Remove watch loops

When source generators write into the unmanaged source directory, bad
things can happen. Continuous builds will loop indefinitely and
compiling will fail because the generated sources get added to the
source list twice, causing the incremental compiler to complain about
compiling classes it has already seen. My two-pronged solution is to
de-duplicate the sources task and to filter out managed source files in
watch sources. The drawback to the latter is that it causes the source
generation task to be executed twice per compile.

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

@@ -329,7 +329,18 @@ object Defaults extends BuildCommon {
       val baseDir = baseDirectory.value
       val bases = unmanagedSourceDirectories.value
       val include = (includeFilter in unmanagedSources).value
-      val exclude = (excludeFilter in unmanagedSources).value
+      val exclude = (excludeFilter in unmanagedSources).value match {
+        case e =>
+          (managedSources in ThisScope).value match {
+            case l if l.nonEmpty =>
+              e || new FileFilter {
+                private val files = l.toSet
+                override def accept(pathname: File): Boolean = files.contains(pathname)
+                override def toString = s"ManagedSourcesFilter($files)"
+              }
+            case _ => e
+          }
+      }
       val baseSources =
         if (sourcesInBase.value) Seq(new Source(baseDir, include, exclude, recursive = false))
         else Nil
@@ -341,7 +352,7 @@ object Defaults extends BuildCommon {
     sourceDirectories := Classpaths
       .concatSettings(unmanagedSourceDirectories, managedSourceDirectories)
       .value,
-    sources := Classpaths.concat(unmanagedSources, managedSources).value
+    sources := Classpaths.concatDistinct(unmanagedSources, managedSources).value
   )
   lazy val resourceConfigPaths = Seq(
     resourceDirectory := sourceDirectory.value / "resources",

sbt/src/sbt-test/tests/watch-loop/build.sbt

@@ -0,0 +1,17 @@
+import java.nio.file.Files
+
+lazy val watchLoopTest = taskKey[Unit]("Check that managed sources are filtered")
+
+sourceGenerators in Compile += Def.task {
+  val path = baseDirectory.value.toPath.resolve("src/main/scala/Foo.scala")
+  Files.write(path, "object Foo".getBytes).toFile :: Nil
+}
+
+watchLoopTest := {
+  val watched = watchSources.value
+  val managedSource = (managedSources in Compile).value.head
+  assert(!SourceWrapper.accept(watched, managedSource))
+  assert((sources in Compile).value.foldLeft((true, Set.empty[File])) {
+    case ((res, set), f) => (res && !set.contains(f), set + f)
+  }._1)
+}

sbt/src/sbt-test/tests/watch-loop/project/SourceWrapper.scala

@@ -0,0 +1,6 @@
+package sbt
+
+object SourceWrapper {
+  def accept(sources: Seq[sbt.internal.io.Source], file: File): Boolean =
+    sources.exists(_.accept(file.toPath))
+}

sbt/src/sbt-test/tests/watch-loop/src/main/scala/Bar.scala

@@ -0,0 +1 @@
+object Bar

sbt/src/sbt-test/tests/watch-loop/src/main/scala/Foo.scala

@@ -0,0 +1 @@
+object Foo

sbt/src/sbt-test/tests/watch-loop/test

@@ -0,0 +1 @@
+> watchLoopTest