Move and refactor LoginRestServlet helper methods (#8182)

This is split out from https://github.com/matrix-org/synapse/pull/7438, which had gotten rather large.

`LoginRestServlet` has a couple helper methods, `login_submission_legacy_convert` and `login_id_thirdparty_from_phone`. They're primarily used for converting legacy user login submissions to "identifier" dicts ([see spec](https://matrix.org/docs/spec/client_server/r0.6.1#post-matrix-client-r0-login)). Identifying information such as usernames or 3PID information used to be top-level in the login body. They're now supposed to be put inside an [identifier](https://matrix.org/docs/spec/client_server/r0.6.1#identifier-types) parameter instead.

#7438's purpose is to allow using the new identifier parameter during User-Interactive Authentication, which is currently handled in AuthHandler. That's why I've moved these helper methods there. I also moved the refactoring of these method from #7438 as they're relevant.

3 files changed, 94 insertions, 55 deletions

diff --git a/changelog.d/8182.misc b/changelog.d/8182.misc
new file mode 100644
index 0000000000..4fcdf1c452
--- /dev/null
+++ b/changelog.d/8182.misc
@@ -0,0 +1 @@
+Refactor some of `LoginRestServlet`'s helper methods, and move them to `AuthHandler` for easier reuse.
\ No newline at end of file
diff --git a/synapse/handlers/auth.py b/synapse/handlers/auth.py
index 654f58ddae..f0b0a4d76a 100644
--- a/synapse/handlers/auth.py
+++ b/synapse/handlers/auth.py
@@ -42,8 +42,9 @@ from synapse.http.site import SynapseRequest
 from synapse.logging.context import defer_to_thread
 from synapse.metrics.background_process_metrics import run_as_background_process
 from synapse.module_api import ModuleApi
-from synapse.types import Requester, UserID
+from synapse.types import JsonDict, Requester, UserID
 from synapse.util import stringutils as stringutils
+from synapse.util.msisdn import phone_number_to_msisdn
 from synapse.util.threepids import canonicalise_email
 
 from ._base import BaseHandler
@@ -51,6 +52,91 @@ from ._base import BaseHandler
 logger = logging.getLogger(__name__)
 
 
+def convert_client_dict_legacy_fields_to_identifier(
+    submission: JsonDict,
+) -> Dict[str, str]:
+    """
+    Convert a legacy-formatted login submission to an identifier dict.
+
+    Legacy login submissions (used in both login and user-interactive authentication)
+    provide user-identifying information at the top-level instead.
+
+    These are now deprecated and replaced with identifiers:
+    https://matrix.org/docs/spec/client_server/r0.6.1#identifier-types
+
+    Args:
+        submission: The client dict to convert
+
+    Returns:
+        The matching identifier dict
+
+    Raises:
+        SynapseError: If the format of the client dict is invalid
+    """
+    identifier = submission.get("identifier", {})
+
+    # Generate an m.id.user identifier if "user" parameter is present
+    user = submission.get("user")
+    if user:
+        identifier = {"type": "m.id.user", "user": user}
+
+    # Generate an m.id.thirdparty identifier if "medium" and "address" parameters are present
+    medium = submission.get("medium")
+    address = submission.get("address")
+    if medium and address:
+        identifier = {
+            "type": "m.id.thirdparty",
+            "medium": medium,
+            "address": address,
+        }
+
+    # We've converted valid, legacy login submissions to an identifier. If the
+    # submission still doesn't have an identifier, it's invalid
+    if not identifier:
+        raise SynapseError(400, "Invalid login submission", Codes.INVALID_PARAM)
+
+    # Ensure the identifier has a type
+    if "type" not in identifier:
+        raise SynapseError(
+            400, "'identifier' dict has no key 'type'", errcode=Codes.MISSING_PARAM,
+        )
+
+    return identifier
+
+
+def login_id_phone_to_thirdparty(identifier: JsonDict) -> Dict[str, str]:
+    """
+    Convert a phone login identifier type to a generic threepid identifier.
+
+    Args:
+        identifier: Login identifier dict of type 'm.id.phone'
+
+    Returns:
+        An equivalent m.id.thirdparty identifier dict
+    """
+    if "country" not in identifier or (
+        # The specification requires a "phone" field, while Synapse used to require a "number"
+        # field. Accept both for backwards compatibility.
+        "phone" not in identifier
+        and "number" not in identifier
+    ):
+        raise SynapseError(
+            400, "Invalid phone-type identifier", errcode=Codes.INVALID_PARAM
+        )
+
+    # Accept both "phone" and "number" as valid keys in m.id.phone
+    phone_number = identifier.get("phone", identifier["number"])
+
+    # Convert user-provided phone number to a consistent representation
+    msisdn = phone_number_to_msisdn(identifier["country"], phone_number)
+
+    return {
+        "type": "m.id.thirdparty",
+        "medium": "msisdn",
+        "address": msisdn,
+    }
+
+
 class AuthHandler(BaseHandler):
     SESSION_EXPIRE_MS = 48 * 60 * 60 * 1000
 
