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The cache wrappers had a habit of leaking the logcontext into the reactor while
the lookup function was running, and then not restoring it correctly when the
lookup function had completed. It's all the fault of
`preserve_context_over_{fn,deferred}` which are basically a bit broken.
447 lines
17 KiB
ReStructuredText
447 lines
17 KiB
ReStructuredText
Log contexts
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============
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.. contents::
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To help track the processing of individual requests, synapse uses a
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'log context' to track which request it is handling at any given moment. This
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is done via a thread-local variable; a ``logging.Filter`` is then used to fish
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the information back out of the thread-local variable and add it to each log
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record.
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Logcontexts are also used for CPU and database accounting, so that we can track
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which requests were responsible for high CPU use or database activity.
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The ``synapse.util.logcontext`` module provides a facilities for managing the
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current log context (as well as providing the ``LoggingContextFilter`` class).
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Deferreds make the whole thing complicated, so this document describes how it
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all works, and how to write code which follows the rules.
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Logcontexts without Deferreds
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-----------------------------
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In the absence of any Deferred voodoo, things are simple enough. As with any
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code of this nature, the rule is that our function should leave things as it
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found them:
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.. code:: python
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from synapse.util import logcontext # omitted from future snippets
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def handle_request(request_id):
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request_context = logcontext.LoggingContext()
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calling_context = logcontext.LoggingContext.current_context()
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logcontext.LoggingContext.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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logcontext.LoggingContext.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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LoggingContext implements the context management methods, so the above can be
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written much more succinctly as:
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.. code:: python
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def handle_request(request_id):
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with logcontext.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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Using logcontexts with Deferreds
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--------------------------------
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Deferreds — and in particular, ``defer.inlineCallbacks`` — break
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the linear flow of code so that there is no longer a single entry point where
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we should set the logcontext and a single exit point where we should remove it.
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Consider the example above, where ``do_request_handling`` needs to do some
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blocking operation, and returns a deferred:
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.. code:: python
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@defer.inlineCallbacks
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def handle_request(request_id):
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with logcontext.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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In the above flow:
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* The logcontext is set
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* ``do_request_handling`` is called, and returns a deferred
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* ``handle_request`` yields the deferred
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* The ``inlineCallbacks`` wrapper of ``handle_request`` returns a deferred
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So we have stopped processing the request (and will probably go on to start
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processing the next), without clearing the logcontext.
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To circumvent this problem, synapse code assumes that, wherever you have a
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deferred, you will want to yield on it. To that end, whereever functions return
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a deferred, we adopt the following conventions:
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**Rules for functions returning deferreds:**
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* If the deferred is already complete, the function returns with the same
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logcontext it started with.
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* If the deferred is incomplete, the function clears the logcontext before
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returning; when the deferred completes, it restores the logcontext before
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running any callbacks.
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That sounds complicated, but actually it means a lot of code (including the
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example above) "just works". There are two cases:
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* If ``do_request_handling`` returns a completed deferred, then the logcontext
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will still be in place. In this case, execution will continue immediately
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after the ``yield``; the "finished" line will be logged against the right
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context, and the ``with`` block restores the original context before we
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return to the caller.
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* If the returned deferred is incomplete, ``do_request_handling`` clears the
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logcontext before returning. The logcontext is therefore clear when
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``handle_request`` yields the deferred. At that point, the ``inlineCallbacks``
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wrapper adds a callback to the deferred, and returns another (incomplete)
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deferred to the caller, and it is safe to begin processing the next request.
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Once ``do_request_handling``'s deferred completes, it will reinstate the
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logcontext, before running the callback added by the ``inlineCallbacks``
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wrapper. That callback runs the second half of ``handle_request``, so again
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the "finished" line will be logged against the right
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context, and the ``with`` block restores the original context.
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As an aside, it's worth noting that ``handle_request`` follows our rules -
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though that only matters if the caller has its own logcontext which it cares
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about.
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The following sections describe pitfalls and helpful patterns when implementing
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these rules.
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Always yield your deferreds
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---------------------------
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Whenever you get a deferred back from a function, you should ``yield`` on it
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as soon as possible. (Returning it directly to your caller is ok too, if you're
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not doing ``inlineCallbacks``.) Do not pass go; do not do any logging; do not
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call any other functions.
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.. code:: python
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@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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defer.returnValue(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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Provided this pattern is followed all the way back up to the callchain to where
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the logcontext was set, this will make things work out ok: provided
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``do_some_stuff`` and ``more_stuff`` follow the rules above, then so will
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``fun`` (as wrapped by ``inlineCallbacks``) and ``nonInlineCallbacksFun``.
