zulip/zerver/actions/streams.py

import hashlib
from collections import defaultdict
from typing import Any, Collection, Dict, Iterable, List, Mapping, Optional, Set, Tuple

from django.conf import settings
from django.db import transaction
from django.db.models import Q, QuerySet
from django.utils.timezone import now as timezone_now
from django.utils.translation import gettext as _
from django.utils.translation import override as override_language
from django_stubs_ext import ValuesQuerySet
from typing_extensions import TypeAlias

from zerver.actions.default_streams import (
    do_remove_default_stream,
    do_remove_streams_from_default_stream_group,
)
from zerver.actions.message_send import internal_send_stream_message
from zerver.lib.cache import (
    cache_delete_many,
    cache_set,
    display_recipient_cache_key,
    to_dict_cache_key_id,
)
from zerver.lib.exceptions import JsonableError
from zerver.lib.mention import silent_mention_syntax_for_user
from zerver.lib.message import get_last_message_id
from zerver.lib.queue import queue_event_on_commit, queue_json_publish
from zerver.lib.stream_color import pick_colors
from zerver.lib.stream_subscription import (
    SubInfo,
    SubscriberPeerInfo,
    bulk_get_subscriber_peer_info,
    get_active_subscriptions_for_stream_id,
    get_bulk_stream_subscriber_info,
    get_used_colors_for_user_ids,
    get_user_ids_for_streams,
    get_users_for_streams,
)
from zerver.lib.stream_traffic import get_streams_traffic
from zerver.lib.streams import (
    can_access_stream_user_ids,
    check_basic_stream_access,
    get_occupied_streams,
    get_stream_permission_policy_name,
    render_stream_description,
    send_stream_creation_event,
    stream_to_dict,
)
from zerver.lib.subscription_info import get_subscribers_query
from zerver.lib.types import APISubscriptionDict
from zerver.lib.users import (
    get_subscribers_of_target_user_subscriptions,
    get_users_involved_in_dms_with_target_users,
)
from zerver.models import (
    ArchivedAttachment,
    Attachment,
    DefaultStream,
    DefaultStreamGroup,
    Message,
    Realm,
    RealmAuditLog,
    Recipient,
    Stream,
    Subscription,
    UserGroup,
    UserProfile,
)
from zerver.models.groups import SystemGroups
from zerver.models.users import active_non_guest_user_ids, active_user_ids, get_system_bot
from zerver.tornado.django_api import send_event, send_event_on_commit


def send_user_remove_events_on_stream_deactivation(
    stream: Stream, subscribed_users: List[UserProfile]
) -> None:
    guest_subscribed_users = [user for user in subscribed_users if user.is_guest]

    if len(guest_subscribed_users) == 0:  # nocoverage
        # Save a few database queries in the case that the stream
        # didn't contain any guest users.
        return

    other_subscriptions_of_subscribed_users = get_subscribers_of_target_user_subscriptions(
        guest_subscribed_users
    )
    users_involved_in_dms_dict = get_users_involved_in_dms_with_target_users(
        guest_subscribed_users, stream.realm
    )

    subscriber_ids = {user.id for user in subscribed_users}
    inaccessible_user_dict: Dict[int, Set[int]] = defaultdict(set)
    for guest_user in guest_subscribed_users:
        users_accessible_by_guest = (
            {guest_user.id}
            | other_subscriptions_of_subscribed_users[guest_user.id]
            | users_involved_in_dms_dict[guest_user.id]
        )
        users_inaccessible_to_guest = subscriber_ids - users_accessible_by_guest
        for user_id in users_inaccessible_to_guest:
            inaccessible_user_dict[user_id].add(guest_user.id)

    for user_id, notify_user_ids in inaccessible_user_dict.items():
        event_remove_user = dict(
            type="realm_user",
            op="remove",
            person=dict(user_id=user_id, full_name=str(UserProfile.INACCESSIBLE_USER_NAME)),
        )
        send_event_on_commit(stream.realm, event_remove_user, list(notify_user_ids))


@transaction.atomic(savepoint=False)
def do_deactivate_stream(stream: Stream, *, acting_user: Optional[UserProfile]) -> None:
    # If the stream is already deactivated, this is a no-op
    if stream.deactivated is True:
        raise JsonableError(_("Channel is already deactivated"))

    # We want to mark all messages in the to-be-deactivated stream as
    # read for all users; otherwise they will pollute queries like
    # "Get the user's first unread message".  Since this can be an
    # expensive operation, we do it via the deferred_work queue
    # processor.
    deferred_work_event = {
        "type": "mark_stream_messages_as_read_for_everyone",
        "stream_recipient_id": stream.recipient_id,
    }
    transaction.on_commit(lambda: queue_json_publish("deferred_work", deferred_work_event))

    # Get the affected user ids *before* we deactivate everybody.
    affected_user_ids = can_access_stream_user_ids(stream)

    stream_subscribers = get_active_subscriptions_for_stream_id(
        stream.id, include_deactivated_users=True
    ).select_related("user_profile")
    subscribed_users = [sub.user_profile for sub in stream_subscribers]
    stream_subscribers.update(active=False)

    was_invite_only = stream.invite_only
    was_public = stream.is_public()
    was_web_public = stream.is_web_public

    # We do not use do_change_stream_permission because no users need to
    # be notified, and we do not want to create audit log entries for
    # changing stream privacy. And due to this we also need to duplicate
    # the code to unset is_web_public field on attachments below.
    stream.deactivated = True
    stream.invite_only = True
    # Preserve as much as possible the original stream name while giving it a
    # special prefix that both indicates that the stream is deactivated and
    # frees up the original name for reuse.
    old_name = stream.name

    # Prepend a substring of the hashed stream ID to the new stream name
    streamID = str(stream.id)
    stream_id_hash_object = hashlib.sha512(streamID.encode())
    hashed_stream_id = stream_id_hash_object.hexdigest()[0:7]

    new_name = (hashed_stream_id + "!DEACTIVATED:" + old_name)[: Stream.MAX_NAME_LENGTH]

    stream.name = new_name[: Stream.MAX_NAME_LENGTH]
    stream.save(update_fields=["name", "deactivated", "invite_only"])

    assert stream.recipient_id is not None
    if was_web_public:
        assert was_public
        # Unset the is_web_public and is_realm_public cache on attachments,
        # since the stream is now private.
        Attachment.objects.filter(messages__recipient_id=stream.recipient_id).update(
            is_web_public=None, is_realm_public=None
        )
        ArchivedAttachment.objects.filter(messages__recipient_id=stream.recipient_id).update(
            is_web_public=None, is_realm_public=None
        )
    elif was_public:
        # Unset the is_realm_public cache on attachments, since the stream is now private.
        Attachment.objects.filter(messages__recipient_id=stream.recipient_id).update(
            is_realm_public=None
        )
        ArchivedAttachment.objects.filter(messages__recipient_id=stream.recipient_id).update(
            is_realm_public=None
        )

