Add documentation for cancellation of request processing (#12761)

Signed-off-by: Sean Quah <seanq@matrix.org>

3 files changed, 394 insertions, 0 deletions

diff --git a/changelog.d/12761.doc b/changelog.d/12761.doc
new file mode 100644
index 0000000000..2eb2c0976f
--- /dev/null
+++ b/changelog.d/12761.doc
@@ -0,0 +1 @@
+Add documentation for cancellation of request processing.
diff --git a/docs/SUMMARY.md b/docs/SUMMARY.md
index 65570cefbe..8400a6539a 100644
--- a/docs/SUMMARY.md
+++ b/docs/SUMMARY.md
@@ -89,6 +89,7 @@
   - [Database Schemas](development/database_schema.md)
   - [Experimental features](development/experimental_features.md)
   - [Synapse Architecture]()
+    - [Cancellation](development/synapse_architecture/cancellation.md)
     - [Log Contexts](log_contexts.md)
     - [Replication](replication.md)
     - [TCP Replication](tcp_replication.md)
diff --git a/docs/development/synapse_architecture/cancellation.md b/docs/development/synapse_architecture/cancellation.md
new file mode 100644
index 0000000000..ef9e022635
--- /dev/null
+++ b/docs/development/synapse_architecture/cancellation.md
@@ -0,0 +1,392 @@
+# Cancellation
+Sometimes, requests take a long time to service and clients disconnect
+before Synapse produces a response. To avoid wasting resources, Synapse
+can cancel request processing for select endpoints marked with the
+`@cancellable` decorator.
+
+Synapse makes use of Twisted's `Deferred.cancel()` feature to make
+cancellation work. The `@cancellable` decorator does nothing by itself
+and merely acts as a flag, signalling to developers and other code alike
+that a method can be cancelled.
+
+## Enabling cancellation for an endpoint
+1. Check that the endpoint method, and any `async` functions in its call
+   tree handle cancellation correctly. See
+   [Handling cancellation correctly](#handling-cancellation-correctly)
+   for a list of things to look out for.
+2. Add the `@cancellable` decorator to the `on_GET/POST/PUT/DELETE`
+   method. It's not recommended to make non-`GET` methods cancellable,
+   since cancellation midway through some database updates is less
+   likely to be handled correctly.
+
+## Mechanics
+There are two stages to cancellation: downward propagation of a
+`cancel()` call, followed by upwards propagation of a `CancelledError`
+out of a blocked `await`.
+Both Twisted and asyncio have a cancellation mechanism.
+
+|               | Method              | Exception                               | Exception inherits from |
+|---------------|---------------------|-----------------------------------------|-------------------------|
+| Twisted       | `Deferred.cancel()` | `twisted.internet.defer.CancelledError` | `Exception` (!)         |
+| asyncio       | `Task.cancel()`     | `asyncio.CancelledError`                | `BaseException`         |
+
+### Deferred.cancel()
+When Synapse starts handling a request, it runs the async method
+responsible for handling it using `defer.ensureDeferred`, which returns
+a `Deferred`. For example:
+
+```python
+def do_something() -> Deferred[None]:
+    ...
+
+@cancellable
+async def on_GET() -> Tuple[int, JsonDict]:
+    d = make_deferred_yieldable(do_something())
+    await d
+    return 200, {}
+
+request = defer.ensureDeferred(on_GET())
+```
+
+When a client disconnects early, Synapse checks for the presence of the
+`@cancellable` decorator on `on_GET`. Since `on_GET` is cancellable,
+`Deferred.cancel()` is called on the `Deferred` from
+`defer.ensureDeferred`, ie. `request`. Twisted knows which `Deferred`
+`request` is waiting on and passes the `cancel()` call on to `d`.
+
+The `Deferred` being waited on, `d`, may have its own handling for
+`cancel()` and pass the call on to other `Deferred`s.
+
+Eventually, a `Deferred` handles the `cancel()` call by resolving itself
+with a `CancelledError`.
+
+### CancelledError
+The `CancelledError` gets raised out of the `await` and bubbles up, as
+per normal Python exception handling.
+
+## Handling cancellation correctly
+In general, when writing code that might be subject to cancellation, two
+things must be considered:
+ * The effect of `CancelledError`s raised out of `await`s.
+ * The effect of `Deferred`s being `cancel()`ed.
+
+Examples of code that handles cancellation incorrectly include:
+ * `try-except` blocks which swallow `CancelledError`s.
