Node-postgres: Client re-connection doesn't work

[sibedge]

I'm running a simple test against the latest pg, under Node.js 8.1.2:

const pg = require('pg');

var client = new pg.Client(connection);

function test() {
    client.connect(err => {
        if (err) {
            console.log(err);
            return;
        }
        client.query('select 123', [], (err, data) => {
            if (err) {
                console.log(err);
            } else {
                console.log('DATA:', data.rows[0]);
            }
            client.end();
        });
    });
}

setInterval(test, 1000);

And it outputs the following, every time:

DATA: anonymous { '?column?': 123 }
DATA: undefined
DATA: undefined
DATA: undefined
DATA: undefined
DATA: undefined
DATA: undefined
DATA: undefined
events.js:182
      throw er; // Unhandled 'error' event
      ^

error: invalid frontend message type 0
    at Connection.parseE (D:\NodeJS\tests\node_modules\pg\lib\connection.js:567:11)
    at Connection.parseMessage (D:\NodeJS\tests\node_modules\pg\lib\connection.js:391:17)
    at Socket.<anonymous> (D:\NodeJS\tests\node_modules\pg\lib\connection.js:129:22)
    at emitOne (events.js:120:20)
    at Socket.emit (events.js:210:7)
    at addChunk (_stream_readable.js:252:12)
    at readableAddChunk (_stream_readable.js:239:11)
    at Socket.Readable.push (_stream_readable.js:197:10)
    at TCP.onread (net.js:588:20)

After I successfully call client.end(), it will successfully re-connect, successfully execute the query, just won't return any data. This looks odd.

And this - error: invalid frontend message type 0 doesn't make any sense.

Could somebody, please explain, why we get such an odd output?

UPDATE

And if just after creating the client, I add the following:

client.on('error', e => {
    console.log(e);
});

The output gets absolutely flooded with a huge error dumb, so big, I can't even paste it here.

[charmander]

I’m almost certain clients aren’t meant to be connected more than once (@brianc, could you confirm this?); treat them as unusable after calling .end().

- var client = new pg.Client(connection);
-
 function test() {
+    var client = new pg.Client(connection);
+
     client.connect(err => {
         if (err) {
             console.log(err);
             return;
         }
         client.query('select 123', [], (err, data) => {
             if (err) {
                 console.log(err);
             } else {
                 console.log('DATA:', data.rows[0]);
             }
             client.end();
         });
     });
 }

[sibedge]

I’m almost certain clients aren’t meant to be connected more than once...

And what is the reason for that?

[charmander]

They’re wrappers around a connection and should be associated with one connection.

[brianc]

Yeah though it's not really documented clients are cheap to instantiate and should be considered 'used up' once they've been disconnected. There's a bit of a state machine inside the client w/ a bunch of event handlers being established after the connect event. Your best bet is going to be to throw the old one away & make a new one. The connection handshake over tcp is the part that takes a little bit of time, but reusing an existing client wouldn't save any time there as reconnecting would still need to happen - it would also introduce additional complexity in ensuring the old event handlers were disposed and the new ones set up correctly. I'll re-open this & add it to the 7.0 milestone to return an error if a client has connect called on it more than once - hopefully that will make things more clear.

[sibedge]

It would be nice! The Client should throw an error when someone is trying to reconnect or call .query after calling .end(), the same as pg-pool should be throwing an error when someone calls .connect or .query after pool.end() has been called.

[brianc]

Closed with #1365

[waruwaruwaru]

I have a similar problem.

`const jwtLogin = new JwtStrategy(jwtOptions, function(payload, done) {

console.log('after connect');

client.connect();
client.query("SELECT FROM users where id = $1", [payload.sub], function(err, data) {

if (err) { return done(err, false); }
if (data) {
  console.log('inside data');
  client.end((err) => {
    console.log('client has been disconnected');
    if (err) {
      console.log('there has been error disconnecting from client: ', err);
    }
  });
  return done(null, data);
} else {
  console.log('hereeeeeeeeeeeeee?');
  client.end();
  return done(null, false);
}

});
});`

The first time I hit my endpoint that runs this code, everything works fine. I get my data back from the database fetch and it console.logged "client has been disconnected" but when I hit this endpoint again, my code only console.log('after connect') and none of my other code runs. I get
Unhandled promise rejection (rejection id: 1): Error: Client has already been connected. You cannot reuse a client.

Am I using client.end() incorrectly?

[charmander]

@waruwaruwaru Don’t end the client if you want to continue using it. If you want to create a new client each time instead of keeping one open, you’ll have to do just that (client = new Client(…); client.connect(…) in function (payload, done)). If you want a connection pool (this is the usual case), use Pool instead.

[waruwaruwaru]

@charmander Thank you.

[goodinmech]

Someone might get misunderstood the comment above, therefore I shall note:

1) Create a new client each time is slow
2) Keep one open client is unreliable
3) Use a Pool is the right way (https://node-postgres.com/features/pooling)