    # If this is a default stream, remove it, properly sending a
    # notification to browser clients.
    if DefaultStream.objects.filter(realm_id=stream.realm_id, stream_id=stream.id).exists():
        do_remove_default_stream(stream)

    default_stream_groups_for_stream = DefaultStreamGroup.objects.filter(streams__id=stream.id)
    for group in default_stream_groups_for_stream:
        do_remove_streams_from_default_stream_group(stream.realm, group, [stream])

    stream_dict = stream_to_dict(stream)
    stream_dict.update(dict(name=old_name, invite_only=was_invite_only))
    event = dict(type="stream", op="delete", streams=[stream_dict])
    send_event_on_commit(stream.realm, event, affected_user_ids)

    if stream.realm.can_access_all_users_group.named_user_group.name != SystemGroups.EVERYONE:
        send_user_remove_events_on_stream_deactivation(stream, subscribed_users)

    event_time = timezone_now()
    RealmAuditLog.objects.create(
        realm=stream.realm,
        acting_user=acting_user,
        modified_stream=stream,
        event_type=RealmAuditLog.STREAM_DEACTIVATED,
        event_time=event_time,
    )


def deactivated_streams_by_old_name(realm: Realm, stream_name: str) -> QuerySet[Stream]:
    fixed_length_prefix = ".......!DEACTIVATED:"
    truncated_name = stream_name[0 : Stream.MAX_NAME_LENGTH - len(fixed_length_prefix)]

    old_names: List[str] = [
        ("!" * bang_length + "DEACTIVATED:" + stream_name)[: Stream.MAX_NAME_LENGTH]
        for bang_length in range(1, 21)
    ]

    possible_streams = Stream.objects.filter(realm=realm, deactivated=True).filter(
        # We go looking for names as they are post-1b6f68bb59dc; 8
        # characters, followed by `!DEACTIVATED:`, followed by at
        # most MAX_NAME_LENGTH-(length of the prefix) of the name
        # they provided:
        Q(name__regex=rf"^{fixed_length_prefix}{truncated_name}")
        # Finally, we go looking for the pre-1b6f68bb59dc version,
        # which is any number of `!` followed by `DEACTIVATED:`
        # and a prefix of the old stream name
        | Q(name__in=old_names),
    )

    return possible_streams


@transaction.atomic(savepoint=False)
def do_unarchive_stream(
    stream: Stream, new_name: str, *, acting_user: Optional[UserProfile]
) -> None:
    realm = stream.realm
    if not stream.deactivated:
        raise JsonableError(_("Channel is not currently deactivated"))
    if Stream.objects.filter(realm=realm, name=new_name).exists():
        raise JsonableError(
            _("Channel named {channel_name} already exists").format(channel_name=new_name)
        )
    assert stream.recipient_id is not None

    stream.deactivated = False
    stream.name = new_name

    # We only set invite_only=True during deactivation, which can lead
    # to the invalid state of to invite-only but also web-public
    # streams.  Explicitly reset the access; we do not use
    # do_change_stream_permission because no users need be notified,
    # and it cannot handle the broken state that may currently exist.
    stream.is_web_public = False
    stream.invite_only = True
    stream.history_public_to_subscribers = True
    stream.save(
        update_fields=[
            "name",
            "deactivated",
            "is_web_public",
            "invite_only",
            "history_public_to_subscribers",
        ]
    )

    # Update caches
    cache_set(display_recipient_cache_key(stream.recipient_id), new_name)
    messages = Message.objects.filter(
        # Uses index: zerver_message_realm_recipient_id
        realm_id=realm.id,
        recipient_id=stream.recipient_id,
    ).only("id")
    cache_delete_many(to_dict_cache_key_id(message.id) for message in messages)

    # Unset the is_web_public and is_realm_public cache on attachments,
    # since the stream is now private.
    Attachment.objects.filter(messages__recipient_id=stream.recipient_id).update(
        is_web_public=None, is_realm_public=None
    )
    ArchivedAttachment.objects.filter(messages__recipient_id=stream.recipient_id).update(
        is_web_public=None, is_realm_public=None
    )

    RealmAuditLog.objects.create(
        realm=realm,
        acting_user=acting_user,
        modified_stream=stream,
        event_type=RealmAuditLog.STREAM_REACTIVATED,
        event_time=timezone_now(),
    )

    recent_traffic = get_streams_traffic({stream.id}, realm)

    # All admins always get to know about private streams' existence,
    # but we only subscribe the realm owners.
    send_stream_creation_event(
        realm, stream, [user.id for user in realm.get_admin_users_and_bots()], recent_traffic
    )
    bulk_add_subscriptions(
        realm=realm,
        streams=[stream],
        users=realm.get_human_owner_users(),
        acting_user=acting_user,
    )

    sender = get_system_bot(settings.NOTIFICATION_BOT, stream.realm_id)
    with override_language(stream.realm.default_language):
        internal_send_stream_message(
            sender,
            stream,
            str(Realm.STREAM_EVENTS_NOTIFICATION_TOPIC_NAME),
            _("Channel {channel_name} un-archived.").format(channel_name=new_name),
        )


def bulk_delete_cache_keys(message_ids_to_clear: List[int]) -> None:
    while len(message_ids_to_clear) > 0:
        batch = message_ids_to_clear[0:5000]

        keys_to_delete = [to_dict_cache_key_id(message_id) for message_id in batch]
        cache_delete_many(keys_to_delete)

        message_ids_to_clear = message_ids_to_clear[5000:]


def merge_streams(
    realm: Realm, stream_to_keep: Stream, stream_to_destroy: Stream
) -> Tuple[int, int, int]:
    recipient_to_destroy = stream_to_destroy.recipient
    recipient_to_keep = stream_to_keep.recipient
    assert recipient_to_keep is not None
    assert recipient_to_destroy is not None
    if recipient_to_destroy.id == recipient_to_keep.id:
        return (0, 0, 0)

    # The high-level approach here is to move all the messages to
    # the surviving stream, deactivate all the subscriptions on
    # the stream to be removed and deactivate the stream, and add
    # new subscriptions to the stream to keep for any users who
    # were only on the now-deactivated stream.
    #
    # The order of operations is carefully chosen so that calling this
    # function again is likely to be an effective way to recover if
    # this process is interrupted by an error.

    # Move the Subscription objects.  This algorithm doesn't
    # preserve any stream settings/colors/etc. from the stream
    # being destroyed, but it's convenient.
    existing_subs = Subscription.objects.filter(recipient=recipient_to_keep)
    users_already_subscribed = {sub.user_profile_id: sub.active for sub in existing_subs}

    subs_to_deactivate = Subscription.objects.filter(recipient=recipient_to_destroy, active=True)
    users_to_activate = [
        sub.user_profile
        for sub in subs_to_deactivate
        if not users_already_subscribed.get(sub.user_profile_id, False)
    ]

    if len(users_to_activate) > 0:
        bulk_add_subscriptions(realm, [stream_to_keep], users_to_activate, acting_user=None)

    # Move the messages, and delete the old copies from caches. We do
    # this before removing the subscription objects, to avoid messages
    # "disappearing" if an error interrupts this function.
    message_ids_to_clear = list(
        Message.objects.filter(
            # Uses index: zerver_message_realm_recipient_id
            realm_id=realm.id,
            recipient=recipient_to_destroy,
        ).values_list("id", flat=True)
    )
    count = Message.objects.filter(
        # Uses index: zerver_message_realm_recipient_id (prefix)
        realm_id=realm.id,
        recipient=recipient_to_destroy,
    ).update(recipient=recipient_to_keep)
    bulk_delete_cache_keys(message_ids_to_clear)

