archivmail/internal/pop3/client.go

package pop3

import (
	"bufio"
	"bytes"
	"crypto/tls"
	"fmt"
	"net"
	"strconv"
	"strings"
	"time"
)

const dialTimeout = 30 * time.Second
const rwTimeout = 30 * time.Second

// Client is a minimal POP3 client implemented directly over net.Conn.
// It supports SSL, STARTTLS, and plaintext connections.
type Client struct {
	conn   net.Conn
	reader *bufio.Reader
}

// Dial connects to a POP3 server and reads the server greeting.
// tlsMode must be one of: "ssl", "starttls", "none".
func Dial(host string, port int, tlsMode string, skipVerify bool) (*Client, error) {
	addr := net.JoinHostPort(host, strconv.Itoa(port))
	tlsCfg := &tls.Config{
		ServerName:         host,
		InsecureSkipVerify: skipVerify, //nolint:gosec // user-controlled opt-in
	}

	var conn net.Conn
	var err error

	switch tlsMode {
	case "ssl":
		dialer := &tls.Dialer{
			NetDialer: &net.Dialer{Timeout: dialTimeout},
			Config:    tlsCfg,
		}
		conn, err = dialer.Dial("tcp", addr)
		if err != nil {
			return nil, fmt.Errorf("pop3 dial ssl: %w", err)
		}
	case "starttls":
		plain, err2 := net.DialTimeout("tcp", addr, dialTimeout)
		if err2 != nil {
			return nil, fmt.Errorf("pop3 dial starttls plain: %w", err2)
		}
		c := &Client{conn: plain, reader: bufio.NewReader(plain)}
		// Read server greeting before STLS
		if _, err2 := c.readLine(); err2 != nil {
			plain.Close()
			return nil, fmt.Errorf("pop3 starttls greeting: %w", err2)
		}
		if err2 := c.sendCmd("STLS"); err2 != nil {
			plain.Close()
			return nil, fmt.Errorf("pop3 starttls send: %w", err2)
		}
		if _, err2 := c.readLine(); err2 != nil {
			plain.Close()
			return nil, fmt.Errorf("pop3 starttls response: %w", err2)
		}
		tlsConn := tls.Client(plain, tlsCfg)
		if err2 := tlsConn.Handshake(); err2 != nil {
			tlsConn.Close()
			return nil, fmt.Errorf("pop3 starttls handshake: %w", err2)
		}
		// Return upgraded client — greeting already consumed
		return &Client{conn: tlsConn, reader: bufio.NewReader(tlsConn)}, nil
	default: // "none"
		conn, err = net.DialTimeout("tcp", addr, dialTimeout)
		if err != nil {
			return nil, fmt.Errorf("pop3 dial plain: %w", err)
		}
	}

	c := &Client{conn: conn, reader: bufio.NewReader(conn)}
	// Read and discard the server greeting (+OK ...)
	if _, err := c.readLine(); err != nil {
		conn.Close()
		return nil, fmt.Errorf("pop3 greeting: %w", err)
	}
	return c, nil
}

// Login authenticates with USER and PASS commands.
func (c *Client) Login(user, pass string) error {
	if err := c.sendCmd("USER " + user); err != nil {
		return fmt.Errorf("pop3 login user: %w", err)
	}
	if _, err := c.readLine(); err != nil {
		return fmt.Errorf("pop3 login user response: %w", err)
	}
	if err := c.sendCmd("PASS " + pass); err != nil {
		return fmt.Errorf("pop3 login pass: %w", err)
	}
	if _, err := c.readLine(); err != nil {
		return fmt.Errorf("pop3 login pass response: %w", err)
	}
	return nil
}

// Stat returns the message count and total mailbox size in bytes.
func (c *Client) Stat() (count, size int, err error) {
	if err := c.sendCmd("STAT"); err != nil {
		return 0, 0, fmt.Errorf("pop3 stat send: %w", err)
	}
	line, err := c.readLine()
	if err != nil {
		return 0, 0, fmt.Errorf("pop3 stat read: %w", err)
	}
	// Response: "+OK count size"
	parts := strings.Fields(line)
	if len(parts) < 2 {
		return 0, 0, fmt.Errorf("pop3 stat: unexpected response: %q", line)
	}
	count, err = strconv.Atoi(parts[0])
	if err != nil {
		return 0, 0, fmt.Errorf("pop3 stat count parse: %w", err)
	}
	if len(parts) >= 2 {
		size, _ = strconv.Atoi(parts[1])
	}
	return count, size, nil
}

