within-callback.go

package main

import (
	"database/sql"
	"fmt"
	"io"
	"log"
	"os"
	"strings"
	"time"

	"github.com/ncruces/go-sqlite3"
	"github.com/ncruces/go-sqlite3/driver"
	_ "github.com/ncruces/go-sqlite3/embed"
	"github.com/ncruces/go-sqlite3/ext/blobio"
)

type DB struct {
	db *sql.DB
}

func New(dbName string) (*DB, error) {
	db, err := driver.Open(dbName, blobio.Register)
	if err != nil {
		return nil, err
	}

	_, err = db.Exec(`CREATE TABLE IF NOT EXISTS entries (id TEXT PRIMARY KEY, contents)`)
	if err != nil {
		db.Close()
		return nil, err
	}

	return &DB{db: db}, nil
}

func (d *DB) Close() error {
	return d.db.Close()
}

func (d *DB) WriteEntry(message string) (string, error) {
	reader := strings.NewReader(message)
	id := fmt.Sprintf("id:%s", time.Now().Format(time.RFC3339))

	_, err := d.db.Exec(`INSERT INTO entries VALUES (:id, :data)`,
		sql.Named("id", id),
		sql.Named("data", sqlite3.ZeroBlob(len(message))))
	if err != nil {
		return "", err
	}

	_, err = d.db.Exec(`SELECT writeblob('main', 'entries', 'contents', last_insert_rowid(), :offset, :data)`,
		sql.Named("offset", 0),
		sql.Named("data", sqlite3.Pointer(reader)))
	if err != nil {
		return "", err
	}
	return id, nil
}

func (d *DB) ReadEntry(id string) error {
	_, err := d.db.Exec(`
			SELECT openblob('main', 'entries', 'contents', (SELECT rowid FROM entries WHERE id = :id), :writeMode, :callback)
	`,
		sql.Named("id", id),
		sql.Named("writeMode", false),
		sql.Named("callback",
			sqlite3.Pointer[blobio.OpenCallback](func(b *sqlite3.Blob, _ ...sqlite3.Value) error {
				log.Printf("start callback")
				if _, err := io.Copy(os.Stdout, b); err != nil {
					panic(err)
				}
				fmt.Fprint(os.Stdout, "\n")
				log.Printf("end callback")
				return nil
			})))
	if err != nil {
		return fmt.Errorf("error opening blob for id %s: %w", id, err)
	}

	return nil
}

func main() {
	db, err := New("test.db")
	if err != nil {
		panic(err)
	}
	defer db.Close()

	message := strings.Repeat("A", 50)
	id, err := db.WriteEntry(message)
	if err != nil {
		panic(err)
	}

	err = db.ReadEntry(id)
	if err != nil {
		panic(err)
	}
}