emailsender/pkg/email/transport.go

package email

import (
	"bytes"
	"context"
	"crypto/tls"
	"errors"
	"fmt"
	"io"
	"net"
	"net/smtp"
	"time"
)

// SendRawContext sends caller-built RFC/MIME bytes without rewriting headers,
// boundaries, or Message-ID. The supplied context drives dialing, TLS
// handshakes, and cancellation of blocked SMTP I/O.
func (service *EmailService) SendRawContext(ctx context.Context, message RawMessage) error {
	if service == nil {
		return permanentError("validate configuration", errors.New("email service is nil"))
	}
	if ctx == nil {
		return permanentError("validate context", errors.New("context is nil"))
	}
	if err := ctx.Err(); err != nil {
		return classifyError(ctx, "begin SMTP delivery", err, ErrorKindCanceled)
	}
	if service.configurationErr != nil {
		return service.configurationErr
	}
	if service.sendRawHook != nil {
		return service.sendRawHook(ctx, message)
	}

	envelope, err := service.resolveRawMessage(message)
	if err != nil {
		return permanentError("validate raw message", err)
	}
	address := net.JoinHostPort(service.host, service.port)
	rawConnection, err := service.dialContext(ctx, "tcp", address)
	if err != nil {
		return classifyError(ctx, "dial SMTP", err, ErrorKindTransient)
	}
	defer rawConnection.Close()

	// Closing the actual socket is what interrupts an SMTP command blocked in
	// net/textproto. No detached send goroutine is used.
	stopCancellation := context.AfterFunc(ctx, func() {
		_ = rawConnection.SetDeadline(time.Now())
		_ = rawConnection.Close()
	})
	defer stopCancellation()

	connection := rawConnection
	if service.tlsMode == TLSModeImplicit {
		if err := service.setDeadline(connection, ctx); err != nil {
			return classifyError(ctx, "set implicit TLS deadline", err, ErrorKindTransient)
		}
		secureConnection := tls.Client(connection, service.tlsConfig())
		if err := secureConnection.HandshakeContext(ctx); err != nil {
			return classifyError(ctx, "implicit TLS handshake", err, ErrorKindTransient)
		}
		connection = secureConnection
	}

	if err := service.setDeadline(connection, ctx); err != nil {
		return classifyError(ctx, "set SMTP greeting deadline", err, ErrorKindTransient)
	}
	client, err := smtp.NewClient(connection, service.host)
	if err != nil {
		return classifyError(ctx, "read SMTP greeting", err, ErrorKindTransient)
	}
	defer client.Close()

	if err := service.hello(ctx, connection, client); err != nil {
		return err
	}
	if service.tlsMode == TLSModeSTARTTLS {
		if supported, _ := client.Extension("STARTTLS"); !supported {
			return permanentError("start TLS", errors.New("SMTP server does not advertise STARTTLS"))
		}
		if err := service.setDeadline(connection, ctx); err != nil {
			return classifyError(ctx, "set STARTTLS deadline", err, ErrorKindTransient)
		}
		if err := client.StartTLS(service.tlsConfig()); err != nil {
			return classifyError(ctx, "start TLS", err, ErrorKindTransient)
		}
	}

	if service.auth != nil {
		if service.tlsMode == TLSModeNone {
			return permanentError("authenticate SMTP", errors.New("authentication requires TLS"))
		}
		if err := service.setDeadline(connection, ctx); err != nil {
			return classifyError(ctx, "set SMTP authentication deadline", err, ErrorKindTransient)
		}
		if err := client.Auth(service.auth); err != nil {
			return classifyError(ctx, "authenticate SMTP", err, ErrorKindPermanent)
		}
	}

	if err := service.smtpCommand(ctx, connection, "MAIL FROM", func() error {
		return client.Mail(envelope.from)
	}); err != nil {
		return err
	}
	for _, recipient := range envelope.recipients {
		if err := service.smtpCommand(ctx, connection, "RCPT TO", func() error {
			return client.Rcpt(recipient)
		}); err != nil {
			return err
		}
	}

