zulip/zerver/lib/email_mirror.py

import logging
import re
import secrets
from email.headerregistry import AddressHeader
from email.message import EmailMessage
from typing import Dict, List, Optional, Tuple

from django.conf import settings
from django.utils.timezone import now as timezone_now
from django.utils.timezone import timedelta

from zerver.actions.message_send import (
    check_send_message,
    internal_send_huddle_message,
    internal_send_private_message,
    internal_send_stream_message,
)
from zerver.lib.email_mirror_helpers import (
    ZulipEmailForwardError,
    ZulipEmailForwardUserError,
    decode_email_address,
    get_email_gateway_message_string_from_address,
)
from zerver.lib.email_notifications import convert_html_to_markdown
from zerver.lib.exceptions import JsonableError, RateLimited
from zerver.lib.message import normalize_body, truncate_topic
from zerver.lib.queue import queue_json_publish
from zerver.lib.rate_limiter import RateLimitedObject
from zerver.lib.send_email import FromAddress
from zerver.lib.upload import upload_message_file
from zerver.models import (
    Message,
    MissedMessageEmailAddress,
    Realm,
    Recipient,
    Stream,
    UserProfile,
    get_client,
    get_display_recipient,
    get_realm,
    get_stream_by_id_in_realm,
    get_system_bot,
    get_user,
)
from zproject.backends import is_user_active

logger = logging.getLogger(__name__)


def redact_email_address(error_message: str) -> str:
    if not settings.EMAIL_GATEWAY_EXTRA_PATTERN_HACK:
        domain = settings.EMAIL_GATEWAY_PATTERN.rsplit("@")[-1]
    else:
        # EMAIL_GATEWAY_EXTRA_PATTERN_HACK is of the form '@example.com'
        domain = settings.EMAIL_GATEWAY_EXTRA_PATTERN_HACK[1:]

    address_match = re.search("\\b(\\S*?)@" + domain, error_message)
    if address_match:
        email_address = address_match.group(0)
        # Annotate basic info about the address before scrubbing:
        if is_missed_message_address(email_address):
            redacted_message = error_message.replace(
                email_address, f"{email_address} <Missed message address>"
            )
        else:
            try:
                target_stream_id = decode_stream_email_address(email_address)[0].id
                annotated_address = f"{email_address} <Address to stream id: {target_stream_id}>"
                redacted_message = error_message.replace(email_address, annotated_address)
            except ZulipEmailForwardError:
                redacted_message = error_message.replace(
                    email_address, f"{email_address} <Invalid address>"
                )

        # Scrub the address from the message, to the form XXXXX@example.com:
        string_to_scrub = address_match.groups()[0]
        redacted_message = redacted_message.replace(string_to_scrub, "X" * len(string_to_scrub))
        return redacted_message

    return error_message


def report_to_zulip(error_message: str) -> None:
    if settings.ERROR_BOT is None:
        return
    error_bot_realm = get_realm(settings.STAFF_SUBDOMAIN)
    error_bot = get_system_bot(settings.ERROR_BOT, error_bot_realm.id)
    error_stream = Stream.objects.get(name="errors", realm=error_bot_realm)
    send_zulip(
        error_bot,
        error_stream,
        "email mirror error",
        f"""~~~\n{error_message}\n~~~""",
    )


def log_and_report(email_message: EmailMessage, error_message: str, to: Optional[str]) -> None:
    recipient = to or "No recipient found"
    error_message = "Sender: {}\nTo: {}\n{}".format(
        email_message.get("From"), recipient, error_message
    )

    error_message = redact_email_address(error_message)
    logger.error(error_message)
    report_to_zulip(error_message)


# Temporary missed message addresses


def generate_missed_message_token() -> str:
    return "mm" + secrets.token_hex(16)


def is_missed_message_address(address: str) -> bool:
    try:
        msg_string = get_email_gateway_message_string_from_address(address)
    except ZulipEmailForwardError:
        return False

    return is_mm_32_format(msg_string)


def is_mm_32_format(msg_string: Optional[str]) -> bool:
    """
    Missed message strings are formatted with a little "mm" prefix
    followed by a randomly generated 32-character string.
    """
    return msg_string is not None and msg_string.startswith("mm") and len(msg_string) == 34


def get_missed_message_token_from_address(address: str) -> str:
    msg_string = get_email_gateway_message_string_from_address(address)

    if not is_mm_32_format(msg_string):
        raise ZulipEmailForwardError("Could not parse missed message address")

    return msg_string


def get_usable_missed_message_address(address: str) -> MissedMessageEmailAddress:
    token = get_missed_message_token_from_address(address)
    try:
        mm_address = MissedMessageEmailAddress.objects.select_related().get(
            email_token=token,
            timestamp__gt=timezone_now()
            - timedelta(seconds=MissedMessageEmailAddress.EXPIRY_SECONDS),
        )
    except MissedMessageEmailAddress.DoesNotExist:
        raise ZulipEmailForwardUserError("Missed message address expired or doesn't exist.")

