Handle synchronous exceptions better

[ForbesLindesay]

Same as #83 but with a fixed diff

[simov]

What does finished do? Thanks.

[ForbesLindesay]

It prevents the callback being called multiple times. For example:

cons.temp.render = function (str, opts, cb) {
  try {
    cb(new Error('this never goes well');
  } catch (ex) {
    cb(ex);
  }
};

cons.temp.render('', {}, function (err, res) {
  console.log('There was an error');
  throw err;
});

In there the console.log will be hit twice:

There was an error
There was an error

Error: this never goes well
  Stack Trace
  Stack Trace
  Stack Trace
  Stack Trace

This kind of bug exists in reality in lots of places in the code and it's really hard to debug, so it's easiest to just fix it at the top level.

[simov]

Can we just

try {
  var result = engine.render(str, options);
} catch (err) {
  return fn(err);
}
fn(null, result);

[ForbesLindesay]

You can for synchronous parsers, and that's always sufficient for the synchronous parsers, but it does nothing to prevent poorly implemented async templating libraries.

[simov]

I don't even know how it's possible to make a synchronous parser in node .. and that's why I'm curious why most of the engines don't have a callback for it's compile/render functions.

[ForbesLindesay]

The only thing that would force you to make your template parsing async is if you allow users to do async work inside their templates. Node.js can do pretty much anything synchronously if you want it to (making http web requests takes a bit of work). Most of the libraries just transform strings to functions so there's no real reason for there to be any async work there.

P.S. The only two that actually operate asyncronously are dust and qejs the rest just trick you into thinking they do by providing a node style apie when in fact their syncronous (for an example take a look at the source code for the render function for jade no Async there P.S. it also nicely demonstrates a template library that suffers from the problem finished would fix.)

[doowb]

I know this is old, and with the promise support that has been added, it might not be an issue. If it is, then I think we can add this logic to the promisiify function.