Fix largest rooms Admin API failing when pg_stats has no common room IDs (#20390)

Fixes #19561

`GET /_synapse/admin/v1/statistics/database/rooms` returns a 500 when
`pg_stats` has no common `room_id` values for one of the tables it
checks. This happens in two cases:

- The table is empty or not yet analyzed, so `pg_stats` has no row for
it.
- No value is more common than the others, so `most_common_vals` is
`NULL`. `event_edges.room_id` hits this on servers where all rows were
written after schema 72 stopped filling the column.

The change skips the table in both cases, since it adds nothing to the
estimate.

Thanks to @christianarielli for finding the cause and suggesting the
fix. They're credited as co-author on the commit.

The new test runs only on Postgres, as the endpoint doesn't support
SQLite. It creates a room, runs `ANALYZE` while `event_push_actions` is
empty and `event_edges.room_id` is all `NULL`, and checks for a 200 with
the room listed. I ran it against Postgres 17.

`event_edges.room_id` isn't written any more, so dropping that table
from the list might make sense. I left that out to keep this PR small.

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---------

Co-authored-by: Christian Arielli <903609+christianarielli@users.noreply.github.com>
Co-authored-by: Eric Eastwood <erice@element.io>

changelog.d/20390.bugfix

@@ -0,0 +1 @@
+Fix the `/_synapse/admin/v1/statistics/database/rooms` Admin API failing with a 500 error when Postgres has no statistics, or no most common `room_id` values, for one of the tables it checks.

synapse/storage/controllers/stats.py

@@ -92,9 +92,15 @@ class StatsController:
 
                 txn.execute(column_sql, (table,))
                 row = txn.fetchone()
-                assert row is not None
-                vals, freqs = row
 
+                # There are no stats for a table that is empty or hasn't been
+                # analyzed yet, and Postgres leaves `most_common_vals` NULL when
+                # no value is more common than the others, e.g. if every
+                # `room_id` in the table is NULL.
+                if row is None or row[0] is None:
+                    continue
+
+                vals, freqs = row
                 for room_id, freq in zip(vals, freqs):
                     room_estimates[room_id] += int(freq * table_size)
 

tests/rest/admin/test_statistics.py

@@ -25,13 +25,15 @@ from twisted.web.resource import Resource
 
 import synapse.rest.admin
 from synapse.api.errors import Codes
-from synapse.rest.client import login
+from synapse.rest.client import login, room
 from synapse.server import HomeServer
+from synapse.storage.database import LoggingTransaction
 from synapse.types import JsonDict
 from synapse.util.clock import Clock
 
 from tests import unittest
 from tests.test_utils import SMALL_PNG
+from tests.utils import USE_POSTGRES_FOR_TESTS
 
 
 class UserMediaStatisticsTestCase(unittest.HomeserverTestCase):
@@ -521,3 +523,56 @@ class UserMediaStatisticsTestCase(unittest.HomeserverTestCase):
         returned_order = [row["user_id"] for row in channel.json_body["users"]]
         self.assertListEqual(expected_user_list, returned_order)
         self._check_fields(channel.json_body["users"])
+
+
+class LargestRoomsStatisticsTestCase(unittest.HomeserverTestCase):
+    if not USE_POSTGRES_FOR_TESTS:
+        skip = "Requires Postgres"
+
+    servlets = [
+        synapse.rest.admin.register_servlets,
+        login.register_servlets,
+        room.register_servlets,
+    ]
+
+    def prepare(self, reactor: MemoryReactor, clock: Clock, hs: HomeServer) -> None:
+        self.admin_user = self.register_user("admin", "pass", admin=True)
+        self.admin_user_tok = self.login("admin", "pass")
+
+        self.url = "/_synapse/admin/v1/statistics/database/rooms"
+
+    def test_tables_without_common_room_ids(self) -> None:
+        """
+        Tables without stats in `pg_stats`, or whose `room_id` column is entirely
+        NULL, have no most common values and must not break the estimate.
+        """
+        room_id = self.helper.create_room_as(self.admin_user, tok=self.admin_user_tok)
+
+        # Nothing has been analyzed yet, so there is nothing to estimate from.
+        channel = self.make_request(
+            "GET",
+            self.url,
+            access_token=self.admin_user_tok,
+        )
+        self.assertEqual(200, channel.code, msg=channel.json_body)
+        self.assertNotIn(room_id, [r["room_id"] for r in channel.json_body["rooms"]])
+
+        for i in range(5):
+            self.helper.send(room_id, f"message {i}", tok=self.admin_user_tok)
+
+        # `event_push_actions` is still empty and `event_edges.room_id` is never
+        # written, so neither gets a list of most common values.
+        def analyze_txn(txn: LoggingTransaction) -> None:
+            txn.execute("ANALYZE")
+
+        self.get_success(
+            self.hs.get_datastores().main.db_pool.runInteraction("analyze", analyze_txn)
+        )
+
+        channel = self.make_request(
+            "GET",
+            self.url,
+            access_token=self.admin_user_tok,
+        )
+        self.assertEqual(200, channel.code, msg=channel.json_body)
+        self.assertIn(room_id, [r["room_id"] for r in channel.json_body["rooms"]])