552 lines
36 KiB
HTML
552 lines
36 KiB
HTML
<!DOCTYPE HTML>
|
|
<html lang="en" class="sidebar-visible no-js light">
|
|
<head>
|
|
<!-- Book generated using mdBook -->
|
|
<meta charset="UTF-8">
|
|
<title>Cancellation - Synapse</title>
|
|
<!-- Custom HTML head -->
|
|
<meta content="text/html; charset=utf-8" http-equiv="Content-Type">
|
|
<meta name="description" content="">
|
|
<meta name="viewport" content="width=device-width, initial-scale=1">
|
|
<meta name="theme-color" content="#ffffff" />
|
|
|
|
<link rel="icon" href="../../favicon.svg">
|
|
<link rel="shortcut icon" href="../../favicon.png">
|
|
<link rel="stylesheet" href="../../css/variables.css">
|
|
<link rel="stylesheet" href="../../css/general.css">
|
|
<link rel="stylesheet" href="../../css/chrome.css">
|
|
<link rel="stylesheet" href="../../css/print.css" media="print">
|
|
<!-- Fonts -->
|
|
<link rel="stylesheet" href="../../FontAwesome/css/font-awesome.css">
|
|
<link rel="stylesheet" href="../../fonts/fonts.css">
|
|
<!-- Highlight.js Stylesheets -->
|
|
<link rel="stylesheet" href="../../highlight.css">
|
|
<link rel="stylesheet" href="../../tomorrow-night.css">
|
|
<link rel="stylesheet" href="../../ayu-highlight.css">
|
|
|
|
<!-- Custom theme stylesheets -->
|
|
<link rel="stylesheet" href="../../docs/website_files/table-of-contents.css">
|
|
<link rel="stylesheet" href="../../docs/website_files/remove-nav-buttons.css">
|
|
<link rel="stylesheet" href="../../docs/website_files/indent-section-headers.css">
|
|
<link rel="stylesheet" href="../../docs/website_files/version-picker.css">
|
|
</head>
|
|
<body>
|
|
<!-- Provide site root to javascript -->
|
|
<script type="text/javascript">
|
|
var path_to_root = "../../";
|
|
var default_theme = window.matchMedia("(prefers-color-scheme: dark)").matches ? "navy" : "light";
|
|
</script>
|
|
|
|
<!-- Work around some values being stored in localStorage wrapped in quotes -->
|
|
<script type="text/javascript">
|
|
try {
|
|
var theme = localStorage.getItem('mdbook-theme');
|
|
var sidebar = localStorage.getItem('mdbook-sidebar');
|
|
if (theme.startsWith('"') && theme.endsWith('"')) {
|
|
localStorage.setItem('mdbook-theme', theme.slice(1, theme.length - 1));
|
|
}
|
|
if (sidebar.startsWith('"') && sidebar.endsWith('"')) {
|
|
localStorage.setItem('mdbook-sidebar', sidebar.slice(1, sidebar.length - 1));
|
|
}
|
|
} catch (e) { }
|
|
</script>
|
|
|
|
<!-- Set the theme before any content is loaded, prevents flash -->
|
|
<script type="text/javascript">
|
|
var theme;
|
|
try { theme = localStorage.getItem('mdbook-theme'); } catch(e) { }
|
|
if (theme === null || theme === undefined) { theme = default_theme; }
|
|
var html = document.querySelector('html');
|
|
html.classList.remove('no-js')
|
|
html.classList.remove('light')
|
|
html.classList.add(theme);
|
|
html.classList.add('js');
|
|
</script>
|
|
|
|
<!-- Hide / unhide sidebar before it is displayed -->
|
|
<script type="text/javascript">
|
|
var html = document.querySelector('html');
|
|
var sidebar = 'hidden';
|
|
if (document.body.clientWidth >= 1080) {
|
|
try { sidebar = localStorage.getItem('mdbook-sidebar'); } catch(e) { }
|
|
sidebar = sidebar || 'visible';
|
|
}
|
|
html.classList.remove('sidebar-visible');
|
|
html.classList.add("sidebar-" + sidebar);
|
|
</script>
|
|
|
|
<nav id="sidebar" class="sidebar" aria-label="Table of contents">
|
|
<div class="sidebar-scrollbox">
|
|
<ol class="chapter"><li class="chapter-item expanded affix "><li class="part-title">Introduction</li><li class="chapter-item expanded "><a href="../../welcome_and_overview.html">Welcome and Overview</a></li><li class="chapter-item expanded affix "><li class="part-title">Setup</li><li class="chapter-item expanded "><a href="../../setup/installation.html">Installation</a></li><li class="chapter-item expanded "><a href="../../postgres.html">Using Postgres</a></li><li class="chapter-item expanded "><a href="../../reverse_proxy.html">Configuring a Reverse Proxy</a></li><li class="chapter-item expanded "><a href="../../setup/forward_proxy.html">Configuring a Forward/Outbound Proxy</a></li><li class="chapter-item expanded "><a href="../../turn-howto.html">Configuring a Turn Server</a></li><li class="chapter-item expanded "><a href="../../delegate.html">Delegation</a></li><li class="chapter-item expanded affix "><li class="part-title">Upgrading</li><li class="chapter-item expanded "><a href="../../upgrade.html">Upgrading between Synapse Versions</a></li><li class="chapter-item expanded affix "><li class="part-title">Usage</li><li class="chapter-item expanded "><a href="../../federate.html">Federation</a></li><li class="chapter-item expanded "><a href="../../usage/configuration/index.html">Configuration</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../usage/configuration/config_documentation.html">Configuration Manual</a></li><li class="chapter-item expanded "><a href="../../usage/configuration/homeserver_sample_config.html">Homeserver Sample Config File</a></li><li class="chapter-item expanded "><a href="../../usage/configuration/logging_sample_config.html">Logging Sample Config File</a></li><li class="chapter-item expanded "><a href="../../structured_logging.html">Structured Logging</a></li><li class="chapter-item expanded "><a href="../../templates.html">Templates</a></li><li class="chapter-item expanded "><a href="../../usage/configuration/user_authentication/index.html">User Authentication</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../usage/configuration/user_authentication/single_sign_on/index.html">Single-Sign On</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../openid.html">OpenID Connect</a></li><li class="chapter-item expanded "><a href="../../usage/configuration/user_authentication/single_sign_on/saml.html">SAML</a></li><li class="chapter-item expanded "><a href="../../usage/configuration/user_authentication/single_sign_on/cas.html">CAS</a></li><li class="chapter-item expanded "><a href="../../sso_mapping_providers.html">SSO Mapping Providers</a></li></ol></li><li class="chapter-item expanded "><a href="../../password_auth_providers.html">Password Auth Providers</a></li><li class="chapter-item expanded "><a href="../../jwt.html">JSON Web Tokens</a></li><li class="chapter-item expanded "><a href="../../usage/configuration/user_authentication/refresh_tokens.html">Refresh Tokens</a></li></ol></li><li class="chapter-item expanded "><a href="../../CAPTCHA_SETUP.html">Registration Captcha</a></li><li class="chapter-item expanded "><a href="../../application_services.html">Application Services</a></li><li class="chapter-item expanded "><a href="../../server_notices.html">Server Notices</a></li><li class="chapter-item expanded "><a href="../../consent_tracking.html">Consent Tracking</a></li><li class="chapter-item expanded "><a href="../../development/url_previews.html">URL Previews</a></li><li class="chapter-item expanded "><a href="../../user_directory.html">User Directory</a></li><li class="chapter-item expanded "><a href="../../message_retention_policies.html">Message Retention Policies</a></li><li class="chapter-item expanded "><a href="../../modules/index.html">Pluggable Modules</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../modules/writing_a_module.html">Writing a module</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../modules/spam_checker_callbacks.html">Spam checker callbacks</a></li><li class="chapter-item expanded "><a href="../../modules/third_party_rules_callbacks.html">Third-party rules callbacks</a></li><li class="chapter-item expanded "><a href="../../modules/presence_router_callbacks.html">Presence router callbacks</a></li><li class="chapter-item expanded "><a href="../../modules/account_validity_callbacks.html">Account validity callbacks</a></li><li class="chapter-item expanded "><a href="../../modules/password_auth_provider_callbacks.html">Password auth provider callbacks</a></li><li class="chapter-item expanded "><a href="../../modules/background_update_controller_callbacks.html">Background update controller callbacks</a></li><li class="chapter-item expanded "><a href="../../modules/account_data_callbacks.html">Account data callbacks</a></li><li class="chapter-item expanded "><a href="../../modules/porting_legacy_module.html">Porting a legacy module to the new interface</a></li></ol></li></ol></li><li class="chapter-item expanded "><a href="../../workers.html">Workers</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../synctl_workers.html">Using synctl with Workers</a></li><li class="chapter-item expanded "><a href="../../systemd-with-workers/index.html">Systemd</a></li></ol></li></ol></li><li class="chapter-item expanded "><a href="../../usage/administration/index.html">Administration</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../usage/administration/admin_api/index.html">Admin