From 4a3879ca6f090f3074af6a8919b492e804914e22 Mon Sep 17 00:00:00 2001
From: Andrew Phelps <andrew.phelps@canonical.com>
Date: Tue, 29 Apr 2025 13:33:52 -0400
Subject: [PATCH] Fix data race between consumer of QueryParam.Entries and the
 sender

---
 client.go | 34 +++++++++++++++++++++++++++++++++-
 1 file changed, 33 insertions(+), 1 deletion(-)

diff --git a/client.go b/client.go
index d4e4f40..4fc8c56 100644
--- a/client.go
+++ b/client.go
@@ -4,6 +4,7 @@
 package mdns
 
 import (
+	"bytes"
 	"context"
 	"fmt"
 	"log"
@@ -380,7 +381,7 @@ func (c *client) query(params *QueryParam) error {
 				}
 				inp.sent = true
 				select {
-				case params.Entries <- inp:
+				case params.Entries <- deepCopyServiceEntry(inp):
 				default:
 				}
 			} else {
@@ -398,6 +399,37 @@ func (c *client) query(params *QueryParam) error {
 	}
 }
 
+// deepCopyServiceEntry returns a deep copy of the given [ServiceEntry]. Since
+// we send pointers back to callers via [QueryParam.Entries], they need to be
+// copied so that neither of us are mutating/reading from the same pointer at
+// the same time.
+func deepCopyServiceEntry(se *ServiceEntry) *ServiceEntry {
+	info := make([]string, len(se.InfoFields))
+	copy(info, se.InfoFields)
+
+	var ipAddrV6 *net.IPAddr
+	if se.AddrV6IPAddr != nil {
+		ipAddrV6 = &net.IPAddr{
+			IP:   bytes.Clone(se.AddrV6IPAddr.IP),
+			Zone: se.AddrV6IPAddr.Zone,
+		}
+	}
+
+	return &ServiceEntry{
+		hasTXT:       se.hasTXT,
+		sent:         se.sent,
+		Name:         se.Name,
+		Host:         se.Host,
+		AddrV4:       bytes.Clone(se.AddrV4),
+		AddrV6:       bytes.Clone(se.AddrV6),
+		Addr:         bytes.Clone(se.Addr),
+		Port:         se.Port,
+		Info:         se.Info,
+		AddrV6IPAddr: ipAddrV6,
+		InfoFields:   info,
+	}
+}
+
 // sendQuery is used to multicast a query out
 func (c *client) sendQuery(q *dns.Msg) error {
 	buf, err := q.Pack()