Fix MailboxSelectorSpec race (#30429)

The comment already said there could be 2 dead letters, but the log
assertion failed when there was a second 'excess' one. Add a flag to
skip checking for excess log lines.

akka-actor-testkit-typed/src/main/mima-filters/2.6.15.backwards.excludes/excess-checking.excludes

@@ -0,0 +1,6 @@
+# Extra methods on interfaces not for user extension:
+ProblemFilters.exclude[ReversedMissingMethodProblem]("akka.actor.testkit.typed.javadsl.LoggingTestKit.withCheckExcess")
+ProblemFilters.exclude[ReversedMissingMethodProblem]("akka.actor.testkit.typed.scaladsl.LoggingTestKit.withCheckExcess")
+
+# Impl
+ProblemFilters.exclude[Problem]("akka.actor.testkit.typed.internal.LoggingTestKitImpl.*")

akka-actor-testkit-typed/src/main/scala/akka/actor/testkit/typed/internal/LoggingTestKitImpl.scala

@@ -24,7 +24,8 @@ import akka.testkit.TestKit
  * INTERNAL API
  */
 @InternalApi private[akka] object LoggingTestKitImpl {
-  def empty: LoggingTestKitImpl = new LoggingTestKitImpl(1, None, None, None, None, None, None, Map.empty, None)
+  def empty: LoggingTestKitImpl =
+    new LoggingTestKitImpl(1, None, None, None, None, None, None, Map.empty, checkExcess = true, None)
 }
 
 /**
@@ -39,6 +40,7 @@ import akka.testkit.TestKit
     messageRegex: Option[Regex],
     cause: Option[Class[_ <: Throwable]],
     mdc: Map[String, String],
+    checkExcess: Boolean,
     custom: Option[Function[LoggingEvent, Boolean]])
     extends javadsl.LoggingTestKit
     with scaladsl.LoggingTestKit {
@@ -92,14 +94,14 @@ import akka.testkit.TestKit
     try {
       val result = code
 
-      // wait some more when occurrences=0 to find asynchronous exceess messages
+      // wait some more when occurrences=0 to find asynchronous excess messages
       if (occurrences == 0)
         awaitNoExcess(settings.ExpectNoMessageDefaultTimeout)
 
       if (!awaitDone(settings.FilterLeeway))
         if (todo > 0)
           throw new AssertionError(s"Timeout (${settings.FilterLeeway}) waiting for $todo messages on $this.")
-        else
+        else if (checkExcess)
           throw new AssertionError(s"Received ${-todo} excess messages on $this.")
       result
     } finally {
@@ -152,6 +154,9 @@ import akka.testkit.TestKit
     withMdc(newMdc.asScala.toMap)
   }
 
+  override def withCheckExcess(check: Boolean): LoggingTestKitImpl =
+    copy(checkExcess = check)
+
   override def withCustom(newCustom: Function[LoggingEvent, Boolean]): LoggingTestKitImpl =
     copy(custom = Option(newCustom))
 

akka-actor-testkit-typed/src/main/scala/akka/actor/testkit/typed/javadsl/LoggingTestKit.scala

@@ -76,6 +76,11 @@ import akka.annotation.DoNotInherit
    */
   def withMdc(newMdc: java.util.Map[String, String]): LoggingTestKit
 
+  /**
+   * After matching the expected number of hits, check for excess messages
+   */
+  def withCheckExcess(check: Boolean): LoggingTestKit
+
   /**
    * Matching events for which the supplied function returns `true`.
    */

akka-actor-testkit-typed/src/main/scala/akka/actor/testkit/typed/scaladsl/LoggingTestKit.scala

@@ -76,6 +76,11 @@ import akka.annotation.DoNotInherit
    */
   def withMdc(newMdc: Map[String, String]): LoggingTestKit
 
+  /**
+   * After matching the expected number of hits, check for excess messages
+   */
+  def withCheckExcess(checkExcess: Boolean): LoggingTestKit
+
   /**
    * Matching events for which the supplied function returns`true`.
    */

akka-actor-typed-tests/src/test/scala/akka/actor/typed/MailboxSelectorSpec.scala

@@ -74,7 +74,7 @@ class MailboxSelectorSpec extends ScalaTestWithActorTestKit("""
       }, MailboxSelector.bounded(2))
       actor ! "one" // actor will block here
       actor ! "two"
-      LoggingTestKit.deadLetters().expect {
+      LoggingTestKit.deadLetters().withCheckExcess(false).expect {
         // one or both of these doesn't fit in mailbox
         // depending on race with how fast actor consumes
         actor ! "three"