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wrapLambdaHandler.go
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wrapLambdaHandler.go
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// Copyright The OpenTelemetry Authors
// SPDX-License-Identifier: Apache-2.0
package otellambda // import "go.opentelemetry.io/contrib/instrumentation/github.com/aws/aws-lambda-go/otellambda"
import (
"context"
"encoding/json"
"fmt"
"reflect"
)
// wrappedHandlerFunction is a struct which only holds an instrumentor and is
// able to instrument invocations of the user's lambda handler function.
type wrappedHandlerFunction struct {
instrumentor instrumentor
}
func errorHandler(e error) func(context.Context, interface{}) (interface{}, error) {
return func(context.Context, interface{}) (interface{}, error) {
return nil, e
}
}
// Ensure handler takes 0-2 values, with context
// as its first value if two arguments exist.
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
}
// Ensure handler returns 0-2 values, with an error
// as its first value if any exist.
func validateReturns(handler reflect.Type) error {
errorType := reflect.TypeOf((*error)(nil)).Elem()
switch n := handler.NumOut(); {
case n > 2:
return fmt.Errorf("handler may not return more than two values")
case n == 2:
if !handler.Out(1).Implements(errorType) {
return fmt.Errorf("handler returns two values, but the second does not implement error")
}
case n == 1:
if !handler.Out(0).Implements(errorType) {
return fmt.Errorf("handler returns a single value, but it does not implement error")
}
}
return nil
}
// Wraps and calls customer lambda handler then unpacks response as necessary.
func (whf *wrappedHandlerFunction) wrapperInternals(ctx context.Context, handlerFunc interface{}, eventJSON []byte, event reflect.Value, takesContext bool) (interface{}, error) {
wrappedLambdaHandler := reflect.ValueOf(whf.wrapper(handlerFunc))
argsWrapped := []reflect.Value{reflect.ValueOf(ctx), reflect.ValueOf(eventJSON), event, reflect.ValueOf(takesContext)}
response := wrappedLambdaHandler.Call(argsWrapped)[0].Interface().([]reflect.Value)
// 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
}
// InstrumentHandler Provides a lambda handler which wraps customer lambda handler with OTel Tracing.
func InstrumentHandler(handlerFunc interface{}, options ...Option) interface{} {
whf := wrappedHandlerFunction{instrumentor: newInstrumentor(options...)}
if handlerFunc == nil {
return errorHandler(fmt.Errorf("handler is nil"))
}
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)
}
// note we will always take context to capture lambda context,
// regardless of whether customer takes context
if handlerType.NumIn() == 0 || handlerType.NumIn() == 1 && takesContext {
return func(ctx context.Context) (interface{}, error) {
var temp *interface{}
event := reflect.ValueOf(temp)
return whf.wrapperInternals(ctx, handlerFunc, []byte{}, event, takesContext)
}
}
// customer either takes both context and payload or just payload
return func(ctx context.Context, payload interface{}) (interface{}, error) {
event := reflect.New(handlerType.In(handlerType.NumIn() - 1))
// lambda SDK normally unmarshalls to customer event type, however
// with the wrapper the SDK unmarshalls to map[string]interface{}
// due to our use of reflection. Therefore we must convert this map
// to customer's desired event, we do so by simply re-marshaling then
// unmarshalling to the desired event type. The remarshalledPayload
// will also be used by users using custom propagators
remarshalledPayload, err := json.Marshal(payload)
if err != nil {
return nil, err
}
if err := json.Unmarshal(remarshalledPayload, event.Interface()); err != nil {
return nil, err
}
return whf.wrapperInternals(ctx, handlerFunc, remarshalledPayload, event.Elem(), takesContext)
}
}
// Adds OTel span surrounding customer handler call.
func (whf *wrappedHandlerFunction) wrapper(handlerFunc interface{}) func(ctx context.Context, eventJSON []byte, event interface{}, takesContext bool) []reflect.Value {
return func(ctx context.Context, eventJSON []byte, event interface{}, takesContext bool) []reflect.Value {
ctx, span := whf.instrumentor.tracingBegin(ctx, eventJSON)
defer whf.instrumentor.tracingEnd(ctx, span)
handler := reflect.ValueOf(handlerFunc)
var args []reflect.Value
if takesContext {
args = append(args, reflect.ValueOf(ctx))
}
if eventExists(event) {
args = append(args, reflect.ValueOf(event))
}
response := handler.Call(args)
return response
}
}
// Determine if an interface{} is nil or the
// if the reflect.Value of the event is nil.
func eventExists(event interface{}) bool {
if event == nil {
return false
}
// reflect.Value.isNil() can only be called on
// Values of certain Kinds. Unsupported Kinds
// will panic rather than return false
switch reflect.TypeOf(event).Kind() {
case reflect.Chan, reflect.Func, reflect.Map, reflect.Ptr, reflect.UnsafePointer, reflect.Interface, reflect.Slice:
return !reflect.ValueOf(event).IsNil()
}
return true
}