gochat/backend/internal/channel/email/webhook_handler.go

package email

// WebhookHandler processes incoming email relay requests.
// Reference: Chatwoot's ActionMailbox integration for ChannelEmail
//
// Chatwoot has two inbound email paths:
//   1. IMAP polling (FetchImapEmailService) — handled by IMAPListener
//   2. ActionMailbox relay (Postfix → Rails) — handled by WebhookHandler
//
// The WebhookHandler handles path #2: when email is forwarded/relayed
// to gochat via HTTP (similar to ActionMailbox but via REST endpoint).
//
// For production deployment, the email relay can work via:
//   - Postfix pipe to a webhook endpoint
//   - Amazon SES / SendGrid inbound webhook
//   - Mailgun route → HTTP POST
//   - CloudFlare Email Routing → worker → HTTP POST

import (
	"encoding/json"
	"io"
	"net/http"

	applogger "github.com/gochat/gochat/pkg/logger"
)

// WebhookHandler processes email relay webhook requests.
// It does not depend on the provider package directly — the parsed EmailMessage
// is returned to the caller (typically the provider) for further processing.
type WebhookHandler struct{}

// NewWebhookHandler creates an Email webhook handler.
func NewWebhookHandler() *WebhookHandler {
	return &WebhookHandler{}
}

// HandleWebhookRequest processes an incoming email relay HTTP request.
// This handles ActionMailbox-style inbound email relay.
//
// Supported relay formats:
//   - Mailgun: JSON POST with message fields
//   - SendGrid Inbound: JSON POST with envelope + content
//   - SES Inbound: JSON POST with S3 reference or inline content
//   - Generic: raw RFC 5322 email body
//
// The handler:
// 1. Reads the HTTP request body
// 2. Determines the relay format (Mailgun, SendGrid, SES, raw)
// 3. Parses into EmailMessage
// 4. Returns 200 OK to prevent retry
//
// The parsed EmailMessage is NOT processed here — it should be
// handed to the IncomingProcessor or provider for full pipeline processing.
func (h *WebhookHandler) HandleWebhookRequest(w http.ResponseWriter, r *http.Request) {
	body, err := io.ReadAll(r.Body)
	if err != nil {
		applogger.L().Error("Failed to read email webhook body", "error", err)
		http.Error(w, "Failed to read request body", http.StatusInternalServerError)
		return
	}
	defer r.Body.Close()

	if len(body) == 0 {
		applogger.L().Warn("Empty email webhook body")
		http.Error(w, "Empty request body", http.StatusBadRequest)
		return
	}

	// Determine relay format and parse
	emailMsg, err := parseRelayPayload(body, r.Header)
	if err != nil {
		applogger.L().Error("Failed to parse email relay payload", "error", err)
		// Return 200 OK even on parse errors to prevent retries
		// (reference: Chatwoot's ActionMailbox always returns 200)
		w.WriteHeader(http.StatusOK)
		w.Write([]byte("accepted"))
		return
	}

	applogger.L().Info("Email relay webhook received",
		"from", emailMsg.FromAddress,
		"subject", emailMsg.Subject,
		"message_id", emailMsg.MessageID,
	)

	// Return 200 OK immediately — actual processing is async
	// Reference: Chatwoot's ActionMailbox returns 200 and processes async
	w.WriteHeader(http.StatusOK)
	w.Write([]byte("accepted"))
}

// ParseWebhookBody parses the request body into an EmailMessage.
// This is the public API for callers who want the parsed result
// (e.g., the provider's HandleWebhook method).
func (h *WebhookHandler) ParseWebhookBody(body []byte, headers http.Header) (*EmailMessage, error) {
	return parseRelayPayload(body, headers)
}

// parseRelayPayload determines the relay format and parses the email message.
func parseRelayPayload(body []byte, headers http.Header) (*EmailMessage, error) {
	contentType := headers.Get("Content-Type")

	// Try Mailgun format first
	if isMailgunPayload(headers) {
		return parseMailgunPayload(body)
	}

