Make future permissions restore room defaults

If we change permission to write=false, then schedule a future that
restores write=true, we end up (after arriving at the future) with an
explicit write=true permission for the user.

This seems slightly wrong because the positive permission is actually
*more* permissive than what the user *probably* had which was just write
permission inherited from the room.  This won't matter hugely right
away, but could show up later if the room default_write value gets
changed (at which point this disciplined user would have write access).

This changes the future to set the override to null if the future would
be setting the room default as a permission.  So, in the described
situation, we'd set `write=null` in the override row rather than
`write=true`, and so the user would end up with write permissions via
the default, rather than via a specific permission grant.

Author: jagerman

sogs/cleanup.py

@@ -94,17 +94,31 @@ def expire_nonce_history():
 def apply_permission_updates():
     with db.transaction():
         now = time.time()
+        # This update gets a bit complicated; basically what we want to do is:
+        # - if a future permission row is to be applied, then any `null` permission should not
+        #   change the current permission (i.e. preserve the current override value if an override
+        #   row exists, otherwise insert a null).
+        # - for non-null permissions if the permission being applied equals the room's default then
+        #   we want to update the override value to null regardless of what it is now, because that
+        #   is almost always what was intended.
+        # The: `CASE WHEN f.perm IS NULL THEN o.perm ELSE NULLIF(f.perm, r.perm) END`s below make
+        # this happen: if our future is null we use the current override value (null if we have no
+        # override), otherwise if the future and room defaults and equal we specifically set null;
+        # otherwise we set the future value.
         num_perms = query(
             """
             INSERT INTO user_permission_overrides (room, "user", read, write, upload)
-            SELECT room, "user", read, write, upload
-                FROM user_permission_futures
+            SELECT f.room, f."user",
+                    CASE WHEN f.read IS NULL THEN o.read ELSE NULLIF(f.read, r.read) END,
+                    CASE WHEN f.write IS NULL THEN o.write ELSE NULLIF(f.write, r.write) END,
+                    CASE WHEN f.upload IS NULL THEN o.upload ELSE NULLIF(f.upload, r.upload) END
+                FROM user_permission_futures f
+                    JOIN rooms r ON f.room = r.id
+                    LEFT JOIN user_permission_overrides o ON f.room = o.room AND f."user" = o."user"
                 WHERE at <= :now
                 ORDER BY at
             ON CONFLICT (room, "user") DO UPDATE SET
-                read = COALESCE(excluded.read, user_permission_overrides.read),
-                write = COALESCE(excluded.write, user_permission_overrides.write),
-                upload = COALESCE(excluded.upload, user_permission_overrides.upload)
+                read = excluded.read, write = excluded.write, upload = excluded.upload
             """,
             now=now,
         ).rowcount