Fix submissions_status RPC when UNION ALL doesn't keep the order (#1201)

UNION ALL is not guaranteed to keep the order of the results. In fact
with (at least) Postgres 13 the order is not well defined and can change
between executions.

This patch adds an extra column to the query, tagging each statistics
with its key, so that we can reconstruct the correct order of the
results.

cms/server/admin/server.py

@@ -27,7 +27,7 @@
 
 import logging
 
-from sqlalchemy import func, not_
+from sqlalchemy import func, not_, literal_column
 
 from cms import config, ServiceCoord, get_service_shards
 from cms.db import SessionGen, Dataset, Submission, SubmissionResult, Task
@@ -176,13 +176,17 @@ def submissions_status(contest_id):
                     .filter(Task.contest_id == contest_id)
             queries['total'] = total_query
 
-            stats = {}
+            # Add a "key" column for keeping track of each stats, in case they
+            # get shuffled.
+            for key, query in queries.items():
+                key_column = literal_column(f"'{key}'").label("key")
+                queries[key] = query.add_columns(key_column)
+
             keys = list(queries.keys())
             results = queries[keys[0]].union_all(
                 *(queries[key] for key in keys[1:])).all()
 
-        for i, k in enumerate(keys):
-            stats[k] = results[i][0]
+        stats = {key: value for value, key in results}
         stats['compiling'] += 2 * stats['total'] - sum(stats.values())
 
         return stats