I'm implementing a functionality which should upon a click on a button extend an user session lifespan by sending a request to the backend and to update the database using MySQL2.
In order to do that I wrote the following front-end code:
onClose: function (oAction) {
try {
if (oAction == "YES") {
let reqURL = "/sessionExtend";
let reqData = {
session_id: sessionStorage.getItem("SessionId"),
user_id: sessionStorage.getItem("UserId")
};
let callbackOK = function (responseData) {
curr.onSuccessfulResponse(curr, responseData, "sessionExtendSuccess", "sessionExtendFail", "", false);
};
let callbackErr = function (responseData) {
curr.onErrorResponse(curr, responseData, "sessionExtendFail");
};
curr.performRequest(reqURL, reqData, callbackOK, callbackErr);
}
} catch (err) {
console.log(err);
MessageToast.show(sMsg);
}
}
The request is received by app.js, which establishes the database connection using MySQL2 and forwards the request to the DAL:
app.post("/sessionExtend", async function (req, res) {
let session_id = req.body.session_id;
let user_id = req.body.user_id;
let con = DAL.getConnection();
res.setHeader("Content-Type", "application/json");
try {
const response = await DAL.sessionExtend(con, session_id, user_id);
res.send(JSON.stringify({
"result": true,
"message": "session extended"
}));
} catch (e) {
res.send(JSON.stringify({
"result": false,
"message": "can not extend session"
}));
}
con.close();
});
The DAL module executes the SQL-query and should return the result either success or error:
sessionExtend: async function sessionExtend(con, session_id, user_id) {
con.connect(function (err) {
try {
if (err) throw err;
con.query(qryDict.SQL_QUERIES.setUpdateExtendSession, [session_id, user_id], function (err) {
let result;
if (err) {
result = JSON.stringify({
"result": false,
"message": "failure"
});
} else {
result = JSON.stringify({
"result": true,
"message": "success"
});
}
return result;
});
} catch (err) {
let result = JSON.stringify({
"result": false,
"message": err
});
return result;
}
});
},
The problem is that when I execute this code in debugger, I get an exception:
ERR_STREAM_WRITE_AFTER_END Error [ERR_STREAM_WRITE_AFTER_END]: write after end at Socket.Writable.write (_stream_writable.js:297:11) at Connection.write (C:\Users\User\IdeaProjects\TST\node_modules\mysql2\lib\connection.js:226:17) at Connection.writePacket (C:\Users\User\IdeaProjects\TST\node_modules\mysql2\lib\connection.js:271:12) at ClientHandshake.sendCredentials (C:\Users\User\IdeaProjects\TST\node_modules\mysql2\lib\commands\client_handshake.js:64:16) at ClientHandshake.handshakeInit (C:\Users\User\IdeaProjects\TST\node_modules\mysql2\lib\commands\client_handshake.js:137:12) at ClientHandshake.execute (C:\Users\User\IdeaProjects\TST\node_modules\mysql2\lib\commands\command.js:39:22) at Connection.handlePacket (C:\Users\User\IdeaProjects\TST\node_modules\mysql2\lib\connection.js:417:32) at PacketParser.onPacket (C:\Users\User\IdeaProjects\TST\node_modules\mysql2\lib\connection.js:75:12) at PacketParser.executeStart (C:\Users\User\IdeaProjects\TST\node_modules\mysql2\lib\packet_parser.js:75:16) at Socket. (C:\Users\User\IdeaProjects\TST\node_modules\mysql2\lib\connection.js:82:25)
I also paid attention that during the debugging, I firstly get on a frontend a response from the backend and only then I reach the breakpoints in DAL withing con.query(qryDict.SQL_QUERIES.setUpdateExtendSession, [session_id, user_id], function (err) {…}.
My questions:
Why do I get ERR_STREAM_WRITE_AFTER_END and how to avoid it?
Why do I firstly get on the frontend a response from the backend and only then I reach the breakpoints in DAL? I assumed that await DAL.sessionExtend(con, session_id, user_id) should wait until the task on a DAL will be done and a promise will be resolved.
In short: You are not awaiting the con.connection and con.query, so the outer code continues and calls con.close and returns the frontend result, and later on con.query attempts to send a query through the now-closed connection, leading to this exception.
You are writing async functions but you only made them "half-async".
For example, this won't work:
async function getStuff () {
stuff.get(function (err, data) {
if (err) throw err // kills your process if it happend!
return data.stuff // returns to nowhere
})
}
// later on:
const stuff = await getStuff()
console.log(stuff) // prints undefined!
...because essentially, your async function just synchronously calls another function( not waiting for it) then immediately returns nothing (i.e. undefined):
async function getStuff () {
stuff.get(...)
// as you can see, no return inside getStuff
}
And later on, the callback you passed will run, but then your outer code's train has already left the platform.
What you'd want to do instead is have stuff.get return a promise (most modern libraries will do that even if they additionally expose a callback promise for compatibility with older codebases) and await it:
async function getStuff () {
const data = await stuff.get() // waits for the stuff to come back
return data.stuff // actually returns the stuff
// The `if (err) throw err` now became unnecessary as well
}
// later on:
const stuff = await getStuff()
console.log(stuff) // prints the stuff!
If your SQL library would expose a promise interface, you could simply await it. You wrote that you are using mysql2. This library has a promise interface if required with require('mysql2/promise'). I'd suggest switching to the promise interface instead of the callback interface!
There is also a way to "upgrade" an existing con connection to the promise interface: con.promise(). So you'd simply do con = DAL.getConnection().promise() instead of con = DAL.getConnection().
Then, you could rewrite the code like this (or equivalent, depending on which library you pick):
async function sessionExtend(con, session_id, user_id) {
try {
await con.connect()
await con.query(qryDict.SQL_QUERIES.setUpdateExtendSession, [session_id, user_id])
return JSON.stringify({ result: true, message: 'success' })
} catch (err) {
return JSON.stringify({ result: false, message: err.toString() })
}
}
EDIT: The following part is actually obsolete because mysql2 allows upgrading an existing connection to the promise interface, but I'll leave this here anyway in case it helps someone else in a similar situation!
If you can't switch to the promise interface, you could instead promisify the existing calls (it just looks a bit more convoluted though):
const { promisify } = require('util')
async function sessionExtend(con, session_id, user_id) {
try {
await promisify(con.connect).call(con)
await promisify(con.query).call(con, qryDict.SQL_QUERIES.setUpdateExtendSession, [session_id, user_id])
return JSON.stringify({ result: true, message: 'success' })
} catch (err) {
return JSON.stringify({ result: false, message: err.toString() })
}
}
util.promisify wraps a function that expects an (err, data) callback, converting it into an async function that returns a promise instead. Since con.query et al. are methods on con though, they need to keep that context, that's why I wrote promisify(con.query).call(con, ...) instead of just promisify(con.query)(...).