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+# Log Contexts
+
+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.logging.context` 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:
+
+```python
+from synapse.logging import context         # omitted from future snippets
+
+def handle_request(request_id):
+    request_context = context.LoggingContext()
+
+    calling_context = context.LoggingContext.current_context()
+    context.LoggingContext.set_current_context(request_context)
+    try:
+        request_context.request = request_id
+        do_request_handling()
+        logger.debug("finished")
+    finally:
+        context.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:
+
+```python
+def handle_request(request_id):
+    with context.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:
+
+```python
+@defer.inlineCallbacks
+def handle_request(request_id):
+    with context.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:
+
+**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.
+
+```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
+
+    return 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 context.
+(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 `context.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:
+
+```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:
+
+```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.
+
+You can also use `context.make_deferred_yieldable`, which just does the
+boilerplate for you, so the above could be written:
+
+```python
+def sleep(seconds):
+    return context.make_deferred_yieldable(get_sleep_deferred(seconds))
+```
+
+## 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:
+
+```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:
+
+```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 `context.run_in_background`, 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:
+
+```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")
+```
+
+## 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`:
+
+```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](#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](#where-you-create-a-new-deferred-make-it-follow-the-rules).
+
+So, option one: reset the logcontext before starting the operations to
+be gathered:
+
+```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
+`context.preserve_fn` almost certainly makes more sense, so that
+`operation1` and `operation2` are both logged against the original
+logcontext. This looks like:
+
+```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])
+```
+
+## 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
+academic 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:
+
+```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
+```
+
+(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:
+
+```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
+`context._PreservingContextDeferred`:
+
+```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:
+
+```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.
+
+## A note on garbage-collection of Deferred chains
+
+It turns out that our logcontext rules do not play nicely with Deferred
+chains which get orphaned and garbage-collected.
+
+Imagine we have some code that looks like this:
+
+```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
+```
+
+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.)
+
+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 *even more* interesting happens.)
+
+Secondly, let's suppose that we decide that the interesting thing is
+never going to happen, and we reset the listener queue:
+
+```python
+def reset_listener_queue():
+    listener_queue.clear()
+```
+
+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 `await_something_interesting` is a generator
+function, and when Python garbage-collects generator functions, it gives
+them a chance to clean up by making the `yield` raise a `GeneratorExit`
+exception. In our case, that means that the `__exit__` handler of
+`PreserveLoggingContext` will carefully restore the request context, but
+there is now nothing waiting for its return, so the request context is
+never cleared.
+
+To reiterate, this problem only arises when *both* 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.