import: Fix import of Stream.creator field.

When importing a Stream, UserProfiles don't yet exist. so trying to
import stream.creator fails with something like:

```
psycopg2.errors.ForeignKeyViolation: insert or update on table
"zerver_stream" violates foreign key constraint
"zerver_stream_creator_id_65aeba7e_fk_zerver_userprofile_id"
DETAIL:  Key (creator_id)=(5) is not present in table "zerver_userprofile".
```

zerver/lib/import_realm.py

@@ -1081,7 +1081,18 @@ def do_import_realm(import_dir: Path, subdomain: str, processes: int = 1) -> Rea
         # Stream objects are created by Django.
         fix_datetime_fields(data, "zerver_stream")
         re_map_foreign_keys(data, "zerver_stream", "realm", related_table="realm")
+
         re_map_foreign_keys(data, "zerver_stream", "creator", related_table="user_profile")
+        # There's a circular dependency between Stream and UserProfile due to
+        # the .creator attribute. We untangle it by first remembering the creator_id
+        # for all the streams and then removing those fields from the data.
+        # That allows us to successfully import streams, and then later after users
+        # are imported, we can set the .creator_id for all these streams correctly.
+        stream_id_to_creator_id = {}
+        for stream in data["zerver_stream"]:
+            creator_id = stream.pop("creator_id", None)
+            stream_id_to_creator_id[stream["id"]] = creator_id
+
         if role_system_groups_dict is not None:
             # Because the system user groups are missing, we manually set up
             # the defaults for can_remove_subscribers_group for all the
@@ -1136,6 +1147,13 @@ def do_import_realm(import_dir: Path, subdomain: str, processes: int = 1) -> Rea
         user_profile.tos_version = UserProfile.TOS_VERSION_BEFORE_FIRST_LOGIN
     UserProfile.objects.bulk_create(user_profiles)
 
+    # UserProfiles have been loaded, so now we're ready to set .creator_id
+    # for streams based on the mapping we saved earlier.
+    streams = Stream.objects.filter(id__in=stream_id_to_creator_id.keys())
+    for stream in streams:
+        stream.creator_id = stream_id_to_creator_id[stream.id]
+    Stream.objects.bulk_update(streams, ["creator_id"])
+
     re_map_foreign_keys(data, "zerver_defaultstream", "stream", related_table="stream")
     re_map_foreign_keys(data, "zerver_realmemoji", "author", related_table="user_profile")
 

zerver/tests/test_import_export.py

@@ -763,6 +763,10 @@ def test_import_realm(self) -> None:
         cordelia = self.example_user("cordelia")
         othello = self.example_user("othello")
 
+        denmark_stream = get_stream("Denmark", original_realm)
+        denmark_stream.creator = hamlet
+        denmark_stream.save()
+
         internal_realm = get_realm(settings.SYSTEM_BOT_REALM)
         cross_realm_bot = get_system_bot(settings.WELCOME_BOT, internal_realm.id)
 
@@ -1012,6 +1016,12 @@ def assert_realm_values(f: Callable[[Realm], object]) -> None:
             f'<p><span class="user-mention" data-user-id="{imported_polonius_user.id}">@Polonius</span></p>',
         )
 
+        imported_hamlet_user = UserProfile.objects.get(
+            delivery_email=self.example_email("hamlet"), realm=imported_realm
+        )
+        imported_denmark_stream = Stream.objects.get(name="Denmark", realm=imported_realm)
+        self.assertEqual(imported_denmark_stream.creator, imported_hamlet_user)
+
         # Check recipient_id was generated correctly for the imported users and streams.
         for user_profile in UserProfile.objects.filter(realm=imported_realm):
             self.assertEqual(