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                        <h1 id="spam-checker-callbacks"><a class="header" href="#spam-checker-callbacks">Spam checker callbacks</a></h1>
<p>Spam checker callbacks allow module developers to implement spam mitigation actions for
Synapse instances. Spam checker callbacks can be registered using the module API's
<code>register_spam_checker_callbacks</code> method.</p>
<h2 id="callbacks"><a class="header" href="#callbacks">Callbacks</a></h2>
<p>The available spam checker callbacks are:</p>
<h3 id="check_event_for_spam"><a class="header" href="#check_event_for_spam"><code>check_event_for_spam</code></a></h3>
<p><em>First introduced in Synapse v1.37.0</em></p>
<p><em>Changed in Synapse v1.60.0: <code>synapse.module_api.NOT_SPAM</code> and <code>synapse.module_api.errors.Codes</code> can be returned by this callback. Returning a boolean or a string is now deprecated.</em> </p>
<pre><code class="language-python">async def check_event_for_spam(event: &quot;synapse.module_api.EventBase&quot;) -&gt; Union[&quot;synapse.module_api.NOT_SPAM&quot;, &quot;synapse.module_api.errors.Codes&quot;, str, bool]
</code></pre>
<p>Called when receiving an event from a client or via federation. The callback must return one of:</p>
<ul>
<li><code>synapse.module_api.NOT_SPAM</code>, to allow the operation. Other callbacks may still 
decide to reject it.</li>
<li><code>synapse.module_api.errors.Codes</code> to reject the operation with an error code. In case
of doubt, <code>synapse.module_api.errors.Codes.FORBIDDEN</code> is a good error code.</li>
<li>(deprecated) a non-<code>Codes</code> <code>str</code> to reject the operation and specify an error message. Note that clients
typically will not localize the error message to the user's preferred locale.</li>
<li>(deprecated) <code>False</code>, which is the same as returning <code>synapse.module_api.NOT_SPAM</code>.</li>
<li>(deprecated) <code>True</code>, which is the same as returning <code>synapse.module_api.errors.Codes.FORBIDDEN</code>.</li>
</ul>
<p>If multiple modules implement this callback, they will be considered in order. If a
callback returns <code>synapse.module_api.NOT_SPAM</code>, Synapse falls through to the next one.
The value of the first callback that does not return <code>synapse.module_api.NOT_SPAM</code> will
be used. If this happens, Synapse will not call any of the subsequent implementations of
this callback.</p>
<h3 id="user_may_join_room"><a class="header" href="#user_may_join_room"><code>user_may_join_room</code></a></h3>
<p><em>First introduced in Synapse v1.37.0</em></p>
<p><em>Changed in Synapse v1.61.0: <code>synapse.module_api.NOT_SPAM</code> and <code>synapse.module_api.errors.Codes</code> can be returned by this callback. Returning a boolean is now deprecated.</em> </p>
<pre><code class="language-python">async def user_may_join_room(user: str, room: str, is_invited: bool) -&gt; Union[&quot;synapse.module_api.NOT_SPAM&quot;, &quot;synapse.module_api.errors.Codes&quot;, bool]
</code></pre>
<p>Called when a user is trying to join a room. The user is represented by their Matrix user ID (e.g.
<code>@alice:example.com</code>) and the room is represented by its Matrix ID (e.g.
<code>!room:example.com</code>). The module is also given a boolean to indicate whether the user
currently has a pending invite in the room.</p>
<p>This callback isn't called if the join is performed by a server administrator, or in the
context of a room creation.</p>
<p>The callback must return one of:</p>
<ul>
<li><code>synapse.module_api.NOT_SPAM</code>, to allow the operation. Other callbacks may still 
decide to reject it.</li>
<li><code>synapse.module_api.errors.Codes</code> to reject the operation with an error code. In case
of doubt, <code>synapse.module_api.errors.Codes.FORBIDDEN</code> is a good error code.</li>
<li>(deprecated) <code>False</code>, which is the same as returning <code>synapse.module_api.NOT_SPAM</code>.</li>
<li>(deprecated) <code>True</code>, which is the same as returning <code>synapse.module_api.errors.Codes.FORBIDDEN</code>.</li>
</ul>
<p>If multiple modules implement this callback, they will be considered in order. If a
callback returns <code>synapse.module_api.NOT_SPAM</code>, Synapse falls through to the next one.
The value of the first callback that does not return <code>synapse.module_api.NOT_SPAM</code> will
be used. If this happens, Synapse will not call any of the subsequent implementations of
this callback.</p>
<h3 id="user_may_invite"><a class="header" href="#user_may_invite"><code>user_may_invite</code></a></h3>
<p><em>First introduced in Synapse v1.37.0</em></p>
<p><em>Changed in Synapse v1.62.0: <code>synapse.module_api.NOT_SPAM</code> and <code>synapse.module_api.errors.Codes</code> can be returned by this callback. Returning a boolean is now deprecated.</em> </p>
<pre><code class="language-python">async def user_may_invite(inviter: str, invitee: str, room_id: str) -&gt; Union[&quot;synapse.module_api.NOT_SPAM&quot;, &quot;synapse.module_api.errors.Codes&quot;, bool]
</code></pre>
<p>Called when processing an invitation. Both inviter and invitee are
represented by their Matrix user ID (e.g. <code>@alice:example.com</code>).</p>
<p>The callback must return one of:</p>
