Refactor code for calculating registration flows (#6106)

because, frankly, it looked like it was written by an axe-murderer.

This should be a non-functional change, except that where `m.login.dummy` was
previously advertised *before* `m.login.terms`, it will now be advertised
afterwards. AFAICT that should have no effect, and will be more consistent with
the flows that involve passing a 3pid.

4 files changed, 145 insertions, 83 deletions

diff --git a/changelog.d/6106.misc b/changelog.d/6106.misc
new file mode 100644
index 0000000000..d732091779
--- /dev/null
+++ b/changelog.d/6106.misc
@@ -0,0 +1 @@
+Refactor code for calculating registration flows.
diff --git a/synapse/rest/client/v2_alpha/register.py b/synapse/rest/client/v2_alpha/register.py
index 135a70808f..e3f3d9126f 100644
--- a/synapse/rest/client/v2_alpha/register.py
+++ b/synapse/rest/client/v2_alpha/register.py
@@ -16,6 +16,7 @@
 
 import hmac
 import logging
+from typing import List, Union
 
 from six import string_types
 
@@ -31,8 +32,11 @@ from synapse.api.errors import (
     ThreepidValidationError,
     UnrecognizedRequestError,
 )
+from synapse.config.captcha import CaptchaConfig
+from synapse.config.consent_config import ConsentConfig
 from synapse.config.emailconfig import ThreepidBehaviour
 from synapse.config.ratelimiting import FederationRateLimitConfig
+from synapse.config.registration import RegistrationConfig
 from synapse.config.server import is_threepid_reserved
 from synapse.http.server import finish_request
 from synapse.http.servlet import (
@@ -371,6 +375,8 @@ class RegisterRestServlet(RestServlet):
         self.ratelimiter = hs.get_registration_ratelimiter()
         self.clock = hs.get_clock()
 
+        self._registration_flows = _calculate_registration_flows(hs.config)
+
     @interactive_auth_handler
     @defer.inlineCallbacks
     def on_POST(self, request):
@@ -491,69 +497,8 @@ class RegisterRestServlet(RestServlet):
                 assigned_user_id=registered_user_id,
             )
 
-        # FIXME: need a better error than "no auth flow found" for scenarios
-        # where we required 3PID for registration but the user didn't give one
-        require_email = "email" in self.hs.config.registrations_require_3pid
-        require_msisdn = "msisdn" in self.hs.config.registrations_require_3pid
-
-        show_msisdn = True
-        if self.hs.config.disable_msisdn_registration:
-            show_msisdn = False
-            require_msisdn = False
-
-        flows = []
-        if self.hs.config.enable_registration_captcha:
-            # only support 3PIDless registration if no 3PIDs are required
-            if not require_email and not require_msisdn:
-                # Also add a dummy flow here, otherwise if a client completes
-                # recaptcha first we'll assume they were going for this flow
-                # and complete the request, when they could have been trying to
-                # complete one of the flows with email/msisdn auth.
-                flows.extend([[LoginType.RECAPTCHA, LoginType.DUMMY]])
-            # only support the email-only flow if we don't require MSISDN 3PIDs
-            if not require_msisdn:
-                flows.extend([[LoginType.RECAPTCHA, LoginType.EMAIL_IDENTITY]])
-
-            if show_msisdn:
-                # only support the MSISDN-only flow if we don't require email 3PIDs
-                if not require_email:
-                    flows.extend([[LoginType.RECAPTCHA, LoginType.MSISDN]])
-                # always let users provide both MSISDN & email
-                flows.extend(
-                    [[LoginType.RECAPTCHA, LoginType.MSISDN, LoginType.EMAIL_IDENTITY]]
-                )
-        else:
-            # only support 3PIDless registration if no 3PIDs are required
-            if not require_email and not require_msisdn:
-                flows.extend([[LoginType.DUMMY]])
-            # only support the email-only flow if we don't require MSISDN 3PIDs
-            if not require_msisdn:
-                flows.extend([[LoginType.EMAIL_IDENTITY]])
-
-            if show_msisdn:
-                # only support the MSISDN-only flow if we don't require email 3PIDs
-                if not require_email or require_msisdn:
-                    flows.extend([[LoginType.MSISDN]])
-                # always let users provide both MSISDN & email
-                flows.extend([[LoginType.MSISDN, LoginType.EMAIL_IDENTITY]])
-
-        # Append m.login.terms to all flows if we're requiring consent
-        if self.hs.config.user_consent_at_registration:
-            new_flows = []
-            for flow in flows:
-                inserted = False
-                # m.login.terms should go near the end but before msisdn or email auth
-                for i, stage in enumerate(flow):
-                    if stage == LoginType.EMAIL_IDENTITY or stage == LoginType.MSISDN:
-                        flow.insert(i, LoginType.TERMS)
-                        inserted = True
-                        break
-                if not inserted:
-                    flow.append(LoginType.TERMS)
-            flows.extend(new_flows)
-
         auth_result, params, session_id = yield self.auth_handler.check_auth(
-            flows, body, self.hs.get_ip_from_request(request)
+            self._registration_flows, body, self.hs.get_ip_from_request(request)
         )
 
