fix: reloading the Schema Cache unnecessarily on a PGRST002 error

When 503 errors happen if the Schema Cache is empty,
it should not retrigger the connection worker since
there's no Schema Cache loaded yet.

test/io/test_io.py

@@ -1892,6 +1892,28 @@ def test_log_error_when_empty_schema_cache_on_startup_to_stderr(defaultenv):
         assert any(log_err_message in line for line in output_start)
 
 
+def test_no_double_schema_cache_reload_on_empty_schema(defaultenv):
+    "Should only load the schema cache once on a 503 error when there's an empty schema cache on startup"
+
+    env = {
+        **defaultenv,
+        "PGRST_INTERNAL_SCHEMA_CACHE_QUERY_SLEEP": "300",
+    }
+
+    with run(env=env, port=freeport(), wait_for_readiness=False) as postgrest:
+        postgrest.wait_until_scache_starts_loading()
+
+        response = postgrest.session.get("/projects")
+        assert response.status_code == 503
+
+        # Should wait enough time to load the schema cache twice to guarantee that the test is valid
+        time.sleep(1)
+
+        response = postgrest.admin.get("/metrics")
+        assert response.status_code == 200
+        assert 'pgrst_schema_cache_loads_total{status="SUCCESS"} 1.0' in response.text
+
+
 @pytest.mark.parametrize("level", ["crit", "error", "warn", "info", "debug"])
 def test_log_pool_req_observation(level, defaultenv):
     "PostgREST should log PoolRequest and PoolRequestFullfilled observation when log-level=debug"