Refactor request sending to have better excpetions (#4358)

* Correctly retry and back off if we get a HTTPerror response

* Refactor request sending to have better excpetions

MatrixFederationHttpClient blindly reraised exceptions to the caller
without differentiating "expected" failures (e.g. connection timeouts
etc) versus more severe problems (e.g. programming errors).

This commit adds a RequestSendFailed exception that is raised when
"expected" failures happen, allowing the TransactionQueue to log them as
warnings while allowing us to log other exceptions as actual exceptions.

1 files changed, 10 insertions, 3 deletions

diff --git a/tests/http/test_fedclient.py b/tests/http/test_fedclient.py
index f3cb1423f0..b2e38276d8 100644
--- a/tests/http/test_fedclient.py
+++ b/tests/http/test_fedclient.py
@@ -20,6 +20,7 @@ from twisted.internet.error import ConnectingCancelledError, DNSLookupError
 from twisted.web.client import ResponseNeverReceived
 from twisted.web.http import HTTPChannel
 
+from synapse.api.errors import RequestSendFailed
 from synapse.http.matrixfederationclient import (
     MatrixFederationHttpClient,
     MatrixFederationRequest,
@@ -49,7 +50,8 @@ class FederationClientTests(HomeserverTestCase):
         self.pump()
 
         f = self.failureResultOf(d)
-        self.assertIsInstance(f.value, DNSLookupError)
+        self.assertIsInstance(f.value, RequestSendFailed)
+        self.assertIsInstance(f.value.inner_exception, DNSLookupError)
 
     def test_client_never_connect(self):
         """
@@ -76,7 +78,11 @@ class FederationClientTests(HomeserverTestCase):
         self.reactor.advance(10.5)
         f = self.failureResultOf(d)
 
-        self.assertIsInstance(f.value, (ConnectingCancelledError, TimeoutError))
+        self.assertIsInstance(f.value, RequestSendFailed)
+        self.assertIsInstance(
+            f.value.inner_exception,
+            (ConnectingCancelledError, TimeoutError),
+        )
 
     def test_client_connect_no_response(self):
         """
@@ -107,7 +113,8 @@ class FederationClientTests(HomeserverTestCase):
         self.reactor.advance(10.5)
         f = self.failureResultOf(d)
 
-        self.assertIsInstance(f.value, ResponseNeverReceived)
+        self.assertIsInstance(f.value, RequestSendFailed)
+        self.assertIsInstance(f.value.inner_exception, ResponseNeverReceived)
 
     def test_client_gets_headers(self):
         """