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+
+                        <h1 id="log-contexts"><a class="header" href="#log-contexts">Log Contexts</a></h1>
+<p>To help track the processing of individual requests, synapse uses a
+'<code>log context</code>' to track which request it is handling at any given
+moment. This is done via a thread-local variable; a <code>logging.Filter</code> is
+then used to fish the information back out of the thread-local variable
+and add it to each log record.</p>
+<p>Logcontexts are also used for CPU and database accounting, so that we
+can track which requests were responsible for high CPU use or database
+activity.</p>
+<p>The <code>synapse.logging.context</code> module provides facilities for managing
+the current log context (as well as providing the <code>LoggingContextFilter</code>
+class).</p>
+<p>Asynchronous functions make the whole thing complicated, so this document describes
+how it all works, and how to write code which follows the rules.</p>
+<p>In this document, &quot;awaitable&quot; refers to any object which can be <code>await</code>ed. In the context of
+Synapse, that normally means either a coroutine or a Twisted 
+<a href="https://twistedmatrix.com/documents/current/api/twisted.internet.defer.Deferred.html"><code>Deferred</code></a>.</p>
+<h2 id="logcontexts-without-asynchronous-code"><a class="header" href="#logcontexts-without-asynchronous-code">Logcontexts without asynchronous code</a></h2>
+<p>In the absence of any asynchronous voodoo, things are simple enough. As with
+any code of this nature, the rule is that our function should leave
+things as it found them:</p>
+<pre><code class="language-python">from synapse.logging import context         # omitted from future snippets
+
+def handle_request(request_id):
+    request_context = context.LoggingContext()
+
+    calling_context = context.set_current_context(request_context)
+    try:
+        request_context.request = request_id
+        do_request_handling()
+        logger.debug(&quot;finished&quot;)
+    finally:
+        context.set_current_context(calling_context)
+
+def do_request_handling():
+    logger.debug(&quot;phew&quot;)  # this will be logged against request_id
+</code></pre>
+<p>LoggingContext implements the context management methods, so the above
+can be written much more succinctly as:</p>
+<pre><code class="language-python">def handle_request(request_id):
+    with context.LoggingContext() as request_context:
+        request_context.request = request_id
+        do_request_handling()
+        logger.debug(&quot;finished&quot;)
+
+def do_request_handling():
+    logger.debug(&quot;phew&quot;)
+</code></pre>
+<h2 id="using-logcontexts-with-awaitables"><a class="header" href="#using-logcontexts-with-awaitables">Using logcontexts with awaitables</a></h2>
+<p>Awaitables break the linear flow of code so that there is no longer a single entry point
+where we should set the logcontext and a single exit point where we should remove it.</p>
+<p>Consider the example above, where <code>do_request_handling</code> needs to do some
+blocking operation, and returns an awaitable:</p>
+<pre><code class="language-python">async def handle_request(request_id):
+    with context.LoggingContext() as request_context:
+        request_context.request = request_id
+        await do_request_handling()
+        logger.debug(&quot;finished&quot;)
+</code></pre>
+<p>In the above flow:</p>
+<ul>
+<li>The logcontext is set</li>
+<li><code>do_request_handling</code> is called, and returns an awaitable</li>
+<li><code>handle_request</code> awaits the awaitable</li>
+<li>Execution of <code>handle_request</code> is suspended</li>
+</ul>
+<p>So we have stopped processing the request (and will probably go on to
+start processing the next), without clearing the logcontext.</p>
+<p>To circumvent this problem, synapse code assumes that, wherever you have
+an awaitable, you will want to <code>await</code> it. To that end, whereever
+functions return awaitables, we adopt the following conventions:</p>
+<p><strong>Rules for functions returning awaitables:</strong></p>
+<blockquote>
+<ul>
+<li>If the awaitable is already complete, the function returns with the
+same logcontext it started with.</li>
+<li>If the awaitable is incomplete, the function clears the logcontext
+before returning; when the awaitable completes, it restores the
+logcontext before running any callbacks.</li>
+</ul>
+</blockquote>
+<p>That sounds complicated, but actually it means a lot of code (including
+the example above) &quot;just works&quot;. There are two cases:</p>
+<ul>
+<li>
+<p>If <code>do_request_handling</code> returns a completed awaitable, then the
+logcontext will still be in place. In this case, execution will
+continue immediately after the <code>await</code>; the &quot;finished&quot; line will
+be logged against the right context, and the <code>with</code> block restores
+the original context before we return to the caller.</p>
+</li>
+<li>
+<p>If the returned awaitable is incomplete, <code>do_request_handling</code> clears
+the logcontext before returning. The logcontext is therefore clear
+when <code>handle_request</code> <code>await</code>s the awaitable.</p>
+<p>Once <code>do_request_handling</code>'s awaitable completes, it will reinstate
+the logcontext, before running the second half of <code>handle_request</code>,
+so again the &quot;finished&quot; line will be logged against the right context,
+and the <code>with</code> block restores the original context.</p>
+</li>
+</ul>
+<p>As an aside, it's worth noting that <code>handle_request</code> follows our rules</p>
+<ul>
+<li>though that only matters if the caller has its own logcontext which it
+cares about.</li>
+</ul>
+<p>The following sections describe pitfalls and helpful patterns when
+implementing these rules.</p>
+<h2 id="always-await-your-awaitables"><a class="header" href="#always-await-your-awaitables">Always await your awaitables</a></h2>
+<p>Whenever you get an awaitable back from a function, you should <code>await</code> on
+it as soon as possible. Do not pass go; do not do any logging; do not
+call any other functions.</p>
+<pre><code class="language-python">async def fun():
+    logger.debug(&quot;starting&quot;)
+    await do_some_stuff()       # just like this
+
+    coro = more_stuff()
+    result = await coro         # also fine, of course
+
+    return result
+</code></pre>
+<p>Provided this pattern is followed all the way back up to the callchain
+to where the logcontext was set, this will make things work out ok:
+provided <code>do_some_stuff</code> and <code>more_stuff</code> follow the rules above, then
+so will <code>fun</code>.</p>
+<p>It's all too easy to forget to <code>await</code>: for instance if we forgot that
+<code>do_some_stuff</code> returned an awaitable, we might plough on regardless. This
+leads to a mess; it will probably work itself out eventually, but not
+before a load of stuff has been logged against the wrong context.
