refactor(errorsx): directly return ErrWrapper when needed

The utility of SafeErrWrapperBuilder is low. Let us instead change the
code to always create ErrWrapper when we're in this package.

While there, also note some TODO-next items.

Part of https://github.com/ooni/probe/issues/1505.

internal/errorsx/dialer_test.go

@@ -18,66 +18,171 @@ func TestErrorWrapperDialerFailure(t *testing.T) {
 		},
 	}}
 	conn, err := d.DialContext(ctx, "tcp", "www.google.com:443")
+	var ew *ErrWrapper
+	if !errors.As(err, &ew) {
+		t.Fatal("cannot convert to ErrWrapper")
+	}
+	if ew.Operation != ConnectOperation {
+		t.Fatal("unexpected operation", ew.Operation)
+	}
+	if ew.Failure != FailureEOFError {
+		t.Fatal("unexpected failure", ew.Failure)
+	}
+	if !errors.Is(ew.WrappedErr, io.EOF) {
+		t.Fatal("unexpected underlying error", ew.WrappedErr)
+	}
 	if conn != nil {
 		t.Fatal("expected a nil conn here")
 	}
-	errorWrapperCheckErr(t, err, ConnectOperation)
-}
-
-func errorWrapperCheckErr(t *testing.T, err error, op string) {
-	if !errors.Is(err, io.EOF) {
-		t.Fatal("expected another error here")
-	}
-	var errWrapper *ErrWrapper
-	if !errors.As(err, &errWrapper) {
-		t.Fatal("cannot cast to ErrWrapper")
-	}
-	if errWrapper.Operation != op {
-		t.Fatal("unexpected Operation")
-	}
-	if errWrapper.Failure != FailureEOFError {
-		t.Fatal("unexpected failure")
-	}
 }
 
 func TestErrorWrapperDialerSuccess(t *testing.T) {
+	origConn := &net.TCPConn{}
 	ctx := context.Background()
 	d := &ErrorWrapperDialer{Dialer: &netxmocks.Dialer{
 		MockDialContext: func(ctx context.Context, network string, address string) (net.Conn, error) {
-			return &netxmocks.Conn{
-				MockRead: func(b []byte) (int, error) {
-					return 0, io.EOF
-				},
-				MockWrite: func(b []byte) (int, error) {
-					return 0, io.EOF
-				},
-				MockClose: func() error {
-					return io.EOF
-				},
-				MockLocalAddr: func() net.Addr {
-					return &net.TCPAddr{Port: 12345}
-				},
-			}, nil
+			return origConn, nil
 		},
 	}}
-	conn, err := d.DialContext(ctx, "tcp", "www.google.com")
+	conn, err := d.DialContext(ctx, "tcp", "www.google.com:443")
 	if err != nil {
 		t.Fatal(err)
 	}
-	if conn == nil {
-		t.Fatal("expected non-nil conn here")
+	ewConn, ok := conn.(*errorWrapperConn)
+	if !ok {
+		t.Fatal("cannot cast to errorWrapperConn")
+	}
+	if ewConn.Conn != origConn {
+		t.Fatal("not the connection we expected")
 	}
-	count, err := conn.Read(nil)
-	errorWrapperCheckIOResult(t, count, err, ReadOperation)
-	count, err = conn.Write(nil)
-	errorWrapperCheckIOResult(t, count, err, WriteOperation)
-	err = conn.Close()
-	errorWrapperCheckErr(t, err, CloseOperation)
 }
 
-func errorWrapperCheckIOResult(t *testing.T, count int, err error, op string) {
-	if count != 0 {
-		t.Fatal("expected nil count here")
+func TestErrorWrapperConnReadFailure(t *testing.T) {
+	c := &errorWrapperConn{
+		Conn: &netxmocks.Conn{
+			MockRead: func(b []byte) (int, error) {
+				return 0, io.EOF
+			},
+		},
+	}
+	buf := make([]byte, 1024)
+	cnt, err := c.Read(buf)
+	var ew *ErrWrapper
+	if !errors.As(err, &ew) {
+		t.Fatal("cannot cast error to ErrWrapper")
+	}
+	if ew.Operation != ReadOperation {
+		t.Fatal("invalid operation", ew.Operation)
+	}
+	if ew.Failure != FailureEOFError {
+		t.Fatal("invalid failure", ew.Failure)
+	}
+	if !errors.Is(ew.WrappedErr, io.EOF) {
+		t.Fatal("invalid wrapped error", ew.WrappedErr)
+	}
+	if cnt != 0 {
+		t.Fatal("expected zero here", cnt)
+	}
+}
+
+func TestErrorWrapperConnReadSuccess(t *testing.T) {
+	c := &errorWrapperConn{
+		Conn: &netxmocks.Conn{
+			MockRead: func(b []byte) (int, error) {
+				return len(b), nil
+			},
+		},
+	}
+	buf := make([]byte, 1024)
+	cnt, err := c.Read(buf)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if cnt != len(buf) {
+		t.Fatal("expected len(buf) here", cnt)
+	}
+}
+
+func TestErrorWrapperConnWriteFailure(t *testing.T) {
+	c := &errorWrapperConn{
+		Conn: &netxmocks.Conn{
+			MockWrite: func(b []byte) (int, error) {
+				return 0, io.EOF
+			},
+		},
+	}
+	buf := make([]byte, 1024)
+	cnt, err := c.Write(buf)
+	var ew *ErrWrapper
+	if !errors.As(err, &ew) {
+		t.Fatal("cannot cast error to ErrWrapper")
+	}
+	if ew.Operation != WriteOperation {
+		t.Fatal("invalid operation", ew.Operation)
+	}
+	if ew.Failure != FailureEOFError {
+		t.Fatal("invalid failure", ew.Failure)
+	}
+	if !errors.Is(ew.WrappedErr, io.EOF) {
+		t.Fatal("invalid wrapped error", ew.WrappedErr)
+	}
+	if cnt != 0 {
+		t.Fatal("expected zero here", cnt)
+	}
+}
+
+func TestErrorWrapperConnWriteSuccess(t *testing.T) {
+	c := &errorWrapperConn{
+		Conn: &netxmocks.Conn{
+			MockWrite: func(b []byte) (int, error) {
+				return len(b), nil
+			},
+		},
+	}
+	buf := make([]byte, 1024)
+	cnt, err := c.Write(buf)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if cnt != len(buf) {
+		t.Fatal("expected len(buf) here", cnt)
+	}
+}
+
+func TestErrorWrapperConnCloseFailure(t *testing.T) {
+	c := &errorWrapperConn{
+		Conn: &netxmocks.Conn{
+			MockClose: func() error {
+				return io.EOF
+			},
+		},
+	}
+	err := c.Close()
+	var ew *ErrWrapper
+	if !errors.As(err, &ew) {
+		t.Fatal("cannot cast error to ErrWrapper")
+	}
+	if ew.Operation != CloseOperation {
+		t.Fatal("invalid operation", ew.Operation)
+	}
+	if ew.Failure != FailureEOFError {
+		t.Fatal("invalid failure", ew.Failure)
+	}
+	if !errors.Is(ew.WrappedErr, io.EOF) {
+		t.Fatal("invalid wrapped error", ew.WrappedErr)
+	}
+}
+
+func TestErrorWrapperConnCloseSuccess(t *testing.T) {
+	c := &errorWrapperConn{
+		Conn: &netxmocks.Conn{
+			MockClose: func() error {
+				return nil
+			},
+		},
+	}
+	err := c.Close()
+	if err != nil {
+		t.Fatal(err)
 	}
-	errorWrapperCheckErr(t, err, op)
 }