Convert sending mail to async/await. (matrix-org#7557)

Mainly because sometimes the email push code raises exceptions where the
stack traces have gotten lost, which is hopefully fixed by this.

changelog.d/7557.misc

@@ -0,0 +1 @@
+Convert sending mail to async/await.

synapse/handlers/identity.py

@@ -290,8 +290,7 @@ def try_unbind_threepid_with_id_server(self, mxid, threepid, id_server):
 
         return changed
 
-    @defer.inlineCallbacks
-    def send_threepid_validation(
+    async def send_threepid_validation(
         self,
         email_address,
         client_secret,
@@ -319,7 +318,7 @@ def send_threepid_validation(
         """
         # Check that this email/client_secret/send_attempt combo is new or
         # greater than what we've seen previously
-        session = yield self.store.get_threepid_validation_session(
+        session = await self.store.get_threepid_validation_session(
             "email", client_secret, address=email_address, validated=False
         )
 
@@ -353,7 +352,7 @@ def send_threepid_validation(
         # Send the mail with the link containing the token, client_secret
         # and session_id
         try:
-            yield send_email_func(email_address, token, client_secret, session_id)
+            await send_email_func(email_address, token, client_secret, session_id)
         except Exception:
             logger.exception(
                 "Error sending threepid validation email to %s", email_address
@@ -364,7 +363,7 @@ def send_threepid_validation(
             self.hs.clock.time_msec() + self.hs.config.email_validation_token_lifetime
         )
 
-        yield self.store.start_or_continue_validation_session(
+        await self.store.start_or_continue_validation_session(
             "email",
             email_address,
             session_id,

tests/rest/client/v2_alpha/test_account.py

@@ -46,7 +46,7 @@ def make_homeserver(self, reactor, clock):
         # Email config.
         self.email_attempts = []
 
-        def sendmail(smtphost, from_addr, to_addrs, msg, **kwargs):
+        async def sendmail(smtphost, from_addr, to_addrs, msg, **kwargs):
             self.email_attempts.append(msg)
             return
 
@@ -358,7 +358,7 @@ def make_homeserver(self, reactor, clock):
         # Email config.
         self.email_attempts = []
 
-        def sendmail(smtphost, from_addr, to_addrs, msg, **kwargs):
+        async def sendmail(smtphost, from_addr, to_addrs, msg, **kwargs):
             self.email_attempts.append(msg)
 
         config["email"] = {