<ul>
<li>
<p><code>synapse.module_api.NOT_SPAM</code>, to allow the operation. Other callbacks may still 
decide to reject it.</p>
</li>
<li>
<p><code>synapse.module_api.errors.Codes</code> to reject the operation with an error code. In case
of doubt, <code>synapse.module_api.errors.Codes.FORBIDDEN</code> is a good error code.</p>
</li>
<li>
<p>(deprecated) <code>False</code>, which is the same as returning <code>synapse.module_api.NOT_SPAM</code>.</p>
</li>
<li>
<p>(deprecated) <code>True</code>, which is the same as returning <code>synapse.module_api.errors.Codes.FORBIDDEN</code>.</p>
</li>
</ul>
<p>If multiple modules implement this callback, they will be considered in order. If a
callback returns <code>synapse.module_api.NOT_SPAM</code>, Synapse falls through to the next one.
The value of the first callback that does not return <code>synapse.module_api.NOT_SPAM</code> will
be used. If this happens, Synapse will not call any of the subsequent implementations of
this callback.</p>
<h3 id="user_may_send_3pid_invite"><a class="header" href="#user_may_send_3pid_invite"><code>user_may_send_3pid_invite</code></a></h3>
<p><em>First introduced in Synapse v1.45.0</em></p>
<p><em>Changed in Synapse v1.62.0: <code>synapse.module_api.NOT_SPAM</code> and <code>synapse.module_api.errors.Codes</code> can be returned by this callback. Returning a boolean is now deprecated.</em> </p>
<pre><code class="language-python">async def user_may_send_3pid_invite(
    inviter: str,
    medium: str,
    address: str,
    room_id: str,
) -&gt; Union[&quot;synapse.module_api.NOT_SPAM&quot;, &quot;synapse.module_api.errors.Codes&quot;, bool]
</code></pre>
<p>Called when processing an invitation using a third-party identifier (also called a 3PID,
e.g. an email address or a phone number). </p>
<p>The inviter is represented by their Matrix user ID (e.g. <code>@alice:example.com</code>), and the
invitee is represented by its medium (e.g. &quot;email&quot;) and its address
(e.g. <code>alice@example.com</code>). See <a href="https://matrix.org/docs/spec/appendices#pid-types">the Matrix specification</a>
for more information regarding third-party identifiers.</p>
<p>For example, a call to this callback to send an invitation to the email address
<code>alice@example.com</code> would look like this:</p>
<pre><code class="language-python">await user_may_send_3pid_invite(
    &quot;@bob:example.com&quot;,  # The inviter's user ID
    &quot;email&quot;,  # The medium of the 3PID to invite
    &quot;alice@example.com&quot;,  # The address of the 3PID to invite
    &quot;!some_room:example.com&quot;,  # The ID of the room to send the invite into
)
</code></pre>
<p><strong>Note</strong>: If the third-party identifier is already associated with a matrix user ID,
<a href="#user_may_invite"><code>user_may_invite</code></a> will be used instead.</p>
<p>The callback must return one of:</p>
<ul>
<li>
<p><code>synapse.module_api.NOT_SPAM</code>, to allow the operation. Other callbacks may still 
decide to reject it.</p>
</li>
<li>
<p><code>synapse.module_api.errors.Codes</code> to reject the operation with an error code. In case
of doubt, <code>synapse.module_api.errors.Codes.FORBIDDEN</code> is a good error code.</p>
</li>
<li>
<p>(deprecated) <code>False</code>, which is the same as returning <code>synapse.module_api.NOT_SPAM</code>.</p>
</li>
<li>
<p>(deprecated) <code>True</code>, which is the same as returning <code>synapse.module_api.errors.Codes.FORBIDDEN</code>.</p>
</li>
</ul>
<p>If multiple modules implement this callback, they will be considered in order. If a
callback returns <code>synapse.module_api.NOT_SPAM</code>, Synapse falls through to the next one.
The value of the first callback that does not return <code>synapse.module_api.NOT_SPAM</code> will
be used. If this happens, Synapse will not call any of the subsequent implementations of
this callback.</p>
<h3 id="user_may_create_room"><a class="header" href="#user_may_create_room"><code>user_may_create_room</code></a></h3>
<p><em>First introduced in Synapse v1.37.0</em></p>
<p><em>Changed in Synapse v1.62.0: <code>synapse.module_api.NOT_SPAM</code> and <code>synapse.module_api.errors.Codes</code> can be returned by this callback. Returning a boolean is now deprecated.</em> </p>
<pre><code class="language-python">async def user_may_create_room(user_id: str) -&gt; Union[&quot;synapse.module_api.NOT_SPAM&quot;, &quot;synapse.module_api.errors.Codes&quot;, bool]
</code></pre>
<p>Called when processing a room creation request.</p>
<p>The callback must return one of:</p>
<ul>
<li>
<p><code>synapse.module_api.NOT_SPAM</code>, to allow the operation. Other callbacks may still 
decide to reject it.</p>
</li>
<li>
<p><code>synapse.module_api.errors.Codes</code> to reject the operation with an error code. In case
of doubt, <code>synapse.module_api.errors.Codes.FORBIDDEN</code> is a good error code.</p>
</li>
<li>
<p>(deprecated) <code>False</code>, which is the same as returning <code>synapse.module_api.NOT_SPAM</code>.</p>
</li>
<li>
<p>(deprecated) <code>True</code>, which is the same as returning <code>synapse.module_api.errors.Codes.FORBIDDEN</code>.</p>
</li>
</ul>
<p>If multiple modules implement this callback, they will be considered in order. If a
