Only trigger reload if sources have changed

Running a `~` command in a local build off the latest develop branch
will cause the build to reload even if the build sources were only
touched and not actually modified.

main/src/main/scala/sbt/MainLoop.scala

@@ -218,7 +218,7 @@ object MainLoop {
       }
       state.get(CheckBuildSourcesKey) match {
         case Some(cbs) =>
-          if (!cbs.needsReload(state, exec.commandLine)) process()
+          if (!cbs.needsReload(state, state.globalLogging.full, exec.commandLine)) process()
           else {
             if (exec.commandLine.startsWith(SetTerminal))
               exec +: Exec("reload", None, None) +: state.remove(CheckBuildSourcesKey)

main/src/main/scala/sbt/internal/Continuous.scala

@@ -526,7 +526,14 @@ private[sbt] object Continuous extends DeprecatedContinuous {
       }
 
       if (buildGlobs.exists(_.matches(path))) {
-        getWatchEvent(forceTrigger = false).map(e => e -> Watch.Reload).toSeq
+        getWatchEvent(forceTrigger = false).flatMap { e =>
+          state.get(CheckBuildSources.CheckBuildSourcesKey) match {
+            case Some(cbs) =>
+              if (cbs.needsReload(state, logger, "")) Some(e -> Watch.Reload) else None
+            case None =>
+              Some(e -> Watch.Reload)
+          }
+        }.toSeq
       } else {
         val acceptedConfigParameters = configs.flatMap { config =>
           config.inputs().flatMap {

main/src/main/scala/sbt/internal/nio/CheckBuildSources.scala

@@ -24,6 +24,7 @@ import sbt.nio.Keys._
 import sbt.nio.file.{ FileAttributes, FileTreeView, Glob, ** }
 import sbt.nio.file.syntax._
 import sbt.nio.Settings
+import sbt.util.Logger
 
 import scala.annotation.tailrec
 import scala.concurrent.duration.{ Deadline => SDeadline, _ }
@@ -102,11 +103,10 @@ private[sbt] class CheckBuildSources extends AutoCloseable {
     !resetState
   }
   @inline private def forceCheck = fileTreeRepository.isEmpty
-  private[sbt] def needsReload(state: State, cmd: String) = {
+  private[sbt] def needsReload(state: State, logger: Logger, cmd: String) = {
     (needCheck(state, cmd) && (forceCheck || needUpdate.compareAndSet(true, false))) && {
       val extracted = Project.extract(state)
       val onChanges = extracted.get(Global / onChangedBuildSource)
-      val logger = state.globalLogging.full
       val current = getStamps(force = false)
       val previous = previousStamps.getAndSet(current)
       Settings.changedFiles(previous, current) match {
@@ -160,8 +160,9 @@ private[sbt] object CheckBuildSources {
   private[sbt] def needReloadImpl: Def.Initialize[Task[StateTransform]] = Def.task {
     val st = state.value
     st.get(CheckBuildSourcesKey) match {
-      case Some(cbs) if (cbs.needsReload(st, "")) => StateTransform("reload" :: (_: State))
+      case Some(cbs) if (cbs.needsReload(st, st.globalLogging.full, "")) =>
-      case _                                      => StateTransform(identity)
+        StateTransform("reload" :: (_: State))
+      case _ => StateTransform(identity)
     }
   }
   private[sbt] def buildSourceFileInputs: Def.Initialize[Seq[Glob]] = Def.setting {

sbt/src/sbt-test/watch/on-start-watch/build.sbt

@@ -16,4 +16,11 @@ failingTask := {
   throw new IllegalStateException("failed")
 }
 
-onChangedBuildSource := ReloadOnSourceChanges
+watchOnIteration := { (count, project, commands) =>
+  val extra = baseDirectory.value / "extra.sbt"
+  IO.copyFile(baseDirectory.value / "changes" / "extra.sbt", extra, CopyOptions().withOverwrite(true))
+  Watch.defaultStartWatch(count, project, commands).foreach(_.linesIterator.foreach(l => println(s"[info] $l")))
+  Watch.Ignore
+}
+
+Global / onChangedBuildSource := ReloadOnSourceChanges

sbt/src/sbt-test/watch/on-start-watch/extra.sbt

@@ -1,7 +1,5 @@
 Compile / compile := {
   Count.increment()
   // Trigger a new build by updating the last modified time
-  val extra = baseDirectory.value / "extra.sbt"
-  IO.copyFile(baseDirectory.value / "changes" / "extra.sbt", extra, CopyOptions().withOverwrite(true))
   (Compile / compile).value
 }