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It's all too easy to forget to ``yield``: for instance if we forgot that
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``do_some_stuff`` returned a deferred, we might plough on regardless. This
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leads to a mess; it will probably work itself out eventually, but not before
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a load of stuff has been logged against the wrong content. (Normally, other
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things will break, more obviously, if you forget to ``yield``, so this tends
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not to be a major problem in practice.)
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Of course sometimes you need to do something a bit fancier with your Deferreds
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- not all code follows the linear A-then-B-then-C pattern. Notes on
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implementing more complex patterns are in later sections.
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Where you create a new Deferred, make it follow the rules
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---------------------------------------------------------
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Most of the time, a Deferred comes from another synapse function. Sometimes,
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though, we need to make up a new Deferred, or we get a Deferred back from
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external code. We need to make it follow our rules.
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The easy way to do it is with a combination of ``defer.inlineCallbacks``, and
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``logcontext.PreserveLoggingContext``. Suppose we want to implement ``sleep``,
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which returns a deferred which will run its callbacks after a given number of
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seconds. That might look like:
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.. code:: python
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# not a logcontext-rules-compliant function
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def get_sleep_deferred(seconds):
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d = defer.Deferred()
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reactor.callLater(seconds, d.callback, None)
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return d
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That doesn't follow the rules, but we can fix it by wrapping it with
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``PreserveLoggingContext`` and ``yield`` ing on it:
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.. code:: python
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@defer.inlineCallbacks
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def sleep(seconds):
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with PreserveLoggingContext():
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yield get_sleep_deferred(seconds)
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This technique works equally for external functions which return deferreds,
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or deferreds we have made ourselves.
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You can also use ``logcontext.make_deferred_yieldable``, which just does the
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boilerplate for you, so the above could be written:
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.. code:: python
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def sleep(seconds):
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return logcontext.make_deferred_yieldable(get_sleep_deferred(seconds))
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Fire-and-forget
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---------------
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Sometimes you want to fire off a chain of execution, but not wait for its
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result. That might look a bit like this:
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.. code:: python
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@defer.inlineCallbacks
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def do_request_handling():
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yield foreground_operation()
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# *don't* do this
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background_operation()
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logger.debug("Request handling complete")
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@defer.inlineCallbacks
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def background_operation():
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yield first_background_step()
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logger.debug("Completed first step")
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yield second_background_step()
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logger.debug("Completed second step")
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The above code does a couple of steps in the background after
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``do_request_handling`` has finished. The log lines are still logged against
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the ``request_context`` logcontext, which may or may not be desirable. There
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are two big problems with the above, however. The first problem is that, if
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``background_operation`` returns an incomplete Deferred, it will expect its
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caller to ``yield`` immediately, so will have cleared the logcontext. In this
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example, that means that 'Request handling complete' will be logged without any
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context.
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The second problem, which is potentially even worse, is that when the Deferred
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returned by ``background_operation`` completes, it will restore the original
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logcontext. There is nothing waiting on that Deferred, so the logcontext will
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leak into the reactor and possibly get attached to some arbitrary future
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operation.
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There are two potential solutions to this.
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One option is to surround the call to ``background_operation`` with a
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``PreserveLoggingContext`` call. That will reset the logcontext before
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starting ``background_operation`` (so the context restored when the deferred
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completes will be the empty logcontext), and will restore the current
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logcontext before continuing the foreground process:
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.. code:: python
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@defer.inlineCallbacks
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def do_request_handling():
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yield foreground_operation()
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# start background_operation off in the empty logcontext, to
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# avoid leaking the current context into the reactor.
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with PreserveLoggingContext():
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background_operation()
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# this will now be logged against the request context
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logger.debug("Request handling complete")
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Obviously that option means that the operations done in
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``background_operation`` would be not be logged against a logcontext (though
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that might be fixed by setting a different logcontext via a ``with
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LoggingContext(...)`` in ``background_operation``).
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The second option is to use ``logcontext.preserve_fn``, which wraps a function
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so that it doesn't reset the logcontext even when it returns an incomplete
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deferred, and adds a callback to the returned deferred to reset the
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logcontext. In other words, it turns a function that follows the Synapse rules
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about logcontexts and Deferreds into one which behaves more like an external
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function — the opposite operation to that described in the previous section.
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It can be used like this:
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.. code:: python
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@defer.inlineCallbacks
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def do_request_handling():
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yield foreground_operation()
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logcontext.preserve_fn(background_operation)()
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# this will now be logged against the request context
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logger.debug("Request handling complete")
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XXX: I think ``preserve_context_over_fn`` is supposed to do the first option,
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but the fact that it does ``preserve_context_over_deferred`` on its results
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means that its use is fraught with difficulty.
