mirror of
https://github.com/element-hq/synapse.git
synced 2026-03-31 21:25:48 +00:00
625 lines
37 KiB
HTML
625 lines
37 KiB
HTML
<!DOCTYPE HTML>
|
||
<html lang="en" class="light sidebar-visible" dir="ltr">
|
||
<head>
|
||
<!-- Book generated using mdBook -->
|
||
<meta charset="UTF-8">
|
||
<title>Cancellation - Synapse</title>
|
||
|
||
|
||
<!-- Custom HTML head -->
|
||
|
||
<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-de23e50b.svg">
|
||
<link rel="shortcut icon" href="../../favicon-8114d1fc.png">
|
||
<link rel="stylesheet" href="../../css/variables-8adf115d.css">
|
||
<link rel="stylesheet" href="../../css/general-2459343d.css">
|
||
<link rel="stylesheet" href="../../css/chrome-ae938929.css">
|
||
<link rel="stylesheet" href="../../css/print-9e4910d8.css" media="print">
|
||
|
||
<!-- Fonts -->
|
||
<link rel="stylesheet" href="../../fonts/fonts-9644e21d.css">
|
||
|
||
<!-- Highlight.js Stylesheets -->
|
||
<link rel="stylesheet" id="mdbook-highlight-css" href="../../highlight-493f70e1.css">
|
||
<link rel="stylesheet" id="mdbook-tomorrow-night-css" href="../../tomorrow-night-4c0ae647.css">
|
||
<link rel="stylesheet" id="mdbook-ayu-highlight-css" href="../../ayu-highlight-3fdfc3ac.css">
|
||
|
||
<!-- Custom theme stylesheets -->
|
||
<link rel="stylesheet" href="../../docs/website_files/indent-section-headers-3390f960.css">
|
||
<link rel="stylesheet" href="../../docs/website_files/version-picker-ddc5216e.css">
|
||
|
||
|
||
<!-- Provide site root and default themes to javascript -->
|
||
<script>
|
||
const path_to_root = "../../";
|
||
const default_light_theme = "light";
|
||
const default_dark_theme = "navy";
|
||
window.path_to_searchindex_js = "../../searchindex-31e8a988.js";
|
||
</script>
|
||
<!-- Start loading toc.js asap -->
|
||
<script src="../../toc-4dc10a98.js"></script>
|
||
</head>
|
||
<body>
|
||
<div id="mdbook-help-container">
|
||
<div id="mdbook-help-popup">
|
||
<h2 class="mdbook-help-title">Keyboard shortcuts</h2>
|
||
<div>
|
||
<p>Press <kbd>←</kbd> or <kbd>→</kbd> to navigate between chapters</p>
|
||
<p>Press <kbd>S</kbd> or <kbd>/</kbd> to search in the book</p>
|
||
<p>Press <kbd>?</kbd> to show this help</p>
|
||
<p>Press <kbd>Esc</kbd> to hide this help</p>
|
||
</div>
|
||
</div>
|
||
</div>
|
||
<div id="mdbook-body-container">
|
||
<!-- Work around some values being stored in localStorage wrapped in quotes -->
|
||
<script>
|
||
try {
|
||
let theme = localStorage.getItem('mdbook-theme');
|
||
let 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>
|
||
const default_theme = window.matchMedia("(prefers-color-scheme: dark)").matches ? default_dark_theme : default_light_theme;
|
||
let theme;
|
||
try { theme = localStorage.getItem('mdbook-theme'); } catch(e) { }
|
||
if (theme === null || theme === undefined) { theme = default_theme; }
|
||
const html = document.documentElement;
|
||
html.classList.remove('light')
|
||
html.classList.add(theme);
|
||
html.classList.add("js");
|
||
</script>
|
||
|
||
<input type="checkbox" id="mdbook-sidebar-toggle-anchor" class="hidden">
|
||
|
||
<!-- Hide / unhide sidebar before it is displayed -->
|
||
<script>
|
||
let sidebar = null;
|
||
const sidebar_toggle = document.getElementById("mdbook-sidebar-toggle-anchor");