API</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../admin_api/account_validity.html">Account Validity</a></li><li class="chapter-item expanded "><a href="../../usage/administration/admin_api/background_updates.html">Background Updates</a></li><li class="chapter-item expanded "><a href="../../admin_api/delete_group.html">Delete Group</a></li><li class="chapter-item expanded "><a href="../../admin_api/event_reports.html">Event Reports</a></li><li class="chapter-item expanded "><a href="../../admin_api/media_admin_api.html">Media</a></li><li class="chapter-item expanded "><a href="../../admin_api/purge_history_api.html">Purge History</a></li><li class="chapter-item expanded "><a href="../../admin_api/register_api.html">Register Users</a></li><li class="chapter-item expanded "><a href="../../usage/administration/admin_api/registration_tokens.html">Registration Tokens</a></li><li class="chapter-item expanded "><a href="../../admin_api/room_membership.html">Manipulate Room Membership</a></li><li class="chapter-item expanded "><a href="../../admin_api/rooms.html">Rooms</a></li><li class="chapter-item expanded "><a href="../../admin_api/server_notices.html">Server Notices</a></li><li class="chapter-item expanded "><a href="../../admin_api/statistics.html">Statistics</a></li><li class="chapter-item expanded "><a href="../../admin_api/user_admin_api.html">Users</a></li><li class="chapter-item expanded "><a href="../../admin_api/version_api.html">Server Version</a></li><li class="chapter-item expanded "><a href="../../usage/administration/admin_api/federation.html">Federation</a></li></ol></li><li class="chapter-item expanded "><a href="../../manhole.html">Manhole</a></li><li class="chapter-item expanded "><a href="../../metrics-howto.html">Monitoring</a></li><li class="chapter-item expanded "><a href="../../usage/administration/understanding_synapse_through_grafana_graphs.html">Understanding Synapse Through Grafana Graphs</a></li><li class="chapter-item expanded "><a href="../../usage/administration/useful_sql_for_admins.html">Useful SQL for Admins</a></li><li class="chapter-item expanded "><a href="../../usage/administration/database_maintenance_tools.html">Database Maintenance Tools</a></li><li class="chapter-item expanded "><a href="../../usage/administration/state_groups.html">State Groups</a></li><li class="chapter-item expanded "><a href="../../usage/administration/request_log.html">Request log format</a></li><li class="chapter-item expanded "><a href="../../usage/administration/admin_faq.html">Admin FAQ</a></li><li class="chapter-item expanded "><div>Scripts</div></li></ol></li><li class="chapter-item expanded "><li class="part-title">Development</li><li class="chapter-item expanded "><a href="../../development/contributing_guide.html">Contributing Guide</a></li><li class="chapter-item expanded "><a href="../../code_style.html">Code Style</a></li><li class="chapter-item expanded "><a href="../../development/releases.html">Release Cycle</a></li><li class="chapter-item expanded "><a href="../../development/git.html">Git Usage</a></li><li class="chapter-item expanded "><div>Testing</div></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../development/demo.html">Demo scripts</a></li></ol></li><li class="chapter-item expanded "><a href="../../opentracing.html">OpenTracing</a></li><li class="chapter-item expanded "><a href="../../development/database_schema.html">Database Schemas</a></li><li class="chapter-item expanded "><a href="../../development/experimental_features.html">Experimental features</a></li><li class="chapter-item expanded "><div>Synapse Architecture</div></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../development/synapse_architecture/cancellation.html" class="active">Cancellation</a></li><li class="chapter-item expanded "><a href="../../log_contexts.html">Log Contexts</a></li><li class="chapter-item expanded "><a href="../../replication.html">Replication</a></li><li class="chapter-item expanded "><a href="../../tcp_replication.html">TCP Replication</a></li></ol></li><li class="chapter-item expanded "><a href="../../development/internal_documentation/index.html">Internal Documentation</a></li><li><ol class="section"><li class="chapter-item expanded "><div>Single Sign-On</div></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../development/saml.html">SAML</a></li><li