Revert "Fix #4561 (revert #4430) (#4562)"

This reverts commit b89886a.

release-notes/VERSION-2.x

@@ -8,11 +8,6 @@ Project: jackson-databind
 
 -
 
-2.17.2 (not yet released)
-
-#4561: Issues using jackson-databind 2.17.1 with Reactor
- (reported by @wdallastella)
-
 2.17.1 (04-May-2024)
 
 #4428: `ByteBuddy` scope went beyond `test` in version 2.17.0

src/main/java/com/fasterxml/jackson/databind/deser/DeserializerCache.java

@@ -1,6 +1,7 @@
 package com.fasterxml.jackson.databind.deser;
 
 import java.util.HashMap;
+import java.util.concurrent.locks.ReentrantLock;
 
 import com.fasterxml.jackson.annotation.JsonFormat;
 import com.fasterxml.jackson.databind.*;
@@ -52,6 +53,12 @@ public final class DeserializerCache
     protected final HashMap<JavaType, JsonDeserializer<Object>> _incompleteDeserializers
         = new HashMap<JavaType, JsonDeserializer<Object>>(8);
 
+
+    /**
+     * We hold an explicit lock while creating deserializers to avoid creating duplicates.
+     */
+    private final ReentrantLock _incompleteDeserializersLock = new ReentrantLock();
+
     /*
     /**********************************************************
     /* Life-cycle
@@ -242,10 +249,12 @@ protected JsonDeserializer<Object> _createAndCacheValueDeserializer(Deserializat
             DeserializerFactory factory, JavaType type)
         throws JsonMappingException
     {
-        // Only one thread to construct deserializers at any given point in time;
-        // limitations necessary to ensure that only completely initialized ones
-        // are visible and used.
-        synchronized (_incompleteDeserializers) {
+        /* Only one thread to construct deserializers at any given point in time;
+         * limitations necessary to ensure that only completely initialized ones
+         * are visible and used.
+         */
+        _incompleteDeserializersLock.lock();
+        try {
             // Ok, then: could it be that due to a race condition, deserializer can now be found?
             JsonDeserializer<Object> deser = _findCachedDeserializer(type);
             if (deser != null) {
@@ -268,6 +277,8 @@ protected JsonDeserializer<Object> _createAndCacheValueDeserializer(Deserializat
                     _incompleteDeserializers.clear();
                 }
             }
+        } finally {
+            _incompleteDeserializersLock.unlock();
         }
     }