|
||
if (document.body.clientWidth >= 1080) {
|
||
try { sidebar = localStorage.getItem('mdbook-sidebar'); } catch(e) { }
|
||
sidebar = sidebar || 'visible';
|
||
} else {
|
||
sidebar = 'hidden';
|
||
sidebar_toggle.checked = false;
|
||
}
|
||
if (sidebar === 'visible') {
|
||
sidebar_toggle.checked = true;
|
||
} else {
|
||
html.classList.remove('sidebar-visible');
|
||
}
|
||
</script>
|
||
|
||
<nav id="mdbook-sidebar" class="sidebar" aria-label="Table of contents">
|
||
<!-- populated by js -->
|
||
<mdbook-sidebar-scrollbox class="sidebar-scrollbox"></mdbook-sidebar-scrollbox>
|
||
<noscript>
|
||
<iframe class="sidebar-iframe-outer" src="../../toc.html"></iframe>
|
||
</noscript>
|
||
<div id="mdbook-sidebar-resize-handle" class="sidebar-resize-handle">
|
||
<div class="sidebar-resize-indicator"></div>
|
||
</div>
|
||
</nav>
|
||
|
||
<div id="mdbook-page-wrapper" class="page-wrapper">
|
||
|
||
<div class="page">
|
||
<div id="mdbook-menu-bar-hover-placeholder"></div>
|
||
<div id="mdbook-menu-bar" class="menu-bar sticky">
|
||
<div class="left-buttons">
|
||
<label id="mdbook-sidebar-toggle" class="icon-button" for="mdbook-sidebar-toggle-anchor" title="Toggle Table of Contents" aria-label="Toggle Table of Contents" aria-controls="mdbook-sidebar">
|
||
<span class=fa-svg><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 448 512"><!--! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --><path d="M0 96C0 78.3 14.3 64 32 64H416c17.7 0 32 14.3 32 32s-14.3 32-32 32H32C14.3 128 0 113.7 0 96zM0 256c0-17.7 14.3-32 32-32H416c17.7 0 32 14.3 32 32s-14.3 32-32 32H32c-17.7 0-32-14.3-32-32zM448 416c0 17.7-14.3 32-32 32H32c-17.7 0-32-14.3-32-32s14.3-32 32-32H416c17.7 0 32 14.3 32 32z"/></svg></span>
|
||
</label>
|
||
<button id="mdbook-theme-toggle" class="icon-button" type="button" title="Change theme" aria-label="Change theme" aria-haspopup="true" aria-expanded="false" aria-controls="mdbook-theme-list">
|
||
<span class=fa-svg><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 576 512"><!--! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --><path d="M371.3 367.1c27.3-3.9 51.9-19.4 67.2-42.9L600.2 74.1c12.6-19.5 9.4-45.3-7.6-61.2S549.7-4.4 531.1 9.6L294.4 187.2c-24 18-38.2 46.1-38.4 76.1L371.3 367.1zm-19.6 25.4l-116-104.4C175.9 290.3 128 339.6 128 400c0 3.9 .2 7.8 .6 11.6c1.8 17.5-10.2 36.4-27.8 36.4H96c-17.7 0-32 14.3-32 32s14.3 32 32 32H240c61.9 0 112-50.1 112-112c0-2.5-.1-5-.2-7.5z"/></svg></span>
|
||
</button>
|
||
<ul id="mdbook-theme-list" class="theme-popup" aria-label="Themes" role="menu">
|
||
<li role="none"><button role="menuitem" class="theme" id="mdbook-theme-default_theme">Auto</button></li>
|
||
<li role="none"><button role="menuitem" class="theme" id="mdbook-theme-light">Light</button></li>
|
||
<li role="none"><button role="menuitem" class="theme" id="mdbook-theme-rust">Rust</button></li>
|
||
<li role="none"><button role="menuitem" class="theme" id="mdbook-theme-coal">Coal</button></li>
|
||
<li role="none"><button role="menuitem" class="theme" id="mdbook-theme-navy">Navy</button></li>
|
||
<li role="none"><button role="menuitem" class="theme" id="mdbook-theme-ayu">Ayu</button></li>
|
||
</ul>
|
||
