need a way to prevent write from blocking

Right now the chat server example is trivial to DoS by writing a client that connects to the server but never reads, given that transport.write() blocks when the buffer is full.

Proposed solution: add an optional 'block=' argument to transport.write() which when False buffers data in Python instead of waiting for the kernel buffer to empty. Of course, this just chains the trivial attack from a blocking attack to a memory consumption attack. That means we'll need either a max size on the buffer or a timeout.

The default value of 'block=' is up for debate. Using native Twisted APIs, one could say that the "default" is to not "wait", since you have to go to extra effort to implement the producer API when you do want to wait. I'm tempted to use this to justify a default of False. AFAICT, it's only people who are writing "streaming" protocols, or protocols which send large amounts of data, which ever want to use block=True.