synapse/docs/log_contexts.md

# 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.set_current_context(request_context)
    try:
        request_context.request = request_id
        do_request_handling()
        logger.debug("finished")
    finally:
        context.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.