<button id="mdbook-search-toggle" class="icon-button" type="button" title="Search (`/`)" aria-label="Toggle Searchbar" aria-expanded="false" aria-keyshortcuts="/ s" aria-controls="mdbook-searchbar">
|
||
<span class=fa-svg><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512"><!--! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --><path d="M416 208c0 45.9-14.9 88.3-40 122.7L502.6 457.4c12.5 12.5 12.5 32.8 0 45.3s-32.8 12.5-45.3 0L330.7 376c-34.4 25.2-76.8 40-122.7 40C93.1 416 0 322.9 0 208S93.1 0 208 0S416 93.1 416 208zM208 352c79.5 0 144-64.5 144-144s-64.5-144-144-144S64 128.5 64 208s64.5 144 144 144z"/></svg></span>
|
||
</button>
|
||
</div>
|
||
|
||
<!-- BEGIN CUSTOM SYNAPSE MODIFICATIONS -->
|
||
<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>
|
||
<!-- END CUSTOM SYNAPSE MODIFICATIONS -->
|
||
|
||
<h1 class="menu-title">Synapse</h1>
|
||
|
||
<div class="right-buttons">
|
||
<a href="../../print.html" title="Print this book" aria-label="Print this book">
|
||
<span class=fa-svg id="print-button"><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512"><!--! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --><path d="M128 0C92.7 0 64 28.7 64 64v96h64V64H354.7L384 93.3V160h64V93.3c0-17-6.7-33.3-18.7-45.3L400 18.7C388 6.7 371.7 0 354.7 0H128zM384 352v32 64H128V384 368 352H384zm64 32h32c17.7 0 32-14.3 32-32V256c0-35.3-28.7-64-64-64H64c-35.3 0-64 28.7-64 64v96c0 17.7 14.3 32 32 32H64v64c0 35.3 28.7 64 64 64H384c35.3 0 64-28.7 64-64V384zm-16-88c-13.3 0-24-10.7-24-24s10.7-24 24-24s24 10.7 24 24s-10.7 24-24 24z"/></svg></span>
|
||
</a>
|
||
<a href="https://github.com/element-hq/synapse" title="Git repository" aria-label="Git repository">
|
||
<span class=fa-svg><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 496 512"><!--! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --><path d="M165.9 397.4c0 2-2.3 3.6-5.2 3.6-3.3.3-5.6-1.3-5.6-3.6 0-2 2.3-3.6 5.2-3.6 3-.3 5.6 1.3 5.6 3.6zm-31.1-4.5c-.7 2 1.3 4.3 4.3 4.9 2.6 1 5.6 0 6.2-2s-1.3-4.3-4.3-5.2c-2.6-.7-5.5.3-6.2 2.3zm44.2-1.7c-2.9.7-4.9 2.6-4.6 4.9.3 2 2.9 3.3 5.9 2.6 2.9-.7 4.9-2.6 4.6-4.6-.3-1.9-3-3.2-5.9-2.9zM244.8 8C106.1 8 0 113.3 0 252c0 110.9 69.8 205.8 169.5 239.2 12.8 2.3 17.3-5.6 17.3-12.1 0-6.2-.3-40.4-.3-61.4 0 0-70 15-84.7-29.8 0 0-11.4-29.1-27.8-36.6 0 0-22.9-15.7 1.6-15.4 0 0 24.9 2 38.6 25.8 21.9 38.6 58.6 27.5 72.9 20.9 2.3-16 8.8-27.1 16-33.7-55.9-6.2-112.3-14.3-112.3-110.5 0-27.5 7.6-41.3 23.6-58.9-2.6-6.5-11.1-33.3 2.6-67.9 20.9-6.5 69 27 69 27 20-5.6 41.5-8.5 62.8-8.5s42.8 2.9 62.8 8.5c0 0 48.1-33.6 69-27 13.7 34.7 5.2 61.4 2.6 67.9 16 17.7 25.8 31.5 25.8 58.9 0 96.5-58.9 104.2-114.8 110.5 9.2 7.9 17 22.9 17 46.4 0 33.7-.3 75.4-.3 83.6 0 6.5 4.6 14.4 17.3 12.1C428.2 457.8 496 362.9 496 252 496 113.3 383.5 8 244.8 8zM97.2 352.9c-1.3 1-1 3.3.7 5.2 1.6 1.6 3.9 2.3 5.2 1 1.3-1 1-3.3-.7-5.2-1.6-1.6-3.9-2.3-5.2-1zm-10.8-8.1c-.7 1.3.3 2.9 2.3 3.9 1.6 1 3.6.7 4.3-.7.7-1.3-.3-2.9-2.3-3.9-2-.6-3.6-.3-4.3.7zm32.4 35.6c-1.6 1.3-1 4.3 1.3 6.2 2.3 2.3 5.2 2.6 6.5 1 1.3-1.3.7-4.3-1.3-6.2-2.2-2.3-5.2-2.6-6.5-1zm-11.4-14.7c-1.6 1-1.6 3.6 0 5.9 1.6 2.3 4.3 3.3 5.6 2.3 1.6-1.3 1.6-3.9 0-6.2-1.4-2.3-4-3.3-5.6-2z"/></svg></span>
|
||
</a>
|
||
<a href="https://github.com/element-hq/synapse/edit/develop/docs/development/synapse_architecture/cancellation.md" title="Suggest an edit" aria-label="Suggest an edit" rel="edit">
