Do not call the callback function twice when image size detection failed

[shinnn]

Currently image-size calls the callback function twice when the function throws an error.
This causes a confusion. When the callback function itself fails though size detection succeed, image-size tries to run the callback again.

For example, this code shows the log twice, unfortunately.

var sizeOf = require('image-size');

sizeOf('img.jpg', function() {
  console.log('This message should be shown only once!');
  throw new Error();
})

> This message should be shown only once!
> This message should be shown only once!

And besides, it causes a problem on asynchronous iteration using async or each-async etc.

For example,

var async = require('async');
var sizeOf = require('image-size');

async.each(imagePathArray, function(imagePath, i, next) {
  imageSize(imagePath, next);
}, function() {
  assert.equal(0, 100); // it throws an error
});

When we run the code above, we expect an error message such as Assertion errror: 0 != 100.
However, the code throws an iteration error like Callback already called. instead of assertion error, because the callback function is called twice.

This PR fixes these problem. When the callback function is called, image-size will finish immediately.

[coveralls]

Coverage increased (+0.09%) when pulling fd9a799 on shinnn:master into c2a859f on netroy:master.

[shinnn]

@netroy What do you think?

[netroy]

since lookup is a sync function, only one of the callback methods would be ever called...
can you add a test that is broken before this change & fixed after it ?

[shinnn]

OK, I'll do it. Done. #25 (comment)

[netroy]

ah, took my slow brain a while to understand... Thanks for doing this.

Essentially we are handling errors from the callback function itself, which kinda feels dirty (handling someone else's errors)..

would it make sense if I change this to be something like this

var dimensions, err;
try {
  dimensions = lookup(buffer, filepath);
} catch (e) {
  err = e;
}
callback(err, dimensions);

solves the same issue without handling external errors.

PS: sorry for not being responsive on this PR, been crazy busy lately :(

[shinnn]

err has been already defined here.
We need not to define it again.

var dimensions;
try {
  dimensions = lookup(buffer, filepath);
} catch (e) {
  err = e;
}
callback(err, dimensions);

[coveralls]

Coverage increased (+0.11%) when pulling db56559 on shinnn:master into 43fc9ad on netroy:master.

[shinnn]

@netroy

1. I added the test to check if the callback function is called only once before fixing index.js. As we can see, it failed.

   https://travis-ci.org/netroy/image-size/jobs/34908820#L97

   Error: done() called multiple times
   

2. After fixing index.js, it passed the test.

[shinnn]

Fixed.

[coveralls]

Coverage increased (+0.08%) when pulling a600057 on shinnn:master into 43fc9ad on netroy:master.

[netroy]

thanks for doing this..

[shinnn]

You're welcome :)