Adding upstream version 3.6.2.

Signed-off-by: Daniel Baumann <daniel@debian.org>

sender.go

@@ -0,0 +1,108 @@
+package mailyak
+
+import (
+	"bufio"
+	"crypto/tls"
+	"io"
+	"net"
+	"net/smtp"
+)
+
+// emailSender abstracts the connection and protocol conversation required to
+// send an email with a remote SMTP server.
+type emailSender interface {
+	Send(m sendableMail) error
+}
+
+// sendableMail provides a set of methods to describe an email to a SMTP server.
+type sendableMail interface {
+	// getLocalName should return the sender domain to be used in the EHLO/HELO
+	// command.
+	getLocalName() string
+
+	// getToAddrs should return a slice of email addresses to be added to the
+	// RCPT TO command.
+	getToAddrs() []string
+
+	// getFromAddr should return the address to be used in the MAIL FROM
+	// command.
+	getFromAddr() string
+
+	// getAuth should return the smtp.Auth if configured, nil if not.
+	getAuth() smtp.Auth
+
+	// buildMime should write the generated MIME to w.
+	//
+	// The emailSender implementation is responsible for providing appropriate
+	// buffering of writes.
+	buildMime(w io.Writer) error
+}
+
+// smtpExchange performs the SMTP protocol conversation necessary to send m over
+// conn.
+//
+// serverName must be the hostname (or IP address) of the remote endpoint.
+func smtpExchange(m sendableMail, conn net.Conn, serverName string, tryTLSUpgrade bool) error {
+	// Connect to the SMTP server
+	c, err := smtp.NewClient(conn, serverName)
+	if err != nil {
+		return err
+	}
+	defer func() { _ = c.Quit() }()
+
+	if localName := m.getLocalName(); localName != "" {
+		if err := c.Hello(localName); err != nil {
+			return err
+		}
+	}
+
+	if tryTLSUpgrade {
+		if ok, _ := c.Extension("STARTTLS"); ok {
+			//nolint:gosec
+			config := &tls.Config{
+				ServerName: serverName,
+			}
+			if err = c.StartTLS(config); err != nil {
+				return err
+			}
+		}
+	}
+
+	// Attempt to authenticate if credentials were provided
+	var nilAuth smtp.Auth
+	if auth := m.getAuth(); auth != nilAuth {
+		if err = c.Auth(auth); err != nil {
+			return err
+		}
+	}
+
+	// Set the from address
+	if err = c.Mail(m.getFromAddr()); err != nil {
+		return err
+	}
+
+	// Add all the recipients
+	for _, to := range m.getToAddrs() {
+		if err = c.Rcpt(to); err != nil {
+			return err
+		}
+	}
+
+	// Start the data session and write the email body
+	dataSession, err := c.Data()
+	if err != nil {
+		return err
+	}
+
+	// Wrap the socket in a small buffer (~4k) to avoid making lots of small
+	// syscalls and therefore reducing CPU usage.
+	buf := bufio.NewWriter(dataSession)
+	if err := m.buildMime(buf); err != nil {
+		return err
+	}
+	if err := buf.Flush(); err != nil {
+		return err
+	}
+
+	return dataSession.Close()
+}