LocalCache: Fix compute() to report COLLECTED removal cause for GC'd entries

When compute() returns null for an entry whose value has been garbage
collected, the removal cause should be COLLECTED, not EXPLICIT.

The fix checks if the original value was null and the reference is still
active (not loading), which indicates the value was garbage collected
rather than explicitly removed by the user.

Fixes #7985

Author: nickita-khylkouski

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

@@ -1077,6 +1077,44 @@ public void testRemovalListener_collected() {
     assertThat(listener.isEmpty()).isTrue();
   }
 
+  // Test for https://github.com/google/guava/issues/7985
+  // When compute() returns null for a collected entry, the removal cause should be COLLECTED.
+  public void testComputeRemovalCause_collected() {
+    QueuingRemovalListener<Object, Object> listener = queuingRemovalListener();
+    LocalCache<Object, Object> map =
+        makeLocalCache(
+            createCacheBuilder().concurrencyLevel(1).softValues().removalListener(listener));
+    Segment<Object, Object> segment = map.segments[0];
+    assertThat(listener.isEmpty()).isTrue();
+
+    Object key = new Object();
+    Object value = new Object();
+
+    // Put an entry into the cache
+    map.put(key, value);
+    assertThat(listener.isEmpty()).isTrue();
+
+    // Get the entry and clear its value reference to simulate garbage collection
+    int hash = map.hash(key);
+    ReferenceEntry<Object, Object> entry = segment.getEntry(key, hash);
+    ValueReference<Object, Object> valueReference = entry.getValueReference();
+
+    // Simulate garbage collection by clearing the value reference
+    ((java.lang.ref.Reference<?>) valueReference).clear();
+
+    // Now call compute() with a function that returns null
+    // Since the value was collected, the removal cause should be COLLECTED, not EXPLICIT
+    map.compute(key, (k, v) -> {
+      // The value should be null since it was collected
+      assertThat(v).isNull();
+      return null;
+    });
+
+    // Verify the removal notification has COLLECTED cause
+    assertNotified(listener, key, null, RemovalCause.COLLECTED);
+    assertThat(listener.isEmpty()).isTrue();
+  }
+
   public void testRemovalListener_expired() {
     FakeTicker ticker = new FakeTicker();
     QueuingRemovalListener<Object, Object> listener = queuingRemovalListener();

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

@@ -2287,7 +2287,13 @@ V waitForLoadingValue(ReferenceEntry<K, V> e, K key, ValueReference<K, V> valueR
           removeLoadingValue(key, hash, computingValueReference);
           return null;
         } else {
-          removeEntry(e, hash, RemovalCause.EXPLICIT);
+          // If the value was garbage collected, report COLLECTED; otherwise EXPLICIT.
+          V oldValue = valueReference.get();
+          RemovalCause cause =
+              (oldValue == null && valueReference.isActive())
+                  ? RemovalCause.COLLECTED
+                  : RemovalCause.EXPLICIT;
+          removeEntry(e, hash, cause);
           return null;
         }
       } finally {