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<h1 id="log-contexts"><a class="header" href="#log-contexts">Log Contexts</a></h1>
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<p>To help track the processing of individual requests, synapse uses a
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'<code>log context</code>' to track which request it is handling at any given
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moment. This is done via a thread-local variable; a <code>logging.Filter</code> is
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then used to fish the information back out of the thread-local variable
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and add it to each log record.</p>
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<p>Logcontexts are also used for CPU and database accounting, so that we
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can track which requests were responsible for high CPU use or database
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activity.</p>
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<p>The <code>synapse.logging.context</code> module provides a facilities for managing
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the current log context (as well as providing the <code>LoggingContextFilter</code>
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class).</p>
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<p>Deferreds make the whole thing complicated, so this document describes
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how it all works, and how to write code which follows the rules.</p>
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<p>##Logcontexts without Deferreds</p>
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<p>In the absence of any Deferred voodoo, things are simple enough. As with
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any code of this nature, the rule is that our function should leave
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things as it found them:</p>
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<pre><code class="language-python">from synapse.logging import context # omitted from future snippets
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def handle_request(request_id):
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request_context = context.LoggingContext()
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calling_context = context.set_current_context(request_context)
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try:
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request_context.request = request_id
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do_request_handling()
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logger.debug("finished")
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finally:
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context.set_current_context(calling_context)
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def do_request_handling():
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logger.debug("phew") # this will be logged against request_id
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</code></pre>
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<p>LoggingContext implements the context management methods, so the above
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can be written much more succinctly as:</p>
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<pre><code class="language-python">def handle_request(request_id):
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with context.LoggingContext() as request_context:
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request_context.request = request_id
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do_request_handling()
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logger.debug("finished")
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def do_request_handling():
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logger.debug("phew")
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</code></pre>
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<h2 id="using-logcontexts-with-deferreds"><a class="header" href="#using-logcontexts-with-deferreds">Using logcontexts with Deferreds</a></h2>
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<p>Deferreds --- and in particular, <code>defer.inlineCallbacks</code> --- break the
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linear flow of code so that there is no longer a single entry point
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where we should set the logcontext and a single exit point where we
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should remove it.</p>
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<p>Consider the example above, where <code>do_request_handling</code> needs to do some
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blocking operation, and returns a deferred:</p>
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<pre><code class="language-python">@defer.inlineCallbacks
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def handle_request(request_id):
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with context.LoggingContext() as request_context:
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request_context.request = request_id
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yield do_request_handling()
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logger.debug("finished")
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</code></pre>
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<p>In the above flow:</p>
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<ul>
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<li>The logcontext is set</li>
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<li><code>do_request_handling</code> is called, and returns a deferred</li>
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<li><code>handle_request</code> yields the deferred</li>
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<li>The <code>inlineCallbacks</code> wrapper of <code>handle_request</code> returns a deferred</li>
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</ul>
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<p>So we have stopped processing the request (and will probably go on to
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start processing the next), without clearing the logcontext.</p>
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<p>To circumvent this problem, synapse code assumes that, wherever you have
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a deferred, you will want to yield on it. To that end, whereever
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functions return a deferred, we adopt the following conventions:</p>
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<p><strong>Rules for functions returning deferreds:</strong></p>
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<blockquote>
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<ul>
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<li>If the deferred is already complete, the function returns with the
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same logcontext it started with.</li>
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<li>If the deferred is incomplete, the function clears the logcontext
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before returning; when the deferred completes, it restores the
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logcontext before running any callbacks.</li>
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</ul>
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</blockquote>
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<p>That sounds complicated, but actually it means a lot of code (including
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the example above) "just works". There are two cases:</p>
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<ul>
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<li>
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<p>If <code>do_request_handling</code> returns a completed deferred, then the
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logcontext will still be in place. In this case, execution will
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continue immediately after the <code>yield</code>; the "finished" line will
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be logged against the right context, and the <code>with</code> block restores
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the original context before we return to the caller.</p>
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</li>
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<li>
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<p>If the returned deferred is incomplete, <code>do_request_handling</code> clears
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the logcontext before returning. The logcontext is therefore clear
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when <code>handle_request</code> yields the deferred. At that point, the
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<code>inlineCallbacks</code> wrapper adds a callback to the deferred, and
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returns another (incomplete) deferred to the caller, and it is safe
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to begin processing the next request.</p>
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<p>Once <code>do_request_handling</code>'s deferred completes, it will reinstate
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the logcontext, before running the callback added by the
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<code>inlineCallbacks</code> wrapper. That callback runs the second half of
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<code>handle_request</code>, so again the "finished" line will be logged
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against the right context, and the <code>with</code> block restores the
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original context.</p>
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</li>
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</ul>
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<p>As an aside, it's worth noting that <code>handle_request</code> follows our rules
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-though that only matters if the caller has its own logcontext which it
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cares about.</p>
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<p>The following sections describe pitfalls and helpful patterns when
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implementing these rules.</p>
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<h2 id="always-yield-your-deferreds"><a class="header" href="#always-yield-your-deferreds">Always yield your deferreds</a></h2>
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<p>Whenever you get a deferred back from a function, you should <code>yield</code> on
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it as soon as possible. (Returning it directly to your caller is ok too,
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if you're not doing <code>inlineCallbacks</code>.) Do not pass go; do not do any
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logging; do not call any other functions.</p>
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<pre><code class="language-python">@defer.inlineCallbacks
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def fun():
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logger.debug("starting")
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yield do_some_stuff() # just like this
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d = more_stuff()
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result = yield d # also fine, of course
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return result
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def nonInlineCallbacksFun():
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logger.debug("just a wrapper really")
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return do_some_stuff() # this is ok too - the caller will yield on
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# it anyway.