    # Remove subscriptions to the old stream.
    if len(subs_to_deactivate) > 0:
        bulk_remove_subscriptions(
            realm,
            [sub.user_profile for sub in subs_to_deactivate],
            [stream_to_destroy],
            acting_user=None,
        )

    do_deactivate_stream(stream_to_destroy, acting_user=None)

    return (len(users_to_activate), count, len(subs_to_deactivate))


def get_subscriber_ids(
    stream: Stream, requesting_user: Optional[UserProfile] = None
) -> ValuesQuerySet[Subscription, int]:
    subscriptions_query = get_subscribers_query(stream, requesting_user)
    return subscriptions_query.values_list("user_profile_id", flat=True)


def send_subscription_add_events(
    realm: Realm,
    sub_info_list: List[SubInfo],
    subscriber_dict: Dict[int, Set[int]],
) -> None:
    info_by_user: Dict[int, List[SubInfo]] = defaultdict(list)
    for sub_info in sub_info_list:
        info_by_user[sub_info.user.id].append(sub_info)

    stream_ids = {sub_info.stream.id for sub_info in sub_info_list}
    recent_traffic = get_streams_traffic(stream_ids=stream_ids, realm=realm)

    # We generally only have a few streams, so we compute subscriber
    # data in its own loop.
    stream_subscribers_dict: Dict[int, List[int]] = {}
    for sub_info in sub_info_list:
        stream = sub_info.stream
        if stream.id not in stream_subscribers_dict:
            if stream.is_in_zephyr_realm and not stream.invite_only:
                subscribers = []
            else:
                subscribers = list(subscriber_dict[stream.id])
            stream_subscribers_dict[stream.id] = subscribers

    for user_id, sub_infos in info_by_user.items():
        sub_dicts: List[APISubscriptionDict] = []
        for sub_info in sub_infos:
            stream = sub_info.stream
            stream_subscribers = stream_subscribers_dict[stream.id]
            subscription = sub_info.sub
            stream_dict = stream_to_dict(stream, recent_traffic)
            # This is verbose as we cannot unpack existing TypedDict
            # to initialize another TypedDict while making mypy happy.
            # https://github.com/python/mypy/issues/5382
            sub_dict = APISubscriptionDict(
                # Fields from Subscription.API_FIELDS
                audible_notifications=subscription.audible_notifications,
                color=subscription.color,
                desktop_notifications=subscription.desktop_notifications,
                email_notifications=subscription.email_notifications,
                is_muted=subscription.is_muted,
                pin_to_top=subscription.pin_to_top,
                push_notifications=subscription.push_notifications,
                wildcard_mentions_notify=subscription.wildcard_mentions_notify,
                # Computed fields not present in Subscription.API_FIELDS
                in_home_view=not subscription.is_muted,
                stream_weekly_traffic=stream_dict["stream_weekly_traffic"],
                subscribers=stream_subscribers,
                # Fields from Stream.API_FIELDS
                can_remove_subscribers_group=stream_dict["can_remove_subscribers_group"],
                creator_id=stream_dict["creator_id"],
                date_created=stream_dict["date_created"],
                description=stream_dict["description"],
                first_message_id=stream_dict["first_message_id"],
                history_public_to_subscribers=stream_dict["history_public_to_subscribers"],
                invite_only=stream_dict["invite_only"],
                is_web_public=stream_dict["is_web_public"],
                message_retention_days=stream_dict["message_retention_days"],
                name=stream_dict["name"],
                rendered_description=stream_dict["rendered_description"],
                stream_id=stream_dict["stream_id"],
                stream_post_policy=stream_dict["stream_post_policy"],
                # Computed fields not present in Stream.API_FIELDS
                is_announcement_only=stream_dict["is_announcement_only"],
            )

            sub_dicts.append(sub_dict)

        # Send a notification to the user who subscribed.
        event = dict(type="subscription", op="add", subscriptions=sub_dicts)
        send_event(realm, event, [user_id])


# This function contains all the database changes as part of
# subscribing users to streams; we use a transaction to ensure that
# the RealmAuditLog entries are created atomically with the
# Subscription object creation (and updates).
@transaction.atomic(savepoint=False)
def bulk_add_subs_to_db_with_logging(
    realm: Realm,
    acting_user: Optional[UserProfile],
    subs_to_add: List[SubInfo],
    subs_to_activate: List[SubInfo],
) -> None:
    Subscription.objects.bulk_create(info.sub for info in subs_to_add)
    sub_ids = [info.sub.id for info in subs_to_activate]
    Subscription.objects.filter(id__in=sub_ids).update(active=True)

    # Log subscription activities in RealmAuditLog
    event_time = timezone_now()
    event_last_message_id = get_last_message_id()

    all_subscription_logs = [
        RealmAuditLog(
            realm=realm,
            acting_user=acting_user,
            modified_user=sub_info.user,
            modified_stream=sub_info.stream,
            event_last_message_id=event_last_message_id,
            event_type=event_type,
            event_time=event_time,
        )
        for event_type, subs in [
            (RealmAuditLog.SUBSCRIPTION_CREATED, subs_to_add),
            (RealmAuditLog.SUBSCRIPTION_ACTIVATED, subs_to_activate),
        ]
        for sub_info in subs
    ]
    # Now since we have all log objects generated we can do a bulk insert
    RealmAuditLog.objects.bulk_create(all_subscription_logs)


def send_stream_creation_events_for_previously_inaccessible_streams(
    realm: Realm,
    stream_dict: Dict[int, Stream],
    altered_user_dict: Dict[int, Set[int]],
    altered_guests: Set[int],
) -> None:
    stream_ids = set(altered_user_dict.keys())
    recent_traffic = get_streams_traffic(stream_ids, realm)

    for stream_id, stream_users_ids in altered_user_dict.items():
        stream = stream_dict[stream_id]

        notify_user_ids = []
        if not stream.is_public():
            # Users newly added to invite-only streams
            # need a `create` notification.  The former, because
            # they need the stream to exist before
            # they get the "subscribe" notification, and the latter so
            # they can manage the new stream.
            # Realm admins already have all created private streams.
            realm_admin_ids = {user.id for user in realm.get_admin_users_and_bots()}
            notify_user_ids = list(stream_users_ids - realm_admin_ids)
        elif not stream.is_web_public:
            # Guese users need a `create` notification for
            # public streams as well because they need the stream
            # to exist before they get the "subscribe" notification.
            notify_user_ids = list(stream_users_ids & altered_guests)

        if notify_user_ids:
            send_stream_creation_event(realm, stream, notify_user_ids, recent_traffic)


def send_peer_subscriber_events(
    op: str,
    realm: Realm,
    stream_dict: Dict[int, Stream],
    altered_user_dict: Dict[int, Set[int]],
    subscriber_peer_info: SubscriberPeerInfo,
) -> None:
    # Send peer_add/peer_remove events to other users who are tracking the
    # subscribers lists of streams in their browser; everyone for
    # public streams and only existing subscribers for private streams.