class="chapter-item expanded "><a href="../../development/cas.html">CAS</a></li></ol></li><li class="chapter-item expanded "><a href="../../development/room-dag-concepts.html">Room DAG concepts</a></li><li class="chapter-item expanded "><div>State Resolution</div></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../auth_chain_difference_algorithm.html">The Auth Chain Difference Algorithm</a></li></ol></li><li class="chapter-item expanded "><a href="../../media_repository.html">Media Repository</a></li><li class="chapter-item expanded "><a href="../../room_and_user_statistics.html">Room and User Statistics</a></li></ol></li><li class="chapter-item expanded "><div>Scripts</div></li><li class="chapter-item expanded affix "><li class="part-title">Other</li><li class="chapter-item expanded "><a href="../../deprecation_policy.html">Dependency Deprecation Policy</a></li><li class="chapter-item expanded "><a href="../../other/running_synapse_on_single_board_computers.html">Running Synapse on a Single-Board Computer</a></li></ol>
|
|
</div>
|
|
<div id="sidebar-resize-handle" class="sidebar-resize-handle"></div>
|
|
</nav>
|
|
|
|
<div id="page-wrapper" class="page-wrapper">
|
|
|
|
<div class="page">
|
|
<div id="menu-bar-hover-placeholder"></div>
|
|
<div id="menu-bar" class="menu-bar sticky bordered">
|
|
<div class="left-buttons">
|
|
<button id="sidebar-toggle" class="icon-button" type="button" title="Toggle Table of Contents" aria-label="Toggle Table of Contents" aria-controls="sidebar">
|
|
<i class="fa fa-bars"></i>
|
|
</button>
|
|
<button id="theme-toggle" class="icon-button" type="button" title="Change theme" aria-label="Change theme" aria-haspopup="true" aria-expanded="false" aria-controls="theme-list">
|
|
<i class="fa fa-paint-brush"></i>
|
|
</button>
|
|
<ul id="theme-list" class="theme-popup" aria-label="Themes" role="menu">
|
|
<li role="none"><button role="menuitem" class="theme" id="light">Light (default)</button></li>
|
|
<li role="none"><button role="menuitem" class="theme" id="rust">Rust</button></li>
|
|
<li role="none"><button role="menuitem" class="theme" id="coal">Coal</button></li>
|
|
<li role="none"><button role="menuitem" class="theme" id="navy">Navy</button></li>
|
|
<li role="none"><button role="menuitem" class="theme" id="ayu">Ayu</button></li>
|
|
</ul>
|
|
<button id="search-toggle" class="icon-button" type="button" title="Search. (Shortkey: s)" aria-label="Toggle Searchbar" aria-expanded="false" aria-keyshortcuts="S" aria-controls="searchbar">
|
|
<i class="fa fa-search"></i>
|
|
</button>
|
|
<div class="version-picker">
|
|
<div class="dropdown">
|
|
<div class="select">
|
|
<span></span>
|
|
<i class="fa fa-chevron-down"></i>
|
|
</div>
|
|
<input type="hidden" name="version">
|
|
<ul class="dropdown-menu">
|
|
<!-- Versions will be added dynamically in version-picker.js -->
|
|
</ul>
|
|
</div>
|
|
</div>
|
|
</div>
|
|
|
|
<h1 class="menu-title">Synapse</h1>
|
|
|
|
<div class="right-buttons">
|
|
<a href="../../print.html" title="Print this book" aria-label="Print this book">
|
|
<i id="print-button" class="fa fa-print"></i>
|
|
</a>
|
|
<a href="https://github.com/matrix-org/synapse" title="Git repository" aria-label="Git repository">
|
|
<i id="git-repository-button" class="fa fa-github"></i>
|
|
</a>
|
|
<a href="https://github.com/matrix-org/synapse/edit/develop/docs/development/synapse_architecture/cancellation.md" title="Suggest an edit" aria-label="Suggest an edit">
|
|
<i id="git-edit-button" class="fa fa-edit"></i>
|
|
</a>
|
|
</div>
|
|
</div>
|
|
|
|
<div id="search-wrapper" class="hidden">
|
|
<form id="searchbar-outer" class="searchbar-outer">
|
|
<input type="search" id="searchbar" name="searchbar" placeholder="Search this book ..." aria-controls="searchresults-outer" aria-describedby="searchresults-header">
|
|
</form>
|
|
<div id="searchresults-outer" class="searchresults-outer hidden">
|
|
<div id="searchresults-header" class="searchresults-header"></div>
|
|
<ul id="searchresults">
|
|
</ul>
|
|
</div>
|
|
</div>
|
|
<!-- Apply ARIA attributes after the sidebar and the sidebar toggle button are added to the DOM -->
|
|
<script type="text/javascript">
|
|
document.getElementById('sidebar-toggle').setAttribute('aria-expanded', sidebar === 'visible');
|
|
document.getElementById('sidebar').setAttribute('aria-hidden', sidebar !== 'visible');
|
|
