ENG-2783 Fix bug in ZREM which causes it to create a redis hash type.

Summary:
If the number of keys to delete in ZREM is 0, it inadvertently creates a redis hash data
type due to the SubDocument being empty. As a result, if we try a ZADD on that same set later on, it
returns a wrong type error.

The fix in this case was to not attempt the deletion altogether if the number of keys is 0.

Test Plan: unit test.

Reviewers: rahuldesirazu, amitanand, kannan, akashnil

Reviewed By: amitanand, kannan, akashnil

Subscribers: bogdan, ram, kannan, ybase

Differential Revision: https://phabricator.dev.yugabyte.com/D4011

src/yb/docdb/doc_operation.cc

@@ -772,6 +772,7 @@ Status RedisWriteOperation::ApplyDel(const DocOperationApplyData& data) {
         RETURN_NOT_OK(PrimitiveValueFromSubKeyStrict(kv.subkey(i), *data_type, &primitive_value));
         values.SetChild(primitive_value, SubDocument(ValueType::kTombstone));
       }
+      num_keys = kv.subkey_size();
       break;
     }
     case REDIS_TYPE_SORTEDSET: {
@@ -839,9 +840,12 @@ Status RedisWriteOperation::ApplyDel(const DocOperationApplyData& data) {
       break;
     }
   }
-  DocPath doc_path = DocPath::DocPathFromRedisKey(kv.hash_code(), kv.key());
-  RETURN_NOT_OK(data.doc_write_batch->ExtendSubDocument(
-      doc_path, values, redis_query_id()));
+
+  if (num_keys != 0) {
+    DocPath doc_path = DocPath::DocPathFromRedisKey(kv.hash_code(), kv.key());
+    RETURN_NOT_OK(data.doc_write_batch->ExtendSubDocument(doc_path, values, redis_query_id()));
+  }
+
   response_.set_code(RedisResponsePB_RedisStatusCode_OK);
   if (EmulateRedisResponse(kv.type())) {
     // If the flag is true, we respond with the number of keys actually being deleted. We don't

src/yb/yql/redis/redisserver/redisserver-test.cc

@@ -1339,6 +1339,17 @@ TEST_F(TestRedisService, TestSortedSets) {
   // Cannot have incr with multiple score value pairs.
   DoRedisTestExpectError(__LINE__, {"ZADD", "z_key", "INCR", "0", "v1", "1", "v2"});
 
+  // Test ZREM on non-existent key and then add the same key.
+  DoRedisTestInt(__LINE__, {"ZREM", "my_z_set", "v1"}, 0);
+  SyncClient();
+  DoRedisTestInt(__LINE__, {"ZCARD", "my_z_set"}, 0);
+  SyncClient();
+  DoRedisTestInt(__LINE__, {"ZADD", "my_z_set", "1", "v1"}, 1);
+  SyncClient();
+  DoRedisTestInt(__LINE__, {"ZCARD", "my_z_set"}, 1);
+  SyncClient();
+  DoRedisTestArray(__LINE__, {"ZRANGEBYSCORE", "my_z_set", "1", "1"}, {"v1"});
+
   SyncClient();
   VerifyCallbacks();
 }