Flip testForkedParallel to true

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

@@ -201,7 +201,7 @@ object Defaults extends BuildCommon {
       discoveredJavaHomes := CrossJava.discoverJavaHomes,
       javaHomes :== ListMap.empty,
       fullJavaHomes := CrossJava.expandJavaHomes(discoveredJavaHomes.value ++ javaHomes.value),
-      testForkedParallel :== false,
+      testForkedParallel :== true,
       javaOptions :== Nil,
       sbtPlugin :== false,
       isMetaBuild :== false,

sbt-app/src/sbt-test/tests/fork-parallel/build.sbt

@@ -1,27 +1,23 @@
 import Tests._
 import Defaults._
 
-ThisBuild / scalaVersion := "2.12.20"
+scalaVersion := "2.12.20"
 val check = taskKey[Unit]("Check that tests are executed in parallel")
 
-lazy val root = (project in file("."))
-  .settings(
-    libraryDependencies += "com.novocode" % "junit-interface" % "0.11" % Test,
-    Test / fork := true,
-    check := {
-      val nbProc = java.lang.Runtime.getRuntime().availableProcessors()
-      val log = streams.value.log
-      if( nbProc < 4 ) {
-        log.warn("With fewer than 4 processors this test is meaningless")
-        // mimic behavior expected by scripted
-        if (!testForkedParallel.value) sys.error("Exiting with error (note: test not performed)")
-      } else {
-        // we've got at least 4 processors, we'll check the upper end but also 3 and 4 as the upper might not
-        // be reached if the system is under heavy load.
-        if( ! (file("max-concurrent-tests_3").exists() || file("max-concurrent-tests_4").exists() ||
-               file("max-concurrent-tests_" + (nbProc -1)).exists() || file("max-concurrent-tests_" + nbProc).exists())) {
-          sys.error("Forked tests were not executed in parallel!")
-        }
-      }
-    }
-  )
+libraryDependencies += "com.novocode" % "junit-interface" % "0.11" % Test
+Test / fork := true
+
+check := {
+  val nbProc = java.lang.Runtime.getRuntime().availableProcessors()
+  val log = streams.value.log
+  if nbProc < 4 then
+    log.warn("With fewer than 4 processors this test is meaningless")
+    // mimic behavior expected by scripted
+    if !testForkedParallel.value then sys.error("Exiting with error (note: test not performed)")
+  else
+    // we've got at least 4 processors, we'll check the upper end but also 3 and 4 as the upper might not
+    // be reached if the system is under heavy load.
+    if ! (file("max-concurrent-tests_3").exists() || file("max-concurrent-tests_4").exists() ||
+           file("max-concurrent-tests_" + (nbProc -1)).exists() || file("max-concurrent-tests_" + nbProc).exists()) then
+      sys.error("Forked tests were not executed in parallel!")
+}

sbt-app/src/sbt-test/tests/fork-parallel/src/test/scala/tests.scala

@@ -9,7 +9,7 @@ object ParallelTest {
 	val maxConcurrentTests = new AtomicInteger(0)
 
 	private def updateMaxConcurrentTests(currentMax: Int, newMax: Int) : Boolean = {
-		if( maxConcurrentTests.compareAndSet(currentMax, newMax) ) {
+		if (maxConcurrentTests.compareAndSet(currentMax, newMax) ) {
 			val f = new File("max-concurrent-tests_" + newMax)
 			f.createNewFile
 			true
@@ -22,7 +22,7 @@ object ParallelTest {
 	def execute(f : => Unit): Unit = {
 		val nb = nbConcurrentTests.incrementAndGet()
 		val max = maxConcurrentTests.get()
-		if( nb <= max || updateMaxConcurrentTests(max, nb)) {
+		if (nb <= max || updateMaxConcurrentTests(max, nb)) {
 			f
 			nbConcurrentTests.getAndDecrement
 		} else {

sbt-app/src/sbt-test/tests/fork-parallel/test

@@ -1,8 +1,13 @@
 # Note: this test is meaningless on less than four cores
 
-> testFull
--> check
-> clean
-> set testForkedParallel := true
 > testFull
 > check
+> clean
+$ delete max-concurrent-tests_1
+$ delete max-concurrent-tests_2
+$ delete max-concurrent-tests_3
+$ delete max-concurrent-tests_4
+
+> set testForkedParallel := false
+> testFull
+-> check