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, 87 insertions, 1 deletions

diff --git a/doc/src/manual/cowboy_handler.ezdoc b/doc/src/manual/cowboy_handler.ezdoc
index 0495f28..b440b60 100644
--- a/doc/src/manual/cowboy_handler.ezdoc
+++ b/doc/src/manual/cowboy_handler.ezdoc
@@ -15,10 +15,96 @@ Environment output:
 
 * result = ok
 
+This module also defines the `cowboy_handler` behaviour that
+defines the basic interface for handlers. All Cowboy handlers
+implement at least the `init/2` callback, and may implement
+the `terminate/3` callback optionally.
+
 :: Types
 
 None.
 
+:: Terminate reasons
+
+The following values may be received as the terminate reason
+in the optional `terminate/3` callback. Different handler types
+may define additional terminate reasons.
+
+: normal
+
+The connection was closed normally.
+
+: {crash, Class, Reason}
+
+A crash occurred in the handler. `Class` and `Reason` can be
+used to obtain more information about the crash. The function
+`erlang:get_stacktrace/0` can also be called to obtain the
+stacktrace of the process when the crash occurred.
+
+:: Callbacks
+
+: init(Req, Opts)
+	-> {ok, Req, State}
+	| {Module, Req, State}
+	| {Module, Req, State, hibernate}
+	| {Module, Req, State, Timeout}
+	| {Module, Req, State, Timeout, hibernate}
+
+Types:
+
+* Req = cowboy_req:req()
+* Opts = any()
+* State = any()
+* Module = module()
+* Timeout = timeout()
+
+Process the request.
+
+This function can be used to switch to an alternate handler
+type by returning the name of the module to be used, along
+with a few options.
+
+For basic handlers this is the function where the response
+should be sent. If no response is sent, Cowboy will ensure
+that a `204 No Content` response is sent.
+
+A crash in this callback will result in `terminate/3` being
+called if it is defined, with the `State` argument set to
+the value of `Opts` originally given to the `init/2` callback.
+
+: terminate(Reason, Req, State) -> ok
+
+Types:
+
+* Reason = any()
+* Req = cowboy_req:req()
+* State = any()
+
+Perform any necessary cleanup of the state.
+
+This callback should release any resource currently in use,
+clear any active timer and reset the process to its original
+state, as it might be reused for future requests sent on the
+same connection. Typical plain HTTP handlers rarely need to
+use it.
+
+A crash in this callback or an invalid return value will
+result in the closing of the connection and the termination
+of the process.
+
 :: Exports
 
-None.
+: terminate(Reason, Req, State, Handler) -> ok
+
+Types:
+
+* Reason = any()
+* Req = cowboy_req:req()
+* State = any()
+* Handler = module()
+
+Call the optional `terminate/3` callback if it exists.
+
+This function should always be called at the end of the execution
+of a handler, to give it a chance to clean up or perform
+miscellaneous operations.