    if not mm_address.is_usable():
        # Technical, this also checks whether the event is expired,
        # but that case is excluded by the logic above.
        raise ZulipEmailForwardUserError("Missed message address out of uses.")

    return mm_address


def create_missed_message_address(user_profile: UserProfile, message: Message) -> str:
    # If the email gateway isn't configured, we specify a reply
    # address, since there's no useful way for the user to reply into
    # Zulip.
    if settings.EMAIL_GATEWAY_PATTERN == "":
        return FromAddress.NOREPLY

    mm_address = MissedMessageEmailAddress.objects.create(
        message=message, user_profile=user_profile, email_token=generate_missed_message_token()
    )
    return str(mm_address)


def construct_zulip_body(
    message: EmailMessage,
    realm: Realm,
    show_sender: bool = False,
    include_quotes: bool = False,
    include_footer: bool = False,
    prefer_text: bool = True,
) -> str:
    body = extract_body(message, include_quotes, prefer_text)
    # Remove null characters, since Zulip will reject
    body = body.replace("\x00", "")
    if not include_footer:
        body = filter_footer(body)

    if not body.endswith("\n"):
        body += "\n"
    body += extract_and_upload_attachments(message, realm)
    if not body.rstrip():
        body = "(No email body)"

    if show_sender:
        sender = str(message.get("From", ""))
        body = f"From: {sender}\n{body}"

    return body


## Sending the Zulip ##


def send_zulip(sender: UserProfile, stream: Stream, topic: str, content: str) -> None:
    internal_send_stream_message(
        sender,
        stream,
        truncate_topic(topic),
        normalize_body(content),
        email_gateway=True,
    )


def send_mm_reply_to_stream(
    user_profile: UserProfile, stream: Stream, topic: str, body: str
) -> None:
    try:
        check_send_message(
            sender=user_profile,
            client=get_client("Internal"),
            message_type_name="stream",
            message_to=[stream.id],
            topic_name=topic,
            message_content=body,
        )
    except JsonableError as error:
        error_message = "Error sending message to stream {stream} via message notification email reply:\n{error}".format(
            stream=stream.name, error=error.msg
        )
        internal_send_private_message(
            get_system_bot(settings.NOTIFICATION_BOT, user_profile.realm_id),
            user_profile,
            error_message,
        )


def get_message_part_by_type(message: EmailMessage, content_type: str) -> Optional[str]:
    charsets = message.get_charsets()

    for idx, part in enumerate(message.walk()):
        if part.get_content_type() == content_type:
            content = part.get_payload(decode=True)
            assert isinstance(content, bytes)
            if charsets[idx]:
                try:
                    return content.decode(charsets[idx], errors="ignore")
                except LookupError:
                    # The RFCs do not define how to handle unknown
                    # charsets, but treating as US-ASCII seems
                    # reasonable; fall through to below.
                    pass

            # If no charset has been specified in the header, assume us-ascii,
            # by RFC6657: https://tools.ietf.org/html/rfc6657
            return content.decode("us-ascii", errors="ignore")

    return None


def extract_body(
    message: EmailMessage, include_quotes: bool = False, prefer_text: bool = True
) -> str:
    plaintext_content = extract_plaintext_body(message, include_quotes)
    html_content = extract_html_body(message, include_quotes)

    if plaintext_content is None and html_content is None:
        logger.warning("Content types: %s", [part.get_content_type() for part in message.walk()])
        raise ZulipEmailForwardUserError("Unable to find plaintext or HTML message body")
    if not plaintext_content and not html_content:
        raise ZulipEmailForwardUserError("Email has no nonempty body sections; ignoring.")

