Logcontext docs

1 files changed, 439 insertions, 8 deletions

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.