fix: reset memory usage on cache clear

foyer-memory/src/raw.rs

@@ -308,7 +308,11 @@ where
             garbages.push(record);
         }
 
+        let usage = std::mem::take(&mut self.usage);
         self.entries = 0;
+        if usage > 0 {
+            self.metrics.memory_usage.decrease(usage as _);
+        }
         if count > 0 {
             self.metrics.memory_entries.decrease(count);
             self.metrics.memory_remove.increase(count);
@@ -1659,6 +1663,28 @@ mod tests {
         assert_eq!(pieces, expected);
     }
 
+    #[test]
+    fn test_clear_resets_usage() {
+        let fifo = fifo_cache_for_test();
+
+        for i in 0..4 {
+            fifo.insert(i, i);
+        }
+        assert_eq!(fifo.usage(), 4);
+        assert_eq!(fifo.entries(), 4);
+
+        fifo.clear();
+        assert_eq!(fifo.usage(), 0);
+        assert_eq!(fifo.entries(), 0);
+
+        // Clearing an empty cache must be idempotent, and accounting must remain
+        // correct when the cache is used again.
+        fifo.clear();
+        fifo.insert(4, 4);
+        assert_eq!(fifo.usage(), 1);
+        assert_eq!(fifo.entries(), 1);
+    }
+
     #[test]
     fn test_insert_size_over_capacity() {
         let cache: RawCache<Fifo<Vec<u8>, Vec<u8>, TestProperties>, ModHasher, HashTableIndexer<_>> =