Fix flaky test TieredSpilloverCacheTests.testComputeIfAbsentConcurrently (#14550)

* Fix flaky test TieredSpilloverCacheTests.testComputeIfAbsentConcurrently

Signed-off-by: Sagar Upadhyaya <sagar.upadhyaya.121@gmail.com>

* Addressing comment

Signed-off-by: Sagar Upadhyaya <upasagar@amazon.com>

---------

Signed-off-by: Sagar Upadhyaya <sagar.upadhyaya.121@gmail.com>
Signed-off-by: Sagar Upadhyaya <upasagar@amazon.com>
Co-authored-by: Sagar Upadhyaya <upasagar@amazon.com>

Author: sgup432

modules/cache-common/src/main/java/org/opensearch/cache/common/tier/TieredSpilloverCache.java

@@ -195,15 +195,24 @@ public V computeIfAbsent(ICacheKey<K> key, LoadAwareCacheLoader<ICacheKey<K>, V>
         // and it only has to be loaded one time, we should report one miss and the rest hits. But, if we do stats in
         // getValueFromTieredCache(),
         // we will see all misses. Instead, handle stats in computeIfAbsent().
-        Tuple<V, String> cacheValueTuple = getValueFromTieredCache(false).apply(key);
+        Tuple<V, String> cacheValueTuple;
+        CompletableFuture<Tuple<ICacheKey<K>, V>> future = null;
+        try (ReleasableLock ignore = readLock.acquire()) {
+            cacheValueTuple = getValueFromTieredCache(false).apply(key);
+            if (cacheValueTuple == null) {
+                // Only one of the threads will succeed putting a future into map for the same key.
+                // Rest will fetch existing future and wait on that to complete.
+                future = completableFutureMap.putIfAbsent(key, new CompletableFuture<>());
+            }
+        }
         List<String> heapDimensionValues = statsHolder.getDimensionsWithTierValue(key.dimensions, TIER_DIMENSION_VALUE_ON_HEAP);
         List<String> diskDimensionValues = statsHolder.getDimensionsWithTierValue(key.dimensions, TIER_DIMENSION_VALUE_DISK);
 
         if (cacheValueTuple == null) {
             // Add the value to the onHeap cache. We are calling computeIfAbsent which does another get inside.
             // This is needed as there can be many requests for the same key at the same time and we only want to load
             // the value once.
-            V value = compute(key, loader);
+            V value = compute(key, loader, future);
             // Handle stats
             if (loader.isLoaded()) {
                 // The value was just computed and added to the cache by this thread. Register a miss for the heap cache, and the disk cache
@@ -232,10 +241,8 @@ public V computeIfAbsent(ICacheKey<K> key, LoadAwareCacheLoader<ICacheKey<K>, V>
         return cacheValueTuple.v1();
     }
 
-    private V compute(ICacheKey<K> key, LoadAwareCacheLoader<ICacheKey<K>, V> loader) throws Exception {
-        // Only one of the threads will succeed putting a future into map for the same key.
-        // Rest will fetch existing future and wait on that to complete.
-        CompletableFuture<Tuple<ICacheKey<K>, V>> future = completableFutureMap.putIfAbsent(key, new CompletableFuture<>());
+    private V compute(ICacheKey<K> key, LoadAwareCacheLoader<ICacheKey<K>, V> loader, CompletableFuture<Tuple<ICacheKey<K>, V>> future)
+        throws Exception {
         // Handler to handle results post processing. Takes a tuple<key, value> or exception as an input and returns
         // the value. Also before returning value, puts the value in cache.
         BiFunction<Tuple<ICacheKey<K>, V>, Throwable, Void> handler = (pair, ex) -> {
@@ -253,7 +260,7 @@ private V compute(ICacheKey<K> key, LoadAwareCacheLoader<ICacheKey<K>, V> loader
                     logger.warn("Exception occurred while trying to compute the value", ex);
                 }
             }
-            completableFutureMap.remove(key); // Remove key from map as not needed anymore.
+            completableFutureMap.remove(key);// Remove key from map as not needed anymore.
             return null;
         };
         V value = null;

modules/cache-common/src/test/java/org/opensearch/cache/common/tier/TieredSpilloverCacheTests.java

@@ -760,7 +760,7 @@ public void testInvalidateAll() throws Exception {
     }
 
     public void testComputeIfAbsentConcurrently() throws Exception {
-        int onHeapCacheSize = randomIntBetween(100, 300);
+        int onHeapCacheSize = randomIntBetween(500, 700);
         int diskCacheSize = randomIntBetween(200, 400);
         int keyValueSize = 50;
 
@@ -782,7 +782,7 @@ public void testComputeIfAbsentConcurrently() throws Exception {
             0
         );
 
-        int numberOfSameKeys = randomIntBetween(10, onHeapCacheSize - 1);
+        int numberOfSameKeys = randomIntBetween(400, onHeapCacheSize - 1);
         ICacheKey<String> key = getICacheKey(UUID.randomUUID().toString());
         String value = UUID.randomUUID().toString();