Fix bugs in GenerationalUtf8Cache constructor and access-time handling

- getUtf8(value, accessTimeMs) was capturing this.accessTimeMs and using
  that for entry timestamps, silently ignoring the parameter contrary
  to its Javadoc.
- The two-arg constructor sized the eden array using tenuredCapacity
  instead of edenCapacity.
- Rename refreshAcessTime to refreshAccessTime (no other callers).
- Correct misleading "evicting the LRU" comments in the LFU paths of
  both SimpleUtf8Cache and GenerationalUtf8Cache.

Adds regression tests for both bugs.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: dougqh

communication/src/main/java/datadog/communication/serialization/GenerationalUtf8Cache.java

@@ -120,7 +120,7 @@ public GenerationalUtf8Cache(int capacity) {
   public GenerationalUtf8Cache(int edenCapacity, int tenuredCapacity) {
     this.accessTimeMs = System.currentTimeMillis();
 
-    int edenSize = Caching.cacheSizeFor(Math.min(tenuredCapacity, MAX_EDEN_CAPACITY));
+    int edenSize = Caching.cacheSizeFor(Math.min(edenCapacity, MAX_EDEN_CAPACITY));
     this.edenEntries = new CacheEntry[edenSize];
     this.edenMarkers = new int[edenSize];
 
@@ -143,7 +143,7 @@ public void updateAccessTime(long accessTimeMs) {
   }
 
   /** Updates access time to the @link {@link System#currentTimeMillis()} */
-  public void refreshAcessTime() {
+  public void refreshAccessTime() {
     this.updateAccessTime(System.currentTimeMillis());
   }
 
@@ -215,14 +215,13 @@ public final byte[] getUtf8(String value, long accessTimeMs) {
     if (value.length() > MAX_ENTRY_LEN) return CacheEntry.utf8(value);
 
     int adjHash = Caching.adjHash(value);
-    long lookupTimeMs = this.accessTimeMs;
 
     CacheEntry[] tenuredEntries = this.tenuredEntries;
     int matchingTenuredIndex = lookupEntryIndex(tenuredEntries, MAX_TENURED_PROBES, adjHash, value);
     if (matchingTenuredIndex != -1) {
       CacheEntry tenuredEntry = tenuredEntries[matchingTenuredIndex];
 
-      tenuredEntry.hit(lookupTimeMs);
+      tenuredEntry.hit(accessTimeMs);
 
       this.tenuredHits += 1;
       return tenuredEntry.utf8();
@@ -233,7 +232,7 @@ public final byte[] getUtf8(String value, long accessTimeMs) {
     if (matchingEdenIndex != -1) {
       CacheEntry edenEntry = edenEntries[matchingEdenIndex];
 
-      double hits = edenEntry.hit(lookupTimeMs);
+      double hits = edenEntry.hit(accessTimeMs);
       if (hits > this.promotionThreshold) {
         // mark promoted first - to avoid racy insertions
         this.promotions += 1;
@@ -256,8 +255,8 @@ public final byte[] getUtf8(String value, long accessTimeMs) {
 
     CacheEntry newEntry = new CacheEntry(adjHash, value);
     // First request was swallowed by marking, so double hit
-    newEntry.hit(lookupTimeMs);
-    newEntry.hit(lookupTimeMs);
+    newEntry.hit(accessTimeMs);
+    newEntry.hit(accessTimeMs);
 
     // search for empty slot or failing that the MFU entry
     int edenMfuIndex = findFirstAvailableOrMfuIndex(edenEntries, MAX_EDEN_PROBES, adjHash);
@@ -346,7 +345,7 @@ static final boolean lfuInsert(CacheEntry[] entries, int numProbes, CacheEntry n
       }
     }
 
-    // If we get here, then we're evicted the LRU
+    // If we get here, then we're evicting the LFU
     entries[lfuIndex] = newEntry;
     return true;
   }

communication/src/main/java/datadog/communication/serialization/SimpleUtf8Cache.java

@@ -160,7 +160,7 @@ static final boolean lfuInsert(CacheEntry[] entries, CacheEntry newEntry) {
       }
     }
 
-    // If we get here, then we're evicting the LRU
+    // If we get here, then we're evicting the LFU
     entries[lfuIndex] = newEntry;
     return true;
   }

communication/src/test/java/datadog/communication/serialization/GenerationalUtf8CacheTest.java

@@ -6,6 +6,7 @@
 import static org.junit.jupiter.api.Assertions.assertNotSame;
 import static org.junit.jupiter.api.Assertions.assertSame;
 
+import java.lang.reflect.Field;
 import java.nio.charset.StandardCharsets;
 import java.util.Random;
 import java.util.concurrent.ThreadLocalRandom;
@@ -36,6 +37,13 @@ public void capacity() {
     assertEquals(128, cache.tenuredCapacity());
   }
 
+  @Test
+  public void capacity_twoArg() {
+    GenerationalUtf8Cache cache = new GenerationalUtf8Cache(64, 256);
+    assertEquals(64, cache.edenCapacity());
+    assertEquals(256, cache.tenuredCapacity());
+  }
+
   @Test
   public void maxCapacity() {
     GenerationalUtf8Cache cache =
@@ -82,6 +90,29 @@ public void caching() {
     assertNotEquals(0, cache.edenHits);
   }
 
+  @Test
+  public void getUtf8_perCallAccessTime_overridesField() throws Exception {
+    GenerationalUtf8Cache cache = create();
+    // The field value should not leak into the entry when an explicit time is supplied.
+    cache.updateAccessTime(0L);
+
+    String value = "bar";
+    long callTime = 12345L;
+
+    // First call only marks; the second call creates the entry.
+    cache.getUtf8(value, callTime);
+    cache.getUtf8(value, callTime);
+
+    assertEquals(callTime, lookupEdenLastUsedMs(cache, value));
+
+    // Drive enough hits to promote into tenured.
+    while (cache.promotions == 0) {
+      cache.getUtf8(value, callTime);
+    }
+
+    assertEquals(callTime, lookupTenuredLastUsedMs(cache, value));
+  }
+
   @Test
   public void promotion() {
     GenerationalUtf8Cache cache = create();
@@ -205,6 +236,29 @@ static final String nextValue() {
     return baseString + valueSuffix;
   }
 
+  static long lookupEdenLastUsedMs(GenerationalUtf8Cache cache, String value) throws Exception {
+    return lookupLastUsedMs(cache, "edenEntries", value);
+  }
+
+  static long lookupTenuredLastUsedMs(GenerationalUtf8Cache cache, String value) throws Exception {
+    return lookupLastUsedMs(cache, "tenuredEntries", value);
+  }
+
+  private static long lookupLastUsedMs(GenerationalUtf8Cache cache, String fieldName, String value)
+      throws Exception {
+    Field arrayField = GenerationalUtf8Cache.class.getDeclaredField(fieldName);
+    arrayField.setAccessible(true);
+    GenerationalUtf8Cache.CacheEntry[] entries =
+        (GenerationalUtf8Cache.CacheEntry[]) arrayField.get(cache);
+
+    for (GenerationalUtf8Cache.CacheEntry entry : entries) {
+      if (entry != null && value.equals(entry.value)) {
+        return entry.lastUsedMs();
+      }
+    }
+    throw new AssertionError("entry for value '" + value + "' not found in " + fieldName);
+  }
+
   static final void printStats(GenerationalUtf8Cache cache) {
     System.out.printf(
         "eden hits: %5d\tpromotion hits: %5d\tpromotions: %5d\tearly: %5d\tlocal evictions: %5d\tglobal evictions: %5d%n",