Draft: Implement watching file paths via oslib.watch

core/api/src/mill/api/Watchable.scala

@@ -8,6 +8,24 @@ package mill.api
  */
 private[mill] sealed trait Watchable
 private[mill] object Watchable {
-  case class Path(p: java.nio.file.Path, quick: Boolean, signature: Int) extends Watchable
-  case class Value(f: () => Long, signature: Long, pretty: String) extends Watchable
+
+  /** A [[Watchable]] that is being watched via polling. */
+  private[mill] sealed trait Pollable extends Watchable
+
+  /** A [[Watchable]] that is being watched via a notification system (like inotify). */
+  private[mill] sealed trait Notifiable extends Watchable
+
+  /**
+   * @param p the path to watch
+   * @param quick if true, only watch file attributes
+   * @param signature the initial hash of the path contents
+   */
+  case class Path(p: java.nio.file.Path, quick: Boolean, signature: Int) extends Notifiable
+
+  /**
+   * @param f the expression to watch, returns some sort of hash
+   * @param signature the initial hash from the first invocation of the expression
+   * @param pretty human-readable name
+   */
+  case class Value(f: () => Long, signature: Long, pretty: String) extends Pollable
 }

integration/invalidation/watch-source-input/src/WatchSourceInputTests.scala

@@ -44,12 +44,9 @@ trait WatchTests extends UtestIntegrationTestSuite {
     val expectedShows0 = mutable.Buffer.empty[String]
     val res = f(expectedOut, expectedErr, expectedShows0)
     val (shows, out) = res.out.linesIterator.toVector.partition(_.startsWith("\""))
-    val err = res.err.linesIterator.toVector
-      .filter(!_.contains("Compiling compiler interface..."))
-      .filter(!_.contains("Watching for changes"))
-      .filter(!_.contains("[info] compiling"))
-      .filter(!_.contains("[info] done compiling"))
-      .filter(!_.contains("mill-server/ exitCode file not found"))
+    val err = res.err.linesIterator.toVector.filter(s =>
+      s.startsWith("Setting up ") || s.startsWith("Running ")
+    )
 
     assert(out == expectedOut)
 

libs/main/src/mill/main/MainModule.scala

@@ -22,8 +22,8 @@ abstract class MainRootModule()(implicit
  * [[show]], [[inspect]], [[plan]], etc.
  */
 trait MainModule extends BaseModule with MainModuleApi {
-  protected[mill] val watchedValues: mutable.Buffer[Watchable] = mutable.Buffer.empty[Watchable]
-  protected[mill] val evalWatchedValues: mutable.Buffer[Watchable] = mutable.Buffer.empty[Watchable]
+  protected[mill] val watchedValues: mutable.Buffer[Watchable] = mutable.Buffer.empty
+  protected[mill] val evalWatchedValues: mutable.Buffer[Watchable] = mutable.Buffer.empty
   object interp {
     def watchValue[T](v0: => T)(implicit fn: sourcecode.FileName, ln: sourcecode.Line): T = {
       os.checker.withValue(os.Checker.Nop) {

mill-build/src/millbuild/Deps.scala

@@ -117,7 +117,9 @@ object Deps {
   val junitInterface = mvn"com.github.sbt:junit-interface:0.13.3"
   val commonsIo = mvn"commons-io:commons-io:2.18.0"
   val log4j2Core = mvn"org.apache.logging.log4j:log4j-core:2.24.3"
-  val osLib = mvn"com.lihaoyi::os-lib:0.11.5-M8"
+  val osLibVersion = "0.11.5-M8"
+  val osLib = mvn"com.lihaoyi::os-lib:$osLibVersion"
+  val osLibWatch = mvn"com.lihaoyi::os-lib-watch:$osLibVersion"
   val pprint = mvn"com.lihaoyi::pprint:0.9.0"
   val mainargs = mvn"com.lihaoyi::mainargs:0.7.6"
   val millModuledefsVersion = "0.11.4"

runner/daemon/package.mill

@@ -19,6 +19,7 @@ object `package` extends MillPublishScalaModule {
   def mvnDeps = Seq(
     Deps.sourcecode,
     Deps.osLib,
+    Deps.osLibWatch,
     Deps.mainargs,
     Deps.upickle,
     Deps.pprint,

runner/daemon/src/mill/daemon/MillBuildBootstrap.scala

@@ -280,7 +280,7 @@ class MillBuildBootstrap(
         // look at the `moduleWatched` of one frame up (`prevOuterFrameOpt`),
         // and not the `moduleWatched` from the current frame (`prevFrameOpt`)
         val moduleWatchChanged =
-          prevOuterFrameOpt.exists(_.moduleWatched.exists(w => !Watching.validate(w)))
+          prevOuterFrameOpt.exists(_.moduleWatched.exists(w => !Watching.validateAnyWatchable(w)))
 
         val classLoader = if (runClasspathChanged || moduleWatchChanged) {
           // Make sure we close the old classloader every time we create a new

runner/daemon/src/mill/daemon/MillCliConfig.scala

@@ -97,6 +97,11 @@ case class MillCliConfig(
       doc = """Watch and re-run the given tasks when when their inputs change."""
     )
     watch: Flag = Flag(),
+    @arg(
+      name = "watch-via-fs-notify",
+      doc = "Use filesystem based file watching instead of polling based one (defaults to true)."
+    )
+    watchViaFsNotify: Boolean = true,
     @arg(
       short = 's',
       doc =

runner/daemon/src/mill/daemon/MillMain.scala

@@ -350,9 +350,13 @@ object MillMain {
                     if (config.watch.value) os.remove(out / OutFiles.millSelectiveExecution)
                     Watching.watchLoop(
                       ringBell = config.ringBell.value,
-                      watch = config.watch.value,
+                      watch = Option.when(config.watch.value)(Watching.WatchArgs(
+                        setIdle = setIdle,
+                        colors,
+                        useNotify = config.watchViaFsNotify,
+                        serverDir = serverDir
+                      )),
                       streams = streams,
-                      setIdle = setIdle,
                       evaluate = (enterKeyPressed: Boolean, prevState: Option[RunnerState]) => {
                         adjustJvmProperties(userSpecifiedProperties, initialSystemProperties)
                         runMillBootstrap(
@@ -361,8 +365,7 @@ object MillMain {
                           config.leftoverArgs.value,
                           streams
                         )
-                      },
-                      colors = colors
+                      }
                     )
                   }
                 }