|
||
<span class=fa-svg id="git-edit-button"><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512"><!--! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --><path d="M421.7 220.3l-11.3 11.3-22.6 22.6-205 205c-6.6 6.6-14.8 11.5-23.8 14.1L30.8 511c-8.4 2.5-17.5 .2-23.7-6.1S-1.5 489.7 1 481.2L38.7 353.1c2.6-9 7.5-17.2 14.1-23.8l205-205 22.6-22.6 11.3-11.3 33.9 33.9 62.1 62.1 33.9 33.9zM96 353.9l-9.3 9.3c-.9 .9-1.6 2.1-2 3.4l-25.3 86 86-25.3c1.3-.4 2.5-1.1 3.4-2l9.3-9.3H112c-8.8 0-16-7.2-16-16V353.9zM453.3 19.3l39.4 39.4c25 25 25 65.5 0 90.5l-14.5 14.5-22.6 22.6-11.3 11.3-33.9-33.9-62.1-62.1L314.3 67.7l11.3-11.3 22.6-22.6 14.5-14.5c25-25 65.5-25 90.5 0z"/></svg></span>
|
||
</a>
|
||
|
||
</div>
|
||
</div>
|
||
|
||
<div id="mdbook-search-wrapper" class="hidden">
|
||
<form id="mdbook-searchbar-outer" class="searchbar-outer">
|
||
<div class="search-wrapper">
|
||
<input type="search" id="mdbook-searchbar" name="searchbar" placeholder="Search this book ..." aria-controls="mdbook-searchresults-outer" aria-describedby="searchresults-header">
|
||
<div class="spinner-wrapper">
|
||
<span class=fa-svg id="fa-spin"><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512"><!--! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --><path d="M304 48c0-26.5-21.5-48-48-48s-48 21.5-48 48s21.5 48 48 48s48-21.5 48-48zm0 416c0-26.5-21.5-48-48-48s-48 21.5-48 48s21.5 48 48 48s48-21.5 48-48zM48 304c26.5 0 48-21.5 48-48s-21.5-48-48-48s-48 21.5-48 48s21.5 48 48 48zm464-48c0-26.5-21.5-48-48-48s-48 21.5-48 48s21.5 48 48 48s48-21.5 48-48zM142.9 437c18.7-18.7 18.7-49.1 0-67.9s-49.1-18.7-67.9 0s-18.7 49.1 0 67.9s49.1 18.7 67.9 0zm0-294.2c18.7-18.7 18.7-49.1 0-67.9S93.7 56.2 75 75s-18.7 49.1 0 67.9s49.1 18.7 67.9 0zM369.1 437c18.7 18.7 49.1 18.7 67.9 0s18.7-49.1 0-67.9s-49.1-18.7-67.9 0s-18.7 49.1 0 67.9z"/></svg></span>
|
||
</div>
|
||
</div>
|
||
</form>
|
||
<div id="mdbook-searchresults-outer" class="searchresults-outer hidden">
|
||
<div id="mdbook-searchresults-header" class="searchresults-header"></div>
|
||
<ul id="mdbook-searchresults">
|
||
</ul>
|
||
</div>
|
||
</div>
|
||
|
||
<!-- Apply ARIA attributes after the sidebar and the sidebar toggle button are added to the DOM -->
|
||
<script>
|
||
document.getElementById('mdbook-sidebar-toggle').setAttribute('aria-expanded', sidebar === 'visible');
|
||
document.getElementById('mdbook-sidebar').setAttribute('aria-hidden', sidebar !== 'visible');
|
||
Array.from(document.querySelectorAll('#mdbook-sidebar a')).forEach(function(link) {
|
||
link.setAttribute('tabIndex', sidebar === 'visible' ? 0 : -1);
|
||
});
|
||
</script>
|
||
|
||
<div id="mdbook-content" class="content">
|
||
<main>
|
||
<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>
|
||
<div class="table-wrapper">
|
||
<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>
|
||
</div>
|
||
<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>
|
||
|
||
<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: ObservableDeferred[None] | 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: ObservableDeferred[None] | 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: ObservableDeferred[None] | 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>
