bugfix: make msc3967 idempotent (#16943)

MSC3967 was updated recently to make it more robust to network failures:

> there is an existing cross-signing master key and it exactly matches
the cross-signing master key provided in the request body. If there are
any additional keys provided in the request (self signing key, user
signing key) they MUST also match the existing keys stored on the
server. In other words, the request contains no new keys. If there are
new keys, UIA MUST be performed.


https://github.com/matrix-org/matrix-spec-proposals/blob/hughns/device-signing-upload-uia/proposals/3967-device-signing-upload-uia.md#proposal

This covers the case where the 200 OK is lost in transit so the client
retries the upload, only to then get UIA'd.

Complement tests: https://github.com/matrix-org/complement/pull/713 -
passing example
https://github.com/element-hq/synapse/actions/runs/7976948122/job/21778795094?pr=16943#step:7:8820

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---------

Co-authored-by: reivilibre <oliverw@matrix.org>

1 files changed, 12 insertions, 2 deletions

diff --git a/synapse/rest/client/keys.py b/synapse/rest/client/keys.py
index b6d9ee074a..86c9515854 100644
--- a/synapse/rest/client/keys.py
+++ b/synapse/rest/client/keys.py
@@ -409,7 +409,18 @@ class SigningKeyUploadServlet(RestServlet):
             # But first-time setup is fine
 
         elif self.hs.config.experimental.msc3967_enabled:
-            # If we already have a master key then cross signing is set up and we require UIA to reset
+            # MSC3967 allows this endpoint to 200 OK for idempotency. Resending exactly the same
+            # keys should just 200 OK without doing a UIA prompt.
+            keys_are_different = await self.e2e_keys_handler.has_different_keys(
+                user_id, body
+            )
+            if not keys_are_different:
+                # FIXME: we do not fallthrough to upload_signing_keys_for_user because confusingly
+                # if we do, we 500 as it looks like it tries to INSERT the same key twice, causing a
+                # unique key constraint violation. This sounds like a bug?
+                return 200, {}
+            # the keys are different, is x-signing set up? If no, then the keys don't exist which is
+            # why they are different. If yes, then we need to UIA to change them.
             if is_cross_signing_setup:
                 await self.auth_handler.validate_user_via_ui_auth(
                     requester,
@@ -420,7 +431,6 @@ class SigningKeyUploadServlet(RestServlet):
                     can_skip_ui_auth=False,
                 )
             # Otherwise we don't require UIA since we are setting up cross signing for first time
-
         else:
             # Previous behaviour is to always require UIA but allow it to be skipped
             await self.auth_handler.validate_user_via_ui_auth(