Rework synchronization in ProtobufMapper (#484)

Author: pjfanning

protobuf/src/main/java/com/fasterxml/jackson/dataformat/protobuf/ProtobufMapper.java

@@ -4,6 +4,7 @@
 import java.io.IOException;
 import java.io.InputStream;
 import java.net.URL;
+import java.util.concurrent.locks.ReentrantLock;
 
 import com.fasterxml.jackson.core.Version;
 import com.fasterxml.jackson.core.type.TypeReference;
@@ -42,7 +43,10 @@ public Builder(ProtobufMapper m) {
      *
      * @since 2.9
      */
-    protected DescriptorLoader _descriptorLoader;
+    protected volatile DescriptorLoader _descriptorLoader;
+
+    // @since 2.18
+    private final ReentrantLock _descriptorLock = new ReentrantLock();
 
     /*
     /**********************************************************
@@ -192,11 +196,19 @@ public FileDescriptorSet loadDescriptorSet(InputStream src) throws IOException {
      *
      * @since 2.9
      */
-    public synchronized DescriptorLoader descriptorLoader() throws IOException
+    public DescriptorLoader descriptorLoader() throws IOException
     {
         DescriptorLoader l = _descriptorLoader;
         if (l == null) {
-            _descriptorLoader = l = DescriptorLoader.construct(this);
+            _descriptorLock.lock();
+            try {
+                l = _descriptorLoader;
+                if (l == null) {
+                    _descriptorLoader = l = DescriptorLoader.construct(this);
+                }
+            } finally {
+                _descriptorLock.unlock();
+            }
         }
         return l;
     }

release-notes/CREDITS-2.x

@@ -319,3 +319,7 @@ Thomas de Lange (@thomasdelange5)
  * Contributed fix for #428: (ion) `IonParser.getIntValue()` fails or does not handle
    value overflow checks
   (2.17.0)
+
+PJ Fanning (pjfanning@github)
+ * Contributed #484: Rework synchronization in `ProtobufMapper`
+  (2.18.0)

release-notes/VERSION-2.x

@@ -16,7 +16,8 @@ Active maintainers:
 
 2.18.0 (not yet released)
 
-No changes since 2.17.
+#484: (protobuf) Rework synchronization in `ProtobufMapper`
+ (contributed by @pjfanning)
 
 2.17.0 (12-Mar-2024)