Stop the transaction cache caching failures

The transaction cache has some code which tries to stop it caching failures,
but if the callback function failed straight away, then things would happen
backwards and we'd end up with the failure stuck in the cache.

tests/rest/client/test_transactions.py

@@ -2,6 +2,8 @@ from synapse.rest.client.transactions import HttpTransactionCache
 from synapse.rest.client.transactions import CLEANUP_PERIOD_MS
 from twisted.internet import defer
 from mock import Mock, call
+
+from synapse.util.logcontext import LoggingContext
 from tests import unittest
 from tests.utils import MockClock
 
@@ -39,6 +41,58 @@ class HttpTransactionCacheTestCase(unittest.TestCase):
         # expect only a single call to do the work
         cb.assert_called_once_with("some_arg", keyword="arg", changing_args=0)
 
+    @defer.inlineCallbacks
+    def test_does_not_cache_exceptions(self):
+        """Checks that, if the callback throws an exception, it is called again
+        for the next request.
+        """
+        called = [False]
+
+        def cb():
+            if called[0]:
+                # return a valid result the second time
+                return defer.succeed(self.mock_http_response)
+
+            called[0] = True
+            raise Exception("boo")
+
+        with LoggingContext("test") as test_context:
+            try:
+                yield self.cache.fetch_or_execute(self.mock_key, cb)
+            except Exception as e:
+                self.assertEqual(e.message, "boo")
+            self.assertIs(LoggingContext.current_context(), test_context)
+
+            res = yield self.cache.fetch_or_execute(self.mock_key, cb)
+            self.assertEqual(res, self.mock_http_response)
+            self.assertIs(LoggingContext.current_context(), test_context)
+
+    @defer.inlineCallbacks
+    def test_does_not_cache_failures(self):
+        """Checks that, if the callback returns a failure, it is called again
+        for the next request.
+        """
+        called = [False]
+
+        def cb():
+            if called[0]:
+                # return a valid result the second time
+                return defer.succeed(self.mock_http_response)
+
+            called[0] = True
+            return defer.fail(Exception("boo"))
+
+        with LoggingContext("test") as test_context:
+            try:
+                yield self.cache.fetch_or_execute(self.mock_key, cb)
+            except Exception as e:
+                self.assertEqual(e.message, "boo")
+            self.assertIs(LoggingContext.current_context(), test_context)
+
+            res = yield self.cache.fetch_or_execute(self.mock_key, cb)
+            self.assertEqual(res, self.mock_http_response)
+            self.assertIs(LoggingContext.current_context(), test_context)
+
     @defer.inlineCallbacks
     def test_cleans_up(self):
         cb = Mock(