Introduce SortReferences adjuster

As demonstrated in https://github.com/jaegertracing/jaeger-ui/issues/966,
jaeger-ui expects the first reference to be of `CHILD_OF` type to properly
render the tree. This new adjuster achieves exactly that.

cmd/query/app/querysvc/adjusters.go

@@ -30,5 +30,6 @@ func StandardAdjusters(maxClockSkewAdjust time.Duration) []adjuster.Adjuster {
 		adjuster.IPTagAdjuster(),
 		adjuster.SortLogFields(),
 		adjuster.SpanReferences(),
+		adjuster.SortReferences(),
 	}
 }

model/adjuster/sort_references.go

@@ -0,0 +1,38 @@
+// Copyright (c) 2019 The Jaeger Authors.
+// Copyright (c) 2017 Uber Technologies, Inc.
+//
+// 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 adjuster
+
+import (
+	"sort"
+
+	"github.com/jaegertracing/jaeger/model"
+)
+
+// SortReferences returns an Adjuster that puts CHILD_OF references first.
+// This is necessary to match jaeger-ui expectations:
+// * https://github.com/jaegertracing/jaeger-ui/issues/966
+func SortReferences() Adjuster {
+	return Func(func(trace *model.Trace) (*model.Trace, error) {
+		for _, span := range trace.Spans {
+			sort.SliceStable(span.References, func(i, j int) bool {
+				iType := span.References[i].GetRefType()
+				jType := span.References[j].GetRefType()
+				return iType == model.SpanRefType_CHILD_OF && jType == model.SpanRefType_FOLLOWS_FROM
+			})
+		}
+		return trace, nil
+	})
+}

model/adjuster/sort_references_test.go

@@ -0,0 +1,90 @@
+// Copyright (c) 2019 The Jaeger Authors.
+// Copyright (c) 2017 Uber Technologies, Inc.
+//
+// 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 adjuster
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+
+	"github.com/jaegertracing/jaeger/model"
+)
+
+func TestSortReferences(t *testing.T) {
+	testCases := []struct {
+		incoming []model.SpanRefType
+		expected []model.SpanRefType
+	}{
+		{
+			incoming: []model.SpanRefType{},
+			expected: []model.SpanRefType{},
+		},
+		{
+			incoming: []model.SpanRefType{model.ChildOf},
+			expected: []model.SpanRefType{model.ChildOf},
+		},
+		{
+			incoming: []model.SpanRefType{model.FollowsFrom},
+			expected: []model.SpanRefType{model.FollowsFrom},
+		},
+		{
+			incoming: []model.SpanRefType{model.ChildOf, model.FollowsFrom},
+			expected: []model.SpanRefType{model.ChildOf, model.FollowsFrom},
+		},
+		{
+			incoming: []model.SpanRefType{model.FollowsFrom, model.ChildOf},
+			expected: []model.SpanRefType{model.ChildOf, model.FollowsFrom},
+		},
+		{
+			incoming: []model.SpanRefType{model.FollowsFrom, model.ChildOf, model.FollowsFrom},
+			expected: []model.SpanRefType{model.ChildOf, model.FollowsFrom, model.FollowsFrom},
+		},
+	}
+	for _, testCase := range testCases {
+		trace := &model.Trace{
+			Spans: []*model.Span{
+				{
+					References: makeReferences(testCase.incoming),
+				},
+			},
+		}
+		trace, err := SortReferences().Adjust(trace)
+		assert.NoError(t, err)
+		assert.Equal(t, testCase.expected, extractReferenceTypes(trace.Spans[0].References))
+	}
+}
+
+func makeReferences(types []model.SpanRefType) []model.SpanRef {
+	references := make([]model.SpanRef, len(types))
+
+	for i, refType := range types {
+		references[i] = model.SpanRef{
+			RefType: refType,
+		}
+	}
+
+	return references
+}
+
+func extractReferenceTypes(references []model.SpanRef) []model.SpanRefType {
+	types := make([]model.SpanRefType, len(references))
+
+	for i, reference := range references {
+		types[i] = reference.RefType
+	}
+
+	return types
+}