ErrorResponse ignored during Sync message in `.execute()`

[maddyblue]

fn main() -> Result<(), postgres::Error> {
    use postgres::{Client, NoTls};
    let mut client = Client::connect("host=localhost user=postgres", NoTls)?;

    let _ = client.batch_execute("drop table t");
    client.batch_execute("create table t (i int, unique (i) DEFERRABLE INITIALLY DEFERRED);")?;
    client.execute("insert into t values (1);", &[])?;
    client.execute("insert into t values (1);", &[])?;
    client.execute("insert into t values (1);", &[])?;

    println!("rows: {}", client.query("SELECT * FROM t", &[])?.len());

    Ok(())
}

Here we expect 3 rows to be present because the three execute calls succeed, however only one row is actually present. This happens because the ErrorResponse message is produced only after postgres attempts to commit the transaction in response to the Sync message. Using deferred constraints causes postgres to delay constraint checking until transaction commit time.

See https://github.com/MaterializeInc/database-issues/issues/2673

[sfackler]

Fix is in MaterializeInc/materialize#833 - I'm pretty surprised we've made it this far without anyone running into this issue!