# Log Contexts - Synapse

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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 facilities for managing the current log context (as well as providing the `LoggingContextFilter` class).

Asynchronous functions make the whole thing complicated, so this document describes how it all works, and how to write code which follows the rules.

In this document, “awaitable” refers to any object which can be `await`ed. In the context of Synapse, that normally means either a coroutine or a Twisted `Deferred`.

## Logcontexts without asynchronous code

In the absence of any asynchronous 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:

    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:

    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")

### The `sentinel` context

The default logcontext is `synapse.logging.context.SENTINEL_CONTEXT`, which is an empty sentinel value to represent the root logcontext. This is what is used when there is no other logcontext set. The phrase “clear/reset the logcontext” means to set the current logcontext to the `sentinel` logcontext.

No CPU/database usage metrics are recorded against the `sentinel` logcontext.

Ideally, nothing from the Synapse homeserver would be logged against the `sentinel` logcontext as we want to know which server the logs came from. In practice, this is not always the case yet especially outside of request handling.

Global things outside of Synapse (e.g. Twisted reactor code) should run in the `sentinel` logcontext. It’s only when it calls into application code that a logcontext gets activated. This means the reactor should be started in the `sentinel` logcontext, and any time an awaitable yields control back to the reactor, it should reset the logcontext to be the `sentinel` logcontext. This is important to avoid leaking the current logcontext to the reactor (which would then get picked up and associated with the next thing the reactor does).

## Using logcontexts with awaitables

Awaitables 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 an awaitable:

    async def handle_request(request_id):
        with context.LoggingContext() as request_context:
            request_context.request = request_id
            await do_request_handling()
            logger.debug("finished")

In the above flow:

- The logcontext is set
- `do_request_handling` is called, and returns an awaitable
- `handle_request` awaits the awaitable
- Execution of `handle_request` is suspended

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 an awaitable, you will want to `await` it. To that end, wherever functions return awaitables, we adopt the following conventions:

Rules for functions returning awaitables:

> - If the awaitable is already complete, the function returns with the same logcontext it started with.
> - If the awaitable is incomplete, the function clears the logcontext before returning; when the awaitable 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 awaitable, then the logcontext will still be in place. In this case, execution will continue immediately after the `await`; 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 awaitable is incomplete, `do_request_handling` clears the logcontext before returning. The logcontext is therefore clear when `handle_request` `await`s the awaitable.

  Once `do_request_handling`’s awaitable completes, it will reinstate the logcontext, before running 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 await your awaitables

Whenever you get an awaitable back from a function, you should `await` on it as soon as possible. Do not pass go; do not do any logging; do not call any other functions.

    async def fun():
        logger.debug("starting")
        await do_some_stuff()       # just like this

        coro = more_stuff()
        result = await coro         # also fine, of course

        return result

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`.

It’s all too easy to forget to `await`: for instance if we forgot that `do_some_stuff` returned an awaitable, 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 `await`, so this tends not to be a major problem in practice.)

Of course sometimes you need to do something a bit fancier with your awaitable - 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 awaitable, make it follow the rules

Most of the time, an awaitable comes from another synapse function. Sometimes, though, we need to make up a new awaitable, or we get an awaitable back from external code. We need to make it follow our rules.

The easy way to do it is by using `context.make_deferred_yieldable`. 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:

    # 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 calling it through `context.make_deferred_yieldable`:

    async def sleep(seconds):
        return await context.make_deferred_yieldable(get_sleep_deferred(seconds))

## Deferred callbacks

When a deferred callback is called, it inherits the current logcontext. The deferred callback chain can resume a coroutine, which if following our logcontext rules, will restore its own logcontext, then run:

- until it yields control back to the reactor, setting the sentinel logcontext
- or until it finishes, restoring the logcontext it was started with (calling context)

This behavior creates two specific issues:

Issue 1: The first issue is that the callback may have reset the logcontext to the sentinel before returning. This means our calling function will continue with the sentinel logcontext instead of the logcontext it was started with (bad).

Issue 2: The second issue is that the current logcontext that called the deferred callback could finish before the callback finishes (bad).

