database/sql: allow drivers to override Scan behavior

Implementing RowsColumnScanner allows the driver
to completely control how values are scanned.

Fixes #67546

Change-Id: Id8e7c3a973479c9665e4476fe2d29e1255aee687
GitHub-Last-Rev: ed0cacaec4
GitHub-Pull-Request: golang/go#67648
Reviewed-on: https://go-review.googlesource.com/c/go/+/588435
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Sean Liao <sean@liao.dev>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

api/next/67546.txt

@@ -0,0 +1,5 @@
+pkg database/sql/driver, type RowsColumnScanner interface { Close, Columns, Next, ScanColumn } #67546
+pkg database/sql/driver, type RowsColumnScanner interface, Close() error #67546
+pkg database/sql/driver, type RowsColumnScanner interface, Columns() []string #67546
+pkg database/sql/driver, type RowsColumnScanner interface, Next([]Value) error #67546
+pkg database/sql/driver, type RowsColumnScanner interface, ScanColumn(interface{}, int) error #67546

doc/next/6-stdlib/99-minor/database/sql/driver/67546.md

@@ -0,0 +1 @@
+A database driver may implement [RowsColumnScanner] to entirely override `Scan` behavior.

src/database/sql/driver/driver.go

@@ -515,6 +515,18 @@ type RowsColumnTypePrecisionScale interface {
 	ColumnTypePrecisionScale(index int) (precision, scale int64, ok bool)
 }
 
+// RowsColumnScanner may be implemented by [Rows]. It allows the driver to completely
+// take responsibility for how values are scanned and replace the normal [database/sql].
+// scanning path. This allows drivers to directly support types that do not implement
+// [database/sql.Scanner].
+type RowsColumnScanner interface {
+	Rows
+
+	// ScanColumn copies the column in the current row into the value pointed at by
+	// dest. It returns [ErrSkip] to fall back to the normal [database/sql] scanning path.
+	ScanColumn(dest any, index int) error
+}
+
 // Tx is a transaction.
 type Tx interface {
 	Commit() error

src/database/sql/sql.go

@@ -3396,7 +3396,16 @@ func (rs *Rows) scanLocked(dest ...any) error {
 	}
 
 	for i, sv := range rs.lastcols {
-		err := convertAssignRows(dest[i], sv, rs)
+		err := driver.ErrSkip
+
+		if rcs, ok := rs.rowsi.(driver.RowsColumnScanner); ok {
+			err = rcs.ScanColumn(dest[i], i)
+		}
+
+		if err == driver.ErrSkip {
+			err = convertAssignRows(dest[i], sv, rs)
+		}
+
 		if err != nil {
 			return fmt.Errorf(`sql: Scan error on column index %d, name %q: %w`, i, rs.rowsi.Columns()[i], err)
 		}

src/database/sql/sql_test.go

@@ -4201,6 +4201,102 @@ func TestNamedValueCheckerSkip(t *testing.T) {
 	}
 }
 
+type rcsDriver struct {
+	fakeDriver
+}
+
+func (d *rcsDriver) Open(dsn string) (driver.Conn, error) {
+	c, err := d.fakeDriver.Open(dsn)
+	fc := c.(*fakeConn)
+	fc.db.allowAny = true
+	return &rcsConn{fc}, err
+}
+
+type rcsConn struct {
+	*fakeConn
+}
+
+func (c *rcsConn) PrepareContext(ctx context.Context, q string) (driver.Stmt, error) {
+	stmt, err := c.fakeConn.PrepareContext(ctx, q)
+	if err != nil {
+		return stmt, err
+	}
+	return &rcsStmt{stmt.(*fakeStmt)}, nil
+}
+
+type rcsStmt struct {
+	*fakeStmt
+}
+
+func (s *rcsStmt) QueryContext(ctx context.Context, args []driver.NamedValue) (driver.Rows, error) {
+	rows, err := s.fakeStmt.QueryContext(ctx, args)
+	if err != nil {
+		return rows, err
+	}
+	return &rcsRows{rows.(*rowsCursor)}, nil
+}
+
+type rcsRows struct {
+	*rowsCursor
+}
+
+func (r *rcsRows) ScanColumn(dest any, index int) error {
+	switch d := dest.(type) {
+	case *int64:
+		*d = 42
+		return nil
+	}
+
+	return driver.ErrSkip
+}
+
+func TestRowsColumnScanner(t *testing.T) {
+	Register("RowsColumnScanner", &rcsDriver{})
+	db, err := Open("RowsColumnScanner", "")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer db.Close()
+
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	_, err = db.ExecContext(ctx, "CREATE|t|str=string,n=int64")
+	if err != nil {
+		t.Fatal("exec create", err)
+	}
+
+	_, err = db.ExecContext(ctx, "INSERT|t|str=?,n=?", "foo", int64(1))
+	if err != nil {
+		t.Fatal("exec insert", err)
+	}
+	var (
+		str string
+		i64 int64
+		i   int
+		f64 float64
+		ui  uint
+	)
+	err = db.QueryRowContext(ctx, "SELECT|t|str,n,n,n,n|").Scan(&str, &i64, &i, &f64, &ui)
+	if err != nil {
+		t.Fatal("select", err)
+	}
+
+	list := []struct{ got, want any }{
+		{str, "foo"},
+		{i64, int64(42)},
+		{i, int(1)},
+		{f64, float64(1)},
+		{ui, uint(1)},
+	}
+
+	for index, item := range list {
+		if !reflect.DeepEqual(item.got, item.want) {
+			t.Errorf("got %#v wanted %#v for index %d", item.got, item.want, index)
+		}
+	}
+}
+
 func TestOpenConnector(t *testing.T) {
 	Register("testctx", &fakeDriverCtx{})
 	db, err := Open("testctx", "people")