Lock client connections and update deadline handling

Add mutex locking for client connections to ensure thread safety. Introduce `HasConnection` method to check active connections and `UpdateDeadline` method to handle timeout updates. Refactor connection handling in `checkConn` and `tls` methods accordingly.

client.go

@@ -12,6 +12,7 @@ import (
 	"net"
 	"os"
 	"strings"
+	"sync"
 	"time"
 
 	"github.com/wneessen/go-mail/log"
@@ -87,6 +88,7 @@ type DialContextFunc func(ctx context.Context, network, address string) (net.Con
 
 // Client is the SMTP client struct
 type Client struct {
+	mutex sync.RWMutex
 	// connection is the net.Conn that the smtp.Client is based on
 	connection net.Conn
 
@@ -589,6 +591,9 @@ func (c *Client) setDefaultHelo() error {
 
 // DialWithContext establishes a connection to the SMTP server with a given context.Context
 func (c *Client) DialWithContext(dialCtx context.Context) error {
+	c.mutex.Lock()
+	defer c.mutex.Unlock()
+
 	ctx, cancel := context.WithDeadline(dialCtx, time.Now().Add(c.connTimeout))
 	defer cancel()
 
@@ -602,17 +607,16 @@ func (c *Client) DialWithContext(dialCtx context.Context) error {
 			c.dialContextFunc = tlsDialer.DialContext
 		}
 	}
-	var err error
-	c.connection, err = c.dialContextFunc(ctx, "tcp", c.ServerAddr())
+	connection, err := c.dialContextFunc(ctx, "tcp", c.ServerAddr())
 	if err != nil && c.fallbackPort != 0 {
 		// TODO: should we somehow log or append the previous error?
-		c.connection, err = c.dialContextFunc(ctx, "tcp", c.serverFallbackAddr())
+		connection, err = c.dialContextFunc(ctx, "tcp", c.serverFallbackAddr())
 	}
 	if err != nil {
 		return err
 	}
 
-	client, err := smtp.NewClient(c.connection, c.host)
+	client, err := smtp.NewClient(connection, c.host)
 	if err != nil {
 		return err
 	}
@@ -691,7 +695,7 @@ func (c *Client) DialAndSendWithContext(ctx context.Context, messages ...*Msg) e
 // checkConn makes sure that a required server connection is available and extends the
 // connection deadline
 func (c *Client) checkConn() error {
-	if c.connection == nil {
+	if !c.smtpClient.HasConnection() {
 		return ErrNoActiveConnection
 	}
 
@@ -701,7 +705,7 @@ func (c *Client) checkConn() error {
 		}
 	}
 
-	if err := c.connection.SetDeadline(time.Now().Add(c.connTimeout)); err != nil {
+	if err := c.smtpClient.UpdateDeadline(c.connTimeout); err != nil {
 		return ErrDeadlineExtendFailed
 	}
 	return nil
@@ -715,7 +719,7 @@ func (c *Client) serverFallbackAddr() string {
 
 // tls tries to make sure that the STARTTLS requirements are satisfied
 func (c *Client) tls() error {
-	if c.connection == nil {
+	if !c.smtpClient.HasConnection() {
 		return ErrNoActiveConnection
 	}
 	if !c.useSSL && c.tlspolicy != NoTLS {

client_120.go

@@ -13,6 +13,8 @@ import (
 
 // Send sends out the mail message
 func (c *Client) Send(messages ...*Msg) (returnErr error) {
+	c.mutex.Lock()
+	defer c.mutex.Unlock()
 	if err := c.checkConn(); err != nil {
 		returnErr = &SendError{Reason: ErrConnCheck, errlist: []error{err}, isTemp: isTempError(err)}
 		return

smtp/smtp.go

@@ -30,6 +30,7 @@ import (
 	"net/textproto"
 	"os"
 	"strings"
+	"time"
 
 	"github.com/wneessen/go-mail/log"
 )
@@ -472,6 +473,19 @@ func (c *Client) SetDSNRcptNotifyOption(d string) {
 	c.dsnrntype = d
 }
 
+// HasConnection checks if the client has an active connection.
+// Returns true if the `conn` field is not nil, indicating an active connection.
+func (c *Client) HasConnection() bool {
+	return c.conn != nil
+}
+
+func (c *Client) UpdateDeadline(timeout time.Duration) error {
+	if err := c.conn.SetDeadline(time.Now().Add(timeout)); err != nil {
+		return fmt.Errorf("smtp: failed to update deadline: %w", err)
+	}
+	return nil
+}
+
 // debugLog checks if the debug flag is set and if so logs the provided message to
 // the log.Logger interface
 func (c *Client) debugLog(d log.Direction, f string, a ...interface{}) {