Prevent callback from being called twice

Question

I have the following piece of code, but due to it's async behavior (I suppose), the callback is called twice. I'm using node-unzip to unzip a file I download from the internet.

function DownloadAndExtract(file, callback) {
    log.debug("Starting download of "+file);
    fse.ensureDirSync(tempPath);
    var extractor = unzip.Extract({path: tempPath});
    extractor.on("close", function() {
        log.debug("Done downloading "+file);
        return callback(null, file);
    });
    extractor.on("error", function (err) {
        log.error("Extracting download "+file+": "+JSON.stringify(err, null, "\t"));
        return callback(err, null); // THIS IS LINE 274
    });

    var url = "";
    if(file == "WONEN") {
        url = "https://example.com/file1.zip";
    }else if(file == "BOG") {
        url = "https://example.com/file2.zip";
    }

    if(url != "") {
        request
            .get(url)
            .on("error", function (err) {
                return callback(err, null);
            })
            .pipe(extractor);
    }else{
        return callback(new Error("Invalid file indicator: '"+file+"'"), null);
    }
}

I expected return to actually quit all running async functions but that is obviously nonsense. Now, the error I keep getting is the following:

/home/nodeusr/huizenier.nl/node_modules/async/lib/async.js:30
        if (called) throw new Error("Callback was already called.");
                          ^
Error: Callback was already called.
    at /home/nodeusr/huizenier.nl/node_modules/async/lib/async.js:30:31
    at /home/nodeusr/huizenier.nl/node_modules/async/lib/async.js:251:21
    at /home/nodeusr/huizenier.nl/node_modules/async/lib/async.js:575:34
    at Extract.<anonymous> (/home/nodeusr/huizenier.nl/realworks.js:274:10)
    at Extract.emit (events.js:129:20)
    at Parse.<anonymous> (/home/nodeusr/huizenier.nl/node_modules/unzip/lib/extract.js:24:10)
    at Parse.emit (events.js:107:17)
    at /home/nodeusr/huizenier.nl/node_modules/unzip/lib/parse.js:60:12
    at processImmediate [as _immediateCallback] (timers.js:358:17)

The output of the log.error() call is the following: 21-02-2015 03:00:05 - [ERROR] Extracting download WONEN: {} so I'm quite confused. There isn't really an error, then why is the event emitted?

How would I prevent the callback from being called twice here? Contacting the creator of the package or create a work around?

Code calling DownloadAndExtract

async.parallel([
    function (callback) {
        DownloadAndExtract("WONEN", callback);
    },
    function (callback) {
        DownloadAndExtract("BOG", callback);
    }
],
function (err, done) {
    if(err) return log.error("Update entries: "+JSON.stringify(err, null, "\t"));
    // Do different logic if no error
});

Edit

One of my attempts is declaring a var callbackAlreadyCalled = false within the function, and at any given point where I call the callback, I do

if(!callbackAlreadyCalled) {
    callbackAlreadyCalled = true;
    return callback(callback params);
}

Is this a good approach or could I handle it in a better way?

Edit 2

Already found out that the empty error is caused by errors not working properly when using JSON.stringify(), however, problem doesn't change.

Don't stringify Error objects, often the result will be an empty object {}. Console.log or console.error the error object directly instead. — Yuri Zarubin, Feb 21 '15 at 17:16
I've just found a solution to prevent this from happening (see: http://stackoverflow.com/a/18391400/2248679), however, this is not the main problem I have — Ruben Rutten, Feb 21 '15 at 17:17
I think you should provide the async call to DownloadAndExtract — JuniorCompressor, Feb 21 '15 at 17:30
Just to clear this up, the problem ONLY occurs in case of an error, where as in other cases it works fine. The problem is that `DownloadAndExtract` continues to work when the callback is already called, and that is the problem I have to tackle. — Ruben Rutten, Feb 21 '15 at 17:37

score 0 · Answer 1 · answered Aug 16 '17 at 02:36

It looks like you have waterfall error calls. When you call the request, it calls the extractor then if there is an error it calls the on_error of the extractor and then the on_error of the request.

Try to unify your returns from errors or call it only once. Make a test removing one of your "on('error'" validation.

Prevent callback from being called twice

Edit

Edit 2

1 Answers1