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// Copyright 2017 Amazon.com, Inc. or its affiliates. All Rights Reserved.
package lambda
import (
"context"
"encoding/json"
"fmt"
"reflect"
)
type Handler interface {
Invoke(ctx context.Context, payload []byte) ([]byte, error)
}
// lambdaHandler is the generic function type
type lambdaHandler func(context.Context, []byte) (interface{}, error)
// Invoke calls the handler, and serializes the response.
// If the underlying handler returned an error, or an error occurs during serialization, error is returned.
func (handler lambdaHandler) Invoke(ctx context.Context, payload []byte) ([]byte, error) {
response, err := handler(ctx, payload)
if err != nil {
return nil, err
}
responseBytes, err := json.Marshal(response)
if err != nil {
return nil, err
}
return responseBytes, nil
}
func errorHandler(e error) lambdaHandler {
return func(ctx context.Context, event []byte) (interface{}, error) {
return nil, e
}
}
func validateArguments(handler reflect.Type) (bool, error) {
handlerTakesContext := false
if handler.NumIn() > 2 {
return false, fmt.Errorf("handlers may not take more than two arguments, but handler takes %d", handler.NumIn())
} else if handler.NumIn() > 0 {
contextType := reflect.TypeOf((*context.Context)(nil)).Elem()
argumentType := handler.In(0)
handlerTakesContext = argumentType.Implements(contextType)
if handler.NumIn() > 1 && !handlerTakesContext {
return false, fmt.Errorf("handler takes two arguments, but the first is not Context. got %s", argumentType.Kind())
}
}
return handlerTakesContext, nil
}
func validateReturns(handler reflect.Type) error {
errorType := reflect.TypeOf((*error)(nil)).Elem()
if handler.NumOut() > 2 {
return fmt.Errorf("handler may not return more than two values")
} else if handler.NumOut() > 1 {
if !handler.Out(1).Implements(errorType) {
return fmt.Errorf("handler returns two values, but the second does not implement error")
}
} else if handler.NumOut() == 1 {
if !handler.Out(0).Implements(errorType) {
return fmt.Errorf("handler returns a single value, but it does not implement error")
}
}
return nil
}
// NewHandler creates a base lambda handler from the given handler function. The
// returned Handler performs JSON deserialization and deserialization, and
// delegates to the input handler function. The handler function parameter must
// satisfy the rules documented by Start. If handlerFunc is not a valid
// handler, the returned Handler simply reports the validation error.
func NewHandler(handlerFunc interface{}) Handler {
if handlerFunc == nil {
return errorHandler(fmt.Errorf("handler is nil"))
}
handler := reflect.ValueOf(handlerFunc)
handlerType := reflect.TypeOf(handlerFunc)
if handlerType.Kind() != reflect.Func {
return errorHandler(fmt.Errorf("handler kind %s is not %s", handlerType.Kind(), reflect.Func))
}
takesContext, err := validateArguments(handlerType)
if err != nil {
return errorHandler(err)
}
if err := validateReturns(handlerType); err != nil {
return errorHandler(err)
}
return lambdaHandler(func(ctx context.Context, payload []byte) (interface{}, error) {
// construct arguments
var args []reflect.Value
if takesContext {
args = append(args, reflect.ValueOf(ctx))
}
if (handlerType.NumIn() == 1 && !takesContext) || handlerType.NumIn() == 2 {
eventType := handlerType.In(handlerType.NumIn() - 1)
event := reflect.New(eventType)
if err := json.Unmarshal(payload, event.Interface()); err != nil {
return nil, err
}
args = append(args, event.Elem())
}
response := handler.Call(args)
// convert return values into (interface{}, error)
var err error
if len(response) > 0 {
if errVal, ok := response[len(response)-1].Interface().(error); ok {
err = errVal
}
}
var val interface{}
if len(response) > 1 {
val = response[0].Interface()
}
return val, err
})
}