Clone a backend message

[krisdiano]

Is your feature request related to a problem? Please describe.
I used the pgconn to handle low level libpq protocol in my projects which are stream replication proxy. I found that the lifespan of messages which were returned from Receive method is very short. I have to clone the backend messages and keep messages in memory to send in proper time. Now, for clone, I need type assert, encode, define a variable and decode.

Describe the solution you'd like

type BackendMessage interface {
    Message()
    Clone() BackendMessage()
    Backend()
}

or

type Message interface {
    Encode([]byte) []byte
    Decode([]byte) error
    Clone() Message
}

The Message interface are also be contained in FrontedMessage, but the source of FrontedMessage is determined by users, so I think the first method is more reasonable. The simeplest way of Clone:

func (src *CopyData) Clone() BackendMessage {
	var ret CopyData
	_ = ret.Decode(src.Encode(nil)[5:])
	return &ret
}

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.

[Jack Christensen]

Overall idea of making it simple to clone a message seems reasonable. But I'm not sure of the proper interface.

Not sure I totally understood your reasoning for deciding between adding to Message or BackendMessage interface. But adding to either interface is slightly unfortunate. Technically, it would be a breaking change to add to an interface. But practically speaking I don't think it would break any existing code. 🤷

But it is a bit weird for a *CopyData.Clone() to return a BackendMessage instead of another *CopyData. Maybe a CopyBackendMessage function should be added instead for the generic case.