add tests for api/trace/current.go (#82)

* add tests for api/trace/current.go

* add interface validaion for type: mockSpan

* rework for exported NoopSpan and ditch useless tabletest for TestSetCurrentSpan()

api/trace/current_test.go

@@ -0,0 +1,110 @@
+package trace_test
+
+import (
+	"context"
+	"testing"
+
+	"google.golang.org/grpc/codes"
+
+	"go.opentelemetry.io/api/core"
+	"go.opentelemetry.io/api/event"
+	"go.opentelemetry.io/api/tag"
+	"go.opentelemetry.io/api/trace"
+)
+
+func TestSetCurrentSpanOverridesPreviouslySetSpan(t *testing.T) {
+	originalSpan := trace.NoopSpan{}
+	expectedSpan := mockSpan{}
+
+	ctx := context.Background()
+
+	ctx = trace.SetCurrentSpan(ctx, originalSpan)
+	ctx = trace.SetCurrentSpan(ctx, expectedSpan)
+
+	if span := trace.CurrentSpan(ctx); span != expectedSpan {
+		t.Errorf("Want: %v, but have: %v", expectedSpan, span)
+	}
+}
+
+func TestCurrentSpan(t *testing.T) {
+	for _, testcase := range []struct {
+		name string
+		ctx  context.Context
+		want trace.Span
+	}{
+		{
+			name: "CurrentSpan() returns a NoopSpan{} from an empty context",
+			ctx:  context.Background(),
+			want: trace.NoopSpan{},
+		},
+		{
+			name: "CurrentSpan() returns current span if set",
+			ctx:  trace.SetCurrentSpan(context.Background(), mockSpan{}),
+			want: mockSpan{},
+		},
+	} {
+		t.Run(testcase.name, func(t *testing.T) {
+			// proto: CurrentSpan(ctx context.Context) trace.Span
+			have := trace.CurrentSpan(testcase.ctx)
+			if have != testcase.want {
+				t.Errorf("Want: %v, but have: %v", testcase.want, have)
+			}
+		})
+	}
+}
+
+// a duplicate of trace.NoopSpan for testing
+type mockSpan struct{}
+
+var _ trace.Span = mockSpan{}
+
+// SpanContext returns an invalid span context.
+func (mockSpan) SpanContext() core.SpanContext {
+	return core.EmptySpanContext()
+}
+
+// IsRecordingEvents always returns false for mockSpan.
+func (mockSpan) IsRecordingEvents() bool {
+	return false
+}
+
+// SetStatus does nothing.
+func (mockSpan) SetStatus(status codes.Code) {
+}
+
+// SetError does nothing.
+func (mockSpan) SetError(v bool) {
+}
+
+// SetAttribute does nothing.
+func (mockSpan) SetAttribute(attribute core.KeyValue) {
+}
+
+// SetAttributes does nothing.
+func (mockSpan) SetAttributes(attributes ...core.KeyValue) {
+}
+
+// ModifyAttribute does nothing.
+func (mockSpan) ModifyAttribute(mutator tag.Mutator) {
+}
+
+// ModifyAttributes does nothing.
+func (mockSpan) ModifyAttributes(mutators ...tag.Mutator) {
+}
+
+// Finish does nothing.
+func (mockSpan) Finish() {
+}
+
+// Tracer returns noop implementation of Tracer.
+func (mockSpan) Tracer() trace.Tracer {
+	return trace.NoopTracer{}
+}
+
+// AddEvent does nothing.
+func (mockSpan) AddEvent(ctx context.Context, event event.Event) {
+}
+
+// Event does nothing.
+func (mockSpan) Event(ctx context.Context, msg string, attrs ...core.KeyValue) {
+}