Fix NullReferenceException in HashObject.GetExpiration (#955)

fix #954

Co-authored-by: Tal Zaccai <[email protected]>

Author: nightroman

libs/server/Objects/Hash/HashObject.cs

@@ -554,7 +554,7 @@ private long GetExpiration(byte[] key)
                 return -2;
             }
 
-            if (expirationTimes.TryGetValue(key, out var expiration))
+            if (expirationTimes is not null && expirationTimes.TryGetValue(key, out var expiration))
             {
                 return expiration;
             }

test/Garnet.test/RespHashTests.cs

@@ -85,6 +85,22 @@ public async Task CanSetWithExpireAndRemoveExpireByCallingSetAgain()
             ClassicAssert.AreEqual("Tsavorite", r);
         }
 
+        // Covers the fix of #954.
+        [Test]
+        public void CanFieldPersistAndGetTimeToLive()
+        {
+            using var redis = ConnectionMultiplexer.Connect(TestUtils.GetConfig());
+            var db = redis.GetDatabase(0);
+            var key = "user:user1";
+            var field = "field1";
+
+            db.HashSet(key, [new HashEntry(field, "v1")]);
+            db.HashFieldExpire(key, [field], TimeSpan.FromHours(1));
+            db.HashFieldPersist(key, [field]);
+            var ttl = db.HashFieldGetTimeToLive(key, [field]);
+            ClassicAssert.AreEqual(-1, ttl[0]);
+        }
+
         [Test]
         public void CanSetAndGetOnePairLarge()
         {