+ * Code that shares the same `Deferred`, which may be cancelled, between
+   multiple requests.
+ * Code that starts some processing that's exempt from cancellation, but
+   uses a logging context from cancellable code. The logging context
+   will be finished upon cancellation, while the uncancelled processing
+   is still using it.
+
+Some common patterns are listed below in more detail.
+
+### `async` function calls
+Most functions in Synapse are relatively straightforward from a
+cancellation standpoint: they don't do anything with `Deferred`s and
+purely call and `await` other `async` functions.
+
+An `async` function handles cancellation correctly if its own code
+handles cancellation correctly and all the async function it calls
+handle cancellation correctly. For example:
+```python
+async def do_two_things() -> None:
+    check_something()
+    await do_something()
+    await do_something_else()
+```
+`do_two_things` handles cancellation correctly if `do_something` and
+`do_something_else` handle cancellation correctly.
+
+That is, when checking whether a function handles cancellation
+correctly, its implementation and all its `async` function calls need to
+be checked, recursively.
+
+As `check_something` is not `async`, it does not need to be checked.
+
+### CancelledErrors
+Because Twisted's `CancelledError`s are `Exception`s, it's easy to
+accidentally catch and suppress them. Care must be taken to ensure that
+`CancelledError`s are allowed to propagate upwards.
+
+<table width="100%">
+<tr>
+<td width="50%" valign="top">
+
+**Bad**:
+```python
+try:
+    await do_something()
+except Exception:
+    # `CancelledError` gets swallowed here.
+    logger.info(...)
+```
+</td>
+<td width="50%" valign="top">
+
+**Good**:
+```python
+try:
+    await do_something()
+except CancelledError:
+    raise
+except Exception:
+    logger.info(...)
+```
+</td>
+</tr>
+<tr>
+<td width="50%" valign="top">
+
+**OK**:
+```python
+try:
+    check_something()
+    # A `CancelledError` won't ever be raised here.
+except Exception:
+    logger.info(...)
+```
+</td>
+<td width="50%" valign="top">
+
+**Good**:
+```python
+try:
+    await do_something()
+except ValueError:
+    logger.info(...)
+```
+</td>
+</tr>
+</table>
+
+#### defer.gatherResults
+`defer.gatherResults` produces a `Deferred` which:
+ * broadcasts `cancel()` calls to every `Deferred` being waited on.
+ * wraps the first exception it sees in a `FirstError`.
+
+Together, this means that `CancelledError`s will be wrapped in
+a `FirstError` unless unwrapped. Such `FirstError`s are liable to be
+swallowed, so they must be unwrapped.
+
+<table width="100%">
+<tr>
+<td width="50%" valign="top">
+
+**Bad**:
+```python
+async def do_something() -> None:
+    await make_deferred_yieldable(
+        defer.gatherResults([...], consumeErrors=True)
+    )
+
+try:
+    await do_something()
+except CancelledError:
+    raise
+except Exception:
+    # `FirstError(CancelledError)` gets swallowed here.
+    logger.info(...)
+```
+
+</td>
+<td width="50%" valign="top">
+
+**Good**:
+```python
+async def do_something() -> None:
+    await make_deferred_yieldable(
+        defer.gatherResults([...], consumeErrors=True)
+    ).addErrback(unwrapFirstError)
+
+try:
+    await do_something()
+except CancelledError:
+    raise
+except Exception:
+    logger.info(...)
+```
+</td>
+</tr>
+</table>
+
+### Creation of `Deferred`s
+If a function creates a `Deferred`, the effect of cancelling it must be considered. `Deferred`s that get shared are likely to have unintended behaviour when cancelled.
+
+<table width="100%">
+<tr>
+<td width="50%" valign="top">
+
+**Bad**:
+```python
+cache: Dict[str, Deferred[None]] = {}
+
+def wait_for_room(room_id: str) -> Deferred[None]:
+    deferred = cache.get(room_id)
+    if deferred is None:
+        deferred = Deferred()
+        cache[room_id] = deferred
+    # `deferred` can have multiple waiters.
+    # All of them will observe a `CancelledError`
+    # if any one of them is cancelled.