Array.from(document.querySelectorAll('#sidebar a')).forEach(function(link) {
|
|
link.setAttribute('tabIndex', sidebar === 'visible' ? 0 : -1);
|
|
});
|
|
</script>
|
|
|
|
<div id="content" class="content">
|
|
<main>
|
|
<!-- Page table of contents -->
|
|
<div class="sidetoc">
|
|
<nav class="pagetoc"></nav>
|
|
</div>
|
|
|
|
<h1 id="cancellation"><a class="header" href="#cancellation">Cancellation</a></h1>
|
|
<p>Sometimes, requests take a long time to service and clients disconnect
|
|
before Synapse produces a response. To avoid wasting resources, Synapse
|
|
can cancel request processing for select endpoints marked with the
|
|
<code>@cancellable</code> decorator.</p>
|
|
<p>Synapse makes use of Twisted's <code>Deferred.cancel()</code> feature to make
|
|
cancellation work. The <code>@cancellable</code> decorator does nothing by itself
|
|
and merely acts as a flag, signalling to developers and other code alike
|
|
that a method can be cancelled.</p>
|
|
<h2 id="enabling-cancellation-for-an-endpoint"><a class="header" href="#enabling-cancellation-for-an-endpoint">Enabling cancellation for an endpoint</a></h2>
|
|
<ol>
|
|
<li>Check that the endpoint method, and any <code>async</code> functions in its call
|
|
tree handle cancellation correctly. See
|
|
<a href="#handling-cancellation-correctly">Handling cancellation correctly</a>
|
|
for a list of things to look out for.</li>
|
|
<li>Add the <code>@cancellable</code> decorator to the <code>on_GET/POST/PUT/DELETE</code>
|
|
method. It's not recommended to make non-<code>GET</code> methods cancellable,
|
|
since cancellation midway through some database updates is less
|
|
likely to be handled correctly.</li>
|
|
</ol>
|
|
<h2 id="mechanics"><a class="header" href="#mechanics">Mechanics</a></h2>
|
|
<p>There are two stages to cancellation: downward propagation of a
|
|
<code>cancel()</code> call, followed by upwards propagation of a <code>CancelledError</code>
|
|
out of a blocked <code>await</code>.
|
|
Both Twisted and asyncio have a cancellation mechanism.</p>
|
|
<table><thead><tr><th></th><th>Method</th><th>Exception</th><th>Exception inherits from</th></tr></thead><tbody>
|
|
<tr><td>Twisted</td><td><code>Deferred.cancel()</code></td><td><code>twisted.internet.defer.CancelledError</code></td><td><code>Exception</code> (!)</td></tr>
|
|
<tr><td>asyncio</td><td><code>Task.cancel()</code></td><td><code>asyncio.CancelledError</code></td><td><code>BaseException</code></td></tr>
|
|
</tbody></table>
|
|
<h3 id="deferredcancel"><a class="header" href="#deferredcancel">Deferred.cancel()</a></h3>
|
|
<p>When Synapse starts handling a request, it runs the async method
|
|
responsible for handling it using <code>defer.ensureDeferred</code>, which returns
|
|
a <code>Deferred</code>. For example:</p>
|
|
<pre><code class="language-python">def do_something() -> Deferred[None]:
|
|
...
|
|
|
|
@cancellable
|
|
async def on_GET() -> Tuple[int, JsonDict]:
|
|
d = make_deferred_yieldable(do_something())
|
|
await d
|
|
return 200, {}
|
|
|
|
request = defer.ensureDeferred(on_GET())
|
|
</code></pre>
|
|
<p>When a client disconnects early, Synapse checks for the presence of the
|
|
<code>@cancellable</code> decorator on <code>on_GET</code>. Since <code>on_GET</code> is cancellable,
|
|
<code>Deferred.cancel()</code> is called on the <code>Deferred</code> from
|
|
<code>defer.ensureDeferred</code>, ie. <code>request</code>. Twisted knows which <code>Deferred</code>
|
|
<code>request</code> is waiting on and passes the <code>cancel()</code> call on to <code>d</code>.</p>
|
|
<p>The <code>Deferred</code> being waited on, <code>d</code>, may have its own handling for
|
|
<code>cancel()</code> and pass the call on to other <code>Deferred</code>s.</p>
|
|
<p>Eventually, a <code>Deferred</code> handles the <code>cancel()</code> call by resolving itself
|
|
with a <code>CancelledError</code>.</p>
|
|
<h3 id="cancellederror"><a class="header" href="#cancellederror">CancelledError</a></h3>
|
|
<p>The <code>CancelledError</code> gets raised out of the <code>await</code> and bubbles up, as
|
|
per normal Python exception handling.</p>
|
|
<h2 id="handling-cancellation-correctly"><a class="header" href="#handling-cancellation-correctly">Handling cancellation correctly</a></h2>
|
|
<p>In general, when writing code that might be subject to cancellation, two
|
|
things must be considered:</p>
|
|
<ul>
|
