Improve handler interface and documentation

This change simplifies a little more the sub protocols mechanism.
Aliases have been removed. The renaming of loop handlers as long
polling handlers has been reverted.

Plain HTTP handlers now simply do their work in the init/2
callback. There is no specific code for them.

Loop handlers now follow the same return value as Websocket,
they use ok to continue and shutdown to stop.

Terminate reasons for all handler types have been documented.
The terminate callback is now appropriately called in all cases
(or should be).

Behaviors for all handler types have been moved in the module
that implement them. This means that cowboy_handler replaces
the cowboy_http_handler behavior, and similarly cowboy_loop
replaces cowboy_loop_handler, cowboy_websocket replaces
cowboy_websocket_handler. Finally cowboy_rest now has the
start of a behavior in it and will have the full list of
optional callbacks defined once Erlang 18.0 gets released.

The guide has been reorganized and should be easier to follow.

1 files changed, 64 insertions, 0 deletions

diff --git a/doc/src/guide/sub_protocols.ezdoc b/doc/src/guide/sub_protocols.ezdoc
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+::: Sub protocols
+
+Sub protocols are used for creating new types of handlers that
+provide extra functionality in a reusable way. Cowboy uses this
+mechanism to provide its loop, REST and Websocket handlers.
+
+This chapter will explain how to create your own sub protocols
+and handler types.
+
+:: Usage
+
+To switch to a sub protocol, the `init/2` callback must return
+the name of the sub protocol module. Everything past this point
+is handled by the sub protocol.
+
+``` erlang
+init(Req, Opts) ->
+	{cowboy_websocket, Req, Opts}.
+```
+
+The return value may also have a `Timeout` value and/or the
+atom `hibernate`. These options are useful for long living
+connections. When they are not provided, the timeout value
+defaults to `infinity` and the hibernate value to `run`.
+
+The following snippet switches to the `my_protocol` sub
+protocol, sets the timeout value to 5 seconds and enables
+hibernation:
+
+``` erlang
+init(Req, Opts) ->
+	{my_protocol, Req, Opts, 5000, hibernate}.
+```
+
+If a sub protocol does not make use of these options, it should
+crash if it receives anything other than the default values.
+
+:: Upgrade
+
+After the `init/2` function returns, Cowboy will then call the
+`upgrade/6` function. This is the only callback defined by the
+`cowboy_sub_protocol` behavior.
+
+The function is named `upgrade` because it mimics the mechanism
+of HTTP protocol upgrades. For some sub protocols, like Websocket,
+an actual upgrade is performed. For others, like REST, this is
+only an upgrade at Cowboy's level and the client has nothing to
+do about it.
+
+The upgrade callback receives the Req object, the middleware
+environment, the handler and its options, and the aforementioned
+timeout and hibernate values.
+
+``` erlang
+upgrade(Req, Env, Handler, HandlerOpts, Timeout, Hibernate) ->
+	%% Sub protocol code here.
+```
+
+This callback is expected to behave like a middleware and to
+return an updated environment and Req object.
+
+Sub protocols are expected to call the `cowboy_handler:terminate/4`
+function when they terminate. This function will make sure that
+the optional `terminate/3` callback is called, if present.