diff --git a/synapse/rest/client/v1/login.py b/synapse/rest/client/v1/login.py
index 379f668d6f..a14618ac84 100644
--- a/synapse/rest/client/v1/login.py
+++ b/synapse/rest/client/v1/login.py
@@ -18,6 +18,10 @@ from typing import Awaitable, Callable, Dict, Optional
 
 from synapse.api.errors import Codes, LoginError, SynapseError
 from synapse.api.ratelimiting import Ratelimiter
+from synapse.handlers.auth import (
+    convert_client_dict_legacy_fields_to_identifier,
+    login_id_phone_to_thirdparty,
+)
 from synapse.http.server import finish_request
 from synapse.http.servlet import (
     RestServlet,
@@ -28,56 +32,11 @@ from synapse.http.site import SynapseRequest
 from synapse.rest.client.v2_alpha._base import client_patterns
 from synapse.rest.well_known import WellKnownBuilder
 from synapse.types import JsonDict, UserID
-from synapse.util.msisdn import phone_number_to_msisdn
 from synapse.util.threepids import canonicalise_email
 
 logger = logging.getLogger(__name__)
 
 
-def login_submission_legacy_convert(submission):
-    """
-    If the input login submission is an old style object
-    (ie. with top-level user / medium / address) convert it
-    to a typed object.
-    """
-    if "user" in submission:
-        submission["identifier"] = {"type": "m.id.user", "user": submission["user"]}
-        del submission["user"]
-
-    if "medium" in submission and "address" in submission:
-        submission["identifier"] = {
-            "type": "m.id.thirdparty",
-            "medium": submission["medium"],
-            "address": submission["address"],
-        }
-        del submission["medium"]
-        del submission["address"]
-
-
-def login_id_thirdparty_from_phone(identifier):
-    """
-    Convert a phone login identifier type to a generic threepid identifier
-    Args:
-        identifier(dict): Login identifier dict of type 'm.id.phone'
-
-    Returns: Login identifier dict of type 'm.id.threepid'
-    """
-    if "country" not in identifier or (
-        # The specification requires a "phone" field, while Synapse used to require a "number"
-        # field. Accept both for backwards compatibility.
-        "phone" not in identifier
-        and "number" not in identifier
-    ):
-        raise SynapseError(400, "Invalid phone-type identifier")
-
-    # Accept both "phone" and "number" as valid keys in m.id.phone
-    phone_number = identifier.get("phone", identifier["number"])
-
-    msisdn = phone_number_to_msisdn(identifier["country"], phone_number)
-
-    return {"type": "m.id.thirdparty", "medium": "msisdn", "address": msisdn}
-
-
 class LoginRestServlet(RestServlet):
     PATTERNS = client_patterns("/login$", v1=True)
     CAS_TYPE = "m.login.cas"
@@ -194,18 +153,11 @@ class LoginRestServlet(RestServlet):
             login_submission.get("address"),
             login_submission.get("user"),
         )
-        login_submission_legacy_convert(login_submission)
-
-        if "identifier" not in login_submission:
-            raise SynapseError(400, "Missing param: identifier")
-
-        identifier = login_submission["identifier"]
-        if "type" not in identifier:
-            raise SynapseError(400, "Login identifier has no type")
+        identifier = convert_client_dict_legacy_fields_to_identifier(login_submission)
 
         # convert phone type identifiers to generic threepids
         if identifier["type"] == "m.id.phone":
-            identifier = login_id_thirdparty_from_phone(identifier)
+            identifier = login_id_phone_to_thirdparty(identifier)
 
         # convert threepid identifiers to user IDs
         if identifier["type"] == "m.id.thirdparty":