Add delay_cancellation utility function (#12180)

`delay_cancellation` behaves like `stop_cancellation`, except it
delays `CancelledError`s until the original `Deferred` resolves.
This is handy for unifying cleanup paths and ensuring that uncancelled
coroutines don't use finished logcontexts.

Signed-off-by: Sean Quah <seanq@element.io>

synapse/util/async_helpers.py

@@ -686,12 +686,48 @@ def stop_cancellation(deferred: "defer.Deferred[T]") -> "defer.Deferred[T]":
             Synapse logcontext rules.
 
     Returns:
-        A new `Deferred`, which will contain the result of the original `Deferred`,
-        but will not propagate cancellation through to the original. When cancelled,
-        the new `Deferred` will fail with a `CancelledError` and will not follow the
-        Synapse logcontext rules. `make_deferred_yieldable` should be used to wrap
-        the new `Deferred`.
+        A new `Deferred`, which will contain the result of the original `Deferred`.
+        The new `Deferred` will not propagate cancellation through to the original.
+        When cancelled, the new `Deferred` will fail with a `CancelledError`.
+
+        The new `Deferred` will not follow the Synapse logcontext rules and should be
+        wrapped with `make_deferred_yieldable`.
     """
-    new_deferred: defer.Deferred[T] = defer.Deferred()
+    new_deferred: "defer.Deferred[T]" = defer.Deferred()
+    deferred.chainDeferred(new_deferred)
+    return new_deferred
+
+
+def delay_cancellation(deferred: "defer.Deferred[T]") -> "defer.Deferred[T]":
+    """Delay cancellation of a `Deferred` until it resolves.
+
+    Has the same effect as `stop_cancellation`, but the returned `Deferred` will not
+    resolve with a `CancelledError` until the original `Deferred` resolves.
+
+    Args:
+        deferred: The `Deferred` to protect against cancellation. May optionally follow
+            the Synapse logcontext rules.
+
+    Returns:
+        A new `Deferred`, which will contain the result of the original `Deferred`.
+        The new `Deferred` will not propagate cancellation through to the original.
+        When cancelled, the new `Deferred` will wait until the original `Deferred`
+        resolves before failing with a `CancelledError`.
+
+        The new `Deferred` will follow the Synapse logcontext rules if `deferred`
+        follows the Synapse logcontext rules. Otherwise the new `Deferred` should be
+        wrapped with `make_deferred_yieldable`.
+    """
+
+    def handle_cancel(new_deferred: "defer.Deferred[T]") -> None:
+        # before the new deferred is cancelled, we `pause` it to stop the cancellation
+        # propagating. we then `unpause` it once the wrapped deferred completes, to
+        # propagate the exception.
+        new_deferred.pause()
+        new_deferred.errback(Failure(CancelledError()))
+
+        deferred.addBoth(lambda _: new_deferred.unpause())
+
+    new_deferred: "defer.Deferred[T]" = defer.Deferred(handle_cancel)
     deferred.chainDeferred(new_deferred)
     return new_deferred