Make runBackground use process IDs (0.12.x branch) (#5120)

Backport of #5115

---------

Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>

Author: arturaz

.editorconfig

@@ -5,7 +5,7 @@ end_of_line = lf
 insert_final_newline = true
 charset = utf-8
 indent_style = space
-indent_size = 4
+indent_size = 2
 
 [*.scala]
 end_of_line = lf

build.mill

@@ -431,7 +431,7 @@ trait MillPublishJavaModule extends MillJavaModule with PublishModule {
     "info.releaseNotesURL" -> Settings.changelogUrl
   )
   def pomSettings = MillPublishJavaModule.commonPomSettings(artifactName())
-  def javacOptions = Seq("-source", "1.8", "-target", "1.8", "-encoding", "UTF-8")
+  def javacOptions = Seq("-source", "11", "-target", "11", "-encoding", "UTF-8")
 }
 
 object MillPublishJavaModule {
@@ -448,7 +448,6 @@ object MillPublishJavaModule {
       )
     )
   }
-
 }
 
 /**

docs/modules/ROOT/pages/cli/flags.adoc

@@ -131,6 +131,13 @@ forcefully terminating the previous process even though it may be still alive:
 $ mill -w foo.runBackground
 ----
 
+Note that even if you interrupt mill watch via CTRL+C, the server spawned by `runBackground` still runs in the
+background. To actually stop the background server use:
+
+[source,console]
+----
+> mill clean foo.runBackground
+----
 
 === `--jobs`/`-j`
 

integration/invalidation/run-background/src/RunBackgroundTests.scala

@@ -37,6 +37,28 @@ object RunBackgroundTests extends UtestIntegrationTestSuite {
       os.write(stop, "")
       eventually { probeLockAvailable(lock) }
     }
+
+    test("sequential") - integrationTest { tester =>
+      import tester._
+      val lock1 = os.temp()
+      val lock2 = os.temp()
+      val stop = os.temp()
+      os.remove(stop)
+      eval(("foo.runBackground", lock1, stop))
+      eventually { !probeLockAvailable(lock1) }
+      eval(("foo.runBackground", lock2, stop))
+      eventually { !probeLockAvailable(lock2) }
+      Predef.assert(
+        probeLockAvailable(lock1),
+        "first process should be exited after second process is running"
+      )
+
+      if (tester.clientServerMode) eval("shutdown")
+      continually { !probeLockAvailable(lock2) }
+      os.write(stop, "")
+      eventually { probeLockAvailable(lock2) }
+    }
+
     test("clean") - integrationTest { tester =>
       import tester._
       val lock = os.temp()
@@ -45,6 +67,7 @@ object RunBackgroundTests extends UtestIntegrationTestSuite {
       eval(("foo.runBackground", lock, stop))
       eventually {
         !probeLockAvailable(lock)
+
       }
 
       eval(("clean", "foo.runBackground"))

main/define/src/mill/define/Task.scala

@@ -122,19 +122,33 @@ object Task extends TaskBase {
       cls: EnclosingClass
   ): Command[T] = macro Target.Internal.commandImpl[T]
 
+  /** Binary compatibility forwarder. */
+  def Command(
+      t: NamedParameterOnlyDummy,
+      exclusive: Boolean
+  ): CommandFactory = new CommandFactory(exclusive = exclusive, persistent = false)
+
   /**
    * @param exclusive Exclusive commands run serially at the end of an evaluation,
    *                  without any other tasks running parallel, and without the
    *                  terminal logging prefixes that are applied to normal tasks.
    *                  These are normally used for "top level" commands which are
    *                  run directly to perform some action or display some output
    *                  to the user.
+   * @param persistent Persistent comands do not erase the `Task.dest` folder
+   *                   between runs.
    */
   def Command(
       t: NamedParameterOnlyDummy = new NamedParameterOnlyDummy,
-      exclusive: Boolean = false
-  ): CommandFactory = new CommandFactory(exclusive)
-  class CommandFactory private[mill] (val exclusive: Boolean) extends TaskBase.TraverseCtxHolder {
+      exclusive: Boolean = false,
+      persistent: Boolean = false
+  ): CommandFactory = new CommandFactory(exclusive = exclusive, persistent = persistent)
+  class CommandFactory private[mill] (val exclusive: Boolean, val persistent: Boolean)
+      extends TaskBase.TraverseCtxHolder {
+
+    /** Binary compatibility forwarder */
+    private[mill] def this(exclusive: Boolean) = this(exclusive, persistent = false)
+
     def apply[T](t: Result[T])(implicit
         w: W[T],
         ctx: mill.define.Ctx,
@@ -662,7 +676,8 @@ object Target extends TaskBase {
           w.splice,
           cls.splice.value,
           taskIsPrivate.splice,
-          exclusive = c.prefix.splice.asInstanceOf[Task.CommandFactory].exclusive
+          exclusive = c.prefix.splice.asInstanceOf[Task.CommandFactory].exclusive,
+          persistent = c.prefix.splice.asInstanceOf[Task.CommandFactory].persistent
         )
       )
     }
@@ -871,15 +886,30 @@ class Command[+T](
     val writer: W[_],
     val cls: Class[_],
     val isPrivate: Option[Boolean],
-    val exclusive: Boolean
+    val exclusive: Boolean,
+    val persistent: Boolean
 ) extends NamedTask[T] {
+  override def flushDest: Boolean = !persistent
+
+  /** Binary compatibility forwarder. */
   def this(
       t: Task[T],
       ctx0: mill.define.Ctx,
       writer: W[_],
       cls: Class[_],
       isPrivate: Option[Boolean]
-  ) = this(t, ctx0, writer, cls, isPrivate, false)
+  ) = this(t, ctx0, writer, cls, isPrivate, exclusive = false, persistent = false)
+
+  /** Binary compatibility forwarder. */
+  def this(
+      t: Task[T],
+      ctx0: mill.define.Ctx,
+      writer: W[_],
+      cls: Class[_],
+      isPrivate: Option[Boolean],
+      exclusive: Boolean
+  ) = this(t, ctx0, writer, cls, isPrivate, exclusive = exclusive, persistent = false)
+
   override def asCommand: Some[Command[T]] = Some(this)
   // FIXME: deprecated return type: Change to Option
   override def writerOpt: Some[W[_]] = Some(writer)

pythonlib/src/mill/pythonlib/PythonModule.scala

@@ -175,8 +175,8 @@ trait PythonModule extends PipModule with TaskModule { outer =>
    *
    * @see [[mainScript]]
    */
-  def runBackground(args: mill.define.Args) = Task.Command {
-    val (procUuidPath, procLockfile, procUuid) = mill.scalalib.RunModule.backgroundSetup(Task.dest)
+  def runBackground(args: mill.define.Args) = Task.Command(persistent = true) {
+    val backgroundPaths = new mill.scalalib.RunModule.BackgroundPaths(destDir = Task.dest)
     val pwd0 = os.Path(java.nio.file.Paths.get(".").toAbsolutePath)
 
     os.checker.withValue(os.Checker.Nop) {
@@ -185,11 +185,7 @@ trait PythonModule extends PipModule with TaskModule { outer =>
         classPath = mill.scalalib.JvmWorkerModule.backgroundWrapperClasspath().map(_.path).toSeq,
         jvmArgs = Nil,
         env = runnerEnvTask(),
-        mainArgs = Seq(
-          procUuidPath.toString,
-          procLockfile.toString,
-          procUuid,
-          "500",
+        mainArgs = backgroundPaths.toArgs ++ Seq(
           "<subprocess>",
           pythonExe().path.toString,
           mainScript().path.toString