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diff --git a/docs/log_contexts.rst b/docs/log_contexts.rst index 0046e171be..a5f59745e6 100644 --- a/docs/log_contexts.rst +++ b/docs/log_contexts.rst @@ -1,10 +1,441 @@ -What do I do about "Unexpected logging context" debug log-lines everywhere? +Log contexts +============ -<Mjark> The logging context lives in thread local storage -<Mjark> Sometimes it gets out of sync with what it should actually be, usually because something scheduled something to run on the reactor without preserving the logging context. -<Matthew> what is the impact of it getting out of sync? and how and when should we preserve log context? -<Mjark> The impact is that some of the CPU and database metrics will be under-reported, and some log lines will be mis-attributed. -<Mjark> It should happen auto-magically in all the APIs that do IO or otherwise defer to the reactor. -<Erik> Mjark: the other place is if we branch, e.g. using defer.gatherResults +.. contents:: -Unanswered: how and when should we preserve log context? \ No newline at end of file +To help track the processing of individual requests, synapse uses a +'log context' to track which request it is handling at any given moment. This +is done via a thread-local variable; a ``logging.Filter`` is then used to fish +the information back out of the thread-local variable and add it to each log +record. + +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. + +The ``synapse.util.logcontext`` module provides a facilities for managing the +current log context (as well as providing the ``LoggingContextFilter`` class). + +Deferreds make the whole thing complicated, so this document describes how it +all works, and how to write code which follows the rules. + +Logcontexts without Deferreds +----------------------------- + +In the absence of any Deferred 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: + +.. code:: python + + from synapse.util import logcontext # omitted from future snippets + + def handle_request(request_id): + request_context = logcontext.LoggingContext() + + calling_context = logcontext.LoggingContext.current_context() + logcontext.LoggingContext.set_current_context(request_context) + try: + request_context.request = request_id + do_request_handling() + logger.debug("finished") + finally: + logcontext.LoggingContext.set_current_context(calling_context) + + def do_request_handling(): + logger.debug("phew") # this will be logged against request_id + + +LoggingContext implements the context management methods, so the above can be +written much more succinctly as: + +.. code:: python + + def handle_request(request_id): + with logcontext.LoggingContext() as request_context: + request_context.request = request_id + do_request_handling() + logger.debug("finished") + + def do_request_handling(): + logger.debug("phew") + + +Using logcontexts with Deferreds +-------------------------------- + +Deferreds — and in particular, ``defer.inlineCallbacks`` — 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. + +Consider the example above, where ``do_request_handling`` needs to do some +blocking operation, and returns a deferred: + +.. code:: python + + @defer.inlineCallbacks + def handle_request(request_id): + with logcontext.LoggingContext() as request_context: + request_context.request = request_id + yield do_request_handling() + logger.debug("finished") + + +In the above flow: + +* The logcontext is set +* ``do_request_handling`` is called, and returns a deferred +* ``handle_request`` yields the deferred +* The ``inlineCallbacks`` wrapper of ``handle_request`` returns a deferred + +So we have stopped processing the request (and will probably go on to start +processing the next), without clearing the logcontext. + +To circumvent this problem, synapse code assumes that, wherever you have a +deferred, you will want to yield on it. To that end, whereever functions return +a deferred, we adopt the following conventions: + +.. note:: Rules for functions returning deferreds: + + * If the deferred is already complete, the function returns with the same + logcontext it started with. + * If the deferred is incomplete, the function clears the logcontext before + returning; when the deferred completes, it restores the logcontext before + running any callbacks. + +That sounds complicated, but actually it means a lot of code (including the +example above) "just works". There are two cases: + +* If ``do_request_handling`` returns a completed deferred, then the logcontext + will still be in place. In this case, execution will continue immediately + after the ``yield``; the "finished" line will be logged against the right + context, and the ``with`` block restores the original context before we + return to the caller. + +* If the returned deferred is incomplete, ``do_request_handling`` clears the + logcontext before returning. The logcontext is therefore clear when + ``handle_request`` yields the deferred. At that point, the ``inlineCallbacks`` + wrapper adds a callback to the deferred, and returns another (incomplete) + deferred to the caller, and it is safe to begin processing the next request. + + Once ``do_request_handling``'s deferred completes, it will reinstate the + logcontext, before running the callback added by the ``inlineCallbacks`` + wrapper. That callback runs the second half of ``handle_request``, so again + the "finished" line will be logged against the right + context, and the ``with`` block restores the original context. + +As an aside, it's worth noting that ``handle_request`` follows our rules - +though that only matters if the caller has its own logcontext which it cares +about. + +The following sections describe pitfalls and helpful patterns when implementing +these rules. + +Always yield your deferreds +--------------------------- + +Whenever you get a deferred back from a function, you should ``yield`` on it +as soon as possible. (Returning it directly to your caller is ok too, if you're +not doing ``inlineCallbacks``.) Do not pass go; do not do any logging; do not +call any other functions. + +.. code:: python + + @defer.inlineCallbacks + def fun(): + logger.debug("starting") + yield do_some_stuff() # just like this + + d = more_stuff() + result = yield d # also fine, of course + + defer.returnValue(result) + + def nonInlineCallbacksFun(): + logger.debug("just a wrapper really") + return do_some_stuff() # this is ok too - the caller will yield on + # it anyway. + +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 +``do_some_stuff`` and ``more_stuff`` follow the rules above, then so will +``fun`` (as wrapped by ``inlineCallbacks``) and ``nonInlineCallbacksFun``. + +It's all too easy to forget to ``yield``: for instance if we forgot that +``do_some_stuff`` 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 content. (Normally, other +things will break, more obviously, if you forget to ``yield``, so this tends +not to be a major problem in practice.) + +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. + +Where you create a new Deferred, make it follow the rules +--------------------------------------------------------- + +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. + +The easy way to do it is with a combination of ``defer.inlineCallbacks``, and +``logcontext.PreserveLoggingContext``. Suppose we want to implement ``sleep``, +which returns a deferred which will run its callbacks after a given number of +seconds. That might look like: + +.. code:: python + + # not a logcontext-rules-compliant function + def get_sleep_deferred(seconds): + d = defer.Deferred() + reactor.callLater(seconds, d.callback, None) + return d + +That doesn't follow the rules, but we can fix it by wrapping it with +``PreserveLoggingContext`` and ``yield`` ing on it: + +.. code:: python + + @defer.inlineCallbacks + def sleep(seconds): + with PreserveLoggingContext(): + yield get_sleep_deferred(seconds) + +This technique works equally for external functions which return deferreds, +or deferreds we have made ourselves. + +XXX: think this is what ``preserve_context_over_deferred`` is supposed to do, +though it is broken, in that it only restores the logcontext for the duration +of the callbacks, which doesn't comply with the logcontext rules. + +Fire-and-forget +--------------- + +Sometimes you want to fire off a chain of execution, but not wait for its +result. That might look a bit like this: + +.. code:: 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") + +The above code does a couple of steps in the background after +``do_request_handling`` has finished. The log lines are still logged against +the ``request_context`` logcontext, which may or may not be desirable. There +are two big problems with the above, however. The first problem is that, if +``background_operation`` returns an incomplete Deferred, it will expect its +caller to ``yield`` immediately, so will have cleared the logcontext. In this +example, that means that 'Request handling complete' will be logged without any +context. + +The second problem, which is potentially even worse, is that when the Deferred +returned by ``background_operation`` 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. + +There are two potential solutions to this. + +One option is to surround the call to ``background_operation`` with a +``PreserveLoggingContext`` call. That will reset the logcontext before +starting ``background_operation`` (so the context restored when the deferred +completes will be the empty logcontext), and will restore the current +logcontext before continuing the foreground process: + +.. code:: 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") + +Obviously that option means that the operations done in +``background_operation`` would be not be logged against a logcontext (though +that might be fixed by setting a different logcontext via a ``with +LoggingContext(...)