	if err := service.setDeadline(connection, ctx); err != nil {
		return classifyError(ctx, "set SMTP DATA deadline", err, ErrorKindTransient)
	}
	writer, err := client.Data()
	if err != nil {
		return classifyError(ctx, "begin SMTP DATA", err, ErrorKindPermanent)
	}
	if err := service.setDeadline(connection, ctx); err != nil {
		return classifyError(ctx, "set message write deadline", err, ErrorKindTransient)
	}
	written, err := io.Copy(writer, bytes.NewReader(envelope.data))
	if err != nil {
		return classifyError(ctx, "write SMTP DATA", err, ErrorKindTransient)
	}
	if written != int64(len(envelope.data)) {
		return classifyError(ctx, "write SMTP DATA", io.ErrShortWrite, ErrorKindTransient)
	}
	if err := service.setDeadline(connection, ctx); err != nil {
		return classifyError(ctx, "set SMTP acknowledgement deadline", err, ErrorKindTransient)
	}
	if err := writer.Close(); err != nil {
		// The server may already have accepted DATA even when its final response
		// is lost. Retrying can therefore duplicate delivery.
		return classifyError(ctx, "finish SMTP DATA", err, ErrorKindTransient)
	}

	if err := service.setDeadline(connection, ctx); err != nil {
		return classifyError(ctx, "set SMTP quit deadline", err, ErrorKindTransient)
	}
	if err := client.Quit(); err != nil {
		return classifyError(ctx, "quit SMTP", err, ErrorKindTransient)
	}
	return nil
}

type resolvedRawMessage struct {
	from       string
	recipients []string
	data       []byte
}

func (service *EmailService) resolveRawMessage(message RawMessage) (resolvedRawMessage, error) {
	if len(message.EnvelopeRecipients) > 0 && message.To != "" {
		return resolvedRawMessage{}, errors.New("canonical and deprecated recipient fields cannot both be set")
	}
	if len(message.Data) > 0 && message.Body != "" {
		return resolvedRawMessage{}, errors.New("canonical and deprecated message fields cannot both be set")
	}

	from := message.EnvelopeFrom
	if from == "" {
		from = service.from
	}
	recipients := append([]string(nil), message.EnvelopeRecipients...)
	if len(recipients) == 0 && message.To != "" {
		recipients = []string{message.To}
	}
	data := append([]byte(nil), message.Data...)
	if len(data) == 0 && message.Body != "" {
		data = []byte(message.Body)
	}

	if err := validateEnvelopeAddress(from); err != nil {
		return resolvedRawMessage{}, fmt.Errorf("envelope sender: %w", err)
	}
	if len(recipients) == 0 || len(recipients) > maximumEnvelopeRecipients {
		return resolvedRawMessage{}, errors.New("envelope recipient count is invalid")
	}
	for _, recipient := range recipients {
		if err := validateEnvelopeAddress(recipient); err != nil {
			return resolvedRawMessage{}, fmt.Errorf("envelope recipient: %w", err)
		}
	}
	if len(data) == 0 {
		return resolvedRawMessage{}, errors.New("raw RFC message is empty")
	}
	return resolvedRawMessage{from: from, recipients: recipients, data: data}, nil
}

func (service *EmailService) hello(ctx context.Context, connection net.Conn, client *smtp.Client) error {
	if err := service.setDeadline(connection, ctx); err != nil {
		return classifyError(ctx, "set SMTP hello deadline", err, ErrorKindTransient)
	}
	if err := client.Hello("localhost"); err != nil {
		return classifyError(ctx, "SMTP hello", err, ErrorKindTransient)
	}
	return nil
}

func (service *EmailService) smtpCommand(ctx context.Context, connection net.Conn, operation string, command func() error) error {
	if err := service.setDeadline(connection, ctx); err != nil {
		return classifyError(ctx, "set "+operation+" deadline", err, ErrorKindTransient)
	}
	if err := command(); err != nil {
		return classifyError(ctx, operation, err, ErrorKindPermanent)
	}
	return nil
}

func (service *EmailService) setDeadline(connection net.Conn, ctx context.Context) error {
	deadline := time.Now().Add(service.operationTimeout)
	if contextDeadline, ok := ctx.Deadline(); ok && contextDeadline.Before(deadline) {
		deadline = contextDeadline
	}
	return connection.SetDeadline(deadline)
}

func (service *EmailService) tlsConfig() *tls.Config {
	return &tls.Config{
		MinVersion: tls.VersionTLS12,
		ServerName: service.host,
		RootCAs:    service.rootCAs,
	}
}