promql: fix aggregation expression String() method.

Fixes #794.

promql/printer.go

@@ -178,7 +178,11 @@ func (es Expressions) String() (s string) {
 func (node *AggregateExpr) String() string {
 	aggrString := fmt.Sprintf("%s(%s)", node.Op, node.Expr)
 	if len(node.Grouping) > 0 {
-		return fmt.Sprintf("%s BY (%s)", aggrString, node.Grouping)
+		format := "%s BY (%s)"
+		if node.KeepExtraLabels {
+			format += " KEEPING_EXTRA"
+		}
+		return fmt.Sprintf(format, aggrString, node.Grouping)
 	}
 	return aggrString
 }

promql/printer_test.go

@@ -0,0 +1,48 @@
+// Copyright 2015 The Prometheus Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package promql
+
+import (
+	"testing"
+)
+
+func TestExprString(t *testing.T) {
+	// A list of valid expressions that are expected to be
+	// returned as out when calling String(). If out is empty the output
+	// is expected to equal the input.
+	inputs := []struct {
+		in, out string
+	}{
+		{
+			in: `sum(task:errors:rate10s{job="s"}) BY (code)`,
+		},
+		{
+			in: `sum(task:errors:rate10s{job="s"}) BY (code) KEEPING_EXTRA`,
+		},
+	}
+
+	for _, test := range inputs {
+		expr, err := ParseExpr(test.in)
+		if err != nil {
+			t.Fatalf("parsing error for %q: %s", test.in, err)
+		}
+		exp := test.in
+		if test.out != "" {
+			exp = test.out
+		}
+		if expr.String() != exp {
+			t.Fatalf("expected %q to be returned as:\n%s\ngot:\n%s\n", test.in, exp, expr.String())
+		}
+	}
+}