Introduce soju.im/pre-away

The IRCv3 spec is stalled, so let's just ship a vendored extension
for now.

References: https://github.com/ircv3/ircv3-specifications/pull/514

doc/ext/pre-away.md

@@ -0,0 +1,91 @@
+---
+title: "`pre-away` Extension"
+layout: spec
+meta-description: A capability allowing client connections to be marked AWAY during connection registration
+work-in-progress: true
+copyrights:
+  -
+    name: "Shivaram Lingamneni"
+    period: "2023"
+    email: "slingamn@cs.stanford.edu"
+---
+
+## Notes for implementing experimental vendor version
+
+This is an experimental specification for a vendored extension.
+
+No guarantees are made regarding the stability of this extension.
+Backwards-incompatible changes can be made at any time without prior notice.
+
+Software implementing this work-in-progress specification MUST NOT use the
+unprefixed `pre-away` CAP name. Instead, implementations SHOULD use the
+`soju.im/pre-away` CAP name to be interoperable with other software implementing
+a compatible work-in-progress version.
+
+## Introduction
+
+Some IRC server implementations offer a mode of operation where a single
+nickname can be associated with multiple concurrent client connections, or no
+client connections. Typically, such implementations are bouncers (i.e.,
+intermediaries between the client and another server), but some are full server
+implementations.
+
+Such implementations may wish to update publicly visible state depending on the
+status of the user's actual client connections. For example, if the user has no
+active connections, it may be desirable to mark them as AWAY, then mark them
+un-AWAY if they reconnect. However, a client implementation may wish to connect
+without active involvement from the user, e.g. to retrieve
+[chathistory][], in which case it would be undesirable to suggest
+that the user is present. This extension provides a mechanism for such clients
+to flag their connections as automatically initiated, so servers can disregard
+them for this or other purposes related to user presence.
+
+## Implementation
+
+This specification introduces a new capability, `soju.im/pre-away`. Clients
+wishing to make use of this specification MUST negotiate the capability; this
+gives the server more information about the context and meaning of the client's
+`AWAY` commands.
+
+If the capability has been negotiated, servers MUST accept the `AWAY` command
+before connection registration has completed. The `AWAY` command has its normal
+semantics, except that servers SHOULD treat the form:
+
+	AWAY *
+
+i.e. an `AWAY` message consisting of the single character `*`, as indicating
+that the user is not present for an unspecified reason. Clients MAY also send
+`AWAY *` post-registration to indicate that the user is no longer present for an
+unspecified reason.
+
+In its conventional form:
+
+	AWAY :Gone to lunch.  Back in 5
+
+the `AWAY` command MAY be used pre-registration to set a human-readable away
+message associated with the connection as usual. Similarly, `AWAY` with no
+parameters indicates that the user is present.
+
+If the client's nickname was not already present on the server, then `AWAY`
+pre-registration sets the away message but does not inhibit reporting of the
+change in nickname status, e.g. via [monitor][].
+
+Clients that have negotiated this capability and subsequently receive `*` as an
+away message (for example, in `301 RPL_AWAY` or [away-notify][])
+SHOULD treat it as indicating that the user is not present for an unspecified
+reason. Servers MAY substitute a human-readable message for the `*` if it would
+otherwise be relayed as an away message.
+
+## Implementation considerations
+
+This section is non-normative.
+
+In general, the server-side aggregation of away statuses across multiple
+connections is outside the scope of this specification. However, in most cases,
+an away message of `*` should be treated as though the connection did not exist
+at all (for example, it should not supersede a human-readable `AWAY` message set
+by another connection, even if it is more recent).
+
+[chathistory]: https://ircv3.net/specs/extensions/chathistory
+[monitor]: https://ircv3.net/specs/extensions/monitor
+[away-notify]: https://ircv3.net/specs/extensions/away-notify

downstream.go

@@ -229,11 +229,12 @@ var permanentDownstreamCaps = map[string]string{
 
 	"draft/read-marker": "",
 
+	"soju.im/account-required":        "",
 	"soju.im/bouncer-networks":        "",
 	"soju.im/bouncer-networks-notify": "",
 	"soju.im/no-implicit-names":       "",
+	"soju.im/pre-away":                "",
 	"soju.im/read":                    "",
-	"soju.im/account-required":        "",
 	"soju.im/webpush":                 "",
 }
 
@@ -741,6 +742,14 @@ func (dc *downstreamConn) handleMessageUnregistered(ctx context.Context, msg *ir
 				Params:  []string{"BOUNCER", "UNKNOWN_COMMAND", subcommand, "Unknown subcommand"},
 			}}
 		}
+	case "AWAY":
+		if len(msg.Params) > 0 {
+			dc.away = &msg.Params[0]
+		} else {
+			dc.away = nil
+		}
+
+		dc.SendMessage(ctx, generateAwayReply(dc.away != nil))
 	default:
 		dc.logger.Printf("unhandled message: %v", msg)
 		return newUnknownCommandError(msg.Command)
@@ -2575,16 +2584,7 @@ func (dc *downstreamConn) handleMessageRegistered(ctx context.Context, msg *irc.
 			dc.away = nil
 		}
 
-		cmd := irc.RPL_NOWAWAY
-		desc := "You have been marked as being away"
-		if dc.away == nil {
-			cmd = irc.RPL_UNAWAY
-			desc = "You are no longer marked as being away"
-		}
-		dc.SendMessage(ctx, &irc.Message{
-			Command: cmd,
-			Params:  []string{dc.nick, desc},
-		})
+		dc.SendMessage(ctx, generateAwayReply(dc.away != nil))
 
 		uc := dc.upstream()
 		if uc != nil {

irc.go

@@ -278,3 +278,16 @@ func isNumeric(cmd string) bool {
 	}
 	return true
 }
+
+func generateAwayReply(away bool) *irc.Message {
+	cmd := irc.RPL_NOWAWAY
+	desc := "You have been marked as being away"
+	if !away {
+		cmd = irc.RPL_UNAWAY
+		desc = "You are no longer marked as being away"
+	}
+	return &irc.Message{
+		Command: cmd,
+		Params:  []string{"*", desc},
+	}
+}