Zalgo bug

[rogierschouten]

Seen in v2.5.3

The callback to connection.query() is sometimes called synchronously in case of an error. In general, one should not call callbacks both synchronously and a-synchronously (google for "releasing Zalgo").

In my case, it resulted in a type error because I reset some variables inside the callback that I make use of directly after calling the query() method, i.e.

function doQuery() {
  var myVar = { foo: "bar" };
  connection.destroy();
  connection.query("SELECT * FROM mytable", function() {
     myVar = undefined;
  });
  console.log(myVar.foo); // myVar is undefined
}

This does not happen if the callback is always called a-synchronously but apparently there are scenarios where it isn't.

It occurred when I accidentally called query() after calling destroy().

[dougwilson]

Hi @rogierschouten to make sure we're looking at the same Zalgo, can you really quick take your code and swap the myVar = undefined with a console.trace() and paste the trace here?

[rogierschouten]

Here you go...

var mysql = require("mysql");

var connection = mysql.createConnection({
  host     : 'localhost',
  user     : 'me',
  password : 'password',
  database : 'my_db'
});

function doTheQuery() {
  var myVar = { foo: "bar" };
  connection.destroy();
  connection.query("SELECT * FROM mytable", function() {
     myVar = undefined;
  });
  console.trace(myVar.foo); // myVar is undefined
}

connection.connect(function(err) {
  if (err) {
    console.error('error connecting: ' + err.stack);
    return;
  }

  console.log('connected as id ' + connection.threadId);

  doTheQuery();

});

Output trace:

$ node test.js
connected as id 8912

c:\Source\eXLent_master\exlent\node_modules\mysql\lib\protocol\Parser.js:82
        throw err;
              ^
TypeError: Cannot read property 'foo' of undefined
    at doTheQuery (c:\Source\eXLent_master\exlent\temp\test.js:16:22)
    at Handshake._callback (c:\Source\eXLent_master\exlent\temp\test.js:27:3)
    at Handshake.Sequence.end (c:\Source\eXLent_master\exlent\node_modules\mysql\lib\protocol\sequences\Sequence.js:96:24)
    at Handshake.Sequence.OkPacket (c:\Source\eXLent_master\exlent\node_modules\mysql\lib\protocol\sequences\Sequence.js:105:8)
    at Protocol._parsePacket (c:\Source\eXLent_master\exlent\node_modules\mysql\lib\protocol\Protocol.js:271:23)
    at Parser.write (c:\Source\eXLent_master\exlent\node_modules\mysql\lib\protocol\Parser.js:77:12)
    at Protocol.write (c:\Source\eXLent_master\exlent\node_modules\mysql\lib\protocol\Protocol.js:39:16)
    at Socket.<anonymous> (c:\Source\eXLent_master\exlent\node_modules\mysql\lib\Connection.js:96:28)
    at Socket.emit (events.js:95:17)
    at Socket.<anonymous> (_stream_readable.js:765:14)

[dougwilson]

Hi @rogierschouten to make sure we're looking at the same Zalgo, can you really quick take your code and swap the myVar = undefined with a console.trace() and paste the trace here? I see you placed the console.trace() in the wrong place. I need it to be where the myVar = undefined is in the code.

[rogierschouten]

This better?

var mysql = require("mysql");

var connection = mysql.createConnection({
  host     : 'localhost',
  user     : 'me',
  password : 'password',
  database : 'my_db'
});

function doTheQuery() {
  var myVar = { foo: "bar" };
  connection.destroy();
  connection.query("SELECT * FROM mytable", function() {
    console.trace();
    myVar = undefined;
  });
  // console.log(myVar.foo); // myVar is undefined
}

connection.connect(function(err) {
  if (err) {
    console.error('error connecting: ' + err.stack);
    return;
  }

  console.log('connected as id ' + connection.threadId);

  doTheQuery();

});

$ node test.js
connected as id 9945
Trace
    at Query._callback (c:\Source\eXLent_master\exlent\temp\test.js:14:10)
    at Query.Sequence.end (c:\Source\eXLent_master\exlent\node_modules\mysql\lib\protocol\sequences\Sequence.js:96:24)
    at Protocol._validateEnqueue (c:\Source\eXLent_master\exlent\node_modules\mysql\lib\protocol\Protocol.js:218:6)
    at Protocol._enqueue (c:\Source\eXLent_master\exlent\node_modules\mysql\lib\protocol\Protocol.js:129:13)
    at Connection.query (c:\Source\eXLent_master\exlent\node_modules\mysql\lib\Connection.js:201:25)
    at doTheQuery (c:\Source\eXLent_master\exlent\temp\test.js:13:14)
    at Handshake._callback (c:\Source\eXLent_master\exlent\temp\test.js:28:3)
    at Handshake.Sequence.end (c:\Source\eXLent_master\exlent\node_modules\mysql\lib\protocol\sequences\Sequence.js:96:24)
    at Handshake.Sequence.OkPacket (c:\Source\eXLent_master\exlent\node_modules\mysql\lib\protocol\sequences\Sequence.js:105:8)
    at Protocol._parsePacket (c:\Source\eXLent_master\exlent\node_modules\mysql\lib\protocol\Protocol.js:271:23)

[dougwilson]

Yes, thank you!