Merge pull request #3597 from sbt/workaround/3527

Add system property to revert to old polling fs watcher

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

@@ -7,18 +7,19 @@
 
 package sbt
 
-import BasicCommandStrings.ClearOnFailure
-import State.FailureWall
-import annotation.tailrec
 import java.io.File
 import java.nio.file.FileSystems
 
-import scala.concurrent.duration._
-
-import sbt.io.{ AllPassFilter, NothingFilter, FileFilter, WatchService }
+import sbt.BasicCommandStrings.ClearOnFailure
+import sbt.State.FailureWall
 import sbt.internal.io.{ Source, SourceModificationWatch, WatchState }
 import sbt.internal.util.AttributeKey
 import sbt.internal.util.Types.const
+import sbt.io._
+
+import scala.annotation.tailrec
+import scala.concurrent.duration._
+import scala.util.Properties
 
 trait Watched {
 
@@ -39,7 +40,7 @@ trait Watched {
   private[sbt] def triggeredMessage(s: WatchState): String = Watched.defaultTriggeredMessage(s)
 
   /** The `WatchService` to use to monitor the file system. */
-  private[sbt] def watchService(): WatchService = FileSystems.getDefault.newWatchService()
+  private[sbt] def watchService(): WatchService = Watched.createWatchService()
 }
 
 object Watched {
@@ -125,4 +126,18 @@ object Watched {
     AttributeKey[WatchState]("watch state", "Internal: tracks state for continuous execution.")
   val Configuration =
     AttributeKey[Watched]("watched-configuration", "Configures continuous execution.")
+
+  def createWatchService(): WatchService = {
+    sys.props.get("sbt.watch.mode") match {
+      case Some("polling") =>
+        new PollingWatchService(PollDelay)
+      case Some("nio") =>
+        FileSystems.getDefault.newWatchService()
+      case _ if Properties.isMac =>
+        // WatchService is slow on macOS - use old polling mode
+        new PollingWatchService(PollDelay)
+      case _ =>
+        FileSystems.getDefault.newWatchService()
+    }
+  }
 }

main/src/main/scala/sbt/Defaults.scala

@@ -9,7 +9,6 @@ package sbt
 
 import Def.{ Initialize, ScopedKey, Setting, SettingsDefinition }
 import java.io.{ File, PrintWriter }
-import java.nio.file.FileSystems
 import java.net.{ URI, URL }
 import java.util.Optional
 import java.util.concurrent.{ TimeUnit, Callable }
@@ -251,7 +250,7 @@ object Defaults extends BuildCommon {
       parallelExecution :== true,
       pollInterval :== new FiniteDuration(500, TimeUnit.MILLISECONDS),
       watchService :== { () =>
-        FileSystems.getDefault.newWatchService
+        Watched.createWatchService()
       },
       logBuffered :== false,
       commands :== Nil,

notes/1.0.3/watch.md

@@ -0,0 +1,26 @@
+### Fixes with compatibility implications
+
+### Improvements
+
+- Add `sbt.watch.mode` system property to allow switching back to old polling behaviour for watch. See below for more details. [#3597][3597] by [@stringbean][@stringbean]
+
+### Bug fixes
+
+#### Alternative watch mode
+
+sbt 1.0.0 introduced a new mechanism for watching for source changes based on the NIO `WatchService` in Java 1.7. On
+some platforms (namely macOS) this has led to long delays before changes are picked up. An alternative `WatchService`
+for these platforms is planned for sbt 1.1.0 ([#3527][3527]), in the meantime an option to select which watch service
+has been added.
+
+The new `sbt.watch.mode` JVM flag has been added with the following supported values:
+
+- `polling`: (default for macOS) poll the filesystem for changes (mechanism used in sbt 0.13).
+- `nio` (default for other platforms): use the NIO based `WatchService`.
+
+If you are experiencing long delays on a non-macOS machine then try adding `-Dsbt.watch.mode=polling` to your sbt
+options.
+
+[@stringbean]: https://github.com/stringbean
+[3527]: https://github.com/sbt/sbt/issues/3527
+[3597]: https://github.com/sbt/sbt/pull/3597