    assert op in ["peer_add", "peer_remove"]

    private_stream_ids = [
        stream_id for stream_id in altered_user_dict if stream_dict[stream_id].invite_only
    ]
    private_peer_dict = subscriber_peer_info.private_peer_dict

    for stream_id in private_stream_ids:
        altered_user_ids = altered_user_dict[stream_id]
        peer_user_ids = private_peer_dict[stream_id] - altered_user_ids

        if peer_user_ids and altered_user_ids:
            event = dict(
                type="subscription",
                op=op,
                stream_ids=[stream_id],
                user_ids=sorted(altered_user_ids),
            )
            send_event_on_commit(realm, event, peer_user_ids)

    public_stream_ids = [
        stream_id
        for stream_id in altered_user_dict
        if not stream_dict[stream_id].invite_only and not stream_dict[stream_id].is_in_zephyr_realm
    ]

    web_public_stream_ids = [
        stream_id for stream_id in public_stream_ids if stream_dict[stream_id].is_web_public
    ]

    subscriber_dict = subscriber_peer_info.subscribed_ids

    if public_stream_ids:
        user_streams: Dict[int, Set[int]] = defaultdict(set)

        non_guest_user_ids = set(active_non_guest_user_ids(realm.id))

        if web_public_stream_ids:
            web_public_peer_ids = set(active_user_ids(realm.id))

        for stream_id in public_stream_ids:
            altered_user_ids = altered_user_dict[stream_id]

            if stream_id in web_public_stream_ids:
                peer_user_ids = web_public_peer_ids - altered_user_ids
            else:
                peer_user_ids = (non_guest_user_ids | subscriber_dict[stream_id]) - altered_user_ids

            if peer_user_ids and altered_user_ids:
                if len(altered_user_ids) == 1:
                    # If we only have one user, we will try to
                    # find other streams they have (un)subscribed to
                    # (where it's just them).  This optimization
                    # typically works when a single user is subscribed
                    # to multiple default public streams during
                    # new-user registration.
                    #
                    # This optimization depends on all public streams
                    # having the same peers for any single user, which
                    # isn't the case for private streams.
                    [altered_user_id] = altered_user_ids
                    user_streams[altered_user_id].add(stream_id)
                else:
                    event = dict(
                        type="subscription",
                        op=op,
                        stream_ids=[stream_id],
                        user_ids=sorted(altered_user_ids),
                    )
                    send_event_on_commit(realm, event, peer_user_ids)

        for user_id, stream_ids in user_streams.items():
            if stream_id in web_public_stream_ids:
                peer_user_ids = web_public_peer_ids - altered_user_ids
            else:
                peer_user_ids = (non_guest_user_ids | subscriber_dict[stream_id]) - altered_user_ids

            event = dict(
                type="subscription",
                op=op,
                stream_ids=sorted(stream_ids),
                user_ids=[user_id],
            )
            send_event_on_commit(realm, event, peer_user_ids)


def send_user_creation_events_on_adding_subscriptions(
    realm: Realm,
    altered_user_dict: Dict[int, Set[int]],
    altered_streams_dict: Dict[UserProfile, Set[int]],
    subscribers_of_altered_user_subscriptions: Dict[int, Set[int]],
) -> None:
    altered_users = list(altered_streams_dict.keys())
    non_guest_user_ids = active_non_guest_user_ids(realm.id)

    users_involved_in_dms = get_users_involved_in_dms_with_target_users(altered_users, realm)

    altered_stream_ids = altered_user_dict.keys()
    subscribers_dict = get_users_for_streams(set(altered_stream_ids))

    subscribers_user_id_map: Dict[int, UserProfile] = {}
    subscriber_ids_dict: Dict[int, Set[int]] = defaultdict(set)
    for stream_id, subscribers in subscribers_dict.items():
        for user in subscribers:
            subscriber_ids_dict[stream_id].add(user.id)
            subscribers_user_id_map[user.id] = user

    from zerver.actions.create_user import notify_created_user

    for user in altered_users:
        streams_for_user = altered_streams_dict[user]
        subscribers_in_altered_streams: Set[int] = set()
        for stream_id in streams_for_user:
            subscribers_in_altered_streams |= subscriber_ids_dict[stream_id]

        users_already_with_access_to_altered_user = (
            set(non_guest_user_ids)
            | subscribers_of_altered_user_subscriptions[user.id]
            | users_involved_in_dms[user.id]
            | {user.id}
        )

        users_to_receive_creation_event = (
            subscribers_in_altered_streams - users_already_with_access_to_altered_user
        )
        if users_to_receive_creation_event:
            notify_created_user(user, list(users_to_receive_creation_event))

        if user.is_guest:
            # If the altered user is a guest, then the user may receive
            # user creation events for subscribers of the new stream.
            users_already_accessible_to_altered_user = (
                subscribers_of_altered_user_subscriptions[user.id]
                | users_involved_in_dms[user.id]
                | {user.id}
            )

            new_accessible_user_ids = (
                subscribers_in_altered_streams - users_already_accessible_to_altered_user
            )
            for accessible_user_id in new_accessible_user_ids:
                accessible_user = subscribers_user_id_map[accessible_user_id]
                notify_created_user(accessible_user, [user.id])


SubT: TypeAlias = Tuple[List[SubInfo], List[SubInfo]]


def bulk_add_subscriptions(
    realm: Realm,
    streams: Collection[Stream],
    users: Iterable[UserProfile],
    color_map: Mapping[str, str] = {},
    from_user_creation: bool = False,
    *,
    acting_user: Optional[UserProfile],
) -> SubT:
    users = list(users)
    user_ids = [user.id for user in users]

    # Sanity check out callers
    for stream in streams:
        assert stream.realm_id == realm.id

    for user in users:
        assert user.realm_id == realm.id

    recipient_ids = [stream.recipient_id for stream in streams]
    recipient_id_to_stream = {stream.recipient_id: stream for stream in streams}

    recipient_color_map = {}
    recipient_ids_set: Set[int] = set()
    for stream in streams:
        assert stream.recipient_id is not None
        recipient_ids_set.add(stream.recipient_id)
        color: Optional[str] = color_map.get(stream.name, None)
        if color is not None:
            recipient_color_map[stream.recipient_id] = color

    used_colors_for_user_ids: Dict[int, Set[str]] = get_used_colors_for_user_ids(user_ids)

    existing_subs = Subscription.objects.filter(
        user_profile_id__in=user_ids,
        recipient__type=Recipient.STREAM,
        recipient_id__in=recipient_ids,
    )

    subs_by_user: Dict[int, List[Subscription]] = defaultdict(list)
    for sub in existing_subs:
        subs_by_user[sub.user_profile_id].append(sub)

    already_subscribed: List[SubInfo] = []
    subs_to_activate: List[SubInfo] = []
    subs_to_add: List[SubInfo] = []
    for user_profile in users:
        my_subs = subs_by_user[user_profile.id]

        # Make a fresh set of all new recipient ids, and then we will
        # remove any for which our user already has a subscription
        # (and we'll re-activate any subscriptions as needed).
        new_recipient_ids: Set[int] = recipient_ids_set.copy()

        for sub in my_subs:
            if sub.recipient_id in new_recipient_ids:
                new_recipient_ids.remove(sub.recipient_id)
                stream = recipient_id_to_stream[sub.recipient_id]
                sub_info = SubInfo(user_profile, sub, stream)
                if sub.active:
                    already_subscribed.append(sub_info)
                else:
                    subs_to_activate.append(sub_info)

        used_colors = used_colors_for_user_ids.get(user_profile.id, set())
        user_color_map = pick_colors(used_colors, recipient_color_map, list(new_recipient_ids))

        for recipient_id in new_recipient_ids:
            stream = recipient_id_to_stream[recipient_id]
            color = user_color_map[recipient_id]

            sub = Subscription(
                user_profile=user_profile,
                is_user_active=user_profile.is_active,
                active=True,
                color=color,
                recipient_id=recipient_id,
            )
            sub_info = SubInfo(user_profile, sub, stream)
            subs_to_add.append(sub_info)

    if len(subs_to_add) == 0 and len(subs_to_activate) == 0:
        # We can return early if users are already subscribed to all the streams.
        return ([], already_subscribed)

    altered_user_dict: Dict[int, Set[int]] = defaultdict(set)
    altered_guests: Set[int] = set()
    altered_streams_dict: Dict[UserProfile, Set[int]] = defaultdict(set)
    for sub_info in subs_to_add + subs_to_activate:
        altered_user_dict[sub_info.stream.id].add(sub_info.user.id)
        altered_streams_dict[sub_info.user].add(sub_info.stream.id)
        if sub_info.user.is_guest:
            altered_guests.add(sub_info.user.id)

    if realm.can_access_all_users_group.named_user_group.name != SystemGroups.EVERYONE:
        altered_users = list(altered_streams_dict.keys())
        subscribers_of_altered_user_subscriptions = get_subscribers_of_target_user_subscriptions(
            altered_users
        )

    bulk_add_subs_to_db_with_logging(
        realm=realm,
        acting_user=acting_user,
        subs_to_add=subs_to_add,
        subs_to_activate=subs_to_activate,
    )

    stream_dict = {stream.id: stream for stream in streams}

    new_streams = [stream_dict[stream_id] for stream_id in altered_user_dict]

    subscriber_peer_info = bulk_get_subscriber_peer_info(
        realm=realm,
        streams=new_streams,
    )