|
<li>The effect of <code>CancelledError</code>s raised out of <code>await</code>s.</li>
|
|
<li>The effect of <code>Deferred</code>s being <code>cancel()</code>ed.</li>
|
|
</ul>
|
|
<p>Examples of code that handles cancellation incorrectly include:</p>
|
|
<ul>
|
|
<li><code>try-except</code> blocks which swallow <code>CancelledError</code>s.</li>
|
|
<li>Code that shares the same <code>Deferred</code>, which may be cancelled, between
|
|
multiple requests.</li>
|
|
<li>Code that starts some processing that's exempt from cancellation, but
|
|
uses a logging context from cancellable code. The logging context
|
|
will be finished upon cancellation, while the uncancelled processing
|
|
is still using it.</li>
|
|
</ul>
|
|
<p>Some common patterns are listed below in more detail.</p>
|
|
<h3 id="async-function-calls"><a class="header" href="#async-function-calls"><code>async</code> function calls</a></h3>
|
|
<p>Most functions in Synapse are relatively straightforward from a
|
|
cancellation standpoint: they don't do anything with <code>Deferred</code>s and
|
|
purely call and <code>await</code> other <code>async</code> functions.</p>
|
|
<p>An <code>async</code> function handles cancellation correctly if its own code
|
|
handles cancellation correctly and all the async function it calls
|
|
handle cancellation correctly. For example:</p>
|
|
<pre><code class="language-python">async def do_two_things() -> None:
|
|
check_something()
|
|
await do_something()
|
|
await do_something_else()
|
|
</code></pre>
|
|
<p><code>do_two_things</code> handles cancellation correctly if <code>do_something</code> and
|
|
<code>do_something_else</code> handle cancellation correctly.</p>
|
|
<p>That is, when checking whether a function handles cancellation
|
|
correctly, its implementation and all its <code>async</code> function calls need to
|
|
be checked, recursively.</p>
|
|
<p>As <code>check_something</code> is not <code>async</code>, it does not need to be checked.</p>
|
|
<h3 id="cancellederrors"><a class="header" href="#cancellederrors">CancelledErrors</a></h3>
|
|
<p>Because Twisted's <code>CancelledError</code>s are <code>Exception</code>s, it's easy to
|
|
accidentally catch and suppress them. Care must be taken to ensure that
|
|
<code>CancelledError</code>s are allowed to propagate upwards.</p>
|
|
<table width="100%">
|
|
<tr>
|
|
<td width="50%" valign="top">
|
|
<p><strong>Bad</strong>:</p>
|
|
<pre><code class="language-python">try:
|
|
await do_something()
|
|
except Exception:
|
|
# `CancelledError` gets swallowed here.
|
|
logger.info(...)
|
|
</code></pre>
|
|
</td>
|
|
<td width="50%" valign="top">
|
|
<p><strong>Good</strong>:</p>
|
|
<pre><code class="language-python">try:
|
|
await do_something()
|
|
except CancelledError:
|
|
raise
|
|
except Exception:
|
|
logger.info(...)
|
|
</code></pre>
|
|
</td>
|
|
</tr>
|
|
<tr>
|
|
<td width="50%" valign="top">
|
|
<p><strong>OK</strong>:</p>
|
|
<pre><code class="language-python">try:
|
|
check_something()
|
|
# A `CancelledError` won't ever be raised here.
|
|
except Exception:
|
|
logger.info(...)
|
|
</code></pre>
|
|
</td>
|
|
<td width="50%" valign="top">
|
|
<p><strong>Good</strong>:</p>
|
|
<pre><code class="language-python">try:
|
|
await do_something()
|
|
except ValueError:
|
|
logger.info(...)
|
|
</code></pre>
|
|
</td>
|
|
</tr>
|
|
</table>
|
|
<h4 id="defergatherresults"><a class="header" href="#defergatherresults">defer.gatherResults</a></h4>
|
|
<p><code>defer.gatherResults</code> produces a <code>Deferred</code> which:</p>
|
|
<ul>
|
|
<li>broadcasts <code>cancel()</code> calls to every <code>Deferred</code> being waited on.</li>
|
|
<li>wraps the first exception it sees in a <code>FirstError</code>.</li>
|
|
</ul>
|
|
<p>Together, this means that <code>CancelledError</code>s will be wrapped in
|
|
a <code>FirstError</code> unless unwrapped. Such <code>FirstError</code>s are liable to be
|
|
swallowed, so they must be unwrapped.</p>
|
|
<table width="100%">
|
|
<tr>
|
|
<td width="50%" valign="top">
|
|
<p><strong>Bad</strong>:</p>
|
|
<pre><code class="language-python">async def do_something() -> None:
|
|
await make_deferred_yieldable(
|
|
defer.gatherResults([...], consumeErrors=True)
|
|
)
|
|
|
|
try:
|
|
await do_something()
|
|
except CancelledError:
|
|
raise
|
|
except Exception:
|
|
# `FirstError(CancelledError)` gets swallowed here.
|
|
logger.info(...)
