[backport] fix(measurexlite): add robust RemoteAddr accessors (#1551)

Closes ooni/probe#2707

internal/measurexlite/conn.go

@@ -39,11 +39,29 @@ type connTrace struct {
 
 var _ net.Conn = &connTrace{}
 
+type remoteAddrProvider interface {
+	RemoteAddr() net.Addr
+}
+
+func safeRemoteAddrNetwork(rap remoteAddrProvider) (result string) {
+	if addr := rap.RemoteAddr(); addr != nil {
+		result = addr.Network()
+	}
+	return result
+}
+
+func safeRemoteAddrString(rap remoteAddrProvider) (result string) {
+	if addr := rap.RemoteAddr(); addr != nil {
+		result = addr.String()
+	}
+	return result
+}
+
 // Read implements net.Conn.Read and saves network events.
 func (c *connTrace) Read(b []byte) (int, error) {
 	// collect preliminary stats when the connection is surely active
-	network := c.RemoteAddr().Network()
-	addr := c.RemoteAddr().String()
+	network := safeRemoteAddrNetwork(c)
+	addr := safeRemoteAddrString(c)
 	started := c.tx.TimeSince(c.tx.ZeroTime())
 
 	// perform the underlying network operation
@@ -99,8 +117,8 @@ func (tx *Trace) CloneBytesReceivedMap() (out map[string]int64) {
 
 // Write implements net.Conn.Write and saves network events.
 func (c *connTrace) Write(b []byte) (int, error) {
-	network := c.RemoteAddr().Network()
-	addr := c.RemoteAddr().String()
+	network := safeRemoteAddrNetwork(c)
+	addr := safeRemoteAddrString(c)
 	started := c.tx.TimeSince(c.tx.ZeroTime())
 
 	count, err := c.Conn.Write(b)

internal/measurexlite/conn_test.go

@@ -12,6 +12,43 @@ import (
 	"github.com/ooni/probe-cli/v3/internal/testingx"
 )
 
+func TestRemoteAddrProvider(t *testing.T) {
+	t.Run("for nil address", func(t *testing.T) {
+		conn := &mocks.Conn{
+			MockRemoteAddr: func() net.Addr {
+				return nil
+			},
+		}
+		if safeRemoteAddrNetwork(conn) != "" {
+			t.Fatal("expected empty network")
+		}
+		if safeRemoteAddrString(conn) != "" {
+			t.Fatal("expected empty string")
+		}
+	})
+
+	t.Run("for common case", func(t *testing.T) {
+		conn := &mocks.Conn{
+			MockRemoteAddr: func() net.Addr {
+				return &mocks.Addr{
+					MockString: func() string {
+						return "1.1.1.1:443"
+					},
+					MockNetwork: func() string {
+						return "tcp"
+					},
+				}
+			},
+		}
+		if safeRemoteAddrNetwork(conn) != "tcp" {
+			t.Fatal("unexpected network")
+		}
+		if safeRemoteAddrString(conn) != "1.1.1.1:443" {
+			t.Fatal("unexpected string")
+		}
+	})
+}
+
 func TestMaybeClose(t *testing.T) {
 	t.Run("with nil conn", func(t *testing.T) {
 		var conn net.Conn = nil