3 files changed, 165 insertions, 1 deletions
diff --git a/pydis_site/apps/home/__init__.py b/pydis_site/apps/home/__init__.py
index e69de29b..ecfab449 100644
--- a/pydis_site/apps/home/__init__.py
+++ b/pydis_site/apps/home/__init__.py
@@ -0,0 +1 @@
+default_app_config = "pydis_site.apps.home.apps.HomeConfig"
diff --git a/pydis_site/apps/home/apps.py b/pydis_site/apps/home/apps.py
index 9a3d213c..055d721b 100644
--- a/pydis_site/apps/home/apps.py
+++ b/pydis_site/apps/home/apps.py
@@ -4,4 +4,11 @@ from django.apps import AppConfig
 class HomeConfig(AppConfig):
     """Django AppConfig for the home app."""
 
-    name = 'home'
+    name = 'pydis_site.apps.home'
+    signal_listener = None
+
+    def ready(self) -> None:
+        """Run when the app has been loaded and is ready to serve requests."""
+        from pydis_site.apps.home.signals import SignalListener
+
+        self.signal_listener = SignalListener()
diff --git a/pydis_site/apps/home/signals.py b/pydis_site/apps/home/signals.py
new file mode 100644
index 00000000..687a99fe
--- /dev/null
+++ b/pydis_site/apps/home/signals.py
@@ -0,0 +1,156 @@
+from typing import List, Optional, Type
+
+from allauth.account.signals import user_logged_in
+from allauth.socialaccount.models import SocialAccount, SocialLogin
+from allauth.socialaccount.providers.base import Provider
+from allauth.socialaccount.providers.discord.provider import DiscordProvider
+from allauth.socialaccount.signals import (
+    pre_social_login, social_account_added, social_account_removed,
+    social_account_updated)
+from django.contrib.auth.models import Group, User as DjangoUser
+from django.db.models.signals import post_save
+
+from pydis_site.apps.api.models import User as DiscordUser
+from pydis_site.apps.staff.models import RoleMapping
+
+
+class SignalListener:
+    """
+    Listens to and processes events via the Django Signals system.
+
+    Django Signals is basically an event dispatcher. It consists of Signals (which are the events)
+    and Receivers, which listen for and handle those events. Signals are triggered by Senders,
+    which are essentially just any class at all, and Receivers can filter the Signals they listen
+    for by choosing a Sender, if required.
+
+    Signals themselves define a set of arguments that they will provide to Receivers when the
+    Signal is sent. They are always keyword arguments, and Django recommends that all Receiver
+    functions accept them as `**kwargs` (and will supposedly error if you don't do this),
+    supposedly because Signals can change in the future and your receivers should still work.
+
+    Signals do provide a list of their arguments when they're initially constructed, but this
+    is purely for documentation purposes only and Django does not enforce it.
+
+    The Django Signals docs are here: https://docs.djangoproject.com/en/2.2/topics/signals/
+    """
+
+    def __init__(self):
+        post_save.connect(self.model_updated, sender=DiscordUser)
+
+        pre_social_login.connect(self.social_account_updated)
+        social_account_added.connect(self.social_account_updated)
+        social_account_updated.connect(self.social_account_updated)
+        social_account_removed.connect(self.social_account_removed)
+
+        user_logged_in.connect(self.user_logged_in)
+
+    def user_logged_in(self, sender: Type[DjangoUser], **kwargs) -> None:
+        """Handles signals relating to Allauth logins."""
+        user: DjangoUser = kwargs["user"]
+
+        try:
+            account: SocialAccount = SocialAccount.objects.get(
+                user=user, provider=DiscordProvider.id
+            )
+        except SocialAccount.DoesNotExist:
+            return  # User's never linked a Discord account
+        try:
+            discord_user: DiscordUser = DiscordUser.objects.get(id=int(account.uid))
+        except DiscordUser.DoesNotExist:
+            return
+
+        self._apply_groups(discord_user, account)
+
+    def social_account_updated(self, sender: Type[SocialLogin], **kwargs) -> None:
+        """Handle signals relating to new/existing social accounts."""
+        social_login: SocialLogin = kwargs["sociallogin"]
+
+        account: SocialAccount = social_login.account
+        provider: Provider = account.get_provider()
+
+        if not isinstance(provider, DiscordProvider):
+            return
+
+        try:
+            user: DiscordUser = DiscordUser.objects.get(id=int(account.uid))
+        except DiscordUser.DoesNotExist:
+            return
+
+        self._apply_groups(user, account)
+
+    def social_account_removed(self, sender: Type[SocialLogin], **kwargs) -> None:
+        """Handle signals relating to removal of social accounts."""
+        account: SocialAccount = kwargs["socialaccount"]
+        provider: Provider = account.get_provider()
+
+        if not isinstance(provider, DiscordProvider):
+            return
+
+        try:
+            user: DiscordUser = DiscordUser.objects.get(id=int(account.uid))
+        except DiscordUser.DoesNotExist:
+            return
+
+        self._apply_groups(user, account, True)
+
+    def model_updated(self, sender: Type[DiscordUser], **kwargs) -> None:
+        """Handle signals related to the updating of Discord User model entries."""
+        instance: DiscordUser = kwargs["instance"]
+        raw: bool = kwargs["raw"]
+
+        # `update_fields` could be used for checking changes, but it's None here due to how the
+        # model is saved without using that argument - so we can't use it.
+
+        if raw:
+            # Fixtures are being loaded, so don't touch anything
+            return
+
+        try:
+            account: SocialAccount = SocialAccount.objects.get(uid=str(instance.id))
+        except SocialAccount.DoesNotExist:
+            return  # User has never logged in with Discord on the site
+
+        self._apply_groups(instance, account)
+
+    def _apply_groups(
+            self, user: DiscordUser, account: Optional[SocialAccount], deletion: bool = False
+    ) -> None:
+        mappings = RoleMapping.objects.all()
+
+        try:
+            current_groups: List[Group] = list(account.user.groups.all())
+        except SocialAccount.user.RelatedObjectDoesNotExist:
+            return  # There's no user account yet, this will be handled by another receiver
+
+        if not user.in_guild:
+            deletion = True
+
+        if deletion:
+            # They've unlinked Discord or left the server, so we have to remove their groups
+
+            if not current_groups:
+                return  # They have no groups anyway, no point in processing
+
+            account.user.groups.remove(
+                *(mapping.group for mapping in mappings)
+            )
+        else:
+            new_groups = []
+
+            for role in user.roles.all():
+                try:
+                    new_groups.append(mappings.get(role=role).group)
+                except RoleMapping.DoesNotExist:
+                    continue  # No mapping exists
+
+            remove_groups = [
+                mapping.group for mapping in mappings if mapping.group not in new_groups
+            ]
+
+            add_groups = [group for group in new_groups if group not in current_groups]
+
+            if remove_groups:
+                account.user.groups.remove(*remove_groups)
+
+            if add_groups:
+                account.user.groups.add(*add_groups)