|
|
</code></pre>
|
|
</td>
|
|
<td width="50%" valign="top">
|
|
<p><strong>Good</strong>:</p>
|
|
<pre><code class="language-python">async def do_something() -> None:
|
|
await make_deferred_yieldable(
|
|
defer.gatherResults([...], consumeErrors=True)
|
|
).addErrback(unwrapFirstError)
|
|
|
|
try:
|
|
await do_something()
|
|
except CancelledError:
|
|
raise
|
|
except Exception:
|
|
logger.info(...)
|
|
</code></pre>
|
|
</td>
|
|
</tr>
|
|
</table>
|
|
<h3 id="creation-of-deferreds"><a class="header" href="#creation-of-deferreds">Creation of <code>Deferred</code>s</a></h3>
|
|
<p>If a function creates a <code>Deferred</code>, the effect of cancelling it must be considered. <code>Deferred</code>s that get shared are likely to have unintended behaviour when cancelled.</p>
|
|
<table width="100%">
|
|
<tr>
|
|
<td width="50%" valign="top">
|
|
<p><strong>Bad</strong>:</p>
|
|
<pre><code class="language-python">cache: Dict[str, Deferred[None]] = {}
|
|
|
|
def wait_for_room(room_id: str) -> Deferred[None]:
|
|
deferred = cache.get(room_id)
|
|
if deferred is None:
|
|
deferred = Deferred()
|
|
cache[room_id] = deferred
|
|
# `deferred` can have multiple waiters.
|
|
# All of them will observe a `CancelledError`
|
|
# if any one of them is cancelled.
|
|
return make_deferred_yieldable(deferred)
|
|
|
|
# Request 1
|
|
await wait_for_room("!aAAaaAaaaAAAaAaAA:matrix.org")
|
|
# Request 2
|
|
await wait_for_room("!aAAaaAaaaAAAaAaAA:matrix.org")
|
|
</code></pre>
|
|
</td>
|
|
<td width="50%" valign="top">
|
|
<p><strong>Good</strong>:</p>
|
|
<pre><code class="language-python">cache: Dict[str, Deferred[None]] = {}
|
|
|
|
def wait_for_room(room_id: str) -> Deferred[None]:
|
|
deferred = cache.get(room_id)
|
|
if deferred is None:
|
|
deferred = Deferred()
|
|
cache[room_id] = deferred
|
|
# `deferred` will never be cancelled now.
|
|
# A `CancelledError` will still come out of
|
|
# the `await`.
|
|
# `delay_cancellation` may also be used.
|
|
return make_deferred_yieldable(stop_cancellation(deferred))
|
|
|
|
# Request 1
|
|
await wait_for_room("!aAAaaAaaaAAAaAaAA:matrix.org")
|
|
# Request 2
|
|
await wait_for_room("!aAAaaAaaaAAAaAaAA:matrix.org")
|
|
</code></pre>
|
|
</td>
|
|
</tr>
|
|
<tr>
|
|
<td width="50%" valign="top">
|
|
</td>
|
|
<td width="50%" valign="top">
|
|
<p><strong>Good</strong>:</p>
|
|
<pre><code class="language-python">cache: Dict[str, List[Deferred[None]]] = {}
|
|
|
|
def wait_for_room(room_id: str) -> Deferred[None]:
|
|
if room_id not in cache:
|
|
cache[room_id] = []
|
|
# Each request gets its own `Deferred` to wait on.
|
|
deferred = Deferred()
|
|
cache[room_id]].append(deferred)
|
|
return make_deferred_yieldable(deferred)
|
|
|
|
# Request 1
|
|
await wait_for_room("!aAAaaAaaaAAAaAaAA:matrix.org")
|
|
# Request 2
|
|
await wait_for_room("!aAAaaAaaaAAAaAaAA:matrix.org")
|
|
</code></pre>
|
|
</td>
|
|
</table>
|
|
<h3 id="uncancelled-processing"><a class="header" href="#uncancelled-processing">Uncancelled processing</a></h3>
|
|
<p>Some <code>async</code> functions may kick off some <code>async</code> processing which is
|
|
intentionally protected from cancellation, by <code>stop_cancellation</code> or
|
|
other means. If the <code>async</code> processing inherits the logcontext of the
|
|
request which initiated it, care must be taken to ensure that the
|
|
logcontext is not finished before the <code>async</code> processing completes.</p>
|
|
<table width="100%">
|
|
<tr>
|
|
<td width="50%" valign="top">
|
|
<p><strong>Bad</strong>:</p>
|
|
<pre><code class="language-python">cache: Optional[ObservableDeferred[None]] = None
|
|
|
|
async def do_something_else(
|
|
to_resolve: Deferred[None]
|
|
) -> None:
|
|
await ...
|
|
logger.info("done!")