    # We now send several types of events to notify browsers.  The
    # first batches of notifications are sent only to the user(s)
    # being subscribed; we can skip these notifications when this is
    # being called from the new user creation flow.
    if not from_user_creation:
        send_stream_creation_events_for_previously_inaccessible_streams(
            realm=realm,
            stream_dict=stream_dict,
            altered_user_dict=altered_user_dict,
            altered_guests=altered_guests,
        )

        send_subscription_add_events(
            realm=realm,
            sub_info_list=subs_to_add + subs_to_activate,
            subscriber_dict=subscriber_peer_info.subscribed_ids,
        )

    if realm.can_access_all_users_group.named_user_group.name != SystemGroups.EVERYONE:
        send_user_creation_events_on_adding_subscriptions(
            realm,
            altered_user_dict,
            altered_streams_dict,
            subscribers_of_altered_user_subscriptions,
        )

    send_peer_subscriber_events(
        op="peer_add",
        realm=realm,
        altered_user_dict=altered_user_dict,
        stream_dict=stream_dict,
        subscriber_peer_info=subscriber_peer_info,
    )

    return (
        subs_to_add + subs_to_activate,
        already_subscribed,
    )


def send_peer_remove_events(
    realm: Realm,
    streams: List[Stream],
    altered_user_dict: Dict[int, Set[int]],
) -> None:
    subscriber_peer_info = bulk_get_subscriber_peer_info(
        realm=realm,
        streams=streams,
    )
    stream_dict = {stream.id: stream for stream in streams}

    send_peer_subscriber_events(
        op="peer_remove",
        realm=realm,
        stream_dict=stream_dict,
        altered_user_dict=altered_user_dict,
        subscriber_peer_info=subscriber_peer_info,
    )


def notify_subscriptions_removed(
    realm: Realm, user_profile: UserProfile, streams: Iterable[Stream]
) -> None:
    payload = [dict(name=stream.name, stream_id=stream.id) for stream in streams]
    event = dict(type="subscription", op="remove", subscriptions=payload)
    send_event_on_commit(realm, event, [user_profile.id])


SubAndRemovedT: TypeAlias = Tuple[
    List[Tuple[UserProfile, Stream]], List[Tuple[UserProfile, Stream]]
]


def send_subscription_remove_events(
    realm: Realm,
    users: List[UserProfile],
    streams: List[Stream],
    removed_subs: List[Tuple[UserProfile, Stream]],
) -> None:
    altered_user_dict: Dict[int, Set[int]] = defaultdict(set)
    streams_by_user: Dict[int, List[Stream]] = defaultdict(list)
    for user, stream in removed_subs:
        streams_by_user[user.id].append(stream)
        altered_user_dict[stream.id].add(user.id)

    for user_profile in users:
        if len(streams_by_user[user_profile.id]) == 0:
            continue
        notify_subscriptions_removed(realm, user_profile, streams_by_user[user_profile.id])

        event = {
            "type": "mark_stream_messages_as_read",
            "user_profile_id": user_profile.id,
            "stream_recipient_ids": [
                stream.recipient_id for stream in streams_by_user[user_profile.id]
            ],
        }
        queue_event_on_commit("deferred_work", event)

        if not user_profile.is_realm_admin:
            inaccessible_streams = [
                stream
                for stream in streams_by_user[user_profile.id]
                if not check_basic_stream_access(
                    user_profile, stream, sub=None, allow_realm_admin=True
                )
            ]

            if inaccessible_streams:
                payload = [stream_to_dict(stream) for stream in inaccessible_streams]
                event = dict(type="stream", op="delete", streams=payload)
                send_event_on_commit(realm, event, [user_profile.id])

    send_peer_remove_events(
        realm=realm,
        streams=streams,
        altered_user_dict=altered_user_dict,
    )


def send_user_remove_events_on_removing_subscriptions(
    realm: Realm, altered_user_dict: Dict[UserProfile, Set[int]]
) -> None:
    altered_stream_ids: Set[int] = set()
    altered_users = list(altered_user_dict.keys())
    for stream_ids in altered_user_dict.values():
        altered_stream_ids |= stream_ids

    users_involved_in_dms = get_users_involved_in_dms_with_target_users(altered_users, realm)
    subscribers_of_altered_user_subscriptions = get_subscribers_of_target_user_subscriptions(
        altered_users
    )

    non_guest_user_ids = active_non_guest_user_ids(realm.id)

    subscribers_dict = get_user_ids_for_streams(altered_stream_ids)

    for user in altered_users:
        users_in_unsubscribed_streams: Set[int] = set()
        for stream_id in altered_user_dict[user]:
            users_in_unsubscribed_streams = (
                users_in_unsubscribed_streams | subscribers_dict[stream_id]
            )

        users_who_can_access_altered_user = (
            set(non_guest_user_ids)
            | subscribers_of_altered_user_subscriptions[user.id]
            | users_involved_in_dms[user.id]
            | {user.id}
        )

        subscribers_without_access_to_altered_user = (
            users_in_unsubscribed_streams - users_who_can_access_altered_user
        )

        if subscribers_without_access_to_altered_user:
            event_remove_user = dict(
                type="realm_user",
                op="remove",
                person=dict(user_id=user.id, full_name=str(UserProfile.INACCESSIBLE_USER_NAME)),
            )
            send_event_on_commit(
                realm, event_remove_user, list(subscribers_without_access_to_altered_user)
            )

        if user.is_guest:
            users_inaccessible_to_altered_user = users_in_unsubscribed_streams - (
                subscribers_of_altered_user_subscriptions[user.id]
                | users_involved_in_dms[user.id]
                | {user.id}
            )

            for user_id in users_inaccessible_to_altered_user:
                event_remove_user = dict(
                    type="realm_user",
                    op="remove",
                    person=dict(user_id=user_id, full_name=str(UserProfile.INACCESSIBLE_USER_NAME)),
                )
                send_event_on_commit(realm, event_remove_user, [user.id])


def bulk_remove_subscriptions(
    realm: Realm,
    users: Iterable[UserProfile],
    streams: Iterable[Stream],
    *,
    acting_user: Optional[UserProfile],
) -> SubAndRemovedT:
    users = list(users)
    streams = list(streams)