callback returns <code>synapse.module_api.NOT_SPAM</code>, Synapse falls through to the next one.
The value of the first callback that does not return <code>synapse.module_api.NOT_SPAM</code> will
be used. If this happens, Synapse will not call any of the subsequent implementations of
this callback.</p>
<h3 id="user_may_create_room_alias"><a class="header" href="#user_may_create_room_alias"><code>user_may_create_room_alias</code></a></h3>
<p><em>First introduced in Synapse v1.37.0</em></p>
<p><em>Changed in Synapse v1.62.0: <code>synapse.module_api.NOT_SPAM</code> and <code>synapse.module_api.errors.Codes</code> can be returned by this callback. Returning a boolean is now deprecated.</em> </p>
<pre><code class="language-python">async def user_may_create_room_alias(user_id: str, room_alias: &quot;synapse.module_api.RoomAlias&quot;) -&gt; Union[&quot;synapse.module_api.NOT_SPAM&quot;, &quot;synapse.module_api.errors.Codes&quot;, bool]
</code></pre>
<p>Called when trying to associate an alias with an existing room.</p>
<p>The callback must return one of:</p>
<ul>
<li>
<p><code>synapse.module_api.NOT_SPAM</code>, to allow the operation. Other callbacks may still 
decide to reject it.</p>
</li>
<li>
<p><code>synapse.module_api.errors.Codes</code> to reject the operation with an error code. In case
of doubt, <code>synapse.module_api.errors.Codes.FORBIDDEN</code> is a good error code.</p>
</li>
<li>
<p>(deprecated) <code>False</code>, which is the same as returning <code>synapse.module_api.NOT_SPAM</code>.</p>
</li>
<li>
<p>(deprecated) <code>True</code>, which is the same as returning <code>synapse.module_api.errors.Codes.FORBIDDEN</code>.</p>
</li>
</ul>
<p>If multiple modules implement this callback, they will be considered in order. If a
callback returns <code>synapse.module_api.NOT_SPAM</code>, Synapse falls through to the next one.
The value of the first callback that does not return <code>synapse.module_api.NOT_SPAM</code> will
be used. If this happens, Synapse will not call any of the subsequent implementations of
this callback.</p>
<h3 id="user_may_publish_room"><a class="header" href="#user_may_publish_room"><code>user_may_publish_room</code></a></h3>
<p><em>First introduced in Synapse v1.37.0</em></p>
<p><em>Changed in Synapse v1.62.0: <code>synapse.module_api.NOT_SPAM</code> and <code>synapse.module_api.errors.Codes</code> can be returned by this callback. Returning a boolean is now deprecated.</em> </p>
<pre><code class="language-python">async def user_may_publish_room(user_id: str, room_id: str) -&gt; Union[&quot;synapse.module_api.NOT_SPAM&quot;, &quot;synapse.module_api.errors.Codes&quot;, bool]
</code></pre>
<p>Called when trying to publish a room to the homeserver's public rooms directory.</p>
<p>The callback must return one of:</p>
<ul>
<li>
<p><code>synapse.module_api.NOT_SPAM</code>, to allow the operation. Other callbacks may still 
decide to reject it.</p>
</li>
<li>
<p><code>synapse.module_api.errors.Codes</code> to reject the operation with an error code. In case
of doubt, <code>synapse.module_api.errors.Codes.FORBIDDEN</code> is a good error code.</p>
</li>
<li>
<p>(deprecated) <code>False</code>, which is the same as returning <code>synapse.module_api.NOT_SPAM</code>.</p>
</li>
<li>
<p>(deprecated) <code>True</code>, which is the same as returning <code>synapse.module_api.errors.Codes.FORBIDDEN</code>.</p>
</li>
</ul>
<p>If multiple modules implement this callback, they will be considered in order. If a
callback returns <code>synapse.module_api.NOT_SPAM</code>, Synapse falls through to the next one.
The value of the first callback that does not return <code>synapse.module_api.NOT_SPAM</code> will
be used. If this happens, Synapse will not call any of the subsequent implementations of
this callback.</p>
<h3 id="check_username_for_spam"><a class="header" href="#check_username_for_spam"><code>check_username_for_spam</code></a></h3>
<p><em>First introduced in Synapse v1.37.0</em></p>
<pre><code class="language-python">async def check_username_for_spam(user_profile: synapse.module_api.UserProfile) -&gt; bool
</code></pre>
<p>Called when computing search results in the user directory. The module must return a
<code>bool</code> indicating whether the given user should be excluded from user directory 
searches. Return <code>True</code> to indicate that the user is spammy and exclude them from 
search results; otherwise return <code>False</code>.</p>
<p>The profile is represented as a dictionary with the following keys:</p>
<ul>
<li><code>user_id: str</code>. The Matrix ID for this user.</li>
<li><code>display_name: Optional[str]</code>. The user's display name, or <code>None</code> if this user
has not set a display name.</li>
<li><code>avatar_url: Optional[str]</code>. The <code>mxc://</code> URL to the user's avatar, or <code>None</code>
if this user has not set an avatar.</li>
</ul>
<p>The module is given a copy of the original dictionary, so modifying it from within the
module cannot modify a user's profile when included in user directory search results.</p>
<p>If multiple modules implement this callback, they will be considered in order. If a
callback returns <code>False</code>, Synapse falls through to the next one. The value of the first
