WriteToSendmailWithContext tests revealed a race condition which has been fixed

msg.go

@@ -1,7 +1,6 @@
 package mail
 
 import (
-	"bufio"
 	"bytes"
 	"context"
 	"errors"
@@ -29,18 +28,24 @@ type Msg struct {
 	// addrHeader is a slice of strings that the different mail AddrHeader fields
 	addrHeader map[AddrHeader][]*mail.Address
 
+	// attachments represent the different attachment File of the Msg
+	attachments []*File
+
 	// boundary is the MIME content boundary
 	boundary string
 
 	// charset represents the charset of the mail (defaults to UTF-8)
 	charset Charset
 
-	// encoding represents the message encoding (the encoder will be a corresponding WordEncoder)
+	// embeds represent the different embedded File of the Msg
-	encoding Encoding
+	embeds []*File
 
 	// encoder represents a mime.WordEncoder from the std lib
 	encoder mime.WordEncoder
 
+	// encoding represents the message encoding (the encoder will be a corresponding WordEncoder)
+	encoding Encoding
+
 	// genHeader is a slice of strings that the different generic mail Header fields
 	genHeader map[Header][]string
 
@@ -49,12 +54,6 @@ type Msg struct {
 
 	// parts represent the different parts of the Msg
 	parts []*Part
-
-	// attachments represent the different attachment File of the Msg
-	attachments []*File
-
-	// embeds represent the different embedded File of the Msg
-	embeds []*File
 }
 
 // SendmailPath is the default system path to the sendmail binary
@@ -511,14 +510,21 @@ func (m *Msg) WriteToSendmailWithContext(ctx context.Context, sp string, a ...st
 	ec.Args = append(ec.Args, "-oi", "-t")
 	ec.Args = append(ec.Args, a...)
 
-	var ebuf bytes.Buffer
+	se, err := ec.StderrPipe()
-	ec.Stderr = &ebuf
+	if err != nil {
-	ses := bufio.NewScanner(&ebuf)
+		return fmt.Errorf("failed to set STDERR pipe: %w", err)
+	}
+	defer func() {
+		_ = se.Close()
+	}()
 
 	si, err := ec.StdinPipe()
 	if err != nil {
 		return fmt.Errorf("failed to set STDIN pipe: %w", err)
 	}
+	defer func() {
+		_ = si.Close()
+	}()
 
 	// Start the execution and write to STDIN
 	if err := ec.Start(); err != nil {
@@ -526,23 +532,17 @@ func (m *Msg) WriteToSendmailWithContext(ctx context.Context, sp string, a ...st
 	}
 	_, err = m.Write(si)
 	if err != nil {
-		_ = si.Close()
 		return fmt.Errorf("failed to write mail to buffer: %w", err)
 	}
-	if err := si.Close(); err != nil {
-		return fmt.Errorf("failed to close STDIN pipe: %w", err)
-	}
 
-	// Read the stderr buffer for possible errors
+	// Read the stderr pipe for possible errors
-	var serr string
+	var serr []byte
-	for ses.Scan() {
+	en, err := se.Read(serr)
-		serr += ses.Text()
+	if err != nil {
-	}
-	if err := ses.Err(); err != nil {
 		return fmt.Errorf("failed to read STDERR pipe: %w", err)
 	}
-	if serr != "" {
+	if en > 0 {
-		return fmt.Errorf("sendmail execution failed: %s", serr)
+		return fmt.Errorf("sendmail command failed: %s", serr)
 	}
 
 	// Wait for completion or cancellation of the sendmail executable

msg_test.go

@@ -1137,5 +1137,28 @@ func TestMsg_Write(t *testing.T) {
 		t.Errorf("Write() failed: expected written byte length: %d, got: %d", n, wbuf.Len())
 		fmt.Printf("%s", wbuf.String())
 	}
-
+}
+
+// TestMsg_WriteToSendmailWithCommand tests the WriteToSendmailWithCommand() method of the Msg
+func TestMsg_WriteToSendmailWithCommand(t *testing.T) {
+	tests := []struct {
+		name string
+		sp   string
+		sf   bool
+	}{
+		{"Sendmail path: /dev/null", "/dev/null", true},
+		{"Sendmail path: /bin/cat", "/bin/cat", true},
+		{"Sendmail path: /is/invalid", "/is/invalid", true},
+		{"Sendmail path: /usr/bin/true", "/usr/bin/true", false},
+	}
+	m := NewMsg()
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			m.SetBodyString(TypeTextPlain, "Plain")
+			if err := m.WriteToSendmailWithCommand(tt.sp); err != nil && !tt.sf {
+				t.Errorf("WriteToSendmailWithCommand() failed: %s", err)
+			}
+			m.Reset()
+		})
+	}
 }