+(Normally, other things will break, more obviously, if you forget to
+<code>await</code>, so this tends not to be a major problem in practice.)</p>
+<p>Of course sometimes you need to do something a bit fancier with your
+awaitable - not all code follows the linear A-then-B-then-C pattern.
+Notes on implementing more complex patterns are in later sections.</p>
+<h2 id="where-you-create-a-new-awaitable-make-it-follow-the-rules"><a class="header" href="#where-you-create-a-new-awaitable-make-it-follow-the-rules">Where you create a new awaitable, make it follow the rules</a></h2>
+<p>Most of the time, an awaitable comes from another synapse function.
+Sometimes, though, we need to make up a new awaitable, or we get an awaitable
+back from external code. We need to make it follow our rules.</p>
+<p>The easy way to do it is by using <code>context.make_deferred_yieldable</code>. Suppose we want to implement
+<code>sleep</code>, which returns a deferred which will run its callbacks after a
+given number of seconds. That might look like:</p>
+<pre><code class="language-python"># not a logcontext-rules-compliant function
+def get_sleep_deferred(seconds):
+    d = defer.Deferred()
+    reactor.callLater(seconds, d.callback, None)
+    return d
+</code></pre>
+<p>That doesn't follow the rules, but we can fix it by calling it through
+<code>context.make_deferred_yieldable</code>:</p>
+<pre><code class="language-python">async def sleep(seconds):
+    return await context.make_deferred_yieldable(get_sleep_deferred(seconds))
+</code></pre>
+<h2 id="fire-and-forget"><a class="header" href="#fire-and-forget">Fire-and-forget</a></h2>
+<p>Sometimes you want to fire off a chain of execution, but not wait for
+its result. That might look a bit like this:</p>
+<pre><code class="language-python">async def do_request_handling():
+    await foreground_operation()
+
+    # *don't* do this
+    background_operation()
+
+    logger.debug(&quot;Request handling complete&quot;)
+
+async def background_operation():
+    await first_background_step()
+    logger.debug(&quot;Completed first step&quot;)
+    await second_background_step()
+    logger.debug(&quot;Completed second step&quot;)
+</code></pre>
+<p>The above code does a couple of steps in the background after
+<code>do_request_handling</code> has finished. The log lines are still logged
+against the <code>request_context</code> logcontext, which may or may not be
+desirable. There are two big problems with the above, however. The first
+problem is that, if <code>background_operation</code> returns an incomplete
+awaitable, it will expect its caller to <code>await</code> immediately, so will have
+cleared the logcontext. In this example, that means that 'Request
+handling complete' will be logged without any context.</p>
+<p>The second problem, which is potentially even worse, is that when the
+awaitable returned by <code>background_operation</code> completes, it will restore
+the original logcontext. There is nothing waiting on that awaitable, so
+the logcontext will leak into the reactor and possibly get attached to
+some arbitrary future operation.</p>
+<p>There are two potential solutions to this.</p>
+<p>One option is to surround the call to <code>background_operation</code> with a
+<code>PreserveLoggingContext</code> call. That will reset the logcontext before
+starting <code>background_operation</code> (so the context restored when the
+deferred completes will be the empty logcontext), and will restore the
+current logcontext before continuing the foreground process:</p>
+<pre><code class="language-python">async def do_request_handling():
+    await foreground_operation()
+
+    # start background_operation off in the empty logcontext, to
+    # avoid leaking the current context into the reactor.