callback that does not return <code>False</code> will be used. If this happens, Synapse will not call
any of the subsequent implementations of this callback.</p>
<h3 id="check_registration_for_spam"><a class="header" href="#check_registration_for_spam"><code>check_registration_for_spam</code></a></h3>
<p><em>First introduced in Synapse v1.37.0</em></p>
<pre><code class="language-python">async def check_registration_for_spam(
    email_threepid: Optional[dict],
    username: Optional[str],
    request_info: Collection[Tuple[str, str]],
    auth_provider_id: Optional[str] = None,
) -&gt; &quot;synapse.spam_checker_api.RegistrationBehaviour&quot;
</code></pre>
<p>Called when registering a new user. The module must return a <code>RegistrationBehaviour</code>
indicating whether the registration can go through or must be denied, or whether the user
may be allowed to register but will be shadow banned.</p>
<p>The arguments passed to this callback are:</p>
<ul>
<li><code>email_threepid</code>: The email address used for registering, if any.</li>
<li><code>username</code>: The username the user would like to register. Can be <code>None</code>, meaning that
Synapse will generate one later.</li>
<li><code>request_info</code>: A collection of tuples, which first item is a user agent, and which
second item is an IP address. These user agents and IP addresses are the ones that were
used during the registration process.</li>
<li><code>auth_provider_id</code>: The identifier of the SSO authentication provider, if any.</li>
</ul>
<p>If multiple modules implement this callback, they will be considered in order. If a
callback returns <code>RegistrationBehaviour.ALLOW</code>, Synapse falls through to the next one.
The value of the first callback that does not return <code>RegistrationBehaviour.ALLOW</code> will
be used. If this happens, Synapse will not call any of the subsequent implementations of
this callback.</p>
<h3 id="check_media_file_for_spam"><a class="header" href="#check_media_file_for_spam"><code>check_media_file_for_spam</code></a></h3>
<p><em>First introduced in Synapse v1.37.0</em></p>
<p><em>Changed in Synapse v1.62.0: <code>synapse.module_api.NOT_SPAM</code> and <code>synapse.module_api.errors.Codes</code> can be returned by this callback. Returning a boolean is now deprecated.</em> </p>
<pre><code class="language-python">async def check_media_file_for_spam(
    file_wrapper: &quot;synapse.rest.media.v1.media_storage.ReadableFileWrapper&quot;,
    file_info: &quot;synapse.rest.media.v1._base.FileInfo&quot;,
) -&gt; Union[&quot;synapse.module_api.NOT_SPAM&quot;, &quot;synapse.module_api.errors.Codes&quot;, bool]
</code></pre>
<p>Called when storing a local or remote file.</p>
<p>The callback must return one of:</p>
<ul>
<li>
<p><code>synapse.module_api.NOT_SPAM</code>, to allow the operation. Other callbacks may still 
decide to reject it.</p>
</li>
<li>
<p><code>synapse.module_api.errors.Codes</code> to reject the operation with an error code. In case
of doubt, <code>synapse.module_api.errors.Codes.FORBIDDEN</code> is a good error code.</p>
</li>
<li>
<p>(deprecated) <code>False</code>, which is the same as returning <code>synapse.module_api.NOT_SPAM</code>.</p>
</li>
<li>
<p>(deprecated) <code>True</code>, which is the same as returning <code>synapse.module_api.errors.Codes.FORBIDDEN</code>.</p>
</li>
</ul>
<p>If multiple modules implement this callback, they will be considered in order. If a
callback returns <code>synapse.module_api.NOT_SPAM</code>, Synapse falls through to the next one.
The value of the first callback that does not return <code>synapse.module_api.NOT_SPAM</code> will
be used. If this happens, Synapse will not call any of the subsequent implementations of
this callback.</p>
<h3 id="should_drop_federated_event"><a class="header" href="#should_drop_federated_event"><code>should_drop_federated_event</code></a></h3>
<p><em>First introduced in Synapse v1.60.0</em></p>
<pre><code class="language-python">async def should_drop_federated_event(event: &quot;synapse.events.EventBase&quot;) -&gt; bool
</code></pre>
<p>Called when checking whether a remote server can federate an event with us. <strong>Returning
<code>True</code> from this function will silently drop a federated event and split-brain our view
of a room's DAG, and thus you shouldn't use this callback unless you know what you are
doing.</strong></p>
<p>If multiple modules implement this callback, they will be considered in order. If a
callback returns <code>False</code>, Synapse falls through to the next one. The value of the first
callback that does not return <code>False</code> will be used. If this happens, Synapse will not call
any of the subsequent implementations of this callback.</p>
<h2 id="example"><a class="header" href="#example">Example</a></h2>
<p>The example below is a module that implements the spam checker callback
<code>check_event_for_spam</code> to deny any message sent by users whose Matrix user IDs are
mentioned in a configured list, and registers a web resource to the path
<code>/_synapse/client/list_spam_checker/is_evil</code> that returns a JSON object indicating
whether the provided user appears in that list.</p>
<pre><code class="language-python">import json
from typing import Union