    # Sanity check our callers
    for stream in streams:
        assert stream.realm_id == realm.id

    for user in users:
        assert user.realm_id == realm.id

    stream_dict = {stream.id: stream for stream in streams}

    existing_subs_by_user = get_bulk_stream_subscriber_info(users, streams)

    def get_non_subscribed_subs() -> List[Tuple[UserProfile, Stream]]:
        stream_ids = {stream.id for stream in streams}

        not_subscribed: List[Tuple[UserProfile, Stream]] = []

        for user_profile in users:
            user_sub_stream_info = existing_subs_by_user[user_profile.id]

            subscribed_stream_ids = {sub_info.stream.id for sub_info in user_sub_stream_info}
            not_subscribed_stream_ids = stream_ids - subscribed_stream_ids

            not_subscribed.extend(
                (user_profile, stream_dict[stream_id]) for stream_id in not_subscribed_stream_ids
            )

        return not_subscribed

    not_subscribed = get_non_subscribed_subs()

    # This loop just flattens out our data into big lists for
    # bulk operations.
    subs_to_deactivate = [
        sub_info for sub_infos in existing_subs_by_user.values() for sub_info in sub_infos
    ]

    if len(subs_to_deactivate) == 0:
        # We can return early if users are not subscribed to any of the streams.
        return ([], not_subscribed)

    sub_ids_to_deactivate = [sub_info.sub.id for sub_info in subs_to_deactivate]
    streams_to_unsubscribe = [sub_info.stream for sub_info in subs_to_deactivate]
    # We do all the database changes in a transaction to ensure
    # RealmAuditLog entries are atomically created when making changes.
    with transaction.atomic():
        Subscription.objects.filter(
            id__in=sub_ids_to_deactivate,
        ).update(active=False)
        occupied_streams_after = list(get_occupied_streams(realm))

        # Log subscription activities in RealmAuditLog
        event_time = timezone_now()
        event_last_message_id = get_last_message_id()
        all_subscription_logs = [
            RealmAuditLog(
                realm=sub_info.user.realm,
                acting_user=acting_user,
                modified_user=sub_info.user,
                modified_stream=sub_info.stream,
                event_last_message_id=event_last_message_id,
                event_type=RealmAuditLog.SUBSCRIPTION_DEACTIVATED,
                event_time=event_time,
            )
            for sub_info in subs_to_deactivate
        ]

        # Now since we have all log objects generated we can do a bulk insert
        RealmAuditLog.objects.bulk_create(all_subscription_logs)

    removed_sub_tuples = [(sub_info.user, sub_info.stream) for sub_info in subs_to_deactivate]
    send_subscription_remove_events(realm, users, streams, removed_sub_tuples)

    if realm.can_access_all_users_group.named_user_group.name != SystemGroups.EVERYONE:
        altered_user_dict: Dict[UserProfile, Set[int]] = defaultdict(set)
        for user, stream in removed_sub_tuples:
            altered_user_dict[user].add(stream.id)
        send_user_remove_events_on_removing_subscriptions(realm, altered_user_dict)

    new_vacant_streams = set(streams_to_unsubscribe) - set(occupied_streams_after)
    new_vacant_private_streams = [stream for stream in new_vacant_streams if stream.invite_only]

    if new_vacant_private_streams:
        # Deactivate any newly-vacant private streams
        for stream in new_vacant_private_streams:
            do_deactivate_stream(stream, acting_user=acting_user)

    return (
        removed_sub_tuples,
        not_subscribed,
    )


def do_change_subscription_property(
    user_profile: UserProfile,
    sub: Subscription,
    stream: Stream,
    property_name: str,
    value: Any,
    *,
    acting_user: Optional[UserProfile],
) -> None:
    database_property_name = property_name
    database_value = value

    # For this property, is_muted is used in the database, but
    # in_home_view is still in the API, since we haven't fully
    # migrated to the new name yet.
    if property_name == "in_home_view":
        database_property_name = "is_muted"
        database_value = not value

    old_value = getattr(sub, database_property_name)
    setattr(sub, database_property_name, database_value)
    sub.save(update_fields=[database_property_name])
    event_time = timezone_now()
    RealmAuditLog.objects.create(
        realm=user_profile.realm,
        event_type=RealmAuditLog.SUBSCRIPTION_PROPERTY_CHANGED,
        event_time=event_time,
        modified_user=user_profile,
        acting_user=acting_user,
        modified_stream=stream,
        extra_data={
            RealmAuditLog.OLD_VALUE: old_value,
            RealmAuditLog.NEW_VALUE: database_value,
            "property": database_property_name,
        },
    )

    # This first in_home_view event is deprecated and will be removed
    # once clients are migrated to handle the subscription update event
    # with is_muted as the property name.
    if database_property_name == "is_muted":
        event_value = not database_value
        in_home_view_event = dict(
            type="subscription",
            op="update",
            property="in_home_view",
            value=event_value,
            stream_id=stream.id,
        )

        send_event(user_profile.realm, in_home_view_event, [user_profile.id])

    event = dict(
        type="subscription",
        op="update",
        property=database_property_name,
        value=database_value,
        stream_id=stream.id,
    )
    send_event(user_profile.realm, event, [user_profile.id])


def send_change_stream_permission_notification(
    stream: Stream,
    *,
    old_policy_name: str,
    new_policy_name: str,
    acting_user: UserProfile,
) -> None:
    sender = get_system_bot(settings.NOTIFICATION_BOT, acting_user.realm_id)
    user_mention = silent_mention_syntax_for_user(acting_user)

    with override_language(stream.realm.default_language):
        notification_string = _(
            "{user} changed the [access permissions]({help_link}) "
            "for this channel from **{old_policy}** to **{new_policy}**."
        )
        notification_string = notification_string.format(
            user=user_mention,
            help_link="/help/channel-permissions",
            old_policy=old_policy_name,
            new_policy=new_policy_name,
        )
        internal_send_stream_message(
            sender, stream, str(Realm.STREAM_EVENTS_NOTIFICATION_TOPIC_NAME), notification_string
        )


def do_change_stream_permission(
    stream: Stream,
    *,
    invite_only: bool,
    history_public_to_subscribers: bool,
    is_web_public: bool,
    acting_user: UserProfile,
) -> None:
    old_invite_only_value = stream.invite_only
    old_history_public_to_subscribers_value = stream.history_public_to_subscribers
    old_is_web_public_value = stream.is_web_public

    stream.is_web_public = is_web_public
    stream.invite_only = invite_only
    stream.history_public_to_subscribers = history_public_to_subscribers

    realm = stream.realm

    with transaction.atomic():
        stream.save(update_fields=["invite_only", "history_public_to_subscribers", "is_web_public"])

        event_time = timezone_now()
        if old_invite_only_value != stream.invite_only:
            # Reset the Attachment.is_realm_public cache for all
            # messages in the stream whose permissions were changed.
            assert stream.recipient_id is not None
            Attachment.objects.filter(messages__recipient_id=stream.recipient_id).update(
                is_realm_public=None
            )
            # We need to do the same for ArchivedAttachment to avoid
            # bugs if deleted attachments are later restored.
            ArchivedAttachment.objects.filter(messages__recipient_id=stream.recipient_id).update(
                is_realm_public=None
            )

