Add POST /_matrix/client/r0/account/3pid/unbind (MSC2140) (#5980)

Implements `POST /_matrix/client/r0/account/3pid/unbind` from [MSC2140](https://github.com/matrix-org/matrix-doc/blob/dbkr/tos_2/proposals/2140-terms-of-service-2.md#post-_matrixclientr0account3pidunbind).

changelog.d/5980.feature

@@ -0,0 +1 @@
+Add POST /_matrix/client/r0/account/3pid/unbind endpoint from MSC2140 for unbinding a 3PID from an identity server without removing it from the homeserver user account.

synapse/handlers/identity.py

@@ -137,7 +137,8 @@ class IdentityHandler(BaseHandler):
 
     @defer.inlineCallbacks
     def try_unbind_threepid(self, mxid, threepid):
-        """Removes a binding from an identity server
+        """Attempt to remove a 3PID from an identity server, or if one is not provided, all
+        identity servers we're aware the binding is present on
 
         Args:
             mxid (str): Matrix user ID of binding to be removed

synapse/rest/client/v2_alpha/account.py

@@ -571,6 +571,38 @@ class ThreepidRestServlet(RestServlet):
         return 200, {}
 
 
+class ThreepidUnbindRestServlet(RestServlet):
+    PATTERNS = client_patterns("/account/3pid/unbind$")
+
+    def __init__(self, hs):
+        super(ThreepidUnbindRestServlet, self).__init__()
+        self.hs = hs
+        self.identity_handler = hs.get_handlers().identity_handler
+        self.auth = hs.get_auth()
+        self.datastore = self.hs.get_datastore()
+
+    @defer.inlineCallbacks
+    def on_POST(self, request):
+        """Unbind the given 3pid from a specific identity server, or identity servers that are
+        known to have this 3pid bound
+        """
+        requester = yield self.auth.get_user_by_req(request)
+        body = parse_json_object_from_request(request)
+        assert_params_in_dict(body, ["medium", "address"])
+
+        medium = body.get("medium")
+        address = body.get("address")
+        id_server = body.get("id_server")
+
+        # Attempt to unbind the threepid from an identity server. If id_server is None, try to
+        # unbind from all identity servers this threepid has been added to in the past
+        result = yield self.identity_handler.try_unbind_threepid(
+            requester.user.to_string(),
+            {"address": address, "medium": medium, "id_server": id_server},
+        )
+        return 200, {"id_server_unbind_result": "success" if result else "no-support"}
+
+
 class ThreepidDeleteRestServlet(RestServlet):
     PATTERNS = client_patterns("/account/3pid/delete$")
 
@@ -629,5 +661,6 @@ def register_servlets(hs, http_server):
     EmailThreepidRequestTokenRestServlet(hs).register(http_server)
     MsisdnThreepidRequestTokenRestServlet(hs).register(http_server)
     ThreepidRestServlet(hs).register(http_server)
+    ThreepidUnbindRestServlet(hs).register(http_server)
     ThreepidDeleteRestServlet(hs).register(http_server)
     WhoamiRestServlet(hs).register(http_server)