#81: Fix error handling in body writer

The error handling in the msgWriter.writeBody() method was not working properly. We basically overwrote the mw.err with nil if the function that followed after a failed write attempt was successful again

This patch fixes #81

msg_test.go

@@ -8,6 +8,7 @@ import (
 	"bufio"
 	"bytes"
 	"embed"
+	"errors"
 	"fmt"
 	htpl "html/template"
 	"io"
@@ -1652,7 +1653,7 @@ func TestMsg_WriteTo(t *testing.T) {
 	}
 }
 
-// TestMsg_WriteTo tests the WriteTo() method of the Msg
+// TestMsg_WriteToSkipMiddleware tests the WriteTo() method of the Msg
 func TestMsg_WriteToSkipMiddleware(t *testing.T) {
 	m := NewMsg(WithMiddleware(encodeMiddleware{}), WithMiddleware(uppercaseMiddleware{}))
 	m.Subject("This is a test")
@@ -1684,6 +1685,30 @@ func TestMsg_WriteToSkipMiddleware(t *testing.T) {
 	}
 }
 
+// TestMsg_WriteTo_fails tests the WriteTo() method of the Msg but with a failing body writer function
+func TestMsg_WriteTo_fails(t *testing.T) {
+	m := NewMsg()
+	m.SetBodyWriter(TypeTextPlain, func(io.Writer) (int64, error) {
+		return 0, errors.New("failed")
+	})
+	_, err := m.WriteTo(io.Discard)
+	if err == nil {
+		t.Errorf("WriteTo() with failing BodyWriter function was supposed to fail, but didn't")
+		return
+	}
+
+	// NoEncoding handles the errors separately
+	m = NewMsg(WithEncoding(NoEncoding))
+	m.SetBodyWriter(TypeTextPlain, func(io.Writer) (int64, error) {
+		return 0, errors.New("failed")
+	})
+	_, err = m.WriteTo(io.Discard)
+	if err == nil {
+		t.Errorf("WriteTo() (no encoding) with failing BodyWriter function was supposed to fail, but didn't")
+		return
+	}
+}
+
 // TestMsg_Write tests the Write() method of the Msg
 func TestMsg_Write(t *testing.T) {
 	m := NewMsg()

msgwriter.go

@@ -282,6 +282,7 @@ func (mw *msgWriter) writeBody(f func(io.Writer) (int64, error), e Encoding) {
 	var w io.Writer
 	var ew io.WriteCloser
 	var n int64
+	var err error
 	if mw.d == 0 {
 		w = mw.w
 	}
@@ -298,8 +299,14 @@ func (mw *msgWriter) writeBody(f func(io.Writer) (int64, error), e Encoding) {
 	case EncodingB64:
 		ew = base64.NewEncoder(base64.StdEncoding, &lb)
 	case NoEncoding:
-		_, mw.err = f(&wbuf)
-		n, mw.err = io.Copy(w, &wbuf)
+		_, err = f(&wbuf)
+		if err != nil {
+			mw.err = fmt.Errorf("bodyWriter function: %w", err)
+		}
+		n, err = io.Copy(w, &wbuf)
+		if err != nil && mw.err == nil {
+			mw.err = fmt.Errorf("bodyWriter io.Copy: %w", err)
+		}
 		if mw.d == 0 {
 			mw.n += n
 		}
@@ -308,10 +315,22 @@ func (mw *msgWriter) writeBody(f func(io.Writer) (int64, error), e Encoding) {
 		ew = quotedprintable.NewWriter(w)
 	}
 
-	_, mw.err = f(ew)
-	mw.err = ew.Close()
-	mw.err = lb.Close()
-	n, mw.err = io.Copy(w, &wbuf)
+	_, err = f(ew)
+	if err != nil {
+		mw.err = fmt.Errorf("bodyWriter function: %w", err)
+	}
+	err = ew.Close()
+	if err != nil && mw.err == nil {
+		mw.err = fmt.Errorf("bodyWriter close encoded writer: %w", err)
+	}
+	err = lb.Close()
+	if err != nil && mw.err == nil {
+		mw.err = fmt.Errorf("bodyWriter close linebreaker: %w", err)
+	}
+	n, err = io.Copy(w, &wbuf)
+	if err != nil && mw.err == nil {
+		mw.err = fmt.Errorf("bodyWriter io.Copy: %w", err)
+	}
 
 	// Since the part writer uses the WriteTo() method, we don't need to add the
 	// bytes twice