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Implementing XMPP Extensions

The mellium.im/xmpp module contains a number of packages that implement XMPP Extension Protocols (XEPs). When contributing new packages, or publishing your own proprietary extensions separately, it's important to make sure that your package is consistent with existing code. The rules in this post were designed to facilitate consistent extension packages and should always be followed when contributing new packages upstream.

Group XEPs into packages by functionality

Normally this means that each XEP is given its own package. However, for smaller or tightly coupled XEPs this may mean sharing a package. For instance, XEP-0082: XMPP Date and Time Profiles and XEP-0202: Entity Time are both related to the concept of transmitting time over an XMPP connection, and XEP-0082 is effectively just a list of constants. Because of this, they both share a single package, mellium.im/xmpp/xtime (note that xtime would not normally be considered a good package name, time would be better but this conflicts with a package in the standard library). Similarly, the various XEPs that make up the Jingle signaling protocol would likely be grouped together. Meanwhile XEP-0234: Jingle File Transfer and XEP-0047: In-Band Bytestreams are both about file transfer, but they likely would not share a single package as they wouldn't share much code whereas XEP-0234 likely would share code with other Jingle related XEPs.

When in doubt, ask. There are no hard and fast rules to grouping XEPs into packages.

Implement data transmissions with simple functions

Users of your package shouldn't have to think about XML, stanzas, or XMPP. Instead, we should provide our users with simple functions that can be used to perform basic operations over the network, and which result in common types where possible. For example, the xtime.Get function sends an IQ requesting the time to the given address over the given session and returns an ordinary time.Time:

func Get(ctx context.Context, s *xmpp.Session, to jid.JID) (time.Time, error)
    Get sends a request to the provided JID asking for its time.

Under the hood this might be sending an IQ and blocking while it waits for a response, or returning a payload that had been previously decoded by a message handler. The user doesn't need to know the details, all that they care about is asking for a time and getting one in the format they're already used to dealing with.

Use the mux package to handle incoming data

The mellium.im/xmpp/mux package provides an xmpp.Handler that can multiplex various top level elements to custom handlers based on their name or stanza type and the name of a payload. While other multiplexers may exist in the future, they should all use the standard handler types:

Because of this, if your package handles one or more types of payload it should contain a type, generally called Handler (eg. xtime.Handler or ping.Handler) that implements one or more of the above handler interfaces and can be registered with a mux.ServeMux to respond to incoming payloads.

Similarly, your package should contain a mux.Option that registers the handler. Generally this option will be called Handle, but if multiple handlers exist that need to be registered separately more descriptive names may be necessary such as example.HandleData and example.HandleOpen. This can either create a new handler if the handler does not require any configuration as the ping.Handle option does, or take a specific handler to register if further configuration may be required as the xtime.Handle option does. Options configure the mux.ServeMux using fields that are not exported, so they must be constructed in terms of other, existing, options. For example, the handler form xtime only handles a specific type of IQ and is defined as:

// Handle returns an option that registers a Handler for entity time requests.
func Handle(h Handler) mux.Option {
	return mux.IQ(stanza.GetIQ, xml.Name{Local: "time", Space: NS}, h)
}

A handler that needs to handle multiple types can use a raw mux.Option func to group multiple options into one:

// Handle is an option that registers an example handler that implements
// both IQHandler and MessageHandler.
func Handle(h Handler) mux.Option {
	return func(m *ServeMux) {
		mux.IQ(stanza.GetIQ, xml.Name{Local: "example", Space: NS}, h)(m)
		mux.Message(stanza.NormalMessage, xml.Name{Local: "example", Space: NS}, h)(m)
	}
}

Export structs for marshaling and unmarshaling

Normally the user will want to use the functions and handlers defined in your package to transmit and receive data, but sometimes they may need to reimplement your extension to add their own proprietary extensions, or to instrument the code for a required logging or metrics library. So that users can reimplement as little as possible, and for the sake of consistency between implementations, we export structs that can be used to marshal or unmarshal stanzas and payloads.

Many XEPs will likely have a single payload that needs to be exported. In this case, if the payload does not need any custom logic to marshal or unmarshal, a struct representing the full IQ (or other stanza type) should be exported. For example, the mellium.im/xmpp/ping package contains the definition for a ping IQ:

// IQ is encoded as a ping request.
type IQ struct {
	stanza.IQ

	Ping struct{} `xml:"urn:xmpp:ping ping"`
}

Exported stanza types should be named after the type of stanza they implement. The name of the previous examples type when fully qualified is therefore ping.IQ. If multiple stanza types need to be exported from the same package, give them all descriptive names, for example: example.RequestMessage and example.ResponseMessage.

If one or more payloads exist that need custom logic to marshal or unmarshal, export the individual payload type with implementations of xml.Marshaler and xml.Unmarshaler containing the custom logic.

In either case, the exported stanza or payload types should implement the following interfaces:

For an example, see the source for xtime.Time.

Lazily decode large, repeating payloads

When unmarshaling a payload of unknown length that contains many similar children, an iterator should be written instead of unmarshaling them all into a slice. This gives the user the option of keeping memory usage low by reusing values, reducing copying when the length is unknown and a slice would have to be extended, and keeping CPU usage low by only partially decoding the child elements if we can short circuit after finding a particular child.

The iterator type should be called Iter and can be used to lazily decode children into a type the user can deal with. The Iter type should have an API similar to the following example, taken from the roster package:

// Iter is an iterator over roster items.
type Iter struct{}

// Item returns the last roster item parsed by the iterator.
(i *Iter) Item() Item

// Next reports whether there are more items to decode.
(i *Iter) Next() bool

// Err returns the last error encountered by the iterator (if any).
(i *Iter) Err() error

// Close indicates that we are finished with the given iterator and processing
// the stream may continue.
// Calling it multiple times has no effect.
(i *Iter) Close() error

The Item method and its return type are named after the thing being decoded, in this case a roster item, and may be different for every Iter. From the developers perspective, the Iter type can now decode elements from the stream with a simple for loop:

iter := roster.Fetch(context.TODO(), session)
defer iter.Close()

for iter.Next() {
	item := iter.Item()
	// Do something with the roster item
}
if iter.Err() != nil {
	// Handle errors
}

Because iterators are common and all largely share the same logic to decode child elements and return them, much of the logic has been extracted into the xmlstream.Iter type. Instead of operating directly on decoded child elements, the xmlstream.Iter type operates on the token stream and returns access to each top-level start token it finds, letting your custom Iter type do the final decoding into a concrete type of your choosing. In the case of the roster package, this type is a roster.Item.

Most iterators will need to maintain some internal state. This normally comprises any errors that were generated, a value representing the last child element that was decoded, and an underlying xmlstream.Iter.

On your Iter type, most methods need to simply call the similarly named method on the underlying xmlstream.Iter. The exceptions are the Err and Next methods. The Err method should return any errors generated by your decoding first and if no such errors exist return iter.Err(). The Next method is a bit more complicated. It should first check that no previous decoding errors exist and that iter.Next() is true. If this is the case it should decode the tokens provided by iter.Current() and save the result or any errors generated.

For example, the roster package defines Next like so:

// Next returns true if there are more items to decode.
func (i *Iter) Next() bool {
	if i.err != nil || !i.iter.Next() {
		return false
	}
	start, r := i.iter.Current()
	d := xml.NewTokenDecoder(r)
	item := Item{}
	i.err = d.DecodeElement(&item, start)
	if i.err != nil {
		return false
	}
	i.current = item
	return true
}
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