         # Check that we're not trying to register a denied 3pid.
@@ -716,6 +661,61 @@ class RegisterRestServlet(RestServlet):
         )
 
 
+def _calculate_registration_flows(
+    # technically `config` has to provide *all* of these interfaces, not just one
+    config: Union[RegistrationConfig, ConsentConfig, CaptchaConfig],
+) -> List[List[str]]:
+    """Get a suitable flows list for registration
+
+    Args:
+        config: server configuration
+
+    Returns: a list of supported flows
+    """
+    # FIXME: need a better error than "no auth flow found" for scenarios
+    # where we required 3PID for registration but the user didn't give one
+    require_email = "email" in config.registrations_require_3pid
+    require_msisdn = "msisdn" in config.registrations_require_3pid
+
+    show_msisdn = True
+    if config.disable_msisdn_registration:
+        show_msisdn = False
+        require_msisdn = False
+
+    flows = []
+
+    # only support 3PIDless registration if no 3PIDs are required
+    if not require_email and not require_msisdn:
+        # Add a dummy step here, otherwise if a client completes
+        # recaptcha first we'll assume they were going for this flow
+        # and complete the request, when they could have been trying to
+        # complete one of the flows with email/msisdn auth.
+        flows.append([LoginType.DUMMY])
+
+    # only support the email-only flow if we don't require MSISDN 3PIDs
+    if not require_msisdn:
+        flows.append([LoginType.EMAIL_IDENTITY])
+
+    # only support the MSISDN-only flow if we don't require email 3PIDs
+    if show_msisdn and not require_email:
+        flows.append([LoginType.MSISDN])
+
+    if show_msisdn:
+        flows.append([LoginType.MSISDN, LoginType.EMAIL_IDENTITY])
+
+    # Prepend m.login.terms to all flows if we're requiring consent
+    if config.user_consent_at_registration:
+        for flow in flows:
+            flow.insert(0, LoginType.TERMS)
+
+    # Prepend recaptcha to all flows if we're requiring captcha
+    if config.enable_registration_captcha:
+        for flow in flows:
+            flow.insert(0, LoginType.RECAPTCHA)
+
+    return flows
+
+
 def register_servlets(hs, http_server):
     EmailRegisterRequestTokenRestServlet(hs).register(http_server)
     MsisdnRegisterRequestTokenRestServlet(hs).register(http_server)
diff --git a/tests/rest/client/v2_alpha/test_register.py b/tests/rest/client/v2_alpha/test_register.py
index ab4d7d70d0..bc2dc47973 100644
--- a/tests/rest/client/v2_alpha/test_register.py
+++ b/tests/rest/client/v2_alpha/test_register.py
@@ -34,19 +34,12 @@ from tests import unittest
 class RegisterRestServletTestCase(unittest.HomeserverTestCase):
 
     servlets = [register.register_servlets]
+    url = b"/_matrix/client/r0/register"
 
-    def make_homeserver(self, reactor, clock):
-
-        self.url = b"/_matrix/client/r0/register"
-
-        self.hs = self.setup_test_homeserver()
-        self.hs.config.enable_registration = True
-        self.hs.config.registrations_require_3pid = []
-        self.hs.config.auto_join_rooms = []
-        self.hs.config.enable_registration_captcha = False
-        self.hs.config.allow_guest_access = True
-
-        return self.hs
+    def default_config(self, name="test"):
+        config = super().default_config(name)
+        config["allow_guest_access"] = True
+        return config
 
     def test_POST_appservice_registration_valid(self):
         user_id = "@as_user_kermit:test"
@@ -199,6 +192,68 @@ class RegisterRestServletTestCase(unittest.HomeserverTestCase):
 
         self.assertEquals(channel.result["code"], b"200", channel.result)
 
