Sometimes in CI insert_select_connection_leak would fail like this:
```diff
END;
SELECT worker_connection_count(:worker_1_port) - :pre_xact_worker_1_connections AS leaked_worker_1_connections,
worker_connection_count(:worker_2_port) - :pre_xact_worker_2_connections AS leaked_worker_2_connections;
leaked_worker_1_connections | leaked_worker_2_connections
-----------------------------+-----------------------------
- 0 | 0
+ -1 | 0
(1 row)
-- ROLLBACK
BEGIN;
INSERT INTO target_table SELECT * FROM source_table;
INSERT INTO target_table SELECT * FROM source_table;
ROLLBACK;
SELECT worker_connection_count(:worker_1_port) - :pre_xact_worker_1_connections AS leaked_worker_1_connections,
worker_connection_count(:worker_2_port) - :pre_xact_worker_2_connections AS leaked_worker_2_connections;
leaked_worker_1_connections | leaked_worker_2_connections
-----------------------------+-----------------------------
- 0 | 0
+ -1 | 0
(1 row)
\set VERBOSITY TERSE
-- Error on constraint failure
BEGIN;
INSERT INTO target_table SELECT * FROM source_table;
SELECT worker_connection_count(:worker_1_port) AS worker_1_connections,
worker_connection_count(:worker_2_port) AS worker_2_connections \gset
SAVEPOINT s1;
INSERT INTO target_table SELECT a, CASE WHEN a < 50 THEN b ELSE null END FROM source_table;
@@ -89,15 +89,15 @@
leaked_worker_1_connections | leaked_worker_2_connections
-----------------------------+-----------------------------
0 | 0
(1 row)
END;
SELECT worker_connection_count(:worker_1_port) - :pre_xact_worker_1_connections AS leaked_worker_1_connections,
worker_connection_count(:worker_2_port) - :pre_xact_worker_2_connections AS leaked_worker_2_connections;
leaked_worker_1_connections | leaked_worker_2_connections
-----------------------------+-----------------------------
- 0 | 0
+ -1 | 0
(1 row)
```
Source:
https://github.com/citusdata/citus/actions/runs/6718401194/attempts/1#summary-18258258387
A negative amount of leaked connectios is obviously not possible. For
some reason there was a connection open when we checked the initial
amount of connections that was closed afterwards. This could be the
from the maintenance daemon or maybe from the previous test that had not
fully closed its connections just yet.
The change in this PR doesnt't actually fix the cause of the negative
connection, but it simply considers it good as well, by changing the
result to zero for negative values.
With this fix we might sometimes miss a leak, because the negative
number can cancel out the leak and still result in a 0. But since the
negative number only occurs sometimes, we'll still find the leak often
enough.
Jelte Fennema-Nio
Ahmet Gedemenli
Hadi Moshayedi