Fix detection of multithreading usage - it was broken, becouse shutdo…

…wn hooks and weak references registry are using threads (#3856)

scripted-tests/run/detect-multithreading/build.sbt

@@ -0,0 +1,15 @@
+enablePlugins(ScalaNativePlugin)
+
+nativeConfig ~= {
+  _.withMultithreading(None) // force detection
+}
+
+scalaVersion := {
+  val scalaVersion = System.getProperty("scala.version")
+  if (scalaVersion == null)
+    throw new RuntimeException(
+      """|The system property 'scala.version' is not defined.
+         |Specify this property using the scriptedLaunchOpts -D.""".stripMargin
+    )
+  else scalaVersion
+}

scripted-tests/run/detect-multithreading/project/scala-native.sbt

@@ -0,0 +1,9 @@
+{
+  val pluginVersion = System.getProperty("plugin.version")
+  if (pluginVersion == null)
+    throw new RuntimeException(
+      """|The system property 'plugin.version' is not defined.
+         |Specify this property using the scriptedLaunchOpts -D.""".stripMargin
+    )
+  else addSbtPlugin("org.scala-native" % "sbt-scala-native" % pluginVersion)
+}

scripted-tests/run/detect-multithreading/src/main/scala/MultiThreaded.scala

@@ -0,0 +1,10 @@
+import scala.scalanative.unsafe._
+import scala.scalanative.libc.stdio._
+import scala.scalanative.meta.LinktimeInfo.isMultithreadingEnabled
+
+object MultiThreaded {
+  def main(args: Array[String]): Unit = {
+    assert(isMultithreadingEnabled == true)
+    new Thread(() => println("hello world")).start()
+  }
+}

scripted-tests/run/detect-multithreading/src/main/scala/SingleThreaded.scala

@@ -0,0 +1,10 @@
+import scala.scalanative.unsafe._
+import scala.scalanative.libc.stdio._
+import scala.scalanative.meta.LinktimeInfo.isMultithreadingEnabled
+
+object SingleThreaded {
+  def main(args: Array[String]): Unit = {
+    assert(isMultithreadingEnabled == false)
+    println("Hello world")
+  }
+}

scripted-tests/run/detect-multithreading/test

@@ -0,0 +1,5 @@
+> set Compile/mainClass := Some("SingleThreaded")
+> run
+
+> set Compile/mainClass := Some("MultiThreaded")
+> run

tools/src/main/scala/scala/scalanative/build/Build.scala

@@ -186,10 +186,14 @@ object Build {
       if (config.compilerConfig.multithreading.isEmpty) {
         // format: off
         val jlThread = nir.Global.Top("java.lang.Thread")
-        val jlThreadStart = jlThread.member(nir.Sig.Method("start", Seq(nir.Type.Unit)))
-        val jlPlatformContext = nir.Global.Top("java.lang.PlatformThreadContext")
-        val jlPlatformContextStart = jlPlatformContext.member(nir.Sig.Method("start", Seq(nir.Type.Ref(jlThread), nir.Type.Unit)))
-        val usesSystemThreads = analysis.infos.contains(jlThreadStart) || analysis.infos.contains(jlPlatformContextStart)
+        val jlMainThread = nir.Global.Top("java.lang.Thread$MainThread$")
+        val jlVirtualThread = nir.Global.Top("java.lang.VirtualThread")
+        val usesSystemThreads = analysis.infos.get(jlThread).collect{
+          case cls: linker.Class =>
+            cls.subclasses.size > 2 ||
+            cls.subclasses.map(_.name).diff(Set(jlMainThread, jlVirtualThread)).nonEmpty || 
+            cls.allocations > 4 // minimal number of allocations
+        }.getOrElse(false)
         // format: on
         if (!usesSystemThreads) {
           config.logger.info(

tools/src/main/scala/scala/scalanative/linker/Infos.scala

@@ -111,7 +111,8 @@ final class Class(
   val implementors = mutable.SortedSet[Class](this)
   val subclasses = mutable.Set.empty[Class]
   val defaultResponds = mutable.Map.empty[nir.Sig, nir.Global.Member]
-  var allocated = false
+  var allocations = 0
+  def allocated = allocations > 0
 
   lazy val fields: Seq[Field] = {
     val out = mutable.UnrolledBuffer.empty[Field]

tools/src/main/scala/scala/scalanative/linker/Reach.scala

@@ -418,8 +418,9 @@ private[linker] class Reach(
   def reachAllocation(
       info: Class
   )(implicit srcPosition: nir.SourcePosition): Unit =
-    if (!info.allocated) {
-      info.allocated = true
+    if (info.allocated) info.allocations += 1
+    else {
+      info.allocations += 1
 
       // Handle all class and trait virtual calls
       // on this class. This includes virtual calls