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</code></pre>
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<p>Provided this pattern is followed all the way back up to the callchain
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to where the logcontext was set, this will make things work out ok:
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|
provided <code>do_some_stuff</code> and <code>more_stuff</code> follow the rules above, then
|
|
so will <code>fun</code> (as wrapped by <code>inlineCallbacks</code>) and
|
|
<code>nonInlineCallbacksFun</code>.</p>
|
|
<p>It's all too easy to forget to <code>yield</code>: for instance if we forgot that
|
|
<code>do_some_stuff</code> returned a deferred, 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>yield</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
|
|
Deferreds - 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-deferred-make-it-follow-the-rules"><a class="header" href="#where-you-create-a-new-deferred-make-it-follow-the-rules">Where you create a new Deferred, make it follow the rules</a></h2>
|
|
<p>Most of the time, a Deferred comes from another synapse function.
|
|
Sometimes, though, we need to make up a new Deferred, or we get a
|
|
Deferred back from external code. We need to make it follow our rules.</p>
|
|
<p>The easy way to do it is with a combination of <code>defer.inlineCallbacks</code>,
|
|
and <code>context.PreserveLoggingContext</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 wrapping it with
|
|
<code>PreserveLoggingContext</code> and <code>yield</code> ing on it:</p>
|
|
<pre><code class="language-python">@defer.inlineCallbacks
|
|
def sleep(seconds):
|
|
with PreserveLoggingContext():
|
|
yield get_sleep_deferred(seconds)
|
|
</code></pre>
|
|
<p>This technique works equally for external functions which return
|
|
deferreds, or deferreds we have made ourselves.</p>
|
|
<p>You can also use <code>context.make_deferred_yieldable</code>, which just does the
|
|
boilerplate for you, so the above could be written:</p>
|
|
<pre><code class="language-python">def sleep(seconds):
|
|
return 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">@defer.inlineCallbacks
|
|
def do_request_handling():
|
|
yield foreground_operation()
|
|
|
|
# *don't* do this
|
|
background_operation()
|
|
|
|
logger.debug("Request handling complete")
|
|
|
|
@defer.inlineCallbacks
|
|
def background_operation():
|
|
yield first_background_step()
|
|
logger.debug("Completed first step")
|
|
yield second_background_step()
|
|
logger.debug("Completed second step")
|
|
</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
|
|
Deferred, it will expect its caller to <code>yield</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
|
|
Deferred returned by <code>background_operation</code> completes, it will restore
|
|
the original logcontext. There is nothing waiting on that Deferred, 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">@defer.inlineCallbacks
|
|
def do_request_handling():
|
|
yield 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("Request handling complete")
|
|
</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 deferred, and adds a callback to the returned deferred to
|
|
reset the logcontext. In other words, it turns a function that follows
|
|
the Synapse rules about logcontexts and Deferreds 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">@defer.inlineCallbacks
|
|
def do_request_handling():
|
|
yield foreground_operation()
|
|
|
|
context.run_in_background(background_operation)
|
|
|
|
# this will now be logged against the request context
|
|
logger.debug("Request handling complete")
|
|
</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 deferred via <code>defer.gatherResults</code>:</p>
|
|
<pre><code class="language-python">d1 = operation1()
|
|
d2 = operation2()
|
|
d3 = defer.gatherResults([d1, d2])
|
|
</code></pre>
|
|
<p>This is really a variation of the fire-and-forget problem above, in that
|
|
we are firing off <code>d1</code> and <code>d2</code> without yielding 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 Deferred returned by <code>gatherResults</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-deferred-make-it-follow-the-rules">Where you create a new Deferred, 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">@defer.inlineCallbacks
|
|
def do_request_handling():
|
|
with PreserveLoggingContext():
|
|
d1 = operation1()
|
|
d2 = operation2()
|
|
result = yield defer.gatherResults([d1, d2])
|
|
</code></pre>
|
|
<p>In this case particularly, though, option two, of using
|
|
<code>context.preserve_fn</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">@defer.inlineCallbacks
|
|
def do_request_handling():
|
|
d1 = context.preserve_fn(operation1)()
|
|
d2 = context.preserve_fn(operation2)()
|
|
|
|
with PreserveLoggingContext():
|
|
result = yield defer.gatherResults([d1, d2])
|
|
</code></pre>
|
|
<h2 id="was-all-this-really-necessary"><a class="header" href="#was-all-this-really-necessary">Was all this really necessary?</a></h2>
|
|
<p>The conventions used work fine for a linear flow where everything
