Fix b/80241237 to correctly *not* change segment weight, nor report a…

…n eviction, when a compute() call does not change the present value.

RELNOTES=Fix a bug where Cache.asMap.compute* methods could cause nonsensical weights to be stored, breaking cache eviction.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=198618238

guava-tests/test/com/google/common/cache/LocalCacheMapComputeTest.java

@@ -55,6 +55,30 @@ public void testComputeIfAbsent() {
     assertEquals(1, cache.size());
   }
 
+  public void testComputeIfAbsentEviction() {
+    // b/80241237
+
+    Cache<String, String> c = CacheBuilder.newBuilder().maximumSize(1).build();
+
+    assertThat(c.asMap().computeIfAbsent("hash-1", k -> "")).isEqualTo("");
+    assertThat(c.asMap().computeIfAbsent("hash-1", k -> "")).isEqualTo("");
+    assertThat(c.asMap().computeIfAbsent("hash-1", k -> "")).isEqualTo("");
+    assertThat(c.size()).isEqualTo(1);
+    assertThat(c.asMap().computeIfAbsent("hash-2", k -> "")).isEqualTo("");
+  }
+
+  public void testComputeEviction() {
+    // b/80241237
+
+    Cache<String, String> c = CacheBuilder.newBuilder().maximumSize(1).build();
+
+    assertThat(c.asMap().compute("hash-1", (k, v) -> "a")).isEqualTo("a");
+    assertThat(c.asMap().compute("hash-1", (k, v) -> "b")).isEqualTo("b");
+    assertThat(c.asMap().compute("hash-1", (k, v) -> "c")).isEqualTo("c");
+    assertThat(c.size()).isEqualTo(1);
+    assertThat(c.asMap().computeIfAbsent("hash-2", k -> "")).isEqualTo("");
+  }
+
   public void testComputeIfPresent() {
     cache.put(key, "1");
     // simultaneous update for same key, expect count successful updates

guava/src/com/google/common/cache/LocalCache.java

@@ -2243,7 +2243,8 @@ V compute(K key, int hash, BiFunction<? super K, ? super V, ? extends V> functio
         if (newValue != null) {
           if (valueReference != null && newValue == valueReference.get()) {
             loadingValueReference.set(newValue);
-            setValue(e, key, newValue, now);
+            e.setValueReference(valueReference);
+            recordWrite(e, 0, now); // no change in weight
             return newValue;
           }
           try {