            RealmAuditLog.objects.create(
                realm=realm,
                acting_user=acting_user,
                modified_stream=stream,
                event_type=RealmAuditLog.STREAM_PROPERTY_CHANGED,
                event_time=event_time,
                extra_data={
                    RealmAuditLog.OLD_VALUE: old_invite_only_value,
                    RealmAuditLog.NEW_VALUE: stream.invite_only,
                    "property": "invite_only",
                },
            )

        if old_history_public_to_subscribers_value != stream.history_public_to_subscribers:
            RealmAuditLog.objects.create(
                realm=realm,
                acting_user=acting_user,
                modified_stream=stream,
                event_type=RealmAuditLog.STREAM_PROPERTY_CHANGED,
                event_time=event_time,
                extra_data={
                    RealmAuditLog.OLD_VALUE: old_history_public_to_subscribers_value,
                    RealmAuditLog.NEW_VALUE: stream.history_public_to_subscribers,
                    "property": "history_public_to_subscribers",
                },
            )

        if old_is_web_public_value != stream.is_web_public:
            # Reset the Attachment.is_realm_public cache for all
            # messages in the stream whose permissions were changed.
            assert stream.recipient_id is not None
            Attachment.objects.filter(messages__recipient_id=stream.recipient_id).update(
                is_web_public=None
            )
            # We need to do the same for ArchivedAttachment to avoid
            # bugs if deleted attachments are later restored.
            ArchivedAttachment.objects.filter(messages__recipient_id=stream.recipient_id).update(
                is_web_public=None
            )

            RealmAuditLog.objects.create(
                realm=realm,
                acting_user=acting_user,
                modified_stream=stream,
                event_type=RealmAuditLog.STREAM_PROPERTY_CHANGED,
                event_time=event_time,
                extra_data={
                    RealmAuditLog.OLD_VALUE: old_is_web_public_value,
                    RealmAuditLog.NEW_VALUE: stream.is_web_public,
                    "property": "is_web_public",
                },
            )

    notify_stream_creation_ids = set()
    if old_invite_only_value and not stream.invite_only:
        # We need to send stream creation event to users who can access the
        # stream now but were not able to do so previously. So, we can exclude
        # subscribers and realm admins from the non-guest user list.
        stream_subscriber_user_ids = get_active_subscriptions_for_stream_id(
            stream.id, include_deactivated_users=False
        ).values_list("user_profile_id", flat=True)

        old_can_access_stream_user_ids = set(stream_subscriber_user_ids) | {
            user.id for user in stream.realm.get_admin_users_and_bots()
        }
        non_guest_user_ids = set(active_non_guest_user_ids(stream.realm_id))
        notify_stream_creation_ids = non_guest_user_ids - old_can_access_stream_user_ids

        recent_traffic = get_streams_traffic({stream.id}, realm)
        send_stream_creation_event(realm, stream, list(notify_stream_creation_ids), recent_traffic)

        # Add subscribers info to the stream object. We need to send peer_add
        # events to users who were previously subscribed to the streams as
        # they did not had subscribers data.
        old_subscribers_access_user_ids = set(stream_subscriber_user_ids) | {
            user.id for user in stream.realm.get_admin_users_and_bots()
        }
        peer_notify_user_ids = non_guest_user_ids - old_subscribers_access_user_ids
        peer_add_event = dict(
            type="subscription",
            op="peer_add",
            stream_ids=[stream.id],
            user_ids=sorted(stream_subscriber_user_ids),
        )
        send_event(stream.realm, peer_add_event, peer_notify_user_ids)

    event = dict(
        op="update",
        type="stream",
        property="invite_only",
        value=stream.invite_only,
        history_public_to_subscribers=stream.history_public_to_subscribers,
        is_web_public=stream.is_web_public,
        stream_id=stream.id,
        name=stream.name,
    )
    # we do not need to send update events to the users who received creation event
    # since they already have the updated stream info.
    notify_stream_update_ids = can_access_stream_user_ids(stream) - notify_stream_creation_ids
    send_event(stream.realm, event, notify_stream_update_ids)

    old_policy_name = get_stream_permission_policy_name(
        invite_only=old_invite_only_value,
        history_public_to_subscribers=old_history_public_to_subscribers_value,
        is_web_public=old_is_web_public_value,
    )
    new_policy_name = get_stream_permission_policy_name(
        invite_only=stream.invite_only,
        history_public_to_subscribers=stream.history_public_to_subscribers,
        is_web_public=stream.is_web_public,
    )
    send_change_stream_permission_notification(
        stream,
        old_policy_name=old_policy_name,
        new_policy_name=new_policy_name,
        acting_user=acting_user,
    )


def send_change_stream_post_policy_notification(
    stream: Stream, *, old_post_policy: int, new_post_policy: int, acting_user: UserProfile
) -> None:
    sender = get_system_bot(settings.NOTIFICATION_BOT, acting_user.realm_id)
    user_mention = silent_mention_syntax_for_user(acting_user)

    with override_language(stream.realm.default_language):
        notification_string = _(
            "{user} changed the [posting permissions]({help_link}) "
            "for this channel:\n\n"
            "* **Old permissions**: {old_policy}.\n"
            "* **New permissions**: {new_policy}.\n"
        )
        notification_string = notification_string.format(
            user=user_mention,
            help_link="/help/channel-posting-policy",
            old_policy=Stream.POST_POLICIES[old_post_policy],
            new_policy=Stream.POST_POLICIES[new_post_policy],
        )
        internal_send_stream_message(
            sender, stream, str(Realm.STREAM_EVENTS_NOTIFICATION_TOPIC_NAME), notification_string
        )


def do_change_stream_post_policy(
    stream: Stream, stream_post_policy: int, *, acting_user: UserProfile
) -> None:
    old_post_policy = stream.stream_post_policy
    with transaction.atomic():
        stream.stream_post_policy = stream_post_policy
        stream.save(update_fields=["stream_post_policy"])
        RealmAuditLog.objects.create(
            realm=stream.realm,
            acting_user=acting_user,
            modified_stream=stream,
            event_type=RealmAuditLog.STREAM_PROPERTY_CHANGED,
            event_time=timezone_now(),
            extra_data={
                RealmAuditLog.OLD_VALUE: old_post_policy,
                RealmAuditLog.NEW_VALUE: stream_post_policy,
                "property": "stream_post_policy",
            },
        )

    event = dict(
        op="update",
        type="stream",
        property="stream_post_policy",
        value=stream_post_policy,
        stream_id=stream.id,
        name=stream.name,
    )
    send_event(stream.realm, event, can_access_stream_user_ids(stream))

    # Backwards-compatibility code: We removed the
    # is_announcement_only property in early 2020, but we send a
    # duplicate event for legacy mobile clients that might want the
    # data.
    event = dict(
        op="update",
        type="stream",
        property="is_announcement_only",
        value=stream.stream_post_policy == Stream.STREAM_POST_POLICY_ADMINS,
        stream_id=stream.id,
        name=stream.name,
    )
    send_event(stream.realm, event, can_access_stream_user_ids(stream))

    send_change_stream_post_policy_notification(
        stream,
        old_post_policy=old_post_policy,
        new_post_policy=stream_post_policy,
        acting_user=acting_user,
    )


def do_rename_stream(stream: Stream, new_name: str, user_profile: UserProfile) -> None:
    old_name = stream.name
    stream.name = new_name
    stream.save(update_fields=["name"])

    RealmAuditLog.objects.create(
        realm=stream.realm,
        acting_user=user_profile,
        modified_stream=stream,
        event_type=RealmAuditLog.STREAM_NAME_CHANGED,
        event_time=timezone_now(),
        extra_data={
            RealmAuditLog.OLD_VALUE: old_name,
            RealmAuditLog.NEW_VALUE: new_name,
        },
    )

    assert stream.recipient_id is not None
    recipient_id: int = stream.recipient_id
    messages = Message.objects.filter(
        # Uses index: zerver_message_realm_recipient_id
        realm_id=stream.realm_id,
        recipient_id=recipient_id,
    ).only("id")

    cache_set(display_recipient_cache_key(recipient_id), stream.name)

