from ", ",", " ...")}")
+            log.info(
+              s"${files.map(_.getName).mkString(s"Loading settings for project ${p.id} from ", ",", " ...")}"
+            )
           for {
             file <- files
             config <- (memoSettings get file).toSeq

main/src/main/scala/sbt/plugins/JUnitXmlReportPlugin.scala

@@ -30,6 +30,6 @@ object JUnitXmlReportPlugin extends AutoPlugin {
   // It might be a good idea to derive this setting into specific test scopes.
   override lazy val projectSettings: Seq[Setting[_]] =
     Seq(
-      testListeners += new JUnitXmlTestsListener(target.value.getAbsolutePath)
+      testListeners += new JUnitXmlTestsListener(target.value.getAbsolutePath, streams.value.log)
     )
 }

testing/src/main/scala/sbt/JUnitXmlTestsListener.scala

@@ -13,8 +13,10 @@ import java.time.LocalDateTime
 import java.time.format.DateTimeFormatter
 import java.time.temporal.ChronoUnit
 import java.util.Hashtable
+import java.util.concurrent.TimeUnit.NANOSECONDS
 
 import scala.collection.mutable.ListBuffer
+import scala.util.Properties
 import scala.xml.{ Elem, Node => XNode, XML }
 import testing.{
   Event => TEvent,
@@ -23,6 +25,7 @@ import testing.{
   OptionalThrowable,
   TestSelector
 }
+import util.Logger
 import sbt.protocol.testing.TestResult
 
 /**
@@ -30,14 +33,27 @@ import sbt.protocol.testing.TestResult
  * report format.
  * @param outputDir path to the dir in which a folder with results is generated
  */
-class JUnitXmlTestsListener(val outputDir: String) extends TestsListener {
+class JUnitXmlTestsListener(val outputDir: String, logger: Logger) extends TestsListener {
+  // This constructor is for binary compatibility with older versions of sbt.
+  def this(outputDir: String) = this(outputDir, null)
 
   /**Current hostname so we know which machine executed the tests*/
-  val hostname =
-    try InetAddress.getLocalHost.getHostName
+  val hostname = {
+    val start = System.nanoTime
+    val name = try InetAddress.getLocalHost.getHostName
     catch {
       case _: IOException => "localhost"
     }
+    val elapsed = System.nanoTime - start
+    if ((NANOSECONDS.toSeconds(elapsed) >= 4) && Properties.isMac && logger != null) {
+      logger.warn(
+        s"Getting the hostname $name was slow (${elapsed / 1.0e6} ms). " +
+          "This is likely because the computer's hostname is not set. You can set the " +
+          "hostname with the command: scutil --set HostName $(scutil --get LocalHostName)."
+      )
+    }
+    name
+  }
 
   /**The dir in which we put all result files. Is equal to the given dir + "/test-reports"*/
   val targetDir = new File(outputDir + "/test-reports/")