delay: permit Context from either stdlib or golang.org/x/net (#95)

Go 1.7 introduced the Context type into the standard library, but it is
assignable to the golang.org/x/net variant.  The appengine libraries are
written in terms of golang.org/x/net, which can interoperate with the
standard library type most of time, but because delay uses reflection,
it depends directly on the exact type.

The test in this change only failed on Go 1.7 and Go 1.8 but not Go
1.9, since after Go 1.9 the golang.org/x/net type became a type alias
for the standard library type.

delay/delay.go

@@ -81,8 +81,7 @@ var (
 	funcs = make(map[string]*Function)
 
 	// precomputed types
-	contextType = reflect.TypeOf((*context.Context)(nil)).Elem()
-	errorType   = reflect.TypeOf((*error)(nil)).Elem()
+	errorType = reflect.TypeOf((*error)(nil)).Elem()
 
 	// errors
 	errFirstArg         = errors.New("first argument must be context.Context")
@@ -112,7 +111,7 @@ func Func(key string, i interface{}) *Function {
 		f.err = errors.New("not a function")
 		return f
 	}
-	if t.NumIn() == 0 || t.In(0) != contextType {
+	if t.NumIn() == 0 || !isContext(t.In(0)) {
 		f.err = errFirstArg
 		return f
 	}

delay/delay_go17.go

@@ -0,0 +1,23 @@
+// Copyright 2017 Google Inc. All rights reserved.
+// Use of this source code is governed by the Apache 2.0
+// license that can be found in the LICENSE file.
+
+//+build go1.7
+
+package delay
+
+import (
+	stdctx "context"
+	"reflect"
+
+	netctx "golang.org/x/net/context"
+)
+
+var (
+	stdContextType = reflect.TypeOf((*stdctx.Context)(nil)).Elem()
+	netContextType = reflect.TypeOf((*netctx.Context)(nil)).Elem()
+)
+
+func isContext(t reflect.Type) bool {
+	return t == stdContextType || t == netContextType
+}

delay/delay_go17_test.go

@@ -0,0 +1,55 @@
+// Copyright 2017 Google Inc. All rights reserved.
+// Use of this source code is governed by the Apache 2.0
+// license that can be found in the LICENSE file.
+
+//+build go1.7
+
+package delay
+
+import (
+	"bytes"
+	stdctx "context"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	netctx "golang.org/x/net/context"
+	"google.golang.org/appengine/taskqueue"
+)
+
+var (
+	stdCtxRuns = 0
+	stdCtxFunc = Func("stdctx", func(c stdctx.Context) {
+		stdCtxRuns++
+	})
+)
+
+func TestStandardContext(t *testing.T) {
+	// Fake out the adding of a task.
+	var task *taskqueue.Task
+	taskqueueAdder = func(_ netctx.Context, tk *taskqueue.Task, queue string) (*taskqueue.Task, error) {
+		if queue != "" {
+			t.Errorf(`Got queue %q, expected ""`, queue)
+		}
+		task = tk
+		return tk, nil
+	}
+
+	c := newFakeContext()
+	stdCtxRuns = 0 // reset state
+	if err := stdCtxFunc.Call(c.ctx); err != nil {
+		t.Fatal("Function.Call:", err)
+	}
+
+	// Simulate the Task Queue service.
+	req, err := http.NewRequest("POST", path, bytes.NewBuffer(task.Payload))
+	if err != nil {
+		t.Fatalf("Failed making http.Request: %v", err)
+	}
+	rw := httptest.NewRecorder()
+	runFunc(c.ctx, rw, req)
+
+	if stdCtxRuns != 1 {
+		t.Errorf("stdCtxRuns: got %d, want 1", stdCtxRuns)
+	}
+}

delay/delay_pre17.go

@@ -0,0 +1,19 @@
+// Copyright 2017 Google Inc. All rights reserved.
+// Use of this source code is governed by the Apache 2.0
+// license that can be found in the LICENSE file.
+
+//+build !go1.7
+
+package delay
+
+import (
+	"reflect"
+
+	"golang.org/x/net/context"
+)
+
+var contextType = reflect.TypeOf((*context.Context)(nil)).Elem()
+
+func isContext(t reflect.Type) bool {
+	return t == contextType
+}