Domains not properly catching errors while testing nodeJS in mocha

Question

When running tests that utilize domains for error handling, Mocha still appears to be throwing an error even if a domain handler inside a library should have caught the error. If I execute the code outside of Mocha, it functions correctly leading me to believe the problem is Mocha.

Example:

foo.js

module.exports = function(done) {
    var domain = require("domain");
    var d = domain.create();

    d.on("error", function() {
        done();
    });

    d.run(function() {
        throw new Error("foo");
    });
}

test.js - Error thrown inside foo.js is not being caught by the domain.

describe("test", function() {
    it("should succeed", function(done) {
        var foo = require("./foo.js");
        foo(function() {
            console.log("done");
            done();
        });
    });
});

result : error thrown

script.js - error is being properly caught by the domain and bubbled up.

var foo = require("./foo.js");
foo(function() {
    console.log("done");
});
result : done

As you can see above, if I node straight to script.js it functions as desired, the error is caught by the domain handler and the code continues. If I run the same block of code inside a Mocha test, the error halts the test and a failure is given. I believe this is because the error is being sent on an uncaughtException handler, or something like it. The other complication is that it works properly in Mocha, if I have a process.nextTick() around the function call, leading me to believe that Mocha is only unable to handle synchronous errors, but works just fine with async errors.

There is some talk of this problem here: https://groups.google.com/forum/#!msg/nodejs/n-W9BSfxCjI/SElI1DJ_6u0J and https://github.com/joyent/node/issues/4375 .

The confusion I have is that all of this discussion seems to state the problem has been resolved months ago. Anyone know of either a simple work-around for the issue, or a why I'm not seeing a bug fixed which other people seem to believe is fixed at this point in time.

I am running node v0.10.18 and Mocha 1.13.0 on CentOS 6.3 Vagrant VirtualBox on Windows 7.

Answer 1

Found the problem. NodeJS domains catch synchronous errors but the event continues to bubble to a try/catch. If you wrap a domain.run() in a try/catch then the domain error handler AND the catch will be executed.

Thus, it seems the best practice is to use process.nextTick inside all domain.run(). This is shown in the docs example, but isn't expressed as explicitly as I would prefer.

Example:

d.run(function() {
    process.nextTick(function() {
        // do stuff
    });
});

In this case, the flaw is not in Mocha.

Proof of NodeJS domains not catching synchronous errors in try/catch: https://gist.github.com/owenallenaz/7141699

Answer 2

nodejs domains do definitely catch synchronous errors

See this simple test case

var domain = require("domain");
var d = domain.create();

d.on("error", function() {
    console.log("domain caught");
});


d.run(function() {
    throw new Error("foo");
});


// result: domain caught

EDIT: Since writing this answer I have written a blog post describing what is going on with domains and try catch and whether you can use domains as a wholesale replacement for try catch. It summarises most of what has been discussed here.

http://www.lighthouselogic.com/node-domains-as-a-replacement-for-try-catch/

ORIGINAL ANSWER:

Actually there is a problem with Mocha.

I wrote the following test function:

function error(callback){
     var d = domain.create().on('error', function(err){
        console.log("Caught Error in Domain Handler")
        return callback(err);
    });
    d.enter();
    throw new Error("TestError");
    d.exit();
}

Then I wrote a simple test without mocha:

error(function(err){
    if(err)
    {
        console.log("Error was returned");
    }else
    {
        console.log("Error was not returned")
    }
})

The output I received was:

Caught Error in Domain Handler
Error was returned

When I tested using Mocha:

describe('Domain Tests', function(){
    it('Should return an error when testing', function(done){
        error(function(err){
            if(err)
            {
                console.log("Error was returned");
            }else
            {
                console.log("Error was not returned")
            }
            return done();
        })
    });
});

I received the following output:

․

  0 passing (4ms)
  1 failing

  1) Domain Tests Should return an error when testing:
     Error: TestError
      at error (/Users/bensudbury/Documents/node_projects/testMochaDomains/test.js:9:11)
      at Context.<anonymous> (/Users/bensudbury/Documents/node_projects/testMochaDomains/testMocha.js:6:3)
      at Test.Runnable.run (/usr/local/share/npm/lib/node_modules/mocha/lib/runnable.js:194:15)
      at Runner.runTest (/usr/local/share/npm/lib/node_modules/mocha/lib/runner.js:358:10)
      at /usr/local/share/npm/lib/node_modules/mocha/lib/runner.js:404:12
      at next (/usr/local/share/npm/lib/node_modules/mocha/lib/runner.js:284:14)
      at /usr/local/share/npm/lib/node_modules/mocha/lib/runner.js:293:7
      at next (/usr/local/share/npm/lib/node_modules/mocha/lib/runner.js:237:23)
      at Object._onImmediate (/usr/local/share/npm/lib/node_modules/mocha/lib/runner.js:261:5)
      at processImmediate [as _immediateCallback] (timers.js:330:15)

As you can see: the domain error handler was short circuited.

This problem seems to be related to the following issues:

https://github.com/visionmedia/mocha/issues/513

While the Node issue has been closed, the issue in mocha is still open.

The workaround that was suggested at: https://gist.github.com/mcollina/4443963 didn't resolve the issue in this case.

I dug through the code of Mocha and found that the problem occurs because mocha wraps the tests in a try catch block. This means that the exception is caught and never sent to the uncaughtException or _fatalException handler depending upon the version of node that you are using.

Your workaround is good, but nodejs domains do definitely catch synchronous errors so I wouldn't change your code but instead change your test. Your new test should look like:

describe("test", function() {
    it("should succeed", function(done) {
        process.nextTick(function(){
            var foo = require("./foo.js");
            foo(function() {
                console.log("done");
                done();
            });
        })      
    });
});

I haven't tested this code, but similar code for my example works properly:

it('Should return an error when testing', function(done){   
    process.nextTick(function(){
        error(function(err){
            if(err)
            {
                console.log("Error was returned");
            }else
            {
                console.log("Error was not returned")
            }
            return done();
        });
    })
});

I have added a comment to the end of the issue in Mocha to see if it can be resolved:

https://github.com/visionmedia/mocha/issues/513