from twisted.web.resource import Resource
from twisted.web.server import Request

from synapse.module_api import ModuleApi


class IsUserEvilResource(Resource):
    def __init__(self, config):
        super(IsUserEvilResource, self).__init__()
        self.evil_users = config.get(&quot;evil_users&quot;) or []

    def render_GET(self, request: Request):
        user = request.args.get(b&quot;user&quot;)[0].decode()
        request.setHeader(b&quot;Content-Type&quot;, b&quot;application/json&quot;)
        return json.dumps({&quot;evil&quot;: user in self.evil_users}).encode()


class ListSpamChecker:
    def __init__(self, config: dict, api: ModuleApi):
        self.api = api
        self.evil_users = config.get(&quot;evil_users&quot;) or []

        self.api.register_spam_checker_callbacks(
            check_event_for_spam=self.check_event_for_spam,
        )

        self.api.register_web_resource(
            path=&quot;/_synapse/client/list_spam_checker/is_evil&quot;,
            resource=IsUserEvilResource(config),
        )

    async def check_event_for_spam(self, event: &quot;synapse.events.EventBase&quot;) -&gt; Union[Literal[&quot;NOT_SPAM&quot;], Codes]:
        if event.sender in self.evil_users:
          return Codes.FORBIDDEN
        else:
          return synapse.module_api.NOT_SPAM
</code></pre>

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