release the read buffer after an oversized packet

The buffer grew to the largest packet ever received and was never given
back, so a single large ACL, user list or channel comment pinned its full
size, up to the ten megabyte packet limit, for the life of the connection.

Keep a modest buffer between packets and allocate larger ones only for as
long as they are needed.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

gumble/gumble/conn.go

@@ -16,6 +16,10 @@ import (
 // DefaultPort is the default port on which Mumble servers listen.
 const DefaultPort = 64738
 
+// retainedPacketBytes is the largest read buffer kept between packets. Bigger
+// buffers are allocated as needed and released again afterwards.
+const retainedPacketBytes = 64 * 1024
+
 // Conn represents a control protocol connection to a Mumble client/server.
 type Conn struct {
 	sync.Mutex
@@ -54,6 +58,11 @@ func (c *Conn) ReadPacket() (uint16, []byte, error) {
 	}
 	if pLengthInt > len(c.buffer) {
 		c.buffer = make([]byte, pLengthInt)
+	} else if len(c.buffer) > retainedPacketBytes && pLengthInt <= retainedPacketBytes {
+		// One oversized packet — a large ACL, user list or channel comment —
+		// used to pin its full size for the life of the connection. Give the
+		// memory back once ordinary traffic resumes.
+		c.buffer = make([]byte, retainedPacketBytes)
 	}
 	if _, err := io.ReadFull(c.Conn, c.buffer[:pLengthInt]); err != nil {
 		return 0, nil, err

gumble/gumble/conn_buffer_regression_test.go

@@ -0,0 +1,43 @@
+package gumble
+
+import (
+	"encoding/binary"
+	"net"
+	"testing"
+)
+
+// Regression: the read buffer grew to the largest packet ever seen and kept
+// that memory for the life of the connection.
+func TestConnBufferShrinksAfterAnOversizedPacket(t *testing.T) {
+	server, client := net.Pipe()
+	defer server.Close()
+	conn := NewConn(client)
+
+	big := 4 * 1024 * 1024
+	go func() {
+		defer server.Close()
+		writePacket := func(length int) {
+			var header [6]byte
+			binary.BigEndian.PutUint16(header[:], 3)
+			binary.BigEndian.PutUint32(header[2:], uint32(length))
+			server.Write(header[:])
+			server.Write(make([]byte, length))
+		}
+		writePacket(big)
+		writePacket(128)
+	}()
+
+	if _, _, err := conn.ReadPacket(); err != nil {
+		t.Fatalf("reading the oversized packet: %v", err)
+	}
+	if len(conn.buffer) < big {
+		t.Fatalf("oversized packet should have grown the buffer, got %d", len(conn.buffer))
+	}
+	if _, _, err := conn.ReadPacket(); err != nil {
+		t.Fatalf("reading the small packet: %v", err)
+	}
+	if len(conn.buffer) > retainedPacketBytes {
+		t.Fatalf("buffer stayed at %d bytes after a small packet, above the %d retained size",
+			len(conn.buffer), retainedPacketBytes)
+	}
+}