Fix handling of connection timeouts in outgoing http requests (#8400)

* Remove `on_timeout_cancel` from `timeout_deferred`

The `on_timeout_cancel` param to `timeout_deferred` wasn't always called on a
timeout (in particular if the canceller raised an exception), so it was
unreliable. It was also only used in one place, and to be honest it's easier to
do what it does a different way.

* Fix handling of connection timeouts in outgoing http requests

Turns out that if we get a timeout during connection, then a different
exception is raised, which wasn't always handled correctly.

To fix it, catch the exception in SimpleHttpClient and turn it into a
RequestTimedOutError (which is already a documented exception).

Also add a description to RequestTimedOutError so that we can see which stage
it failed at.

* Fix incorrect handling of timeouts reading federation responses

This was trapping the wrong sort of TimeoutError, so was never being hit.

The effect was relatively minor, but we should fix this so that it does the
expected thing.

* Fix inconsistent handling of `timeout` param between methods

`get_json`, `put_json` and `delete_json` were applying a different timeout to
the response body to `post_json`; bring them in line and test.

Co-authored-by: Patrick Cloke <clokep@users.noreply.github.com>
Co-authored-by: Erik Johnston <erik@matrix.org>

1 files changed, 18 insertions, 29 deletions

diff --git a/synapse/util/async_helpers.py b/synapse/util/async_helpers.py
index 67ce9a5f39..382f0cf3f0 100644
--- a/synapse/util/async_helpers.py
+++ b/synapse/util/async_helpers.py
@@ -449,18 +449,8 @@ class ReadWriteLock:
 R = TypeVar("R")
 
 
-def _cancelled_to_timed_out_error(value: R, timeout: float) -> R:
-    if isinstance(value, failure.Failure):
-        value.trap(CancelledError)
-        raise defer.TimeoutError(timeout, "Deferred")
-    return value
-
-
 def timeout_deferred(
-    deferred: defer.Deferred,
-    timeout: float,
-    reactor: IReactorTime,
-    on_timeout_cancel: Optional[Callable[[Any, float], Any]] = None,
+    deferred: defer.Deferred, timeout: float, reactor: IReactorTime,
 ) -> defer.Deferred:
     """The in built twisted `Deferred.addTimeout` fails to time out deferreds
     that have a canceller that throws exceptions. This method creates a new
@@ -469,27 +459,21 @@ def timeout_deferred(
 
     (See https://twistedmatrix.com/trac/ticket/9534)
 
-    NOTE: Unlike `Deferred.addTimeout`, this function returns a new deferred
+    NOTE: Unlike `Deferred.addTimeout`, this function returns a new deferred.
+
+    NOTE: the TimeoutError raised by the resultant deferred is
+    twisted.internet.defer.TimeoutError, which is *different* to the built-in
+    TimeoutError, as well as various other TimeoutErrors you might have imported.
 
     Args:
         deferred: The Deferred to potentially timeout.
         timeout: Timeout in seconds
         reactor: The twisted reactor to use
-        on_timeout_cancel: A callable which is called immediately
-            after the deferred times out, and not if this deferred is
-            otherwise cancelled before the timeout.
 
-            It takes an arbitrary value, which is the value of the deferred at
-            that exact point in time (probably a CancelledError Failure), and
-            the timeout.
-
-            The default callable (if none is provided) will translate a
-            CancelledError Failure into a defer.TimeoutError.
 
     Returns:
-        A new Deferred.
+        A new Deferred, which will errback with defer.TimeoutError on timeout.
     """
-
     new_d = defer.Deferred()
 
     timed_out = [False]
@@ -502,18 +486,23 @@ def timeout_deferred(
         except:  # noqa: E722, if we throw any exception it'll break time outs
             logger.exception("Canceller failed during timeout")
 
+        # the cancel() call should have set off a chain of errbacks which
+        # will have errbacked new_d, but in case it hasn't, errback it now.
+
         if not new_d.called:
-            new_d.errback(defer.TimeoutError(timeout, "Deferred"))
+            new_d.errback(defer.TimeoutError("Timed out after %gs" % (timeout,)))
 
     delayed_call = reactor.callLater(timeout, time_it_out)
 
-    def convert_cancelled(value):
-        if timed_out[0]:
-            to_call = on_timeout_cancel or _cancelled_to_timed_out_error
-            return to_call(value, timeout)
+    def convert_cancelled(value: failure.Failure):
+        # if the orgininal deferred was cancelled, and our timeout has fired, then
+        # the reason it was cancelled was due to our timeout. Turn the CancelledError
+        # into a TimeoutError.
+        if timed_out[0] and value.check(CancelledError):
+            raise defer.TimeoutError("Timed out after %gs" % (timeout,))
         return value
 
-    deferred.addBoth(convert_cancelled)
+    deferred.addErrback(convert_cancelled)
 
     def cancel_timeout(result):
         # stop the pending call to cancel the deferred if it's been fired