From 32227570e5e179e33f15f8614ece51d1a12bc682 Mon Sep 17 00:00:00 2001
From: Simone Bordet <simone.bordet@gmail.com>
Date: Mon, 18 Sep 2023 09:37:03 +0200
Subject: [PATCH] Fixed deadlock in class initialization. (#10540)

* Thread T1 may initialize HttpTester.Message that extends MutableHttpFields, so grabs the lock for the initialization of class MutableHttpFields.
* Thread T2 may initialize HttpFields, so grabs the lock for HttpFields and initializes field EMPTY, which calls new MutableHttpFields.
* To initialize MutableHttpFields, T1 must initialize HttpFields, but sees that its lock is taken and waits.
* To initialize HttpFields, T2 must create an instance and therefore initialize MutableHttpFields, but sees that its lock is taken and waits.
* Deadlock.

The solution is to use another class, EmptyHttpFields, to initialize HttpFields.EMPTY, so that there is no deadlock.

Signed-off-by: Simone Bordet <simone.bordet@gmail.com>
---
 .../eclipse/jetty/http/EmptyHttpFields.java   | 26 +++++++++++++++++++
 .../org/eclipse/jetty/http/HttpFields.java    |  2 +-
 2 files changed, 27 insertions(+), 1 deletion(-)
 create mode 100644 jetty-core/jetty-http/src/main/java/org/eclipse/jetty/http/EmptyHttpFields.java

diff --git a/jetty-core/jetty-http/src/main/java/org/eclipse/jetty/http/EmptyHttpFields.java b/jetty-core/jetty-http/src/main/java/org/eclipse/jetty/http/EmptyHttpFields.java
new file mode 100644
index 00000000000..a03f028671c
--- /dev/null
+++ b/jetty-core/jetty-http/src/main/java/org/eclipse/jetty/http/EmptyHttpFields.java
@@ -0,0 +1,26 @@
+//
+// ========================================================================
+// Copyright (c) 1995 Mort Bay Consulting Pty Ltd and others.
+//
+// This program and the accompanying materials are made available under the
+// terms of the Eclipse Public License v. 2.0 which is available at
+// https://www.eclipse.org/legal/epl-2.0, or the Apache License, Version 2.0
+// which is available at https://www.apache.org/licenses/LICENSE-2.0.
+//
+// SPDX-License-Identifier: EPL-2.0 OR Apache-2.0
+// ========================================================================
+//
+
+package org.eclipse.jetty.http;
+
+import java.util.Collections;
+import java.util.ListIterator;
+
+class EmptyHttpFields implements HttpFields
+{
+    @Override
+    public ListIterator<HttpField> listIterator(int index)
+    {
+        return Collections.emptyListIterator();
+    }
+}
diff --git a/jetty-core/jetty-http/src/main/java/org/eclipse/jetty/http/HttpFields.java b/jetty-core/jetty-http/src/main/java/org/eclipse/jetty/http/HttpFields.java
index 44c6aca679b..baf663b4b61 100644
--- a/jetty-core/jetty-http/src/main/java/org/eclipse/jetty/http/HttpFields.java
+++ b/jetty-core/jetty-http/src/main/java/org/eclipse/jetty/http/HttpFields.java
@@ -64,7 +64,7 @@ public interface HttpFields extends Iterable<HttpField>, Supplier<HttpFields>
     /**
      * <p>A constant for an immutable and empty {@link HttpFields}.</p>
      */
-    HttpFields EMPTY = build().asImmutable();
+    HttpFields EMPTY = new EmptyHttpFields();
 
     /**
      * <p>Returns an empty {@link Mutable} instance.</p>