// List returns the message numbers available on the server.
func (c *Client) List() ([]int, error) {
	if err := c.sendCmd("LIST"); err != nil {
		return nil, fmt.Errorf("pop3 list send: %w", err)
	}
	// Read status line
	if _, err := c.readLine(); err != nil {
		return nil, fmt.Errorf("pop3 list status: %w", err)
	}
	// Read multi-line response
	data, err := c.readMultiLine()
	if err != nil {
		return nil, fmt.Errorf("pop3 list multiline: %w", err)
	}
	var nums []int
	for _, line := range strings.Split(string(data), "\n") {
		line = strings.TrimSpace(line)
		if line == "" {
			continue
		}
		// Each line: "msgnum size"
		parts := strings.Fields(line)
		if len(parts) == 0 {
			continue
		}
		n, err := strconv.Atoi(parts[0])
		if err != nil {
			continue
		}
		nums = append(nums, n)
	}
	return nums, nil
}

// Retr retrieves a message by its number and returns the raw RFC 2822 bytes.
func (c *Client) Retr(num int) ([]byte, error) {
	if err := c.sendCmd(fmt.Sprintf("RETR %d", num)); err != nil {
		return nil, fmt.Errorf("pop3 retr send: %w", err)
	}
	// Read status line
	if _, err := c.readLine(); err != nil {
		return nil, fmt.Errorf("pop3 retr status: %w", err)
	}
	data, err := c.readMultiLine()
	if err != nil {
		return nil, fmt.Errorf("pop3 retr multiline: %w", err)
	}
	return data, nil
}

// Quit sends the QUIT command and waits for the server acknowledgement.
func (c *Client) Quit() error {
	if err := c.sendCmd("QUIT"); err != nil {
		return fmt.Errorf("pop3 quit send: %w", err)
	}
	_, err := c.readLine()
	return err
}

// Close closes the underlying network connection.
func (c *Client) Close() {
	c.conn.Close()
}

// sendCmd writes a POP3 command terminated with CRLF.
func (c *Client) sendCmd(cmd string) error {
	_ = c.conn.SetWriteDeadline(time.Now().Add(rwTimeout))
	_, err := fmt.Fprintf(c.conn, "%s\r\n", cmd)
	if err != nil {
		return fmt.Errorf("pop3 write: %w", err)
	}
	return nil
}

// readLine reads one response line from the server.
// It strips the CRLF, verifies the +OK/-ERR prefix, and returns the
// text after the status indicator (without the "+OK" / "-ERR" prefix).
// An error is returned if the server replies with -ERR.
func (c *Client) readLine() (string, error) {
	_ = c.conn.SetReadDeadline(time.Now().Add(rwTimeout))
	line, err := c.reader.ReadString('\n')
	if err != nil {
		return "", fmt.Errorf("pop3 read line: %w", err)
	}
	line = strings.TrimRight(line, "\r\n")
	if strings.HasPrefix(line, "-ERR") {
		return "", fmt.Errorf("pop3 server error: %s", strings.TrimPrefix(line, "-ERR "))
	}
	// Strip "+OK" prefix
	if strings.HasPrefix(line, "+OK") {
		return strings.TrimPrefix(strings.TrimPrefix(line, "+OK"), " "), nil
	}
	return line, nil
}

// readMultiLine reads a dot-stuffed multi-line POP3 response until the
// terminating ".\r\n" line. Dot-unstuffing is applied: lines beginning
// with ".." are returned with a single leading ".".
func (c *Client) readMultiLine() ([]byte, error) {
	var buf bytes.Buffer
	for {
		_ = c.conn.SetReadDeadline(time.Now().Add(rwTimeout))
		line, err := c.reader.ReadString('\n')
		if err != nil {
			return nil, fmt.Errorf("pop3 read multiline: %w", err)
		}
		// Terminator: a single dot on a line by itself
		if line == ".\r\n" || line == ".\n" {
			break
		}
		// Dot-unstuffing: RFC 1939 §3 — lines beginning with ".." → "."
		if strings.HasPrefix(line, "..") {
			line = line[1:]
		}
		buf.WriteString(line)
	}
	return buf.Bytes(), nil
}