database/sql: prepared statements leak connections with high concurrency #5323
Description
When using database/sql with prepared statements and concurrency higher that MaxIdleConns connections are leaked and recreated until database cannot accept anymore. See https://groups.google.com/d/msg/golang-nuts/-YbB2Qjg41g/Afuaf8nSmGEJ for a related discussion. The problem is that all connections that have ever been used by Stmt are marked as dependent on that Stmt, and that dependency is only removed when Stmt is closed. When connection, that was over MaxIdleConns limit, is returned, it is not actually closed, because it is still used by Stmt. Later, when Stmt tries to find a free connection, is sees that connection is not free and doesn't use it. This causes database/sql to create driver connections over and over again, until underlying database refuses and replies with errors. I think dependency should only be added in Stmt.connStmt and removed in releaseConn callbacks, because prepared statements don't really appear to "use" the connection, merely "cache" it. On the other hand such solution would mean that drivers's Stmt.Close is never called.