feat: add dedicated method to extract error from errRow

[aerialls]

Add a dedicated method to extract error from errRow. This is already present for errRows. This allows the use case to extract the error to retry if necessary.

type errRow interface {
    pgx.Row
    Err() error
}

func (r *RetriableDBTX) QueryRow(ctx context.Context, sql string, args ...any) pgx.Row {
    row := r.dbtx.QueryRow(ctx, sql, args...)
    if er, ok := row.(errRow); ok {
        // Custom logic to retry
    }

    return row
}

[jackc]

I'm not sure I understand the use case. In your example I would expect you to call Scan and handle the returned error.

The reason errRows has an Err() method is to satisfy the Rows interface. And the reason errRow doesn't is because the Row interface doesn't.

[aerialls]

I'm using sqlc and trying to add a global retry mechanism for a read-only connection. sqlc is using the following interface as an entrypoint.

type DBTX interface {
    Exec(context.Context, string, ...interface{}) (pgconn.CommandTag, error)
    Query(context.Context, string, ...interface{}) (pgx.Rows, error)
    QueryRow(context.Context, string, ...interface{}) pgx.Row
}

My initial idea was to create a RetriableDBTX with the same interface as an entry-point for sqlc and to retry the method in case of an error. It's working for Exec and Query but I don't have any error for QueryRow and so my idea to be able to retrieve the error if we have an Err() error method. The Scan method is executed afterward by sqlc for each SQL query so it's much harder to perform the retry logic here.

[jackc]

This change won't do what you want. And what you are doing with Query() isn't reliable either.

Query() will only return an error in limited circumstances such as a network failure sending the request. The returned Rows has to be closed and Err() checked to know if it successfully ran.

QueryRow() is a very simple wrapper around Query(). The result isn't actually read until Scan() is called.