Join worker threads on pool shutdown

build.sbt

@@ -879,7 +879,7 @@ lazy val tests: CrossProject = crossProject(JSPlatform, JVMPlatform, NativePlatf
     scalacOptions ~= { _.filterNot(_.startsWith("-P:scalajs:mapSourceURI")) }
   )
   .jvmSettings(
-    Test / fork := true,
+    fork := true,
     Test / javaOptions += s"-Dsbt.classpath=${(Test / fullClasspath).value.map(_.data.getAbsolutePath).mkString(File.pathSeparator)}"
   )
 

core/jvm/src/main/scala/cats/effect/unsafe/WorkStealingThreadPool.scala

@@ -689,6 +689,7 @@ private[effect] final class WorkStealingThreadPool[P](
   def shutdown(): Unit = {
     // Clear the interrupt flag.
     val interruptCalling = Thread.interrupted()
+    val currentThread = Thread.currentThread()
 
     // Execute the shutdown logic only once.
     if (done.compareAndSet(false, true)) {
@@ -701,27 +702,30 @@ private[effect] final class WorkStealingThreadPool[P](
       // the face of unhandled exceptions or as part of the whole JVM exiting.
       var i = 0
       while (i < threadCount) {
-        workerThreads(i).interrupt()
+        val workerThread = workerThreads(i)
+        if (workerThread ne currentThread) {
+          workerThread.interrupt()
+          workerThread.join()
+        }
         system.closePoller(pollers(i))
         i += 1
       }
 
       system.close()
 
-      // Clear the interrupt flag.
-      Thread.interrupted()
-
       var t: WorkerThread[P] = null
       while ({
         t = cachedThreads.pollFirst()
         t ne null
       }) {
         t.interrupt()
+        // don't join, blocking threads may be uninterruptible blocked.
+        // anyway, they do not have pollers
       }
 
       // Drain the external queue.
       externalQueue.clear()
-      if (interruptCalling) Thread.currentThread().interrupt()
+      if (interruptCalling) currentThread.interrupt()
     }
   }
 

tests/jvm/src/main/scala/catseffect/examplesplatform.scala

@@ -37,7 +37,7 @@ package examples {
       super.runtimeConfig.copy(shutdownHookTimeout = Duration.Zero)
 
     val run: IO[Unit] =
-      IO(System.exit(0)).uncancelable
+      IO.blocking(System.exit(0)).uncancelable
   }
 
   object FatalErrorUnsafeRun extends IOApp {