Fix connection_ping() to check transaction_status before SELECT 1 and return 503 CONNECTION_LOST

- Check transaction_status before executing ping query to avoid disrupting
  long-running queries or active transactions (addresses review feedback).
  Moved the check inside try-except to handle race conditions where the
  connection drops between connected() and transaction_status access.
- Return 503 service_unavailable with CONNECTION_LOST info instead of 500
  internal_server_error when reconnect fails in children() endpoint, so the
  tree_nodes.ts reconnect dialog is properly triggered.

Author: ZoroXL

web/pgadmin/browser/utils.py

@@ -18,7 +18,7 @@
 
 from config import PG_DEFAULT_DRIVER
 from pgadmin.utils.ajax import make_json_response, precondition_required,\
-    internal_server_error
+    internal_server_error, service_unavailable
 from pgadmin.utils.exception import ConnectionLost, SSHTunnelConnectionLost,\
     CryptKeyMissing
 from pgadmin.utils.constants import DATABASE_LAST_SYSTEM_OID
@@ -441,7 +441,9 @@ def children(self, **kwargs):
             if not conn.connection_ping():
                 status, msg = conn.connect()
                 if not status:
-                    return internal_server_error(errormsg=msg)
+                    return service_unavailable(
+                        msg, info="CONNECTION_LOST"
+                    )
         except (ConnectionLost, SSHTunnelConnectionLost, CryptKeyMissing):
             raise
         except Exception:

web/pgadmin/utils/driver/psycopg3/connection.py

@@ -1425,7 +1425,22 @@ def connection_ping(self):
         """
         if not self.connected():
             return False
+
         try:
+            # Check the transaction status before executing the ping
+            # query.  If a query is already in progress (ACTIVE) or we
+            # are inside a transaction block (INTRANS / INERROR), running
+            # SELECT 1 would fail or disrupt the ongoing operation.  In
+            # those states the connection is evidently alive, so just
+            # return True.
+            txn_status = self.conn.info.transaction_status
+            if txn_status != 0:
+                # 0 = IDLE — safe to send a query
+                # 1 = ACTIVE — command in progress, connection is alive
+                # 2 = INTRANS — in transaction block, connection is alive
+                # 3 = INERROR — in failed transaction, connection is alive
+                return True
+
             cur = self.conn.cursor()
             cur.execute("SELECT 1")
             cur.close()