Added error return to ConnectHook and fixed extension example

The ConnectHook field of an SQLiteDriver should return an error in
case something bad happened during the hook.

The extension example needs to load the extension in a ConnectHook,
otherwise the extension is only loaded in a single connection in the pool.
By putting the extension loading in the ConnectHook, its called for every
connection that is opened by the sql.DB.

example/extension/extension.go

@@ -8,10 +8,30 @@ import (
 )
 
 func main() {
+	const (
+		use_hook = true
+		load_query = "SELECT load_extension('sqlite3_mod_regexp.dll')"
+	)
+
 	sql.Register("sqlite3_with_extensions",
 		&sqlite3.SQLiteDriver{
 			EnableLoadExtension: true,
-			ConnectHook: nil,
+			ConnectHook: func(c *sqlite3.SQLiteConn) error {
+				if use_hook {
+					stmt, err := c.Prepare(load_query)
+					if err != nil {
+						return err
+					}
+
+					_, err = stmt.Exec(nil)
+					if err != nil {
+						return err
+					}
+
+					return stmt.Close()
+				}
+				return nil
+			},
 		})
 
 	db, err := sql.Open("sqlite3_with_extensions", ":memory:")
@@ -20,11 +40,21 @@ func main() {
 	}
 	defer db.Close()
 
-	_, err = db.Exec("select load_extension('sqlite3_mod_regexp.dll')")
+	if !use_hook {
+		if _, err = db.Exec(load_query); err != nil {
+			log.Fatal(err)
+		}
+	}
+
+	// Force db to make a new connection in pool
+	// by putting the original in a transaction
+	tx, err := db.Begin()
 	if err != nil {
 		log.Fatal(err)
 	}
+	defer tx.Commit()
 
+	// New connection works (hopefully!)
 	rows, err := db.Query("select 'hello world' where 'hello world' regexp '^hello.*d$'")
 	if err != nil {
 		log.Fatal(err)

sqlite3.go

@@ -78,7 +78,7 @@ func init() {
 // Driver struct.
 type SQLiteDriver struct {
 	EnableLoadExtension bool
-	ConnectHook func(*SQLiteConn)
+	ConnectHook         func(*SQLiteConn) error
 }
 
 // Conn struct.
@@ -194,7 +194,9 @@ func (d *SQLiteDriver) Open(dsn string) (driver.Conn, error) {
 	conn := &SQLiteConn{db}
 
 	if d.ConnectHook != nil {
-		d.ConnectHook(conn)
+		if err := d.ConnectHook(conn); err != nil {
+			return nil, err
+		}
 	}
 
 	return conn, nil