In the following example, the deferred callback is called with the “main” logcontext and runs until we yield control back to the reactor in the `await` inside `clock.sleep(0)`. Since `clock.sleep(0)` follows our logcontext rules, it sets the logcontext to the sentinel before yielding control back to the reactor. Our `main` function continues with the sentinel logcontext (first bad thing) instead of the “main” logcontext. Then the `with LoggingContext("main")` block exits, finishing the “main” logcontext and yielding control back to the reactor again. Finally, later on when `clock.sleep(0)` completes, our `with LoggingContext("competing")` block exits, and restores the previous “main” logcontext which has already finished, resulting in `WARNING: Re-starting finished log context main` and leaking the `main` logcontext into the reactor which will then erronously be associated with the next task the reactor picks up.

    async def competing_callback():
        # Since this is run with the "main" logcontext, when the "competing"
        # logcontext exits, it will restore the previous "main" logcontext which has
        # already finished and results in "WARNING: Re-starting finished log context main"
        # and leaking the `main` logcontext into the reactor.
        with LoggingContext("competing"):
            await clock.sleep(0)

    def main():
        with LoggingContext("main"):
            d = defer.Deferred()
            d.addCallback(lambda _: defer.ensureDeferred(competing_callback()))
            # Call the callback within the "main" logcontext.
            d.callback(None)
            # Bad: This will be logged against sentinel logcontext
            logger.debug("ugh")

    main()

Solution 1: We could of course fix this by following the general rule of “always await your awaitables”:

    async def main():
        with LoggingContext("main"):
            d = defer.Deferred()
            d.addCallback(lambda _: defer.ensureDeferred(competing_callback()))
            d.callback(None)
            # Wait for `d` to finish before continuing so the "main" logcontext is
            # still active. This works because `d` already follows our logcontext
            # rules. If not, we would also have to use `make_deferred_yieldable(d)`.
            await d
            # Good: This will be logged against the "main" logcontext
            logger.debug("phew")

Solution 2: We could also fix this by surrounding the call to `d.callback` with a `PreserveLoggingContext`, which will reset the logcontext to the sentinel before calling the callback, and restore the “main” logcontext afterwards before continuing the `main` function. This solves the problem because when the “competing” logcontext exits, it will restore the sentinel logcontext which is never finished by its nature, so there is no warning and no leakage into the reactor.

    async def main():
        with LoggingContext("main"):
            d = defer.Deferred()
            d.addCallback(lambda _: defer.ensureDeferred(competing_callback()))
            d.callback(None)
            with PreserveLoggingContext():
                # Call the callback with the sentinel logcontext.
                d.callback(None)
            # Good: This will be logged against the "main" logcontext
            logger.debug("phew")

Solution 3: But let’s say you do want to run (fire-and-forget) the deferred callback in the current context without running into issues:

We can solve the first issue by using `run_in_background(...)` to run the callback in the current logcontext and it handles the magic behind the scenes of a) restoring the calling logcontext before returning to the caller and b) resetting the logcontext to the sentinel after the deferred completes and we yield control back to the reactor to avoid leaking the logcontext into the reactor.

To solve the second issue, we can extend the lifetime of the “main” logcontext by avoiding the `LoggingContext`’s context manager lifetime methods (`__enter__`/`__exit__`). We can still set “main” as the current logcontext by using `PreserveLoggingContext` and passing in the “main” logcontext.

    async def main():
        main_context = LoggingContext("main")
        with PreserveLoggingContext(main_context):
            d = defer.Deferred()
            d.addCallback(lambda _: defer.ensureDeferred(competing_callback()))
            # The whole lambda will be run in the "main" logcontext. But we're using
            # a trick to return the deferred `d` itself so that `run_in_background`
            # will wait on that to complete and reset the logcontext to the sentinel
            # when it does to avoid leaking the "main" logcontext into the reactor.
            run_in_background(lambda: (d.callback(None), d)[1])
            # Good: This will be logged against the "main" logcontext
            logger.debug("phew")

    ...