+    with PreserveLoggingContext():
+        background_operation()
+
+    # this will now be logged against the request context
+    logger.debug(&quot;Request handling complete&quot;)
+</code></pre>
+<p>Obviously that option means that the operations done in
+<code>background_operation</code> would be not be logged against a logcontext
+(though that might be fixed by setting a different logcontext via a
+<code>with LoggingContext(...)</code> in <code>background_operation</code>).</p>
+<p>The second option is to use <code>context.run_in_background</code>, which wraps a
+function so that it doesn't reset the logcontext even when it returns
+an incomplete awaitable, and adds a callback to the returned awaitable to
+reset the logcontext. In other words, it turns a function that follows
+the Synapse rules about logcontexts and awaitables into one which behaves
+more like an external function --- the opposite operation to that
+described in the previous section. It can be used like this:</p>
+<pre><code class="language-python">async def do_request_handling():
+    await foreground_operation()
+
+    context.run_in_background(background_operation)
+
+    # this will now be logged against the request context
+    logger.debug(&quot;Request handling complete&quot;)
+</code></pre>
+<h2 id="passing-synapse-deferreds-into-third-party-functions"><a class="header" href="#passing-synapse-deferreds-into-third-party-functions">Passing synapse deferreds into third-party functions</a></h2>
+<p>A typical example of this is where we want to collect together two or
+more awaitables via <code>defer.gatherResults</code>:</p>
+<pre><code class="language-python">a1 = operation1()
+a2 = operation2()
+a3 = defer.gatherResults([a1, a2])
+</code></pre>
+<p>This is really a variation of the fire-and-forget problem above, in that
+we are firing off <code>a1</code> and <code>a2</code> without awaiting on them. The difference
+is that we now have third-party code attached to their callbacks. Anyway
+either technique given in the <a href="#fire-and-forget">Fire-and-forget</a>
+section will work.</p>
+<p>Of course, the new awaitable returned by <code>gather</code> needs to be
+wrapped in order to make it follow the logcontext rules before we can
+yield it, as described in <a href="#where-you-create-a-new-awaitable-make-it-follow-the-rules">Where you create a new awaitable, make it
+follow the
+rules</a>.</p>
+<p>So, option one: reset the logcontext before starting the operations to
+be gathered:</p>
+<pre><code class="language-python">async def do_request_handling():
+    with PreserveLoggingContext():
+        a1 = operation1()
+        a2 = operation2()
+        result = await defer.gatherResults([a1, a2])
+</code></pre>
+<p>In this case particularly, though, option two, of using
+<code>context.run_in_background</code> almost certainly makes more sense, so that
+<code>operation1</code> and <code>operation2</code> are both logged against the original
+logcontext. This looks like:</p>
+<pre><code class="language-python">async def do_request_handling():
+    a1 = context.run_in_background(operation1)
+    a2 = context.run_in_background(operation2)
+
+    result = await make_deferred_yieldable(defer.gatherResults([a1, a2]))
+</code></pre>
+<h2 id="a-note-on-garbage-collection-of-awaitable-chains"><a class="header" href="#a-note-on-garbage-collection-of-awaitable-chains">A note on garbage-collection of awaitable chains</a></h2>
+<p>It turns out that our logcontext rules do not play nicely with awaitable
+chains which get orphaned and garbage-collected.</p>
+<p>Imagine we have some code that looks like this:</p>
+<pre><code class="language-python">listener_queue = []
+
+def on_something_interesting():
+    for d in listener_queue:
+        d.callback(&quot;foo&quot;)
+
+async def await_something_interesting():
+    new_awaitable = defer.Deferred()
+    listener_queue.append(new_awaitable)
+
+    with PreserveLoggingContext():
+        await new_awaitable
+</code></pre>
+<p>Obviously, the idea here is that we have a bunch of things which are
+waiting for an event. (It's just an example of the problem here, but a
+relatively common one.)</p>
+<p>Now let's imagine two further things happen. First of all, whatever was
+waiting for the interesting thing goes away. (Perhaps the request times
+out, or something <em>even more</em> interesting happens.)</p>
+<p>Secondly, let's suppose that we decide that the interesting thing is
+never going to happen, and we reset the listener queue:</p>
+<pre><code class="language-python">def reset_listener_queue():
+    listener_queue.clear()
+</code></pre>
+<p>So, both ends of the awaitable chain have now dropped their references,
+and the awaitable chain is now orphaned, and will be garbage-collected at
+some point. Note that <code>await_something_interesting</code> is a coroutine, 
+which Python implements as a generator function.  When Python
+garbage-collects generator functions, it gives them a chance to 
+clean up by making the <code>await</code> (or <code>yield</code>) raise a <code>GeneratorExit</code>
+exception. In our case, that means that the <code>__exit__</code> handler of
+<code>PreserveLoggingContext</code> will carefully restore the request context, but
+there is now nothing waiting for its return, so the request context is
+never cleared.</p>
+<p>To reiterate, this problem only arises when <em>both</em> ends of a awaitable
+chain are dropped. Dropping the the reference to an awaitable you're
+supposed to be awaiting is bad practice, so this doesn't
+actually happen too much. Unfortunately, when it does happen, it will
+lead to leaked logcontexts which are incredibly hard to track down.</p>
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