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// Copyright 2011 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.
Package delay provides a way to execute code outside the scope of a
user request by using the taskqueue API.
To declare a function that may be executed later, call Func
in a top-level assignment context, passing it an arbitrary string key
and a function whose first argument is of type context.Context.
The key is used to look up the function so it can be called later.
var laterFunc = delay.Func("key", myFunc)
It is also possible to use a function literal.
var laterFunc = delay.Func("key", func(c context.Context, x string) {
// ...
To call a function, invoke its Call method.
laterFunc.Call(c, "something")
A function may be called any number of times. If the function has any
return arguments, and the last one is of type error, the function may
return a non-nil error to signal that the function should be retried.
The arguments to functions may be of any type that is encodable by the gob
package. If an argument is of interface type, it is the client's responsibility
to register with the gob package whatever concrete type may be passed for that
argument; see for details.
Any errors during initialization or execution of a function will be
logged to the application logs. Error logs that occur during initialization will
be associated with the request that invoked the Call method.
The state of a function invocation that has not yet successfully
executed is preserved by combining the file name in which it is declared
with the string key that was passed to the Func function. Updating an app
with pending function invocations is safe as long as the relevant
functions have the (filename, key) combination preserved.
The delay package uses the Task Queue API to create tasks that call the
reserved application path "/_ah/queue/go/delay".
This path must not be marked as "login: required" in app.yaml;
it must be marked as "login: admin" or have no access restriction.
package delay // import ""
import (
// Function represents a function that may have a delayed invocation.
type Function struct {
fv reflect.Value // Kind() == reflect.Func
key string
err error // any error during initialization
const (
// The HTTP path for invocations.
path = "/_ah/queue/go/delay"
// Use the default queue.
queue = ""
type contextKey int
var (
// registry of all delayed functions
funcs = make(map[string]*Function)
// precomputed types
contextType = reflect.TypeOf((*context.Context)(nil)).Elem()
errorType = reflect.TypeOf((*error)(nil)).Elem()
// errors
errFirstArg = errors.New("first argument must be context.Context")
errOutsideDelayFunc = errors.New("request headers are only available inside a delay.Func")
// context keys
headersContextKey contextKey = 0
// Func declares a new Function. The second argument must be a function with a
// first argument of type context.Context.
// This function must be called at program initialization time. That means it
// must be called in a global variable declaration or from an init function.
// This restriction is necessary because the instance that delays a function
// call may not be the one that executes it. Only the code executed at program
// initialization time is guaranteed to have been run by an instance before it
// receives a request.
func Func(key string, i interface{}) *Function {
f := &Function{fv: reflect.ValueOf(i)}
// Derive unique, somewhat stable key for this func.
_, file, _, _ := runtime.Caller(1)
f.key = file + ":" + key
t := f.fv.Type()
if t.Kind() != reflect.Func {
f.err = errors.New("not a function")
return f
if t.NumIn() == 0 || t.In(0) != contextType {
f.err = errFirstArg
return f
// Register the function's arguments with the gob package.
// This is required because they are marshaled inside a []interface{}.
// gob.Register only expects to be called during initialization;
// that's fine because this function expects the same.
for i := 0; i < t.NumIn(); i++ {
// Only concrete types may be registered. If the argument has
// interface type, the client is resposible for registering the
// concrete types it will hold.
if t.In(i).Kind() == reflect.Interface {
if old := funcs[f.key]; old != nil {
old.err = fmt.Errorf("multiple functions registered for %s in %s", key, file)
funcs[f.key] = f
return f
type invocation struct {
Key string
Args []interface{}
// Call invokes a delayed function.
// err := f.Call(c, ...)
// is equivalent to
// t, _ := f.Task(...)
// _, err := taskqueue.Add(c, t, "")
func (f *Function) Call(c context.Context, args ...interface{}) error {
t, err := f.Task(args...)
if err != nil {
return err
_, err = taskqueueAdder(c, t, queue)
return err
// Task creates a Task that will invoke the function.
// Its parameters may be tweaked before adding it to a queue.
