Implement Logger interface

As stated in #102 (comment) it would be beneficial if, instead of forcing the Go stdlib logger on the user to provide a simple interface and use that for logging purposes.

This PR implements this simple log.Logger interface as well as a standard logger that satisfies this interface. If no custom logger is provided, the Stdlog will be used (which makes use of the Go stdlib again).

Accordingly, a `Client.WithLogger` and `Client.SetLogger` have been implemented. Same applies for the smtp counterparts.

client.go

@@ -14,6 +14,7 @@ import (
 	"strings"
 	"time"
 
+	"github.com/wneessen/go-mail/log"
 	"github.com/wneessen/go-mail/smtp"
 )
 
@@ -133,6 +134,9 @@ type Client struct {
 
 	// dl enables the debug logging on the SMTP client
 	dl bool
+
+	// l is a logger that implements the log.Logger interface
+	l log.Logger
 }
 
 // Option returns a function that can be used for grouping Client options
@@ -252,6 +256,14 @@ func WithDebugLog() Option {
 	}
 }
 
+// WithLogger overrides the default log.Logger that is used for debug logging
+func WithLogger(l log.Logger) Option {
+	return func(c *Client) error {
+		c.l = l
+		return nil
+	}
+}
+
 // WithHELO tells the client to use the provided string as HELO/EHLO greeting host
 func WithHELO(h string) Option {
 	return func(c *Client) error {
@@ -417,6 +429,14 @@ func (c *Client) SetDebugLog(v bool) {
 	}
 }
 
+// SetLogger tells the Client which log.Logger to use
+func (c *Client) SetLogger(l log.Logger) {
+	c.l = l
+	if c.sc != nil {
+		c.sc.SetLogger(l)
+	}
+}
+
 // SetTLSConfig overrides the current *tls.Config with the given *tls.Config value
 func (c *Client) SetTLSConfig(co *tls.Config) error {
 	if co == nil {
@@ -481,6 +501,9 @@ func (c *Client) DialWithContext(pc context.Context) error {
 	if err != nil {
 		return err
 	}
+	if c.l != nil {
+		c.sc.SetLogger(c.l)
+	}
 	if c.dl {
 		c.sc.SetDebugLog(true)
 	}

client_test.go

@@ -15,6 +15,7 @@ import (
 	"testing"
 	"time"
 
+	"github.com/wneessen/go-mail/log"
 	"github.com/wneessen/go-mail/smtp"
 )
 
@@ -106,6 +107,7 @@ func TestNewClientWithOptions(t *testing.T) {
 		{"WithDSNRcptNotifyType() wrong option", WithDSNRcptNotifyType("FAIL"), true},
 		{"WithoutNoop()", WithoutNoop(), false},
 		{"WithDebugLog()", WithDebugLog(), false},
+		{"WithLogger()", WithLogger(log.New(os.Stderr, log.LevelDebug)), false},
 
 		{
 			"WithDSNRcptNotifyType() NEVER combination",
@@ -567,6 +569,31 @@ func TestClient_DialWithContext_Debug(t *testing.T) {
 	}
 }
 
+// TestClient_DialWithContext_Debug_custom tests the DialWithContext method for the Client
+// object with debug logging enabled and a custom logger on the SMTP client
+func TestClient_DialWithContext_Debug_custom(t *testing.T) {
+	c, err := getTestClient(true)
+	if err != nil {
+		t.Skipf("failed to create test client: %s. Skipping tests", err)
+	}
+	ctx := context.Background()
+	if err := c.DialWithContext(ctx); err != nil {
+		t.Errorf("failed to dial with context: %s", err)
+		return
+	}
+	if c.co == nil {
+		t.Errorf("DialWithContext didn't fail but no connection found.")
+	}
+	if c.sc == nil {
+		t.Errorf("DialWithContext didn't fail but no SMTP client found.")
+	}
+	c.SetDebugLog(true)
+	c.SetLogger(log.New(os.Stderr, log.LevelDebug))
+	if err := c.Close(); err != nil {
+		t.Errorf("failed to close connection: %s", err)
+	}
+}
+
 // TestClient_DialWithContextInvalidHost tests the DialWithContext method with intentional breaking
 // for the Client object
 func TestClient_DialWithContextInvalidHost(t *testing.T) {

log/log.go

@@ -0,0 +1,14 @@
+// SPDX-FileCopyrightText: Copyright (c) 2022-2023 The go-mail Authors
+//
+// SPDX-License-Identifier: MIT
+
+// Package log implements a logger interface that can be used within the go-mail package
+package log
+
+// Logger is the log interface for go-mail
+type Logger interface {
+	Errorf(format string, v ...interface{})
+	Warnf(format string, v ...interface{})
+	Infof(format string, v ...interface{})
+	Debugf(format string, v ...interface{})
+}

