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Proposal: Implement stanza handlers

Author(s): Sam Whited
Last updated: 2020-02-26
Discussion: https://mellium.im/issue/25

Abstract

An API is needed for multiplexing based on stanzas that allows for matching based on type and quickly replying or re-marshaling the original stanzas.

Background

In the previous IQ mux proposal a new mechanism for routing IQ stanzas based on their type and payload was introduced. In practice, registering the IQ muxer ended up being cumbersome, and the previous proposal did not solve the problem of routing message or presence stanzas (this was deliberately left for a future proposal). To solve both of these problems at once the current multiplexer can be adapted using what we learned from the IQ mux experiment such that it can route message and presence stanzas based on their type, and IQs based on their type and payload.

Requirements

Ability to multiplex stanzas by stanza type and payload name
IQ, message, presence, and general top-level stream elements must have their own distinct handler types
The handlers must be extensible to add functionality such as replying to IQs or other thought of features in the future without making breaking API changes to the handler itself
The entire stanza and all children must be able to be decoded in one pass with Decoder.Decode or Decoder.DecodeElement

Proposal

In addition to the existing Handler and IQHandler types, the following types would be added for messages and presence:

type MessageHandler interface {
        HandleMessage(msg stanza.Message, t xmlstream.TokenReadEncoder) error
}
    MessageHandler responds to message stanzas.

type PresenceHandler interface {
        HandlePresence(p stanza.Presence, t xmlstream.TokenReadEncoder) error
}
    PresenceHandler responds to presence stanzas.

Adapters for functions with the provided signature would also be made available, similar to HandlerFunc.

Decoding the entire payload using an xml.Decoder requires the initial start token which has already been consumed when creating the stanza.Message, stanza.IQ, or stanza.Presence. We can't pass the start element instead of the decoded stanza type because this limits future extensibility (we can add methods to the stanza type related to encoding and decoding, replying, etc.), and it is undesirable to pass both because then we are passing duplicate information to the handler. Instead, we can prepend the start element to the buffered token stream so that every handler has access to the entire stanza.

The existing IQ mux would be removed (this is a backwards incompatible change, however, we are pre-1.0 and the IQ mux was never in a release) and its methods and functionality would be added to ServeMux.

The options related to registering stanzas would then be modified to take the new patterns as follows:

func IQ(typ stanza.IQType, payload xml.Name, h IQHandler) Option
    IQ returns an option that matches IQ stanzas by type and payload name.

func Message(typ stanza.MessageType, payload xml.Name, h MessageHandler) Option
    Message returns an option that matches message stanzas by type and
    payload name.

func Presence(typ stanza.PresenceType, payload xml.Name, h PresenceHandler) Option
    Presence returns an option that matches presence stanzas by type and
    payload name.

Functional versions of these options (taking a HandlerFunc) would also be added.

Registering a handler that matches a stanza using the Handle option will cause a panic, but this behavior is subject to change in the future.

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