    # Delete cache entries for everything else, which is cheaper and
    # clearer than trying to set them. display_recipient is the out of
    # date field in all cases.
    cache_delete_many(to_dict_cache_key_id(message.id) for message in messages)

    # We want to key these updates by id, not name, since id is
    # the immutable primary key, and obviously name is not.
    event = dict(
        op="update",
        type="stream",
        property="name",
        value=new_name,
        stream_id=stream.id,
        name=old_name,
    )
    send_event(stream.realm, event, can_access_stream_user_ids(stream))
    sender = get_system_bot(settings.NOTIFICATION_BOT, stream.realm_id)
    with override_language(stream.realm.default_language):
        internal_send_stream_message(
            sender,
            stream,
            str(Realm.STREAM_EVENTS_NOTIFICATION_TOPIC_NAME),
            _("{user_name} renamed channel {old_channel_name} to {new_channel_name}.").format(
                user_name=silent_mention_syntax_for_user(user_profile),
                old_channel_name=f"**{old_name}**",
                new_channel_name=f"**{new_name}**",
            ),
        )


def send_change_stream_description_notification(
    stream: Stream, *, old_description: str, new_description: str, acting_user: UserProfile
) -> None:
    sender = get_system_bot(settings.NOTIFICATION_BOT, acting_user.realm_id)
    user_mention = silent_mention_syntax_for_user(acting_user)

    with override_language(stream.realm.default_language):
        if new_description == "":
            new_description = "*" + _("No description.") + "*"
        if old_description == "":
            old_description = "*" + _("No description.") + "*"

        notification_string = (
            _("{user} changed the description for this channel.").format(user=user_mention)
            + "\n\n* **"
            + _("Old description")
            + ":**"
            + f"\n```` quote\n{old_description}\n````\n"
            + "* **"
            + _("New description")
            + ":**"
            + f"\n```` quote\n{new_description}\n````"
        )

        internal_send_stream_message(
            sender, stream, str(Realm.STREAM_EVENTS_NOTIFICATION_TOPIC_NAME), notification_string
        )


def do_change_stream_description(
    stream: Stream, new_description: str, *, acting_user: UserProfile
) -> None:
    old_description = stream.description

    with transaction.atomic():
        stream.description = new_description
        stream.rendered_description = render_stream_description(new_description, stream.realm)
        stream.save(update_fields=["description", "rendered_description"])
        RealmAuditLog.objects.create(
            realm=stream.realm,
            acting_user=acting_user,
            modified_stream=stream,
            event_type=RealmAuditLog.STREAM_PROPERTY_CHANGED,
            event_time=timezone_now(),
            extra_data={
                RealmAuditLog.OLD_VALUE: old_description,
                RealmAuditLog.NEW_VALUE: new_description,
                "property": "description",
            },
        )

    event = dict(
        type="stream",
        op="update",
        property="description",
        name=stream.name,
        stream_id=stream.id,
        value=new_description,
        rendered_description=stream.rendered_description,
    )
    send_event(stream.realm, event, can_access_stream_user_ids(stream))

    send_change_stream_description_notification(
        stream,
        old_description=old_description,
        new_description=new_description,
        acting_user=acting_user,
    )


def send_change_stream_message_retention_days_notification(
    user_profile: UserProfile, stream: Stream, old_value: Optional[int], new_value: Optional[int]
) -> None:
    sender = get_system_bot(settings.NOTIFICATION_BOT, user_profile.realm_id)
    user_mention = silent_mention_syntax_for_user(user_profile)

    # If switching from or to the organization's default retention policy,
    # we want to take the realm's default into account.
    if old_value is None:
        old_value = stream.realm.message_retention_days
    if new_value is None:
        new_value = stream.realm.message_retention_days

    with override_language(stream.realm.default_language):
        if old_value == Stream.MESSAGE_RETENTION_SPECIAL_VALUES_MAP["unlimited"]:
            old_retention_period = _("Forever")
            new_retention_period = _("{number_of_days} days").format(number_of_days=new_value)
            summary_line = _(
                "Messages in this channel will now be automatically deleted {number_of_days} days after they are sent."
            ).format(number_of_days=new_value)
        elif new_value == Stream.MESSAGE_RETENTION_SPECIAL_VALUES_MAP["unlimited"]:
            old_retention_period = _("{number_of_days} days").format(number_of_days=old_value)
            new_retention_period = _("Forever")
            summary_line = _("Messages in this channel will now be retained forever.")
        else:
            old_retention_period = _("{number_of_days} days").format(number_of_days=old_value)
            new_retention_period = _("{number_of_days} days").format(number_of_days=new_value)
            summary_line = _(
                "Messages in this channel will now be automatically deleted {number_of_days} days after they are sent."
            ).format(number_of_days=new_value)
        notification_string = _(
            "{user} has changed the [message retention period]({help_link}) for this channel:\n"
            "* **Old retention period**: {old_retention_period}\n"
            "* **New retention period**: {new_retention_period}\n\n"
            "{summary_line}"
        )
        notification_string = notification_string.format(
            user=user_mention,
            help_link="/help/message-retention-policy",
            old_retention_period=old_retention_period,
            new_retention_period=new_retention_period,
            summary_line=summary_line,
        )
        internal_send_stream_message(
            sender, stream, str(Realm.STREAM_EVENTS_NOTIFICATION_TOPIC_NAME), notification_string
        )


def do_change_stream_message_retention_days(
    stream: Stream, acting_user: UserProfile, message_retention_days: Optional[int] = None
) -> None:
    old_message_retention_days_value = stream.message_retention_days

    with transaction.atomic():
        stream.message_retention_days = message_retention_days
        stream.save(update_fields=["message_retention_days"])
        RealmAuditLog.objects.create(
            realm=stream.realm,
            acting_user=acting_user,
            modified_stream=stream,
            event_type=RealmAuditLog.STREAM_MESSAGE_RETENTION_DAYS_CHANGED,
            event_time=timezone_now(),
            extra_data={
                RealmAuditLog.OLD_VALUE: old_message_retention_days_value,
                RealmAuditLog.NEW_VALUE: message_retention_days,
            },
        )

    event = dict(
        op="update",
        type="stream",
        property="message_retention_days",
        value=message_retention_days,
        stream_id=stream.id,
        name=stream.name,
    )
    send_event(stream.realm, event, can_access_stream_user_ids(stream))
    send_change_stream_message_retention_days_notification(
        user_profile=acting_user,
        stream=stream,
        old_value=old_message_retention_days_value,
        new_value=message_retention_days,
    )


def do_change_stream_group_based_setting(
    stream: Stream,
    setting_name: str,
    user_group: UserGroup,
    *,
    acting_user: Optional[UserProfile] = None,
) -> None:
    old_user_group = getattr(stream, setting_name)
    old_user_group_id = None
    if old_user_group is not None:
        old_user_group_id = old_user_group.id

    setattr(stream, setting_name, user_group)
    stream.save()

    RealmAuditLog.objects.create(
        realm=stream.realm,
        acting_user=acting_user,
        modified_stream=stream,
        event_type=RealmAuditLog.STREAM_GROUP_BASED_SETTING_CHANGED,
        event_time=timezone_now(),
        extra_data={
            RealmAuditLog.OLD_VALUE: old_user_group_id,
            RealmAuditLog.NEW_VALUE: user_group.id,
            "property": setting_name,
        },
    )
    event = dict(
        op="update",
        type="stream",
        property=setting_name,
        value=user_group.id,
        stream_id=stream.id,
        name=stream.name,
    )
    send_event_on_commit(stream.realm, event, can_access_stream_user_ids(stream))