+    return make_deferred_yieldable(deferred)
+
+# Request 1
+await wait_for_room("!aAAaaAaaaAAAaAaAA:matrix.org")
+# Request 2
+await wait_for_room("!aAAaaAaaaAAAaAaAA:matrix.org")
+```
+</td>
+<td width="50%" valign="top">
+
+**Good**:
+```python
+cache: Dict[str, Deferred[None]] = {}
+
+def wait_for_room(room_id: str) -> Deferred[None]:
+    deferred = cache.get(room_id)
+    if deferred is None:
+        deferred = Deferred()
+        cache[room_id] = deferred
+    # `deferred` will never be cancelled now.
+    # A `CancelledError` will still come out of
+    # the `await`.
+    # `delay_cancellation` may also be used.
+    return make_deferred_yieldable(stop_cancellation(deferred))
+
+# Request 1
+await wait_for_room("!aAAaaAaaaAAAaAaAA:matrix.org")
+# Request 2
+await wait_for_room("!aAAaaAaaaAAAaAaAA:matrix.org")
+```
+</td>
+</tr>
+<tr>
+<td width="50%" valign="top">
+</td>
+<td width="50%" valign="top">
+
+**Good**:
+```python
+cache: Dict[str, List[Deferred[None]]] = {}
+
+def wait_for_room(room_id: str) -> Deferred[None]:
+    if room_id not in cache:
+        cache[room_id] = []
+    # Each request gets its own `Deferred` to wait on.
+    deferred = Deferred()
+    cache[room_id]].append(deferred)
+    return make_deferred_yieldable(deferred)
+
+# Request 1
+await wait_for_room("!aAAaaAaaaAAAaAaAA:matrix.org")
+# Request 2
+await wait_for_room("!aAAaaAaaaAAAaAaAA:matrix.org")
+```
+</td>
+</table>
+
+### Uncancelled processing
+Some `async` functions may kick off some `async` processing which is
+intentionally protected from cancellation, by `stop_cancellation` or
+other means. If the `async` processing inherits the logcontext of the
+request which initiated it, care must be taken to ensure that the
+logcontext is not finished before the `async` processing completes.
+
+<table width="100%">
+<tr>
+<td width="50%" valign="top">
+
+**Bad**:
+```python
+cache: Optional[ObservableDeferred[None]] = None
+
+async def do_something_else(
+    to_resolve: Deferred[None]
+) -> None:
+    await ...
+    logger.info("done!")
+    to_resolve.callback(None)
+
+async def do_something() -> None:
+    if not cache:
+        to_resolve = Deferred()
+        cache = ObservableDeferred(to_resolve)
+        # `do_something_else` will never be cancelled and
+        # can outlive the `request-1` logging context.
+        run_in_background(do_something_else, to_resolve)
+
+    await make_deferred_yieldable(cache.observe())
+
+with LoggingContext("request-1"):
+    await do_something()
+```
+</td>
+<td width="50%" valign="top">
+
+**Good**:
+```python
+cache: Optional[ObservableDeferred[None]] = None
+
+async def do_something_else(
+    to_resolve: Deferred[None]
+) -> None:
+    await ...
+    logger.info("done!")
+    to_resolve.callback(None)
+
+async def do_something() -> None:
+    if not cache:
+        to_resolve = Deferred()
+        cache = ObservableDeferred(to_resolve)
+        run_in_background(do_something_else, to_resolve)
+        # We'll wait until `do_something_else` is
+        # done before raising a `CancelledError`.
+        await make_deferred_yieldable(
+            delay_cancellation(cache.observe())
+        )
+    else:
+        await make_deferred_yieldable(cache.observe())
+
+with LoggingContext("request-1"):
+    await do_something()
+```
+</td>
+</tr>
+<tr>
+<td width="50%">
+
+**OK**:
+```python
+cache: Optional[ObservableDeferred[None]] = None
+
+async def do_something_else(
+    to_resolve: Deferred[None]
+) -> None:
+    await ...
+    logger.info("done!")
+    to_resolve.callback(None)
+
+async def do_something() -> None:
+    if not cache:
+        to_resolve = Deferred()
+        cache = ObservableDeferred(to_resolve)
+        # `do_something_else` will get its own independent
+        # logging context. `request-1` will not count any
+        # metrics from `do_something_else`.
+        run_as_background_process(
+            "do_something_else",
+            do_something_else,
+            to_resolve,
+        )
+
+    await make_deferred_yieldable(cache.observe())
+
+with LoggingContext("request-1"):
+    await do_something()
+```
+</td>
+<td width="50%">
+</td>
+</tr>
+</table>