`` in ``background_operation``). + +The second option is to use ``logcontext.preserve_fn``, 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: + +.. code:: python + + @defer.inlineCallbacks + def do_request_handling(): + yield foreground_operation() + + logcontext.preserve_fn(background_operation)() + + # this will now be logged against the request context + logger.debug("Request handling complete") + +XXX: I think ``preserve_context_over_fn`` is supposed to do the first option, +but the fact that it does ``preserve_context_over_deferred`` on its results +means that its use is fraught with difficulty. + +Passing synapse deferreds into third-party functions +---------------------------------------------------- + +A typical example of this is where we want to collect together two or more +deferred via ``defer.gatherResults``: + +.. code:: python + + d1 = operation1() + d2 = operation2() + d3 = defer.gatherResults([d1, d2]) + +This is really a variation of the fire-and-forget problem above, in that we are +firing off ``d1`` and ``d2`` without yielding on them. The difference +is that we now have third-party code attached to their callbacks. Anyway either +technique given in the `Fire-and-forget`_ section will work. + +Of course, the new Deferred returned by ``gatherResults`` needs to be wrapped +in order to make it follow the logcontext rules before we can yield it, as +described in `Where you create a new Deferred, make it follow the rules`_. + +So, option one: reset the logcontext before starting the operations to be +gathered: + +.. code:: python + + @defer.inlineCallbacks + def do_request_handling(): + with PreserveLoggingContext(): + d1 = operation1() + d2 = operation2() + result = yield defer.gatherResults([d1, d2]) + +In this case particularly, though, option two, of using +``logcontext.preserve.fn`` almost certainly makes more sense, so that +``operation1`` and ``operation2`` are both logged against the original +logcontext. This looks like: + +.. code:: python + + @defer.inlineCallbacks + def do_request_handling(): + d1 = logcontext.preserve_fn(operation1)() + d2 = logcontext.preserve_fn(operation2)() + + with PreserveLoggingContext(): + result = yield defer.gatherResults([d1, d2]) + + +Was all this really necessary? +------------------------------ + +The conventions used work fine for a linear flow where everything happens in +series via ``defer.inlineCallbacks`` and ``yield``, but are certainly tricky to +follow for any more exotic flows. It's hard not to wonder if we could have done +something else. + +We're not going to rewrite Synapse now, so the following is entirely of +acadamic interest, but I'd like to record some thoughts on an alternative +approach. + +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. + +My alternative rules were: + +* functions always preserve the logcontext of their caller, whether or not they + are returning a Deferred. + +* Deferreds returned by synapse functions run their callbacks in the same + context as the function was orignally called in. + +The main point of this scheme is that everywhere that sets the logcontext is +responsible for clearing it before returning control to the reactor. + +So, for example, if you were the function which started a ``with +LoggingContext`` block, you wouldn't ``yield`` within it — instead you'd start +off the background process, and then leave the ``with`` block to wait for it: + +.. code:: python + + def handle_request(request_id): + with logcontext.LoggingContext() as request_context: + request_context.request = request_id + d = do_request_handling() + + def cb(r): + logger.debug("finished") + + d.addCallback(cb) + return d + +(in general, mixing ``with LoggingContext`` blocks and +``defer.inlineCallbacks`` in the same function leads to slighly +counter-intuitive code, under this scheme). + +Because we leave the original ``with`` 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 ``do_request_handling`` which needs to wait for a Deferred to +complete, there is no need for it to worry about clearing the logcontext before +doing so: + +.. code:: python + + def handle_request(): + r = do_some_stuff() + r.addCallback(do_some_more_stuff) + return r + +— and provided ``do_some_stuff`` follows the rules of returning a Deferred which +runs its callbacks in the original logcontext, all is happy. + +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 +``logcontext._PreservingContextDeferred``: + +.. code:: python + + def do_some_stuff(): + deferred = do_some_io() + pcd = _PreservingContextDeferred(LoggingContext.current_context()) + deferred.chainDeferred(pcd) + return pcd + +It turns out that, thanks to the way that Deferreds chain together, we +automatically get the property of a context-preserving deferred with +``defer.inlineCallbacks``, provided the final Defered the function ``yields`` +on has that property. So we can just write: + +.. code:: python + + @defer.inlineCallbacks + def handle_request(): + yield do_some_stuff() + yield do_some_more_stuff() + +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. |