log/stdlog.go

@@ -0,0 +1,74 @@
+// SPDX-FileCopyrightText: Copyright (c) 2023 The go-mail Authors
+//
+// SPDX-License-Identifier: MIT
+
+package log
+
+import (
+	"fmt"
+	"io"
+	"log"
+)
+
+// Level is a type wrapper for an int
+type Level int
+
+// Stdlog is the default logger that satisfies the Logger interface
+type Stdlog struct {
+	l     Level
+	err   *log.Logger
+	warn  *log.Logger
+	info  *log.Logger
+	debug *log.Logger
+}
+
+const (
+	// LevelError is the Level for only ERROR log messages
+	LevelError Level = iota
+	// LevelWarn is the Level for WARN and higher log messages
+	LevelWarn
+	// LevelInfo is the Level for INFO and higher log messages
+	LevelInfo
+	// LevelDebug is the Level for DEBUG and higher log messages
+	LevelDebug
+)
+
+// New returns a new Stdlog type that satisfies the Logger interface
+func New(o io.Writer, l Level) *Stdlog {
+	lf := log.Lmsgprefix | log.LstdFlags
+	return &Stdlog{
+		l:     l,
+		err:   log.New(o, "ERROR: ", lf),
+		warn:  log.New(o, " WARN: ", lf),
+		info:  log.New(o, " INFO: ", lf),
+		debug: log.New(o, "DEBUG: ", lf),
+	}
+}
+
+// Debugf performs a Printf() on the debug logger
+func (l *Stdlog) Debugf(f string, v ...interface{}) {
+	if l.l >= LevelDebug {
+		_ = l.debug.Output(2, fmt.Sprintf(f, v...))
+	}
+}
+
+// Infof performs a Printf() on the info logger
+func (l *Stdlog) Infof(f string, v ...interface{}) {
+	if l.l >= LevelInfo {
+		_ = l.info.Output(2, fmt.Sprintf(f, v...))
+	}
+}
+
+// Warnf performs a Printf() on the warn logger
+func (l *Stdlog) Warnf(f string, v ...interface{}) {
+	if l.l >= LevelWarn {
+		_ = l.warn.Output(2, fmt.Sprintf(f, v...))
+	}
+}
+
+// Errorf performs a Printf() on the error logger
+func (l *Stdlog) Errorf(f string, v ...interface{}) {
+	if l.l >= LevelError {
+		_ = l.err.Output(2, fmt.Sprintf(f, v...))
+	}
+}

log/stdlog_test.go

@@ -0,0 +1,89 @@
+package log
+
+import (
+	"bytes"
+	"strings"
+	"testing"
+)
+
+func TestNew(t *testing.T) {
+	var b bytes.Buffer
+	l := New(&b, LevelDebug)
+	if l.l != LevelDebug {
+		t.Error("Expected level to be LevelDebug, got ", l.l)
+	}
+	if l.err == nil || l.warn == nil || l.info == nil || l.debug == nil {
+		t.Error("Loggers not initialized")
+	}
+}
+
+func TestDebugf(t *testing.T) {
+	var b bytes.Buffer
+	l := New(&b, LevelDebug)
+
+	l.Debugf("test %s", "foo")
+	expected := "DEBUG: test foo\n"
+	if !strings.HasSuffix(b.String(), expected) {
+		t.Errorf("Expected %q, got %q", expected, b.String())
+	}
+
+	b.Reset()
+	l.l = LevelInfo
+	l.Debugf("test %s", "foo")
+	if b.String() != "" {
+		t.Error("Debug message was not expected to be logged")
+	}
+}
+
+func TestInfof(t *testing.T) {
+	var b bytes.Buffer
+	l := New(&b, LevelInfo)
+
+	l.Infof("test %s", "foo")
+	expected := " INFO: test foo\n"
+	if !strings.HasSuffix(b.String(), expected) {
+		t.Errorf("Expected %q, got %q", expected, b.String())
+	}
+
+	b.Reset()
+	l.l = LevelWarn
+	l.Infof("test %s", "foo")
+	if b.String() != "" {
+		t.Error("Info message was not expected to be logged")
+	}
+}
+
+func TestWarnf(t *testing.T) {
+	var b bytes.Buffer
+	l := New(&b, LevelWarn)
+
+	l.Warnf("test %s", "foo")
+	expected := " WARN: test foo\n"
+	if !strings.HasSuffix(b.String(), expected) {
+		t.Errorf("Expected %q, got %q", expected, b.String())
+	}
+
+	b.Reset()
+	l.l = LevelError
+	l.Warnf("test %s", "foo")
+	if b.String() != "" {
+		t.Error("Warn message was not expected to be logged")
+	}
+}
+
+func TestErrorf(t *testing.T) {
+	var b bytes.Buffer
+	l := New(&b, LevelError)
+
+	l.Errorf("test %s", "foo")
+	expected := "ERROR: test foo\n"
+	if !strings.HasSuffix(b.String(), expected) {
+		t.Errorf("Expected %q, got %q", expected, b.String())
+	}
+	b.Reset()
+	l.l = LevelError - 1
+	l.Warnf("test %s", "foo")
+	if b.String() != "" {
+		t.Error("Error message was not expected to be logged")
+	}
+}