|
||
|
||
</tr>
|
||
</table>
|
||
|
||
</main>
|
||
|
||
<nav class="nav-wrapper" aria-label="Page navigation">
|
||
<!-- Mobile navigation buttons -->
|
||
<a rel="prev" href="../../development/dependencies.html" class="mobile-nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left">
|
||
<span class=fa-svg><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 320 512"><!--! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --><path d="M41.4 233.4c-12.5 12.5-12.5 32.8 0 45.3l160 160c12.5 12.5 32.8 12.5 45.3 0s12.5-32.8 0-45.3L109.3 256 246.6 118.6c12.5-12.5 12.5-32.8 0-45.3s-32.8-12.5-45.3 0l-160 160z"/></svg></span>
|
||
</a>
|
||
|
||
<a rel="next prefetch" href="../../log_contexts.html" class="mobile-nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right">
|
||
<span class=fa-svg><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 320 512"><!--! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --><path d="M278.6 233.4c12.5 12.5 12.5 32.8 0 45.3l-160 160c-12.5 12.5-32.8 12.5-45.3 0s-12.5-32.8 0-45.3L210.7 256 73.4 118.6c-12.5-12.5-12.5-32.8 0-45.3s32.8-12.5 45.3 0l160 160z"/></svg></span>
|
||
</a>
|
||
|
||
<div style="clear: both"></div>
|
||
</nav>
|
||
</div>
|
||
</div>
|
||
|
||
<nav class="nav-wide-wrapper" aria-label="Page navigation">
|
||
<a rel="prev" href="../../development/dependencies.html" class="nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left">
|
||
<span class=fa-svg><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 320 512"><!--! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --><path d="M41.4 233.4c-12.5 12.5-12.5 32.8 0 45.3l160 160c12.5 12.5 32.8 12.5 45.3 0s12.5-32.8 0-45.3L109.3 256 246.6 118.6c12.5-12.5 12.5-32.8 0-45.3s-32.8-12.5-45.3 0l-160 160z"/></svg></span>
|
||
</a>
|
||
|
||
<a rel="next prefetch" href="../../log_contexts.html" class="nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right">
|
||
<span class=fa-svg><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 320 512"><!--! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --><path d="M278.6 233.4c12.5 12.5 12.5 32.8 0 45.3l-160 160c-12.5 12.5-32.8 12.5-45.3 0s-12.5-32.8 0-45.3L210.7 256 73.4 118.6c-12.5-12.5-12.5-32.8 0-45.3s32.8-12.5 45.3 0l160 160z"/></svg></span>
|
||
</a>
|
||
</nav>
|
||
|
||
</div>
|
||
|
||
<template id=fa-eye><span class=fa-svg><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 576 512"><!--! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --><path d="M288 32c-80.8 0-145.5 36.8-192.6 80.6C48.6 156 17.3 208 2.5 243.7c-3.3 7.9-3.3 16.7 0 24.6C17.3 304 48.6 356 95.4 399.4C142.5 443.2 207.2 480 288 480s145.5-36.8 192.6-80.6c46.8-43.5 78.1-95.4 93-131.1c3.3-7.9 3.3-16.7 0-24.6c-14.9-35.7-46.2-87.7-93-131.1C433.5 68.8 368.8 32 288 32zM432 256c0 79.5-64.5 144-144 144s-144-64.5-144-144s64.5-144 144-144s144 64.5 144 144zM288 192c0 35.3-28.7 64-64 64c-11.5 0-22.3-3-31.6-8.4c-.2 2.8-.4 5.5-.4 8.4c0 53 43 96 96 96s96-43 96-96s-43-96-96-96c-2.8 0-5.6 .1-8.4 .4c5.3 9.3 8.4 20.1 8.4 31.6z"/></svg></span></template>
|
||