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Passing synapse deferreds into third-party functions
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----------------------------------------------------
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A typical example of this is where we want to collect together two or more
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deferred via ``defer.gatherResults``:
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.. code:: python
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d1 = operation1()
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d2 = operation2()
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d3 = defer.gatherResults([d1, d2])
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This is really a variation of the fire-and-forget problem above, in that we are
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firing off ``d1`` and ``d2`` without yielding on them. The difference
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is that we now have third-party code attached to their callbacks. Anyway either
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technique given in the `Fire-and-forget`_ section will work.
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Of course, the new Deferred returned by ``gatherResults`` needs to be wrapped
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in order to make it follow the logcontext rules before we can yield it, as
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described in `Where you create a new Deferred, make it follow the rules`_.
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So, option one: reset the logcontext before starting the operations to be
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gathered:
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.. code:: python
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@defer.inlineCallbacks
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def do_request_handling():
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with PreserveLoggingContext():
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d1 = operation1()
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d2 = operation2()
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result = yield defer.gatherResults([d1, d2])
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In this case particularly, though, option two, of using
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``logcontext.preserve_fn`` almost certainly makes more sense, so that
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``operation1`` and ``operation2`` are both logged against the original
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logcontext. This looks like:
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.. code:: python
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@defer.inlineCallbacks
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def do_request_handling():
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d1 = logcontext.preserve_fn(operation1)()
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d2 = logcontext.preserve_fn(operation2)()
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with PreserveLoggingContext():
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result = yield defer.gatherResults([d1, d2])
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Was all this really necessary?
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------------------------------
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The conventions used work fine for a linear flow where everything happens in
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series via ``defer.inlineCallbacks`` and ``yield``, but are certainly tricky to
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follow for any more exotic flows. It's hard not to wonder if we could have done
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something else.
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We're not going to rewrite Synapse now, so the following is entirely of
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academic interest, but I'd like to record some thoughts on an alternative
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approach.
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I briefly prototyped some code following an alternative set of rules. I think
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it would work, but I certainly didn't get as far as thinking how it would
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interact with concepts as complicated as the cache descriptors.
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My alternative rules were:
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* functions always preserve the logcontext of their caller, whether or not they
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are returning a Deferred.
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* Deferreds returned by synapse functions run their callbacks in the same
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context as the function was orignally called in.
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The main point of this scheme is that everywhere that sets the logcontext is
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responsible for clearing it before returning control to the reactor.
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So, for example, if you were the function which started a ``with
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LoggingContext`` block, you wouldn't ``yield`` within it — instead you'd start
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off the background process, and then leave the ``with`` block to wait for it:
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.. code:: python
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def handle_request(request_id):
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with logcontext.LoggingContext() as request_context:
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request_context.request = request_id
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d = do_request_handling()
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def cb(r):
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logger.debug("finished")
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d.addCallback(cb)
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return d
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(in general, mixing ``with LoggingContext`` blocks and
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``defer.inlineCallbacks`` in the same function leads to slighly
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counter-intuitive code, under this scheme).
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Because we leave the original ``with`` block as soon as the Deferred is
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returned (as opposed to waiting for it to be resolved, as we do today), the
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logcontext is cleared before control passes back to the reactor; so if there is
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some code within ``do_request_handling`` which needs to wait for a Deferred to
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complete, there is no need for it to worry about clearing the logcontext before
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doing so:
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.. code:: python
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def handle_request():
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r = do_some_stuff()
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r.addCallback(do_some_more_stuff)
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return r
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— and provided ``do_some_stuff`` follows the rules of returning a Deferred which
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runs its callbacks in the original logcontext, all is happy.
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The business of a Deferred which runs its callbacks in the original logcontext
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isn't hard to achieve — we have it today, in the shape of
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``logcontext._PreservingContextDeferred``:
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.. code:: python
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def do_some_stuff():
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deferred = do_some_io()
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pcd = _PreservingContextDeferred(LoggingContext.current_context())
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deferred.chainDeferred(pcd)
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return pcd
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It turns out that, thanks to the way that Deferreds chain together, we
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automatically get the property of a context-preserving deferred with
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``defer.inlineCallbacks``, provided the final Defered the function ``yields``
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on has that property. So we can just write:
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.. code:: python
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@defer.inlineCallbacks
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def handle_request():
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yield do_some_stuff()
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yield do_some_more_stuff()
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To conclude: I think this scheme would have worked equally well, with less
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danger of messing it up, and probably made some more esoteric code easier to
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write. But again — changing the conventions of the entire Synapse codebase is
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not a sensible option for the marginal improvement offered.
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