|
|
happens in series via <code>defer.inlineCallbacks</code> and <code>yield</code>, but are
|
|
certainly tricky to follow for any more exotic flows. It's hard not to
|
|
wonder if we could have done something else.</p>
|
|
<p>We're not going to rewrite Synapse now, so the following is entirely of
|
|
academic interest, but I'd like to record some thoughts on an
|
|
alternative approach.</p>
|
|
<p>I briefly prototyped some code following an alternative set of rules. I
|
|
think it would work, but I certainly didn't get as far as thinking how
|
|
it would interact with concepts as complicated as the cache descriptors.</p>
|
|
<p>My alternative rules were:</p>
|
|
<ul>
|
|
<li>functions always preserve the logcontext of their caller, whether or
|
|
not they are returning a Deferred.</li>
|
|
<li>Deferreds returned by synapse functions run their callbacks in the
|
|
same context as the function was orignally called in.</li>
|
|
</ul>
|
|
<p>The main point of this scheme is that everywhere that sets the
|
|
logcontext is responsible for clearing it before returning control to
|
|
the reactor.</p>
|
|
<p>So, for example, if you were the function which started a
|
|
<code>with LoggingContext</code> block, you wouldn't <code>yield</code> within it --- instead
|
|
you'd start off the background process, and then leave the <code>with</code> block
|
|
to wait for it:</p>
|
|
<pre><code class="language-python">def handle_request(request_id):
|
|
with context.LoggingContext() as request_context:
|
|
request_context.request = request_id
|
|
d = do_request_handling()
|
|
|
|
def cb(r):
|
|
logger.debug("finished")
|
|
|
|
d.addCallback(cb)
|
|
return d
|
|
</code></pre>
|
|
<p>(in general, mixing <code>with LoggingContext</code> blocks and
|
|
<code>defer.inlineCallbacks</code> in the same function leads to slighly
|
|
counter-intuitive code, under this scheme).</p>
|
|
<p>Because we leave the original <code>with</code> block as soon as the Deferred is
|
|
returned (as opposed to waiting for it to be resolved, as we do today),
|
|
the logcontext is cleared before control passes back to the reactor; so
|
|
if there is some code within <code>do_request_handling</code> which needs to wait
|
|
for a Deferred to complete, there is no need for it to worry about
|
|
clearing the logcontext before doing so:</p>
|
|
<pre><code class="language-python">def handle_request():
|
|
r = do_some_stuff()
|
|
r.addCallback(do_some_more_stuff)
|
|
return r
|
|
</code></pre>
|
|
<p>--- and provided <code>do_some_stuff</code> follows the rules of returning a
|
|
Deferred which runs its callbacks in the original logcontext, all is
|
|
happy.</p>
|
|
<p>The business of a Deferred which runs its callbacks in the original
|
|
logcontext isn't hard to achieve --- we have it today, in the shape of
|
|
<code>context._PreservingContextDeferred</code>:</p>
|
|
<pre><code class="language-python">def do_some_stuff():
|
|
deferred = do_some_io()
|
|
pcd = _PreservingContextDeferred(LoggingContext.current_context())
|
|
deferred.chainDeferred(pcd)
|
|
return pcd
|
|
</code></pre>
|
|
<p>It turns out that, thanks to the way that Deferreds chain together, we
|
|
automatically get the property of a context-preserving deferred with
|
|
<code>defer.inlineCallbacks</code>, provided the final Defered the function
|
|
<code>yields</code> on has that property. So we can just write:</p>
|
|
<pre><code class="language-python">@defer.inlineCallbacks
|
|
def handle_request():
|
|
yield do_some_stuff()
|
|
yield do_some_more_stuff()
|
|
</code></pre>
|
|
<p>To conclude: I think this scheme would have worked equally well, with
|
|
less danger of messing it up, and probably made some more esoteric code
|
|
easier to write. But again --- changing the conventions of the entire
|
|
Synapse codebase is not a sensible option for the marginal improvement
|
|
offered.</p>
|
|
<h2 id="a-note-on-garbage-collection-of-deferred-chains"><a class="header" href="#a-note-on-garbage-collection-of-deferred-chains">A note on garbage-collection of Deferred chains</a></h2>
|
|
<p>It turns out that our logcontext rules do not play nicely with Deferred
|
|
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("foo")
|
|
|
|
@defer.inlineCallbacks
|
|
def await_something_interesting():
|
|
new_deferred = defer.Deferred()
|
|
listener_queue.append(new_deferred)
|
|
|
|
with PreserveLoggingContext():
|
|
yield new_deferred
|
|
</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 deferred chain have now dropped their references,
|
|
and the deferred chain is now orphaned, and will be garbage-collected at
|
|
some point. Note that <code>await_something_interesting</code> is a generator
|
|
function, and when Python garbage-collects generator functions, it gives
|
|
them a chance to clean up by making the <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 deferred
|
|
chain are dropped. Dropping the the reference to a deferred you're
|
|
supposed to be calling is probably 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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