<template id=fa-eye-slash><span class=fa-svg><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 640 512"><!--! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --><path d="M38.8 5.1C28.4-3.1 13.3-1.2 5.1 9.2S-1.2 34.7 9.2 42.9l592 464c10.4 8.2 25.5 6.3 33.7-4.1s6.3-25.5-4.1-33.7L525.6 386.7c39.6-40.6 66.4-86.1 79.9-118.4c3.3-7.9 3.3-16.7 0-24.6c-14.9-35.7-46.2-87.7-93-131.1C465.5 68.8 400.8 32 320 32c-68.2 0-125 26.3-169.3 60.8L38.8 5.1zM223.1 149.5C248.6 126.2 282.7 112 320 112c79.5 0 144 64.5 144 144c0 24.9-6.3 48.3-17.4 68.7L408 294.5c5.2-11.8 8-24.8 8-38.5c0-53-43-96-96-96c-2.8 0-5.6 .1-8.4 .4c5.3 9.3 8.4 20.1 8.4 31.6c0 10.2-2.4 19.8-6.6 28.3l-90.3-70.8zm223.1 298L373 389.9c-16.4 6.5-34.3 10.1-53 10.1c-79.5 0-144-64.5-144-144c0-6.9 .5-13.6 1.4-20.2L83.1 161.5C60.3 191.2 44 220.8 34.5 243.7c-3.3 7.9-3.3 16.7 0 24.6c14.9 35.7 46.2 87.7 93 131.1C174.5 443.2 239.2 480 320 480c47.8 0 89.9-12.9 126.2-32.5z"/></svg></span></template>
|
||
<template id=fa-copy><span class=fa-svg><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512"><!--! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --><path d="M502.6 70.63l-61.25-61.25C435.4 3.371 427.2 0 418.7 0H255.1c-35.35 0-64 28.66-64 64l.0195 256C192 355.4 220.7 384 256 384h192c35.2 0 64-28.8 64-64V93.25C512 84.77 508.6 76.63 502.6 70.63zM464 320c0 8.836-7.164 16-16 16H255.1c-8.838 0-16-7.164-16-16L239.1 64.13c0-8.836 7.164-16 16-16h128L384 96c0 17.67 14.33 32 32 32h47.1V320zM272 448c0 8.836-7.164 16-16 16H63.1c-8.838 0-16-7.164-16-16L47.98 192.1c0-8.836 7.164-16 16-16H160V128H63.99c-35.35 0-64 28.65-64 64l.0098 256C.002 483.3 28.66 512 64 512h192c35.2 0 64-28.8 64-64v-32h-47.1L272 448z"/></svg></span></template>
|
||
<template id=fa-play><span class=fa-svg><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 384 512"><!--! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --><path d="M73 39c-14.8-9.1-33.4-9.4-48.5-.9S0 62.6 0 80V432c0 17.4 9.4 33.4 24.5 41.9s33.7 8.1 48.5-.9L361 297c14.3-8.7 23-24.2 23-41s-8.7-32.2-23-41L73 39z"/></svg></span></template>
|
||
<template id=fa-clock-rotate-left><span class=fa-svg><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512"><!--! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --><path d="M75 75L41 41C25.9 25.9 0 36.6 0 57.9V168c0 13.3 10.7 24 24 24H134.1c21.4 0 32.1-25.9 17-41l-30.8-30.8C155 85.5 203 64 256 64c106 0 192 86 192 192s-86 192-192 192c-40.8 0-78.6-12.7-109.7-34.4c-14.5-10.1-34.4-6.6-44.6 7.9s-6.6 34.4 7.9 44.6C151.2 495 201.7 512 256 512c141.4 0 256-114.6 256-256S397.4 0 256 0C185.3 0 121.3 28.7 75 75zm181 53c-13.3 0-24 10.7-24 24V256c0 6.4 2.5 12.5 7 17l72 72c9.4 9.4 24.6 9.4 33.9 0s9.4-24.6 0-33.9l-65-65V152c0-13.3-10.7-24-24-24z"/></svg></span></template>
|
||
|
||
|
||
|
||
<script>
|
||
window.playground_copyable = true;
|
||
</script>
|
||
|
||
|
||
<script src="../../elasticlunr-ef4e11c1.min.js"></script>
|
||
<script src="../../mark-09e88c2c.min.js"></script>
|
||
<script src="../../searcher-c2a407aa.js"></script>
|
||
|
||
<script src="../../clipboard-1626706a.min.js"></script>
|
||
<script src="../../highlight-abc7f01d.js"></script>
|
||
<script src="../../book-a0b12cfe.js"></script>
|
||
|
||
<!-- Custom JS scripts -->
|
||
<script src="../../docs/website_files/version-picker-a1e40922.js"></script>
|
||
<script src="../../docs/website_files/version-d2c738f4.js"></script>
|
||
|
||
|
||
|
||
</div>
|
||
</body>
|
||
</html>
|