prevent exceptions in callback from locking up the postgres client - #332

When the query emits 'end' the client which owns the query checks it's internal queue of queries to see about issuing another one. If the client has no more queries it will emit a 'drain'. This would have been a much more difficult problem in the past with the weird pauseDrain and resumeDrain stuff I recently removed...but I still think this might introduce weird timing bugs? Basically if you issue a second query to the same client from the callback of the first query the client will have emitted the 'drain' event in between. For example...this:

var client = new Client('blalba');

//automatically close the client when its empty
client.on('drain', client.end.bind(client));

client.query('SELECT * FROM whatever', function() {
  //client has at this point emitted drain and is in the process of closing
  //so this next query will error out
  client.query('SELECT * FROM something_else', function() {

  });
});

I'm not saying the above code example is good code, but I'm using it to demonstrate moving the callback into process.nextTick introduces some subtle backwards compatibile breaks. There are a few options...

1. if your code throws an exception according to node you're supposed to consider the process in an "undetermined state" and close it ASAP. Since your test threw an exception...it failed...so...

2. Run the pulseQueryQueue within the client also in a process.nextTick. I'm wary this not introducing other subtle timing bugs.

3. Put a try/catch/rethrow around the callback. I will have to benchmark this to see the perf impact. Not entirely out of the question, but feels dirty.

I kind of like the "dirty" try/catch idea. It's direct, and it may perform better than putting the callback and pulseQueryQueue on nextTick. Plus, it seems to do a better job (than nextTick) of expressing the intent: to protect the rest of the code from an exception in the callback.

fixed in pg@1.0.2

Thanks for your code - normally I accept & don't reimplement but there were some tweaks I wanted to put in.