    if prefer_text:
        if plaintext_content:
            return plaintext_content
        else:
            assert html_content  # Needed for mypy. Ensured by the validating block above.
            return html_content
    else:
        if html_content:
            return html_content
        else:
            assert plaintext_content  # Needed for mypy. Ensured by the validating block above.
            return plaintext_content


talon_initialized = False


def extract_plaintext_body(message: EmailMessage, include_quotes: bool = False) -> Optional[str]:
    import talon_core

    global talon_initialized
    if not talon_initialized:
        talon_core.init()
        talon_initialized = True

    plaintext_content = get_message_part_by_type(message, "text/plain")
    if plaintext_content is not None:
        if include_quotes:
            return plaintext_content
        else:
            return talon_core.quotations.extract_from_plain(plaintext_content)
    else:
        return None


def extract_html_body(message: EmailMessage, include_quotes: bool = False) -> Optional[str]:
    import talon_core

    global talon_initialized
    if not talon_initialized:  # nocoverage
        talon_core.init()
        talon_initialized = True

    html_content = get_message_part_by_type(message, "text/html")
    if html_content is not None:
        if include_quotes:
            return convert_html_to_markdown(html_content)
        else:
            return convert_html_to_markdown(talon_core.quotations.extract_from_html(html_content))
    else:
        return None


def filter_footer(text: str) -> str:
    # Try to filter out obvious footers.
    possible_footers = [line for line in text.split("\n") if line.strip() == "--"]
    if len(possible_footers) != 1:
        # Be conservative and don't try to scrub content if there
        # isn't a trivial footer structure.
        return text

    return re.split(r"^\s*--\s*$", text, 1, flags=re.MULTILINE)[0].strip()


def extract_and_upload_attachments(message: EmailMessage, realm: Realm) -> str:
    user_profile = get_system_bot(settings.EMAIL_GATEWAY_BOT, realm.id)

    attachment_links = []
    for part in message.walk():
        content_type = part.get_content_type()
        filename = part.get_filename()
        if filename:
            attachment = part.get_payload(decode=True)
            if isinstance(attachment, bytes):
                s3_url = upload_message_file(
                    filename,
                    len(attachment),
                    content_type,
                    attachment,
                    user_profile,
                    target_realm=realm,
                )
                formatted_link = f"[{filename}]({s3_url})"
                attachment_links.append(formatted_link)
            else:
                logger.warning(
                    "Payload is not bytes (invalid attachment %s in message from %s).",
                    filename,
                    message.get("From"),
                )

    return "\n".join(attachment_links)


def decode_stream_email_address(email: str) -> Tuple[Stream, Dict[str, bool]]:
    token, options = decode_email_address(email)

    try:
        stream = Stream.objects.get(email_token=token)
    except Stream.DoesNotExist:
        raise ZulipEmailForwardError("Bad stream token from email recipient " + email)

    return stream, options


def find_emailgateway_recipient(message: EmailMessage) -> str:
    # We can't use Delivered-To; if there is a X-Gm-Original-To
    # it is more accurate, so try to find the most-accurate
    # recipient list in descending priority order
    recipient_headers = [
        "X-Gm-Original-To",
        "Delivered-To",
        "Envelope-To",
        "Resent-To",
        "Resent-CC",
        "To",
        "CC",
    ]

    pattern_parts = [re.escape(part) for part in settings.EMAIL_GATEWAY_PATTERN.split("%s")]
    match_email_re = re.compile(".*?".join(pattern_parts))

    for header_name in recipient_headers:
        for header_value in message.get_all(header_name, []):
            if isinstance(header_value, AddressHeader):
                emails = [addr.addr_spec for addr in header_value.addresses]
            else:
                emails = [str(header_value)]

            for email in emails:
                if match_email_re.match(email):
                    return email

    raise ZulipEmailForwardError("Missing recipient in mirror email")


def strip_from_subject(subject: str) -> str:
    # strips RE and FWD from the subject
    # from: https://stackoverflow.com/questions/9153629/regex-code-for-removing-fwd-re-etc-from-email-subject
    reg = r"([\[\(] *)?\b(RE|FWD?) *([-:;)\]][ :;\])-]*|$)|\]+ *$"
    stripped = re.sub(reg, "", subject, flags=re.IGNORECASE | re.MULTILINE)
    return stripped.strip()


def is_forwarded(subject: str) -> bool:
    # regex taken from strip_from_subject, we use it to detect various forms
    # of FWD at the beginning of the subject.
    reg = r"([\[\(] *)?\b(FWD?) *([-:;)\]][ :;\])-]*|$)|\]+ *$"
    return bool(re.match(reg, subject, flags=re.IGNORECASE))