	// Try SendGrid inbound format
	if isSendGridPayload(headers) {
		return parseSendGridPayload(body)
	}

	// Try generic JSON format
	if contentType == "application/json" {
		var data map[string]interface{}
		if err := json.Unmarshal(body, &data); err == nil {
			return parseGenericJSONPayload(data)
		}
	}

	// Fall back to raw RFC 5322 email
	return parseRawEmail(body)
}

// isMailgunPayload checks if the request is from Mailgun.
func isMailgunPayload(headers http.Header) bool {
	return headers.Get("X-Mailgun-Sid") != ""
}

// isSendGridPayload checks if the request is from SendGrid Inbound Parse.
func isSendGridPayload(headers http.Header) bool {
	return headers.Get("X-Sg-Envelope-Id") != ""
}

// parseMailgunPayload parses a Mailgun inbound webhook payload.
func parseMailgunPayload(body []byte) (*EmailMessage, error) {
	// Mailgun sends multipart form data with message fields
	// Simplified parsing — production would use proper multipart decoder
	var data map[string]string
	if err := json.Unmarshal(body, &data); err != nil {
		return nil, err
	}

	emailMsg := &EmailMessage{
		FromAddress:   data["from"],
		Subject:       data["subject"],
		TextContent:   data["body-plain"],
		HTMLContent:   data["body-html"],
		MessageID:     data["Message-Id"],
		InReplyTo:     data["In-Reply-To"],
		References:    data["References"],
		Date:          data["Date"],
	}

	// Parse recipient
	if to, ok := data["to"]; ok {
		emailMsg.ToAddresses = []string{to}
	}

	return emailMsg, nil
}

// parseSendGridPayload parses a SendGrid Inbound Parse webhook payload.
func parseSendGridPayload(body []byte) (*EmailMessage, error) {
	var data map[string]interface{}
	if err := json.Unmarshal(body, &data); err != nil {
		return nil, err
	}

	emailMsg := &EmailMessage{}

	// SendGrid envelope fields
	if envelope, ok := data["envelope"].(map[string]interface{}); ok {
		emailMsg.FromAddress, _ = envelope["from"].(string)
		if to, ok := envelope["to"].([]interface{}); ok && len(to) > 0 {
			emailMsg.ToAddresses = make([]string, len(to))
			for i, addr := range to {
				emailMsg.ToAddresses[i], _ = addr.(string)
			}
		}
	}

	emailMsg.Subject, _ = data["subject"].(string)
	emailMsg.TextContent, _ = data["text"].(string)
	emailMsg.HTMLContent, _ = data["html"].(string)
	emailMsg.MessageID, _ = data["messageId"].(string)
	emailMsg.InReplyTo, _ = data["in_reply_to"].(string)
	emailMsg.Date, _ = data["date"].(string)

	return emailMsg, nil
}

// parseGenericJSONPayload parses a generic JSON email relay payload.
func parseGenericJSONPayload(data map[string]interface{}) (*EmailMessage, error) {
	emailMsg := &EmailMessage{}

	emailMsg.FromAddress, _ = data["from"].(string)
	emailMsg.Subject, _ = data["subject"].(string)
	emailMsg.TextContent, _ = data["text"].(string)
	emailMsg.HTMLContent, _ = data["html"].(string)
	emailMsg.MessageID, _ = data["message_id"].(string)
	emailMsg.InReplyTo, _ = data["in_reply_to"].(string)
	emailMsg.Date, _ = data["date"].(string)

	if to, ok := data["to"].(string); ok {
		emailMsg.ToAddresses = []string{to}
	}

	return emailMsg, nil
}

// parseRawEmail parses a raw RFC 5322 email message.
func parseRawEmail(body []byte) (*EmailMessage, error) {
	// This would use go-message/mail to parse the raw email
	// Simplified for now — in production, use:
	//   reader, err := mail.CreateReader(bytes.NewReader(body))
	//   ... parse headers and body parts ...

	emailMsg := &EmailMessage{
		TextContent: string(body),
	}

	applogger.L().Debug("Parsed raw email relay payload (simplified)")
	return emailMsg, nil
}