database/sql: add method Err on sql.Row

The Row.Err method is intended to assist wrapping sql.DB.
Because sql.Row is a struct with private fields,
a wrapper in an existing code base cannot easily provide users
with a different implementation without large rewrites.
Adding this method allows query level errors to be handled
centrally.

Fixes #35804

Change-Id: I94e6329de89a7ee1284ce9ef76af4363d2d081f9
Reviewed-on: https://go-review.googlesource.com/c/go/+/214317
Reviewed-by: Daniel Theophanes <kardianos@gmail.com>
Run-TryBot: Daniel Theophanes <kardianos@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/database/sql/sql.go

@@ -3174,6 +3174,14 @@ func (r *Row) Scan(dest ...interface{}) error {
 	return r.rows.Close()
 }
 
+// Err provides a way for wrapping packages to check for
+// query errors without calling Scan.
+// Err returns the error, if any, that was encountered while running the query.
+// If this error is not nil, this error will also be returned from Scan.
+func (r *Row) Err() error {
+	return r.err
+}
+
 // A Result summarizes an executed SQL command.
 type Result interface {
 	// LastInsertId returns the integer generated by the database

src/database/sql/sql_test.go

@@ -788,6 +788,24 @@ func TestQueryRow(t *testing.T) {
 	}
 }
 
+func TestRowErr(t *testing.T) {
+	db := newTestDB(t, "people")
+
+	err := db.QueryRowContext(context.Background(), "SELECT|people|bdate|age=?", 3).Err()
+	if err != nil {
+		t.Errorf("Unexpected err = %v; want %v", err, nil)
+	}
+
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel()
+
+	err = db.QueryRowContext(ctx, "SELECT|people|bdate|age=?", 3).Err()
+	exp := "context canceled"
+	if err == nil || !strings.Contains(err.Error(), exp) {
+		t.Errorf("Expected err = %v; got %v", exp, err)
+	}
+}
+
 func TestTxRollbackCommitErr(t *testing.T) {
 	db := newTestDB(t, "people")
 	defer closeDB(t, db)