    # Wherever possible, it's best to finish the logcontext by calling `__exit__` at some
    # point. This allows us to catch bugs if we later try to erroneously restart a finished
    # logcontext.
    #
    # Since the "main" logcontext stores the `LoggingContext.previous_context` when it is
    # created, we can wrap this call in `PreserveLoggingContext()` to restore the correct
    # previous logcontext. Our goal is to have the calling context remain unchanged after
    # finishing the "main" logcontext.
    with PreserveLoggingContext():
        # Finish the "main" logcontext
        with main_context:
            # Empty block - We're just trying to call `__exit__` on the "main" context
            # manager to finish it. We can't call `__exit__` directly as the code expects us
            # to `__enter__` before calling `__exit__` to `start`/`stop` things
            # appropriately. And in any case, it's probably best not to call the internal
            # methods directly.
            pass

The same thing applies if you have some deferreds stored somewhere which you want to callback in the current logcontext.

### Deferred errbacks and cancellations

The same care should be taken when calling errbacks on deferreds. An errback and callback act the same in this regard (see section above).

    d = defer.Deferred()
    d.addErrback(some_other_function)
    d.errback(failure)

Additionally, cancellation is the same as directly calling the errback with a `twisted.internet.defer.CancelledError`:

    d = defer.Deferred()
    d.addErrback(some_other_function)
    d.cancel()

## 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:

    async def do_request_handling():
        await foreground_operation()

        # *don't* do this
        background_operation()

        logger.debug("Request handling complete")

    async def background_operation():
        await first_background_step()
        logger.debug("Completed first step")
        await 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 awaitable, it will expect its caller to `await` 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 awaitable returned by `background_operation` completes, it will restore the original logcontext. There is nothing waiting on that awaitable, 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:

    async def do_request_handling():
        await 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 awaitable, and adds a callback to the returned awaitable to reset the logcontext. In other words, it turns a function that follows the Synapse rules about logcontexts and awaitables 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:

    async def do_request_handling():
        await 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 awaitables via `defer.gatherResults`:

    a1 = operation1()
    a2 = operation2()
    a3 = defer.gatherResults([a1, a2])

This is really a variation of the fire-and-forget problem above, in that we are firing off `a1` and `a2` without awaiting 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 awaitable returned by `gather` 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 awaitable, make it follow the rules.

So, option one: reset the logcontext before starting the operations to be gathered:

    async def do_request_handling():
        with PreserveLoggingContext():
            a1 = operation1()
            a2 = operation2()
            result = await defer.gatherResults([a1, a2])

In this case particularly, though, option two, of using `context.run_in_background` almost certainly makes more sense, so that `operation1` and `operation2` are both logged against the original logcontext. This looks like:

    async def do_request_handling():
        a1 = context.run_in_background(operation1)
        a2 = context.run_in_background(operation2)

        result = await make_deferred_yieldable(defer.gatherResults([a1, a2]))

## A note on garbage-collection of awaitable chains

It turns out that our logcontext rules do not play nicely with awaitable chains which get orphaned and garbage-collected.

Imagine we have some code that looks like this:

    listener_queue = []

    def on_something_interesting():
        for d in listener_queue:
            d.callback("foo")

    async def await_something_interesting():
        new_awaitable = defer.Deferred()
        listener_queue.append(new_awaitable)

        with PreserveLoggingContext():
            await new_awaitable

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:

    def reset_listener_queue():
        listener_queue.clear()

So, both ends of the awaitable chain have now dropped their references, and the awaitable chain is now orphaned, and will be garbage-collected at some point. Note that `await_something_interesting` is a coroutine, which Python implements as a generator function. When Python garbage-collects generator functions, it gives them a chance to clean up by making the `await` (or `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 awaitable chain are dropped. Dropping the the reference to an awaitable you’re supposed to be awaiting is 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.

## Debugging logcontext issues

Debugging logcontext issues can be tricky as leaking or losing a logcontext will surface downstream and can point to an unrelated part of the codebase. It’s best to enable debug logging for `synapse.logging.context.debug` (needs to be explicitly configured) and go backwards in the logs from the point where the issue is observed to find the root cause.

`log.config.yaml`

    loggers:
        # Unlike other loggers, this one needs to be explicitly configured to see debug logs.
        synapse.logging.context.debug:
            level: DEBUG