Provide conformance tests for tracers (#191)

* Provide conformance tests for tracers

The test harness may be used to ensure that a given tracer behaves
according to the expectations set by the API.

* Add `ToMatchError` matcher

* Use DeepEqual to compare unknown types in matchers

Unlike basic `==`/`!=`, `reflect.DeepEqual` can compare arbitrary
types (e.g., `[]string` to `[]string`)

* Use struct instead of string for test context key

internal/matchers/expectation.go

@@ -0,0 +1,114 @@
+// Copyright 2019, OpenTelemetry 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 matchers
+
+import (
+	"reflect"
+	"testing"
+)
+
+type Expectation struct {
+	t      *testing.T
+	actual interface{}
+}
+
+func (e *Expectation) ToEqual(expected interface{}) {
+	e.verifyExpectedNotNil(expected)
+
+	if !reflect.DeepEqual(e.actual, expected) {
+		e.t.Fatalf("Expected\n\t%v\nto equal\n\t%v\n", e.actual, expected)
+	}
+}
+
+func (e *Expectation) NotToEqual(expected interface{}) {
+	e.verifyExpectedNotNil(expected)
+
+	if reflect.DeepEqual(e.actual, expected) {
+		e.t.Fatalf("Expected\n\t%v\nnot to equal\n\t%v\n", e.actual, expected)
+	}
+}
+
+func (e *Expectation) ToBeNil() {
+	if e.actual != nil {
+		e.t.Fatalf("Expected\n\t%v\nto be nil\n", e.actual)
+	}
+}
+
+func (e *Expectation) NotToBeNil() {
+	if e.actual == nil {
+		e.t.Fatalf("Expected\n\t%v\nnot to be nil\n", e.actual)
+	}
+}
+
+func (e *Expectation) ToBeTrue() {
+	switch a := e.actual.(type) {
+	case bool:
+		if e.actual == false {
+			e.t.Fatalf("Expected\n\t%v\nto be true\n", e.actual)
+		}
+	default:
+		e.t.Fatalf("Cannot check if non-bool value\n\t%v\nis truthy\n", a)
+	}
+}
+
+func (e *Expectation) ToBeFalse() {
+	switch a := e.actual.(type) {
+	case bool:
+		if e.actual == true {
+			e.t.Fatalf("Expected\n\t%v\nto be false\n", e.actual)
+		}
+	default:
+		e.t.Fatalf("Cannot check if non-bool value\n\t%v\nis truthy\n", a)
+	}
+}
+
+func (e *Expectation) ToSucceed() {
+	switch actual := e.actual.(type) {
+	case error:
+		if actual != nil {
+			e.t.Fatalf("Expected error\n\t%v\nto have succeeded\n", actual)
+		}
+	default:
+		e.t.Fatalf("Cannot check if non-error value\n\t%v\nsucceeded\n", actual)
+	}
+}
+
+func (e *Expectation) ToMatchError(expected interface{}) {
+	e.verifyExpectedNotNil(expected)
+
+	actual, ok := e.actual.(error)
+	if !ok {
+		e.t.Fatalf("Cannot check if non-error value\n\t%v\nmatches error\n", e.actual)
+	}
+
+	switch expected := expected.(type) {
+	case error:
+		if !reflect.DeepEqual(actual, expected) {
+			e.t.Fatalf("Expected\n\t%v\nto match error\n\t%v\n", actual, expected)
+		}
+	case string:
+		if actual.Error() != expected {
+			e.t.Fatalf("Expected\n\t%v\nto match error\n\t%v\n", actual, expected)
+		}
+	default:
+		e.t.Fatalf("Cannot match\n\t%v\nagainst non-error\n\t%v\n", actual, expected)
+	}
+}
+
+func (e *Expectation) verifyExpectedNotNil(expected interface{}) {
+	if expected == nil {
+		e.t.Fatal("Refusing to compare with <nil>. Use `ToBeNil` or `NotToBeNil` instead.")
+	}
+}

internal/matchers/expecter.go

@@ -0,0 +1,36 @@
+// Copyright 2019, OpenTelemetry 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 matchers
+
+import (
+	"testing"
+)
+
+type Expecter struct {
+	t *testing.T
+}
+
+func NewExpecter(t *testing.T) *Expecter {
+	return &Expecter{
+		t: t,
+	}
+}
+
+func (a *Expecter) Expect(actual interface{}) *Expectation {
+	return &Expectation{
+		t:      a.t,
+		actual: actual,
+	}
+}

internal/matchers/package.go

@@ -0,0 +1,15 @@
+// Copyright 2019, OpenTelemetry 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 matchers // import "go.opentelemetry.io/internal/matchers"