From 4e32b1b0cfeb0349dae80b5c8228f9836ebb7172 Mon Sep 17 00:00:00 2001
From: IvarHenckel <henckelivar@gmail.com>
Date: Sun, 2 Nov 2025 20:39:14 +0100
Subject: [PATCH] Keep track of pending Redis get calls in client side caching
 to avoid race with invalidations. #3481

---
 .../core/support/caching/CacheAccessor.java   | 16 ++++++-
 .../support/caching/ClientSideCaching.java    |  2 +
 .../support/caching/MapCacheAccessor.java     | 36 --------------
 .../caching/PendingAwareCacheAccessor.java    | 47 +++++++++++++++++++
 4 files changed, 63 insertions(+), 38 deletions(-)
 delete mode 100644 src/main/java/io/lettuce/core/support/caching/MapCacheAccessor.java
 create mode 100644 src/main/java/io/lettuce/core/support/caching/PendingAwareCacheAccessor.java

diff --git a/src/main/java/io/lettuce/core/support/caching/CacheAccessor.java b/src/main/java/io/lettuce/core/support/caching/CacheAccessor.java
index 0e9f789c51..7b5f955c9d 100644
--- a/src/main/java/io/lettuce/core/support/caching/CacheAccessor.java
+++ b/src/main/java/io/lettuce/core/support/caching/CacheAccessor.java
@@ -8,7 +8,7 @@
  *
  * @param <K> Key type.
  * @param <V> Value type.
- * @author Mark Paluch
+ * @author Mark Paluch, Ivar Henckel
  * @since 6.0
  */
 public interface CacheAccessor<K, V> {
@@ -22,7 +22,7 @@ public interface CacheAccessor<K, V> {
      * @return a {@link CacheAccessor} backed by a {@link Map} implementation.
      */
     static <K, V> CacheAccessor<K, V> forMap(Map<K, V> map) {
-        return new MapCacheAccessor<>(map);
+        return new PendingAwareCacheAccessor<>(map);
     }
 
     /**
@@ -59,4 +59,16 @@ static <K, V> CacheAccessor<K, V> forMap(Map<K, V> map) {
      */
     void evict(K key);
 
+    /**
+     * Mark a key as pending.
+     * <p>
+     * By marking keys as pending, i.e. Redis get operation in progress, it's possible to block cache updates that would
+     * otherwise overwrite simultaneous invalidates.
+     *
+     * @param key the key to be marked as pending.
+     */
+    default void setPending(K key) {
+        // Do nothing.
+    }
+
 }
diff --git a/src/main/java/io/lettuce/core/support/caching/ClientSideCaching.java b/src/main/java/io/lettuce/core/support/caching/ClientSideCaching.java
index e512c88f7f..34faa3262d 100644
--- a/src/main/java/io/lettuce/core/support/caching/ClientSideCaching.java
+++ b/src/main/java/io/lettuce/core/support/caching/ClientSideCaching.java
@@ -124,6 +124,7 @@ public V get(K key) {
         V value = cacheAccessor.get(key);
 
         if (value == null) {
+            cacheAccessor.setPending(key);
             value = redisCache.get(key);
 
             if (value != null) {
@@ -140,6 +141,7 @@ public V get(K key, Callable<V> valueLoader) {
         V value = cacheAccessor.get(key);
 
         if (value == null) {
+            cacheAccessor.setPending(key);
             value = redisCache.get(key);
 
             if (value == null) {
diff --git a/src/main/java/io/lettuce/core/support/caching/MapCacheAccessor.java b/src/main/java/io/lettuce/core/support/caching/MapCacheAccessor.java
deleted file mode 100644
index ac9f21f109..0000000000
--- a/src/main/java/io/lettuce/core/support/caching/MapCacheAccessor.java
+++ /dev/null
@@ -1,36 +0,0 @@
-package io.lettuce.core.support.caching;
-
-import java.util.Map;
-
-/**
- * {@link CacheAccessor} implementation for {@link Map}-based cache implementations.
- *
- * @param <K> Key type.
- * @param <V> Value type.
- * @author Mark Paluch
- * @since 6.0
- */
-class MapCacheAccessor<K, V> implements CacheAccessor<K, V> {
-
-    private final Map<K, V> map;
-
-    MapCacheAccessor(Map<K, V> map) {
-        this.map = map;
-    }
-
-    @Override
-    public V get(K key) {
-        return map.get(key);
-    }
-
-    @Override
-    public void put(K key, V value) {
-        map.put(key, value);
-    }
-
-    @Override
-    public void evict(K key) {
-        map.remove(key);
-    }
-
-}
diff --git a/src/main/java/io/lettuce/core/support/caching/PendingAwareCacheAccessor.java b/src/main/java/io/lettuce/core/support/caching/PendingAwareCacheAccessor.java
new file mode 100644
index 0000000000..0fd3796873
--- /dev/null
+++ b/src/main/java/io/lettuce/core/support/caching/PendingAwareCacheAccessor.java
@@ -0,0 +1,47 @@
+package io.lettuce.core.support.caching;
+
+import java.util.Map;
+
+/**
+ * {@link CacheAccessor} implementation for {@link Map}-based cache implementations.
+ *
+ * @param <K> Key type.
+ * @param <V> Value type.
+ * @author Mark Paluch, Ivar Henckel
+ * @since 6.0
+ */
+class PendingAwareCacheAccessor<K, V> implements CacheAccessor<K, V> {
+
+    private static final Object REDIS_IN_PROGRESS = new Object();
+
+    private final Map<K, V> map;
+
+    PendingAwareCacheAccessor(Map<K, V> map) {
+        this.map = map;
+    }
+
+    @Override
+    public V get(K key) {
+        V value = map.get(key);
+        if (value == REDIS_IN_PROGRESS) {
+            return null;
+        }
+        return value;
+    }
+
+    @Override
+    public void put(K key, V value) {
+        map.replace(key, (V) REDIS_IN_PROGRESS, value);
+    }
+
+    @Override
+    public void evict(K key) {
+        map.remove(key);
+    }
+
+    @Override
+    public void setPending(K key) {
+        map.put(key, (V) REDIS_IN_PROGRESS);
+    }
+
+}