+    def test_advertised_flows(self):
+        request, channel = self.make_request(b"POST", self.url, b"{}")
+        self.render(request)
+        self.assertEquals(channel.result["code"], b"401", channel.result)
+        flows = channel.json_body["flows"]
+
+        # with the stock config, we expect all four combinations of 3pid
+        self.assertCountEqual(
+            [
+                ["m.login.dummy"],
+                ["m.login.email.identity"],
+                ["m.login.msisdn"],
+                ["m.login.msisdn", "m.login.email.identity"],
+            ],
+            (f["stages"] for f in flows),
+        )
+
+    @unittest.override_config(
+        {
+            "enable_registration_captcha": True,
+            "user_consent": {
+                "version": "1",
+                "template_dir": "/",
+                "require_at_registration": True,
+            },
+        }
+    )
+    def test_advertised_flows_captcha_and_terms(self):
+        request, channel = self.make_request(b"POST", self.url, b"{}")
+        self.render(request)
+        self.assertEquals(channel.result["code"], b"401", channel.result)
+        flows = channel.json_body["flows"]
+
+        self.assertCountEqual(
+            [
+                ["m.login.recaptcha", "m.login.terms", "m.login.dummy"],
+                ["m.login.recaptcha", "m.login.terms", "m.login.email.identity"],
+                ["m.login.recaptcha", "m.login.terms", "m.login.msisdn"],
+                [
+                    "m.login.recaptcha",
+                    "m.login.terms",
+                    "m.login.msisdn",
+                    "m.login.email.identity",
+                ],
+            ],
+            (f["stages"] for f in flows),
+        )
+
+    @unittest.override_config(
+        {"registrations_require_3pid": ["email"], "disable_msisdn_registration": True}
+    )
+    def test_advertised_flows_no_msisdn_email_required(self):
+        request, channel = self.make_request(b"POST", self.url, b"{}")
+        self.render(request)
+        self.assertEquals(channel.result["code"], b"401", channel.result)
+        flows = channel.json_body["flows"]
+
+        # with the stock config, we expect all four combinations of 3pid
+        self.assertCountEqual(
+            [["m.login.email.identity"]], (f["stages"] for f in flows)
+        )
+
 
 class AccountValidityTestCase(unittest.HomeserverTestCase):
 
diff --git a/tests/test_terms_auth.py b/tests/test_terms_auth.py
index 52739fbabc..5ec5d2b358 100644
--- a/tests/test_terms_auth.py
+++ b/tests/test_terms_auth.py
@@ -28,6 +28,21 @@ from tests import unittest
 class TermsTestCase(unittest.HomeserverTestCase):
     servlets = [register_servlets]
 
+    def default_config(self, name="test"):
+        config = super().default_config(name)
+        config.update(
+            {
+                "public_baseurl": "https://example.org/",
+                "user_consent": {
+                    "version": "1.0",
+                    "policy_name": "My Cool Privacy Policy",
+                    "template_dir": "/",
+                    "require_at_registration": True,
+                },
+            }
+        )
+        return config
+
     def prepare(self, reactor, clock, hs):
         self.clock = MemoryReactorClock()
         self.hs_clock = Clock(self.clock)
@@ -35,17 +50,8 @@ class TermsTestCase(unittest.HomeserverTestCase):
         self.registration_handler = Mock()
         self.auth_handler = Mock()
         self.device_handler = Mock()
-        hs.config.enable_registration = True
-        hs.config.registrations_require_3pid = []
-        hs.config.auto_join_rooms = []
-        hs.config.enable_registration_captcha = False
 
     def test_ui_auth(self):
-        self.hs.config.user_consent_at_registration = True
-        self.hs.config.user_consent_policy_name = "My Cool Privacy Policy"
-        self.hs.config.public_baseurl = "https://example.org/"
-        self.hs.config.user_consent_version = "1.0"
-
         # Do a UI auth request
         request, channel = self.make_request(b"POST", self.url, b"{}")
         self.render(request)