smtp/smtp.go

@@ -26,11 +26,12 @@ import (
 	"errors"
 	"fmt"
 	"io"
-	"log"
 	"net"
 	"net/textproto"
 	"os"
 	"strings"
+
+	"github.com/wneessen/go-mail/log"
 )
 
 // A Client represents a client connection to an SMTP server.
@@ -52,8 +53,8 @@ type Client struct {
 	didHello   bool   // whether we've said HELO/EHLO
 	helloError error  // the error from the hello
 	// debug logging
-	debug  bool        // debug logging is enabled
-	logger *log.Logger // logger will be used for debug logging
+	debug  bool       // debug logging is enabled
+	logger log.Logger // logger will be used for debug logging
 	// DSN support
 	dsnmrtype string // dsnmrtype defines the mail return option in case DSN is enabled
 	dsnrntype string // dsnrntype defines the recipient notify option in case DSN is enabled
@@ -441,12 +442,23 @@ func (c *Client) Quit() error {
 func (c *Client) SetDebugLog(v bool) {
 	c.debug = v
 	if v {
-		c.logger = log.New(os.Stderr, "[DEBUG] ", log.LstdFlags|log.Lmsgprefix)
+		if c.logger == nil {
+			c.logger = log.New(os.Stderr, log.LevelDebug)
+		}
 		return
 	}
 	c.logger = nil
 }
 
+// SetLogger overrides the default log.Stdlog for the debug logging with a logger that
+// satisfies the log.Logger interface
+func (c *Client) SetLogger(l log.Logger) {
+	if l == nil {
+		return
+	}
+	c.logger = l
+}
+
 // SetDSNMailReturnOption sets the DSN mail return option for the Mail method
 func (c *Client) SetDSNMailReturnOption(d string) {
 	c.dsnmrtype = d
@@ -465,7 +477,7 @@ func (c *Client) debugLog(d logDirection, f string, a ...interface{}) {
 			p = "C --> S:"
 		}
 		fs := fmt.Sprintf("%s %s", p, f)
-		c.logger.Printf(fs, a...)
+		c.logger.Debugf(fs, a...)
 	}
 }
 

smtp/smtp_test.go

@@ -23,10 +23,13 @@ import (
 	"io"
 	"net"
 	"net/textproto"
+	"os"
 	"runtime"
 	"strings"
 	"testing"
 	"time"
+
+	"github.com/wneessen/go-mail/log"
 )
 
 type authTest struct {
@@ -661,6 +664,37 @@ func TestClient_SetDebugLog(t *testing.T) {
 	}
 }
 
+// TestClient_SetLogger tests the Client method with the Client.SetLogger method
+// to provide a custom logger
+func TestClient_SetLogger(t *testing.T) {
+	server := strings.Join(strings.Split(newClientServer, "\n"), "\r\n")
+
+	var cmdbuf strings.Builder
+	bcmdbuf := bufio.NewWriter(&cmdbuf)
+	out := func() string {
+		if err := bcmdbuf.Flush(); err != nil {
+			t.Errorf("failed to flush: %s", err)
+		}
+		return cmdbuf.String()
+	}
+	var fake faker
+	fake.ReadWriter = bufio.NewReadWriter(bufio.NewReader(strings.NewReader(server)), bcmdbuf)
+	c, err := NewClient(fake, "fake.host")
+	if err != nil {
+		t.Fatalf("NewClient: %v\n(after %v)", err, out())
+	}
+	defer func() {
+		_ = c.Close()
+	}()
+	c.SetLogger(log.New(os.Stderr, log.LevelDebug))
+	if c.logger == nil {
+		t.Errorf("Expected Logger to be set but received nil")
+	}
+	c.logger.Debugf("test")
+	c.SetLogger(nil)
+	c.logger.Debugf("test")
+}
+
 var newClientServer = `220 hello world
 250-mx.google.com at your service
 250-SIZE 35651584