database/sql: Check errors in QueryRow.Scan

The previous coding did not correctly check for errors from the driver's
Next() or Close(), which could mask genuine errors from the database, as
witnessed in issue #6651.

Even after this change errors from Close() will be ignored if the query
returned no rows (as Rows.Next will have closed the handle already), but it
is a lot easier for the drivers to guard against that.

Fixes #6651.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/41590043

src/pkg/database/sql/sql.go

@@ -1495,10 +1495,12 @@ type Rows struct {
 	closeStmt driver.Stmt // if non-nil, statement to Close on close
 }
 
-// Next prepares the next result row for reading with the Scan method.
-// It returns true on success, false if there is no next result row.
-// Every call to Scan, even the first one, must be preceded by a call
-// to Next.
+// Next prepares the next result row for reading with the Scan method.  It
+// returns true on success, or false if there is no next result row or an error
+// happened while preparing it.  Err should be consulted to distinguish between
+// the two cases.
+//
+// Every call to Scan, even the first one, must be preceded by a call to Next.
 func (rs *Rows) Next() bool {
 	if rs.closed {
 		return false
@@ -1625,12 +1627,19 @@ func (r *Row) Scan(dest ...interface{}) error {
 	}
 
 	if !r.rows.Next() {
+		if err := r.rows.Err(); err != nil {
+			return err
+		}
 		return ErrNoRows
 	}
 	err := r.rows.Scan(dest...)
 	if err != nil {
 		return err
 	}
+	// Make sure the query can be processed to completion with no errors.
+	if err := r.rows.Close(); err != nil {
+		return err
+	}
 
 	return nil
 }