|
|
to_resolve.callback(None)
|
|
|
|
async def do_something() -> None:
|
|
if not cache:
|
|
to_resolve = Deferred()
|
|
cache = ObservableDeferred(to_resolve)
|
|
# `do_something_else` will never be cancelled and
|
|
# can outlive the `request-1` logging context.
|
|
run_in_background(do_something_else, to_resolve)
|
|
|
|
await make_deferred_yieldable(cache.observe())
|
|
|
|
with LoggingContext("request-1"):
|
|
await do_something()
|
|
</code></pre>
|
|
</td>
|
|
<td width="50%" valign="top">
|
|
<p><strong>Good</strong>:</p>
|
|
<pre><code class="language-python">cache: Optional[ObservableDeferred[None]] = None
|
|
|
|
async def do_something_else(
|
|
to_resolve: Deferred[None]
|
|
) -> None:
|
|
await ...
|
|
logger.info("done!")
|
|
to_resolve.callback(None)
|
|
|
|
async def do_something() -> None:
|
|
if not cache:
|
|
to_resolve = Deferred()
|
|
cache = ObservableDeferred(to_resolve)
|
|
run_in_background(do_something_else, to_resolve)
|
|
# We'll wait until `do_something_else` is
|
|
# done before raising a `CancelledError`.
|
|
await make_deferred_yieldable(
|
|
delay_cancellation(cache.observe())
|
|
)
|
|
else:
|
|
await make_deferred_yieldable(cache.observe())
|
|
|
|
with LoggingContext("request-1"):
|
|
await do_something()
|
|
</code></pre>
|
|
</td>
|
|
</tr>
|
|
<tr>
|
|
<td width="50%">
|
|
<p><strong>OK</strong>:</p>
|
|
<pre><code class="language-python">cache: Optional[ObservableDeferred[None]] = None
|
|
|
|
async def do_something_else(
|
|
to_resolve: Deferred[None]
|
|
) -> None:
|
|
await ...
|
|
logger.info("done!")
|
|
to_resolve.callback(None)
|
|
|
|
async def do_something() -> None:
|
|
if not cache:
|
|
to_resolve = Deferred()
|
|
cache = ObservableDeferred(to_resolve)
|
|
# `do_something_else` will get its own independent
|
|
# logging context. `request-1` will not count any
|
|
# metrics from `do_something_else`.
|
|
run_as_background_process(
|
|
"do_something_else",
|
|
do_something_else,
|
|
to_resolve,
|
|
)
|
|
|
|
await make_deferred_yieldable(cache.observe())
|
|
|
|
with LoggingContext("request-1"):
|
|
await do_something()
|
|
</code></pre>
|
|
</td>
|
|
<td width="50%">
|
|
</td>
|
|
</tr>
|
|
</table>
|
|
|
|
</main>
|
|
|
|
<nav class="nav-wrapper" aria-label="Page navigation">
|
|
<!-- Mobile navigation buttons -->
|
|
<a rel="prev" href="../../development/experimental_features.html" class="mobile-nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left">
|
|
<i class="fa fa-angle-left"></i>
|
|
</a>
|
|
<a rel="next" href="../../log_contexts.html" class="mobile-nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right">
|
|
<i class="fa fa-angle-right"></i>
|
|
</a>
|
|
<div style="clear: both"></div>
|
|
</nav>
|
|
</div>
|
|
</div>
|
|
|
|
<nav class="nav-wide-wrapper" aria-label="Page navigation">
|
|
<a rel="prev" href="../../development/experimental_features.html" class="nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left">
|
|
<i class="fa fa-angle-left"></i>
|
|
</a>
|
|
<a rel="next" href="../../log_contexts.html" class="nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right">
|
|
<i class="fa fa-angle-right"></i>
|
|
</a>
|
|
</nav>
|
|
|
|
</div>
|
|
|
|
<script type="text/javascript">
|
|
window.playground_copyable = true;
|
|
</script>
|
|
<script src="../../elasticlunr.min.js" type="text/javascript" charset="utf-8"></script>
|
|
<script src="../../mark.min.js" type="text/javascript" charset="utf-8"></script>
|
|
<script src="../../searcher.js" type="text/javascript" charset="utf-8"></script>
|
|
<script src="../../clipboard.min.js" type="text/javascript" charset="utf-8"></script>
|
|
<script src="../../highlight.js" type="text/javascript" charset="utf-8"></script>
|
|
<script src="../../book.js" type="text/javascript" charset="utf-8"></script>
|
|
|
|
<!-- Custom JS scripts -->
|
|
<script type="text/javascript" src="../../docs/website_files/table-of-contents.js"></script>
|
|
<script type="text/javascript" src="../../docs/website_files/version-picker.js"></script>
|
|
<script type="text/javascript" src="../../docs/website_files/version.js"></script>
|
|
</body>
|
|
</html>
|