| 1 | // Package mail sends the one message CONTRA sends: a signup link. |
| 2 | // |
| 3 | // Stdlib net/smtp against the credentials the platform delivers |
| 4 | // (CARLOS_SMTP_*). No dependency, because one message with one link does not |
| 5 | // need a mail library, and because everything this package touches is an |
| 6 | // address we have promised not to keep. |
| 7 | // |
| 8 | // # What it must not do |
| 9 | // |
| 10 | // The address is used to address one message and is never written down. There |
| 11 | // is no send log, no bounce table, no "last emailed at" column — the blind |
| 12 | // index is the only trace a signup leaves, and adding a second one here would |
| 13 | // undo the reason the first one is blind. |
| 14 | package mail |
| 15 | |
| 16 | import ( |
| 17 | "context" |
| 18 | "crypto/tls" |
| 19 | "errors" |
| 20 | "fmt" |
| 21 | "net" |
| 22 | "net/smtp" |
| 23 | "os" |
| 24 | "strings" |
| 25 | "time" |
| 26 | ) |
| 27 | |
| 28 | // Sender delivers a message. An interface so tests take a fake and never open |
| 29 | // a socket, and so a deployment with no email configured fails loudly at boot |
| 30 | // rather than at the first signup. |
| 31 | type Sender interface { |
| 32 | Send(ctx context.Context, to, subject, body string) error |
| 33 | } |
| 34 | |
| 35 | var ErrNotConfigured = errors.New("mail: no SMTP configuration (CARLOS_SMTP_HOST and friends)") |
| 36 | |
| 37 | type smtpSender struct { |
| 38 | host, port, user, pass, from string |
| 39 | } |
| 40 | |
| 41 | // FromEnv reads the platform's delivered configuration. It returns |
| 42 | // ErrNotConfigured rather than a silent no-op sender: a signup flow whose |
| 43 | // email quietly goes nowhere is worse than one that refuses to start. |
| 44 | func FromEnv() (Sender, error) { |
| 45 | s := smtpSender{ |
| 46 | host: os.Getenv("CARLOS_SMTP_HOST"), |
| 47 | port: os.Getenv("CARLOS_SMTP_PORT"), |
| 48 | user: os.Getenv("CARLOS_SMTP_USER"), |
| 49 | pass: os.Getenv("CARLOS_SMTP_PASS"), |
| 50 | from: os.Getenv("CARLOS_SMTP_FROM"), |
| 51 | } |
| 52 | if s.host == "" || s.from == "" { |
| 53 | return nil, ErrNotConfigured |
| 54 | } |
| 55 | if s.port == "" { |
| 56 | s.port = "587" |
| 57 | } |
| 58 | return s, nil |
| 59 | } |
| 60 | |
| 61 | func (s smtpSender) Send(ctx context.Context, to, subject, body string) error { |
| 62 | // One header set, built here rather than assembled from user input: `to` |
| 63 | // is the only variable, and a newline in it would be header injection. |
| 64 | if strings.ContainsAny(to, "\r\n") || strings.ContainsAny(subject, "\r\n") { |
| 65 | return errors.New("mail: newline in a header") |
| 66 | } |
| 67 | |
| 68 | msg := strings.Join([]string{ |
| 69 | "From: " + s.from, |
| 70 | "To: " + to, |
| 71 | "Subject: " + subject, |
| 72 | "MIME-Version: 1.0", |
| 73 | "Content-Type: text/plain; charset=utf-8", |
| 74 | "", body, |
| 75 | }, "\r\n") |
| 76 | |
| 77 | deadline, ok := ctx.Deadline() |
| 78 | if !ok { |
| 79 | deadline = time.Now().Add(20 * time.Second) |
| 80 | } |
| 81 | conn, err := (&net.Dialer{Deadline: deadline}).DialContext( |
| 82 | ctx, "tcp", net.JoinHostPort(s.host, s.port)) |
| 83 | if err != nil { |
| 84 | return fmt.Errorf("mail: dial: %w", err) |
| 85 | } |
| 86 | defer conn.Close() |
| 87 | |
| 88 | c, err := smtp.NewClient(conn, s.host) |
| 89 | if err != nil { |
| 90 | return fmt.Errorf("mail: smtp: %w", err) |
| 91 | } |
| 92 | defer c.Close() |
| 93 | |
| 94 | // STARTTLS with the server name verified. The credentials are a bearer |
| 95 | // secret and the recipient is an address we have promised not to keep; |
| 96 | // neither goes over the wire in the clear. |
| 97 | if err := c.StartTLS(&tls.Config{ServerName: s.host, MinVersion: tls.VersionTLS12}); err != nil { |
| 98 | return fmt.Errorf("mail: starttls: %w", err) |
| 99 | } |
| 100 | if s.user != "" { |
| 101 | if err := c.Auth(smtp.PlainAuth("", s.user, s.pass, s.host)); err != nil { |
| 102 | return fmt.Errorf("mail: auth: %w", err) |
| 103 | } |
| 104 | } |
| 105 | if err := c.Mail(s.from); err != nil { |
| 106 | return fmt.Errorf("mail: from: %w", err) |
| 107 | } |
| 108 | if err := c.Rcpt(to); err != nil { |
| 109 | return fmt.Errorf("mail: rcpt: %w", err) |
| 110 | } |
| 111 | w, err := c.Data() |
| 112 | if err != nil { |
| 113 | return fmt.Errorf("mail: data: %w", err) |
| 114 | } |
| 115 | if _, err := w.Write([]byte(msg)); err != nil { |
| 116 | return fmt.Errorf("mail: write: %w", err) |
| 117 | } |
| 118 | if err := w.Close(); err != nil { |
| 119 | return fmt.Errorf("mail: close: %w", err) |
| 120 | } |
| 121 | return c.Quit() |
| 122 | } |
| 123 | |
amadan