Reproduce and fix ByteString initialization issue (#29244)

* Reproduce ByteString initialization issue

* scalafmt

* make test reliably fail

* core: fix initialization of `ByteString.empty`

Refs #29263

Co-authored-by: Johannes Rudolph <johannes.rudolph@gmail.com>

akka-actor-tests/src/test/scala/akka/util/ByteStringInitializationSpec.scala

@@ -0,0 +1,52 @@
+/*
+ * Copyright (C) 2020 Lightbend Inc. <https://www.lightbend.com>
+ */
+
+package akka.util
+
+import java.io.InputStream
+
+import org.scalatest.matchers.should.Matchers
+import org.scalatest.wordspec.AnyWordSpec
+
+import scala.collection.mutable
+import scala.collection.mutable.ArrayBuilder
+
+class ByteStringInitializationSpec extends AnyWordSpec with Matchers {
+  "ByteString intialization" should {
+    "not get confused by initializing CompactByteString before ByteString" in {
+      // a classloader that creates a new universe of classes for everything beneath akka
+      // that prevents that this test interacts with any tests
+      val cleanCl = new ClassLoader(null) {
+        val outerCl = ByteStringInitializationSpec.this.getClass.getClassLoader
+        val buffer = new Array[Byte](1000000)
+        override def loadClass(name: String): Class[_] =
+          if (!name.startsWith("akka")) outerCl.loadClass(name)
+          else {
+            val classFile = name.replace(".", "/") + ".class"
+            val is = outerCl.getResourceAsStream(classFile)
+            val res = slurp(is, new mutable.ArrayBuilder.ofByte)
+            defineClass(name, res, 0, res.length)
+          }
+
+        def slurp(is: InputStream, res: ArrayBuilder[Byte]): Array[Byte] = {
+          val read = is.read(buffer)
+          if (read == 0) throw new IllegalStateException
+          else if (read > 0) slurp(is, res ++= buffer.take(read))
+          else res.result()
+        }
+      }
+
+      import scala.language.reflectiveCalls
+      type WithRun = { def run(): Unit }
+      cleanCl.loadClass("akka.util.ByteStringInitTest").newInstance().asInstanceOf[WithRun].run()
+    }
+  }
+}
+
+class ByteStringInitTest {
+  def run(): Unit = {
+    require(CompactByteString.empty ne null)
+    require(ByteString.empty ne null)
+  }
+}

akka-actor/src/main/scala-2.13/akka/util/ByteString.scala

@@ -747,7 +747,7 @@ sealed abstract class ByteString
 
   override protected def fromSpecific(coll: IterableOnce[Byte]): ByteString = ByteString(coll)
   override protected def newSpecificBuilder: mutable.Builder[Byte, ByteString] = ByteString.newBuilder
-  override val empty: ByteString = ByteString.empty
+  override def empty: ByteString = ByteString.empty
 
   def apply(idx: Int): Byte
   private[akka] def byteStringCompanion: ByteString.Companion