From 6ca88c469373a7ef345d05a4b69afe810e240183 Mon Sep 17 00:00:00 2001
From: David Baker <dave@matrix.org>
Date: Tue, 14 May 2019 19:04:59 +0100
Subject: [PATCH 1/3] Only check 3pids not in use when registering

We checked that 3pids were not already in use before we checked if
we were going to return the account previously registered in the
same UI auth session, in which case the 3pids will definitely
be in use.

https://github.com/vector-im/riot-web/issues/9586
---
 synapse/rest/client/v2_alpha/register.py | 40 +++++++++++++-----------
 1 file changed, 22 insertions(+), 18 deletions(-)

diff --git a/synapse/rest/client/v2_alpha/register.py b/synapse/rest/client/v2_alpha/register.py
index dc3e265bcd..ecec610859 100644
--- a/synapse/rest/client/v2_alpha/register.py
+++ b/synapse/rest/client/v2_alpha/register.py
@@ -391,13 +391,6 @@ class RegisterRestServlet(RestServlet):
         # the user-facing checks will probably already have happened in
         # /register/email/requestToken when we requested a 3pid, but that's not
         # guaranteed.
-        #
-        # Also check that we're not trying to register a 3pid that's already
-        # been registered.
-        #
-        # This has probably happened in /register/email/requestToken as well,
-        # but if a user hits this endpoint twice then clicks on each link from
-        # the two activation emails, they would register the same 3pid twice.
 
         if auth_result:
             for login_type in [LoginType.EMAIL_IDENTITY, LoginType.MSISDN]:
@@ -413,17 +406,6 @@ class RegisterRestServlet(RestServlet):
                             Codes.THREEPID_DENIED,
                         )
 
-                    existingUid = yield self.store.get_user_id_by_threepid(
-                        medium, address,
-                    )
-
-                    if existingUid is not None:
-                        raise SynapseError(
-                            400,
-                            "%s is already in use" % medium,
-                            Codes.THREEPID_IN_USE,
-                        )
-
         if registered_user_id is not None:
             logger.info(
                 "Already registered user ID %r for this session",
@@ -446,6 +428,28 @@ class RegisterRestServlet(RestServlet):
             if auth_result:
                 threepid = auth_result.get(LoginType.EMAIL_IDENTITY)
 
+                # Also check that we're not trying to register a 3pid that's already
+                # been registered.
+                #
+                # This has probably happened in /register/email/requestToken as well,
+                # but if a user hits this endpoint twice then clicks on each link from
+                # the two activation emails, they would register the same 3pid twice.
+                for login_type in [LoginType.EMAIL_IDENTITY, LoginType.MSISDN]:
+                    if login_type in auth_result:
+                        medium = auth_result[login_type]['medium']
+                        address = auth_result[login_type]['address']
+
+                        existingUid = yield self.store.get_user_id_by_threepid(
+                            medium, address,
+                        )
+
+                        if existingUid is not None:
+                            raise SynapseError(
+                                400,
+                                "%s is already in use" % medium,
+                                Codes.THREEPID_IN_USE,
+                            )
+
             (registered_user_id, _) = yield self.registration_handler.register(
                 localpart=desired_username,
                 password=new_password,

From efefb5bda2364de6c205a8e3fa2e8be95b2ae89e Mon Sep 17 00:00:00 2001
From: David Baker <dave@matrix.org>
Date: Tue, 14 May 2019 19:18:42 +0100
Subject: [PATCH 2/3] Have I got newsfile for you

---
 changelog.d/5187.bugfix | 1 +
 1 file changed, 1 insertion(+)
 create mode 100644 changelog.d/5187.bugfix

diff --git a/changelog.d/5187.bugfix b/changelog.d/5187.bugfix
new file mode 100644
index 0000000000..03b985265f
--- /dev/null
+++ b/changelog.d/5187.bugfix
@@ -0,0 +1 @@
+Fix register endpoint returning a previously registered account.

From cd0faba7cd8d469533342a35882648051b393744 Mon Sep 17 00:00:00 2001
From: David Baker <dbkr@users.noreply.github.com>
Date: Wed, 15 May 2019 20:53:48 +0100
Subject: [PATCH 3/3] Make newsfile clearer

---
 changelog.d/5187.bugfix | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/changelog.d/5187.bugfix b/changelog.d/5187.bugfix
index 03b985265f..df176cf5b2 100644
--- a/changelog.d/5187.bugfix
+++ b/changelog.d/5187.bugfix
@@ -1 +1 @@
-Fix register endpoint returning a previously registered account.
+Fix a bug where the register endpoint would fail with M_THREEPID_IN_USE instead of returning an account previously registered in the same session.