Printf-debugging around MSDIDN validation

synapse/rest/client/account.py

@@ -468,6 +468,12 @@ async def on_POST(self, request: SynapseRequest) -> Tuple[int, JsonDict]:
 
         msisdn = phone_number_to_msisdn(country, phone_number)
 
+        # Didn't like the sound of logging `client_secret`, but the spec says it is
+        # "A unique string generated by the client, and used to identify the validation
+        # attempt." I.e. something to facilitate deduplication. I don't think it's a
+        # sensitive secret per se.
+        logger.info("Request to verify ownership of %s: %s", msisdn, body)
+
         if not check_3pid_allowed(self.hs, "msisdn", msisdn):
             raise SynapseError(
                 403,
@@ -494,6 +500,7 @@ async def on_POST(self, request: SynapseRequest) -> Tuple[int, JsonDict]:
                 await self.hs.get_clock().sleep(random.randint(1, 10) / 10)
                 return 200, {"sid": random_string(16)}
 
+            logger.info("MSISDN %s is already in use by %s", msisdn, existing_user_id)
             raise SynapseError(400, "MSISDN is already in use", Codes.THREEPID_IN_USE)
 
         if not self.hs.config.registration.account_threepid_delegate_msisdn:
@@ -518,6 +525,7 @@ async def on_POST(self, request: SynapseRequest) -> Tuple[int, JsonDict]:
         threepid_send_requests.labels(type="msisdn", reason="add_threepid").observe(
             send_attempt
         )
+        logger.info("MSISDN %s is already in use by %s", msisdn, existing_user_id)
 
         return 200, ret