implemented ExecuteMultiple

[skoef]

Like mentioned before in earlier PRs, I wrote/maintain a MySQL proxy acting as a gateway between client and server. When a client connects with the CLIENT_MULTI_STATEMENTS capability, it could execute multiple queries at once, for instance SELECT 1; INSERT INTO foo VALUES (1, "foo"). When such a query is executed through the regular Execute() function, the server will return multiple results, one for each query with the SERVER_MORE_RESULTS_EXISTS flag set for all results except the last. The additional results are not handled properly in the current implementation.

ExecuteMultiple allows me to keep handling all results in a user-defined callback, for instance:

func (q MyHandler) HandleQuery(query string) (*mysql.Result, error) {
	// ...

        if clientConn.HasStatus(mysql.CLIENT_MULTIPLE_STATEMENTS) {
            // execute multiple queries and write back the results to the client one by one
	    return backendConn.ExecuteMultiple(queries, func(result *mysql.Result, err error) {
                    // set proper status according to server results
                    clientConn.SetStatus(mysql.SERVER_MORE_RESULTS_EXISTS)
		    // if this is the last result, unset the more_results status
		    if result.Status&mysql.SERVER_MORE_RESULTS_EXISTS == 0 {
                        clientConn.UnsetStatus(mysql.SERVER_MORE_RESULTS_EXISTS)
		    }

         	    // if the query resulted in an error, write that back to the client
		    if err != nil {
		        clientConn.WriteValue(err)
		    } else {
  		        // handle result appropriately
		        clientConn.WriteValue(result)
		    }
	    })
	} else {
            // assume single query command
            return backendConn.Execute(query)
	}
}

ExecuteMultiple returns an empty resultset with the new streamingtype StreamingMulti set, so when it bubbles from HandleQuery towards WriteQuery/writeResultSet it doesn't send an additional OK or EOF to the client.

I have been running this in production for a while (about a year) and I was in the process of backporting my local changes to go-mysql to the repo so I can hopefully one day just use the upstream repo itself again 🤗