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, 18 insertions, 0 deletions

diff --git a/synapse/api/errors.py b/synapse/api/errors.py
index 48b903374d..0b464834ce 100644
--- a/synapse/api/errors.py
+++ b/synapse/api/errors.py
@@ -348,6 +348,24 @@ class IncompatibleRoomVersionError(SynapseError):
         )
 
 
+class RequestSendFailed(RuntimeError):
+    """Sending a HTTP request over federation failed due to not being able to
+    talk to the remote server for some reason.
+
+    This exception is used to differentiate "expected" errors that arise due to
+    networking (e.g. DNS failures, connection timeouts etc), versus unexpected
+    errors (like programming errors).
+    """
+    def __init__(self, inner_exception, can_retry):
+        super(RequestSendFailed, self).__init__(
+            "Failed to send request: %s: %s" % (
+                type(inner_exception).__name__, inner_exception,
+            )
+        )
+        self.inner_exception = inner_exception
+        self.can_retry = can_retry
+
+
 def cs_error(msg, code=Codes.UNKNOWN, **kwargs):
     """ Utility method for constructing an error response for client-server
     interactions.