def process_stream_message(to: str, message: EmailMessage) -> None:
    subject_header = message.get("Subject", "")
    subject = strip_from_subject(subject_header) or "(no topic)"

    stream, options = decode_stream_email_address(to)
    # Don't remove quotations if message is forwarded, unless otherwise specified:
    if "include_quotes" not in options:
        options["include_quotes"] = is_forwarded(subject_header)

    body = construct_zulip_body(message, stream.realm, **options)
    send_zulip(get_system_bot(settings.EMAIL_GATEWAY_BOT, stream.realm_id), stream, subject, body)
    logger.info(
        "Successfully processed email to %s (%s)",
        stream.name,
        stream.realm.string_id,
    )


def process_missed_message(to: str, message: EmailMessage) -> None:
    mm_address = get_usable_missed_message_address(to)
    mm_address.increment_times_used()

    user_profile = mm_address.user_profile
    topic = mm_address.message.topic_name()

    if mm_address.message.recipient.type == Recipient.PERSONAL:
        # We need to reply to the sender so look up their personal recipient_id
        recipient = mm_address.message.sender.recipient
    else:
        recipient = mm_address.message.recipient

    if not is_user_active(user_profile):
        logger.warning("Sending user is not active. Ignoring this message notification email.")
        return

    body = construct_zulip_body(message, user_profile.realm)

    assert recipient is not None
    if recipient.type == Recipient.STREAM:
        stream = get_stream_by_id_in_realm(recipient.type_id, user_profile.realm)
        send_mm_reply_to_stream(user_profile, stream, topic, body)
        recipient_str = stream.name
    elif recipient.type == Recipient.PERSONAL:
        display_recipient = get_display_recipient(recipient)
        assert not isinstance(display_recipient, str)
        recipient_str = display_recipient[0]["email"]
        recipient_user = get_user(recipient_str, user_profile.realm)
        internal_send_private_message(user_profile, recipient_user, body)
    elif recipient.type == Recipient.HUDDLE:
        display_recipient = get_display_recipient(recipient)
        assert not isinstance(display_recipient, str)
        emails = [user_dict["email"] for user_dict in display_recipient]
        recipient_str = ", ".join(emails)
        internal_send_huddle_message(user_profile.realm, user_profile, emails, body)
    else:
        raise AssertionError("Invalid recipient type!")

    logger.info(
        "Successfully processed email from user %s to %s",
        user_profile.id,
        recipient_str,
    )


def process_message(message: EmailMessage, rcpt_to: Optional[str] = None) -> None:
    to: Optional[str] = None

    try:
        if rcpt_to is not None:
            to = rcpt_to
        else:
            to = find_emailgateway_recipient(message)

        if is_missed_message_address(to):
            process_missed_message(to, message)
        else:
            process_stream_message(to, message)
    except ZulipEmailForwardUserError as e:
        # TODO: notify sender of error, retry if appropriate.
        logger.info(e.args[0])
    except ZulipEmailForwardError as e:
        log_and_report(message, e.args[0], to)


def validate_to_address(rcpt_to: str) -> None:
    if is_missed_message_address(rcpt_to):
        get_usable_missed_message_address(rcpt_to)
    else:
        decode_stream_email_address(rcpt_to)


def mirror_email_message(rcpt_to: str, msg_base64: str) -> Dict[str, str]:
    try:
        validate_to_address(rcpt_to)
    except ZulipEmailForwardError as e:
        return {
            "status": "error",
            "msg": f"5.1.1 Bad destination mailbox address: {e}",
        }

    queue_json_publish(
        "email_mirror",
        {
            "rcpt_to": rcpt_to,
            "msg_base64": msg_base64,
        },
    )
    return {"status": "success"}


# Email mirror rate limiter code:


class RateLimitedRealmMirror(RateLimitedObject):
    def __init__(self, realm: Realm) -> None:
        self.realm = realm
        super().__init__()

    def key(self) -> str:
        return f"{type(self).__name__}:{self.realm.string_id}"

    def rules(self) -> List[Tuple[int, int]]:
        return settings.RATE_LIMITING_MIRROR_REALM_RULES


def rate_limit_mirror_by_realm(recipient_realm: Realm) -> None:
    ratelimited, secs_to_freedom = RateLimitedRealmMirror(recipient_realm).rate_limit()

    if ratelimited:
        raise RateLimited(secs_to_freedom)