zulip/zerver/lib/url_decoding.py

from urllib.parse import urlsplit

from django.conf import settings


def is_same_server_message_link(url: str) -> bool:
    split_result = urlsplit(url)
    hostname = split_result.hostname
    fragment = split_result.fragment

    if hostname not in {None, settings.EXTERNAL_HOST_WITHOUT_PORT}:
        return False

    # A message link always has category `narrow`, section `channel`
    # or `dm`, and ends with `/near/<message_id>`, where <message_id>
    # is a sequence of digits. The URL fragment of a message link has
    # at least 5 parts. e.g. '#narrow/dm/9,15-dm/near/43'
    fragment_parts = fragment.split("/")
    if len(fragment_parts) < 5:
        return False

    category = fragment_parts[0]
    section = fragment_parts[1]
    ends_with_near_message_id = fragment_parts[-2] == "near" and fragment_parts[-1].isdigit()

    return category == "narrow" and section in {"channel", "dm"} and ends_with_near_message_id