fix(telnet): stop a short NAWS report from crashing the server

internal/inputhandlers/term_iac_test.go

@@ -3,7 +3,9 @@ package inputhandlers
 import (
 	"testing"
 
+	"github.com/GoMudEngine/GoMud/internal/connections"
 	"github.com/GoMudEngine/GoMud/internal/term"
+	"github.com/stretchr/testify/assert"
 )
 
 // newEnvironPayload builds the bytes that appear between `IAC SB NEW-ENVIRON IS`
@@ -90,3 +92,17 @@ func TestNewEnvironResponseMatcher(t *testing.T) {
 		t.Errorf("expected extracted payload to be detected as Mudlet, payload=%v", payload)
 	}
 }
+
+// TestTelnetIACHandlerShortScreenSizeReport sends a NAWS report cut short after
+// three bytes. It used to index past the payload and panic on the connection
+// goroutine, which has no recover, so the whole server exited.
+func TestTelnetIACHandlerShortScreenSizeReport(t *testing.T) {
+	clientInput := &connections.ClientInput{
+		ConnectionId: 1,
+		DataIn:       []byte{term.TELNET_IAC, term.TELNET_SB, term.TELNET_OPT_NAWS, 0, 80, 0},
+	}
+
+	assert.NotPanics(t, func() {
+		TelnetIACHandler(clientInput, map[string]any{})
+	})
+}

internal/term/telnet.go

@@ -167,7 +167,9 @@ func TelnetDONT(what IACByte) []IACByte {
 
 func TelnetParseScreenSizePayload(info []byte) (width int, height int, err error) {
 
-	if len(info) >= 3 {
+	// Width and height are two bytes each, so a report shorter than four
+	// bytes cannot be read. Any client can send one.
+	if len(info) >= 4 {
 
 		width = (int(info[0]) << 8) | int(info[1])
 		height = (int(info[2]) << 8) | int(info[3])

internal/term/telnet_test.go

@@ -0,0 +1,43 @@
+package term
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// A NAWS report is four bytes: width then height, each high byte first. A
+// client can send fewer, and that must come back as an error rather than a
+// read past the end of the payload.
+func TestTelnetParseScreenSizePayload(t *testing.T) {
+	tests := []struct {
+		name       string
+		payload    []byte
+		wantWidth  int
+		wantHeight int
+		wantErr    bool
+	}{
+		{name: "80x24", payload: []byte{0, 80, 0, 24}, wantWidth: 80, wantHeight: 24},
+		{name: "high bytes", payload: []byte{1, 0, 0, 50}, wantWidth: 256, wantHeight: 50},
+		{name: "trailing IAC SE", payload: []byte{0, 120, 0, 40, TELNET_IAC, TELNET_SE}, wantWidth: 120, wantHeight: 40},
+		{name: "three bytes", payload: []byte{0, 80, 0}, wantErr: true},
+		{name: "two bytes", payload: []byte{0, 80}, wantErr: true},
+		{name: "empty", payload: []byte{}, wantErr: true},
+		{name: "nil", payload: nil, wantErr: true},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			width, height, err := TelnetParseScreenSizePayload(tt.payload)
+			if tt.wantErr {
+				assert.Error(t, err)
+				return
+			}
+
+			require.NoError(t, err)
+			assert.Equal(t, tt.wantWidth, width)
+			assert.Equal(t, tt.wantHeight, height)
+		})
+	}
+}