database/sql: convert SQL null values to []byte as nil.

Also allow string values to scan into []byte.
Fixes #2788.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/5577054

src/pkg/database/sql/convert.go

@@ -40,6 +40,9 @@ func convertAssign(dest, src interface{}) error {
 		case *string:
 			*d = s
 			return nil
+		case *[]byte:
+			*d = []byte(s)
+			return nil
 		}
 	case []byte:
 		switch d := dest.(type) {
@@ -50,6 +53,12 @@ func convertAssign(dest, src interface{}) error {
 			*d = s
 			return nil
 		}
+	case nil:
+		switch d := dest.(type) {
+		case *[]byte:
+			*d = nil
+			return nil
+		}
 	}
 
 	var sv reflect.Value

src/pkg/database/sql/sql.go

@@ -904,6 +904,12 @@ func (rs *Rows) Scan(dest ...interface{}) error {
 		if !ok {
 			continue
 		}
+		if *b == nil {
+			// If the []byte is now nil (for a NULL value),
+			// don't fall through to below which would
+			// turn it into a non-nil 0-length byte slice
+			continue
+		}
 		if _, ok = dp.(*RawBytes); ok {
 			continue
 		}
@@ -945,17 +951,10 @@ func (r *Row) Scan(dest ...interface{}) error {
 	if r.err != nil {
 		return r.err
 	}
-	defer r.rows.Close()
-	if !r.rows.Next() {
-		return ErrNoRows
-	}
-	err := r.rows.Scan(dest...)
-	if err != nil {
-		return err
-	}
 
 	// TODO(bradfitz): for now we need to defensively clone all
-	// []byte that the driver returned, since we're about to close
+	// []byte that the driver returned (not permitting 
+	// *RawBytes in Rows.Scan), since we're about to close
 	// the Rows in our defer, when we return from this function.
 	// the contract with the driver.Next(...) interface is that it
 	// can return slices into read-only temporary memory that's
@@ -970,14 +969,17 @@ func (r *Row) Scan(dest ...interface{}) error {
 		if _, ok := dp.(*RawBytes); ok {
 			return errors.New("sql: RawBytes isn't allowed on Row.Scan")
 		}
-		b, ok := dp.(*[]byte)
-		if !ok {
-			continue
-		}
-		clone := make([]byte, len(*b))
-		copy(clone, *b)
-		*b = clone
 	}
+
+	defer r.rows.Close()
+	if !r.rows.Next() {
+		return ErrNoRows
+	}
+	err := r.rows.Scan(dest...)
+	if err != nil {
+		return err
+	}
+
 	return nil
 }
 

src/pkg/database/sql/sql_test.go

@@ -358,6 +358,34 @@ func TestIssue2542Deadlock(t *testing.T) {
 	}
 }
 
+// Tests fix for issue 2788, that we bind nil to a []byte if the
+// value in the column is sql null
+func TestNullByteSlice(t *testing.T) {
+	db := newTestDB(t, "")
+	defer closeDB(t, db)
+	exec(t, db, "CREATE|t|id=int32,name=nullstring")
+	exec(t, db, "INSERT|t|id=10,name=?", nil)
+
+	var name []byte
+
+	err := db.QueryRow("SELECT|t|name|id=?", 10).Scan(&name)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if name != nil {
+		t.Fatalf("name []byte should be nil for null column value, got: %#v", name)
+	}
+
+	exec(t, db, "INSERT|t|id=11,name=?", "bob")
+	err = db.QueryRow("SELECT|t|name|id=?", 11).Scan(&name)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(name) != "bob" {
+		t.Fatalf("name []byte should be bob, got: %q", string(name))
+	}
+}
+
 func TestQueryRowClosingStmt(t *testing.T) {
 	db := newTestDB(t, "people")
 	defer closeDB(t, db)