// Users should not modify the Path or Payload fields of the returned Task.
func (f *Function) Task(args ...interface{}) (*taskqueue.Task, error) {
if f.err != nil {
return nil, fmt.Errorf("delay: func is invalid: %v", f.err)
nArgs := len(args) + 1 // +1 for the context.Context
ft := f.fv.Type()
minArgs := ft.NumIn()
if ft.IsVariadic() {
if nArgs < minArgs {
return nil, fmt.Errorf("delay: too few arguments to func: %d < %d", nArgs, minArgs)
if !ft.IsVariadic() && nArgs > minArgs {
return nil, fmt.Errorf("delay: too many arguments to func: %d > %d", nArgs, minArgs)
// Check arg types.
for i := 1; i < nArgs; i++ {
at := reflect.TypeOf(args[i-1])
var dt reflect.Type
if i < minArgs {
// not a variadic arg
dt = ft.In(i)
} else {
// a variadic arg
dt = ft.In(minArgs).Elem()
// nil arguments won't have a type, so they need special handling.
if at == nil {
// nil interface
switch dt.Kind() {
case reflect.Chan, reflect.Func, reflect.Interface, reflect.Map, reflect.Ptr, reflect.Slice:
continue // may be nil
return nil, fmt.Errorf("delay: argument %d has wrong type: %v is not nilable", i, dt)
switch at.Kind() {
case reflect.Chan, reflect.Func, reflect.Interface, reflect.Map, reflect.Ptr, reflect.Slice:
av := reflect.ValueOf(args[i-1])
if av.IsNil() {
// nil value in interface; not supported by gob, so we replace it
// with a nil interface value
args[i-1] = nil
if !at.AssignableTo(dt) {
return nil, fmt.Errorf("delay: argument %d has wrong type: %v is not assignable to %v", i, at, dt)
inv := invocation{
Key: f.key,
Args: args,
buf := new(bytes.Buffer)
if err := gob.NewEncoder(buf).Encode(inv); err != nil {
return nil, fmt.Errorf("delay: gob encoding failed: %v", err)
return &taskqueue.Task{
Path: path,
Payload: buf.Bytes(),
}, nil
// Request returns the special task-queue HTTP request headers for the current
// task queue handler. Returns an error if called from outside a delay.Func.
func RequestHeaders(c context.Context) (*taskqueue.RequestHeaders, error) {
if ret, ok := c.Value(headersContextKey).(*taskqueue.RequestHeaders); ok {
return ret, nil
return nil, errOutsideDelayFunc
var taskqueueAdder = taskqueue.Add // for testing
func init() {
http.HandleFunc(path, func(w http.ResponseWriter, req *http.Request) {
runFunc(appengine.NewContext(req), w, req)
func runFunc(c context.Context, w http.ResponseWriter, req *http.Request) {
defer req.Body.Close()
c = context.WithValue(c, headersContextKey, taskqueue.ParseRequestHeaders(req.Header))
var inv invocation
if err := gob.NewDecoder(req.Body).Decode(&inv); err != nil {
log.Errorf(c, "delay: failed decoding task payload: %v", err)
log.Warningf(c, "delay: dropping task")
f := funcs[inv.Key]
if f == nil {
log.Errorf(c, "delay: no func with key %q found", inv.Key)
log.Warningf(c, "delay: dropping task")
ft := f.fv.Type()
in := []reflect.Value{reflect.ValueOf(c)}
for _, arg := range inv.Args {
var v reflect.Value
if arg != nil {
v = reflect.ValueOf(arg)
} else {
// Task was passed a nil argument, so we must construct
// the zero value for the argument here.
n := len(in) // we're constructing the nth argument
var at reflect.Type
if !ft.IsVariadic() || n < ft.NumIn()-1 {
at = ft.In(n)
} else {
at = ft.In(ft.NumIn() - 1).Elem()
v = reflect.Zero(at)
in = append(in, v)
out := f.fv.Call(in)
if n := ft.NumOut(); n > 0 && ft.Out(n-1) == errorType {
if errv := out[n-1]; !errv.IsNil() {
log.Errorf(c, "delay: func failed (will retry): %v", errv.Interface())