We now return a Reader instead of the io.Reader interface

msg.go

@@ -807,24 +807,31 @@ func (m *Msg) WriteToSendmailWithContext(ctx context.Context, sp string, a ...st
 	return nil
 }
 
-// NewReader returns a Msg reader that satisfies the io.Reader interface.
-// **Please note:** when creating a new reader, the current state of the Msg is taken, as
-// basis for the reader. If you perform changes on Msg after creating the reader, you need
-// to perform a call to Msg.UpdateReader first
-func (m *Msg) NewReader() io.Reader {
+// NewReader returns a Reader type that satisfies the io.Reader interface.
+//
+// IMPORTANT: when creating a new Reader, the current state of the Msg is taken, as
+// basis for the Reader. If you perform changes on Msg after creating the Reader, these
+// changes will not be reflected in the Reader. You will have to use Msg.UpdateReader
+// first to update the Reader's buffer with the current Msg content
+func (m *Msg) NewReader() *Reader {
+	r := &Reader{}
 	wbuf := bytes.Buffer{}
-	_, _ = m.Write(&wbuf)
-	r := &reader{buf: wbuf.Bytes()}
+	_, err := m.Write(&wbuf)
+	if err != nil {
+		r.err = fmt.Errorf("failed to write Msg to Reader buffer: %w", err)
+	}
+	r.buf = wbuf.Bytes()
 	return r
 }
 
-// UpdateReader will update a reader with the content of the current Msg and reset the reader position
-// to the start
-func (m *Msg) UpdateReader(r *reader) {
+// UpdateReader will update a Reader with the content of the given Msg and reset the
+// Reader position to the start
+func (m *Msg) UpdateReader(r *Reader) {
 	wbuf := bytes.Buffer{}
-	_, _ = m.Write(&wbuf)
+	_, err := m.Write(&wbuf)
 	r.Reset()
 	r.buf = wbuf.Bytes()
+	r.err = err
 }
 
 // Read outputs the length of p into p to satisfy the io.Reader interface

msg_test.go

@@ -1784,6 +1784,86 @@ func TestMsg_Read_ioCopy(t *testing.T) {
 	}
 }
 
+// TestMsg_NewReader tests the Msg.NewReader method
+func TestMsg_NewReader(t *testing.T) {
+	m := NewMsg()
+	m.SetBodyString(TypeTextPlain, "TEST123")
+	mr := m.NewReader()
+	if mr == nil {
+		t.Errorf("NewReader failed: Reader is nil")
+	}
+}
+
+// TestMsg_NewReader_broken tests the Msg.NewReader method error handling
+func TestMsg_NewReader_broken(t *testing.T) {
+	m := &Msg{}
+	mr := m.NewReader()
+	if mr == nil {
+		t.Errorf("NewReader failed: Reader is nil")
+	}
+}
+
+// TestMsg_NewReader_ioCopy tests the Msg.NewReader method using io.Copy
+func TestMsg_NewReader_ioCopy(t *testing.T) {
+	wbuf1 := bytes.Buffer{}
+	wbuf2 := bytes.Buffer{}
+	m := NewMsg()
+	m.SetBodyString(TypeTextPlain, "TEST123")
+	mr := m.NewReader()
+	if mr == nil {
+		t.Errorf("NewReader failed: Reader is nil")
+	}
+
+	// First we use WriteTo to have something to compare to
+	_, err := m.WriteTo(&wbuf1)
+	if err != nil {
+		t.Errorf("failed to write body to buffer: %s", err)
+	}
+
+	// Then we write to wbuf2 via io.Copy
+	n, err := io.Copy(&wbuf2, mr)
+	if err != nil {
+		t.Errorf("failed to use io.Copy on Reader: %s", err)
+	}
+	if n != int64(wbuf1.Len()) {
+		t.Errorf("message length of WriteTo and io.Copy differ. Expected: %d, got: %d", wbuf1.Len(), n)
+	}
+	if wbuf1.String() != wbuf2.String() {
+		t.Errorf("message content of WriteTo and io.Copy differ")
+	}
+}
+
+// TestMsg_UpdateReader tests the Msg.UpdateReader method
+func TestMsg_UpdateReader(t *testing.T) {
+	m := NewMsg()
+	m.Subject("Subject-Run 1")
+	mr := m.NewReader()
+	if mr == nil {
+		t.Errorf("NewReader failed: Reader is nil")
+	}
+	wbuf1 := bytes.Buffer{}
+	_, err := io.Copy(&wbuf1, mr)
+	if err != nil {
+		t.Errorf("io.Copy on Reader failed: %s", err)
+	}
+	if !strings.Contains(wbuf1.String(), "Subject: Subject-Run 1") {
+		t.Errorf("io.Copy on Reader failed. Expected to find %q but string in Subject was not found",
+			"Subject-Run 1")
+	}
+
+	m.Subject("Subject-Run 2")
+	m.UpdateReader(mr)
+	wbuf2 := bytes.Buffer{}
+	_, err = io.Copy(&wbuf2, mr)
+	if err != nil {
+		t.Errorf("2nd io.Copy on Reader failed: %s", err)
+	}
+	if !strings.Contains(wbuf2.String(), "Subject: Subject-Run 2") {
+		t.Errorf("io.Copy on Reader failed. Expected to find %q but string in Subject was not found",
+			"Subject-Run 2")
+	}
+}
+
 // TestMsg_SetBodyTextTemplate tests the Msg.SetBodyTextTemplate method
 func TestMsg_SetBodyTextTemplate(t *testing.T) {
 	tests := []struct {

reader.go

@@ -4,16 +4,22 @@
 
 package mail
 
-import "io"
+import (
+	"io"
+)
 
-// reader is a type that implements the io.Reader interface for a Msg
-type reader struct {
+// Reader is a type that implements the io.Reader interface for a Msg
+type Reader struct {
 	buf []byte // contents are the bytes buf[off : len(buf)]
 	off int    // read at &buf[off], write at &buf[len(buf)]
+	err error  // initalization error
 }
 
 // Read reads the length of p of the Msg buffer to satisfy the io.Reader interface
-func (r *reader) Read(p []byte) (n int, err error) {
+func (r *Reader) Read(p []byte) (n int, err error) {
+	if r.err != nil {
+		return 0, r.err
+	}
 	if r.empty() {
 		r.Reset()
 		if len(p) == 0 {
@@ -23,15 +29,15 @@ func (r *reader) Read(p []byte) (n int, err error) {
 	}
 	n = copy(p, r.buf[r.off:])
 	r.off += n
-	return n, nil
+	return n, err
 }
 
 // Reset resets the Reader buffer to be empty, but it retains the underlying storage
 // for use by future writes.
-func (r *reader) Reset() {
+func (r *Reader) Reset() {
 	r.buf = r.buf[:0]
 	r.off = 0
 }
 
 // empty reports whether the unread portion of the Reader buffer is empty.
-func (r *reader) empty() bool { return len(r.buf) <= r.off }
+func (r *Reader) empty() bool { return len(r.buf) <= r.off }