From ae9a6345c229356d1ea8b536bab4535ca8e8ead7 Mon Sep 17 00:00:00 2001
From: Sanne Grinovero <sanne@hibernate.org>
Date: Wed, 21 Jul 2021 10:30:46 +0100
Subject: [PATCH] Optimise the strategy we use to reset the classloader in the
 Common ForkJoin ThreadPool

---
 .../bootstrap/ForkJoinClassLoading.java       | 55 +++++++++++--------
 1 file changed, 33 insertions(+), 22 deletions(-)

diff --git a/core/deployment/src/main/java/io/quarkus/runner/bootstrap/ForkJoinClassLoading.java b/core/deployment/src/main/java/io/quarkus/runner/bootstrap/ForkJoinClassLoading.java
index 784242bcdd81a..f9fedc368ebcb 100644
--- a/core/deployment/src/main/java/io/quarkus/runner/bootstrap/ForkJoinClassLoading.java
+++ b/core/deployment/src/main/java/io/quarkus/runner/bootstrap/ForkJoinClassLoading.java
@@ -20,31 +20,42 @@ public class ForkJoinClassLoading {
      * Really we should just not use the common pool at all.
      */
     public static void setForkJoinClassLoader(ClassLoader classLoader) {
-        CountDownLatch allDone = new CountDownLatch(ForkJoinPool.getCommonPoolParallelism());
-        CountDownLatch taskRelease = new CountDownLatch(1);
-        for (int i = 0; i < ForkJoinPool.getCommonPoolParallelism(); ++i) {
-            ForkJoinPool.commonPool().execute(new Runnable() {
-                @Override
-                public void run() {
-                    Thread.currentThread().setContextClassLoader(classLoader);
-                    allDone.countDown();
-                    try {
-                        taskRelease.await();
-                    } catch (InterruptedException e) {
-                        log.error("Failed to set fork join ClassLoader", e);
+        final ForkJoinPool commonPool = ForkJoinPool.commonPool();
+
+        final int activeThreadCount = commonPool.getActiveThreadCount();
+
+        //The following approach is rather heavy, but there's good chances that there are actually no active threads,
+        //so skip it in this case.
+        //Additionally, if we can skip this this will prevent us from actually forcing to start all threads in the pool.
+        if (activeThreadCount > 0) {
+            //TODO we could theoretically improve this by using the system property java.util.concurrent.ForkJoinPool.common.threadFactory
+            //and force the common pool to register created threads so that we can reach and reset them.
+            CountDownLatch allDone = new CountDownLatch(ForkJoinPool.getCommonPoolParallelism());
+            CountDownLatch taskRelease = new CountDownLatch(1);
+            for (int i = 0; i < ForkJoinPool.getCommonPoolParallelism(); ++i) {
+                commonPool.execute(new Runnable() {
+                    @Override
+                    public void run() {
+                        Thread.currentThread().setContextClassLoader(classLoader);
+                        allDone.countDown();
+                        try {
+                            taskRelease.await();
+                        } catch (InterruptedException e) {
+                            log.error("Failed to set fork join ClassLoader", e);
+                        }
                     }
+                });
+            }
+            try {
+                if (!allDone.await(1, TimeUnit.SECONDS)) {
+                    log.error(
+                            "Timed out trying to set fork join ClassLoader, this should never happen unless something has tied up a fork join thread before the app launched");
                 }
-            });
-        }
-        try {
-            if (!allDone.await(1, TimeUnit.SECONDS)) {
-                log.error(
-                        "Timed out trying to set fork join ClassLoader, this should never happen unless something has tied up a fork join thread before the app launched");
+            } catch (InterruptedException e) {
+                log.error("Failed to set fork join ClassLoader", e);
+            } finally {
+                taskRelease.countDown();
             }
-        } catch (InterruptedException e) {
-            log.error("Failed to set fork join ClassLoader", e);
-        } finally {
-            taskRelease.countDown();
         }
     }
 }