From 9a7bdda6a0bf67d434409be3fa3ea4a3f549aa16 Mon Sep 17 00:00:00 2001
From: Sai Asish Y <say.apm35@gmail.com>
Date: Tue, 7 Jul 2026 21:29:36 -0700
Subject: [PATCH] fix #2411: split long CAP ACK/NAK across multiple lines
 (#2414)

Signed-off-by: Sai Asish Y <say.apm35@gmail.com>
Co-authored-by: Shivaram Lingamneni <slingamn@cs.stanford.edu>
---
 irc/handlers.go | 32 +++++++++++++++++++++++++++-----
 1 file changed, 27 insertions(+), 5 deletions(-)

diff --git a/irc/handlers.go b/irc/handlers.go
index ce3eccfa..98731770 100644
--- a/irc/handlers.go
+++ b/irc/handlers.go
@@ -676,7 +676,6 @@ func capHandler(server *Server, client *Client, msg ircmsg.Message, rb *Response
 	subCommand := strings.ToUpper(msg.Params[0])
 	toAdd := caps.NewSet()
 	toRemove := caps.NewSet()
-	var capString string
 
 	config := server.Config()
 	supportedCaps := config.Server.supportedCaps
@@ -687,10 +686,12 @@ func capHandler(server *Server, client *Client, msg ircmsg.Message, rb *Response
 	}
 
 	badCaps := false
+	var capString string
+	var capFields []string
 	if len(msg.Params) > 1 {
 		capString = msg.Params[1]
-		strs := strings.Fields(capString)
-		for _, str := range strs {
+		capFields = strings.Fields(capString)
+		for _, str := range capFields {
 			remove := false
 			if str[0] == '-' {
 				str = str[1:]
@@ -726,6 +727,27 @@ func capHandler(server *Server, client *Client, msg ircmsg.Message, rb *Response
 		}
 	}
 
+	// CAP ACK / CAP NAK may also overflow the 512-byte limit, so we may need
+	// to split it. Unlike LS / LIST, each ACK and NAK is processed independently
+	// and there is no line continuation form
+	sendCapReqResponse := func(response string) {
+		// :server.name CAP nickname ACK :list of caps\r\n
+		maxLen := (MaxLineLen - 2) - 1 - len(server.name) - 5 - len(details.nick) - 6
+		if len(capString) <= maxLen || len(capFields) == 0 {
+			rb.Add(nil, server.name, "CAP", details.nick, response, capString)
+			return
+		}
+		var t utils.TokenLineBuilder
+		t.Initialize(maxLen, " ")
+		for _, token := range capFields {
+			t.Add(token)
+		}
+		capLines := t.Lines()
+		for _, capStr := range capLines {
+			rb.Add(nil, server.name, "CAP", details.nick, response, capStr)
+		}
+	}
+
 	switch subCommand {
 	case "LS":
 		if !client.registered {
@@ -754,7 +776,7 @@ func capHandler(server *Server, client *Client, msg ircmsg.Message, rb *Response
 		// every offered capability. during registration, requesting it produces a quit,
 		// otherwise just a CAP NAK
 		if badCaps || (toAdd.Has(caps.Nope) && client.registered) {
-			rb.Add(nil, server.name, "CAP", details.nick, "NAK", capString)
+			sendCapReqResponse("NAK")
 			return false
 		} else if toAdd.Has(caps.Nope) && !client.registered {
 			client.Quit(fmt.Sprintf(client.t("Requesting the %s client capability is forbidden"), caps.Nope.Name()), rb.session, nil)
@@ -763,7 +785,7 @@ func capHandler(server *Server, client *Client, msg ircmsg.Message, rb *Response
 
 		rb.session.capabilities.Union(toAdd)
 		rb.session.capabilities.Subtract(toRemove)
-		rb.Add(nil, server.name, "CAP", details.nick, "ACK", capString)
+		sendCapReqResponse("ACK")
 
 		// XXX compatibility hack, if draft/metadata-2 was requested, silently enable draft/metadata-3
 		// this way we only have to check for one cap at runtime