Ability to return an iterator on rows

[viewsharp]

Example of generated query:

const iterCities = `-- name: IterCities :iter
SELECT slug, name
FROM city
ORDER BY name
`

func (q *Queries) IterCities(ctx context.Context) IterCitiesRows {
	rows, err := q.query(ctx, q.iterCitiesStmt, iterCities)
	if err != nil {
		return IterCitiesRows{err: err}
	}
	return IterCitiesRows{rows: rows}
}

type IterCitiesRows struct {
	rows *sql.Rows
	err  error
}

func (r *IterCitiesRows) Iterate() iter.Seq[City] {
	if r.rows == nil {
		return func(yield func(City) bool) {}
	}

	return func(yield func(City) bool) {
		for r.rows.Next() {
			var i City
			err := r.rows.Scan(&i.Slug, &i.Name)
			if err != nil {
				r.err = err
				return
			}

			if !yield(i) {
				if err = r.rows.Close(); err != nil {
					r.err = err
				}
				return
			}
		}
	}
}

func (r *IterCitiesRows) Close() error {
	return r.rows.Close()
}

func (r *IterCitiesRows) Err() error {
	if r.err != nil {
		return r.err
	}
	return r.rows.Err()
}

Example of use:

func example(ctx context.Context, q *Queries) {
	rows := q.IterCities(ctx)
	for city := range rows.Iterate() {
		// do something with city
	}
	if err := rows.Err(); err != nil {
		// handle error
	}
}

[kyleconroy]

Thank you so much for the first pass on iterator support. Could you update your PR description with an example of how to use the Iterate() method? I looked over the changes and didn't see an end-to-end test of the new method.

[kyleconroy]

Fixes #720

[viewsharp]

@kyleconroy
I've placed end-to-end tests here https://github.com/sqlc-dev/sqlc/pull/3631/files#diff-ef71fe84cdfc32f51a1cf70a80a17506a422b5ccc42e714a56a6a2334e6a4bc5
Do you need some other kind of tests? Please give an example

[viewsharp]

@kyleconroy
What do you think about the Close method? I still haven't decided if it's necessary.

[viewsharp]

I found some example tests here https://github.com/sqlc-dev/sqlc/tree/main/examples and added iterator tests there

[viewsharp]

@kyleconroy, hi
I updated my pull request, check it please