Report to the RuntimeAPI any errors that occur when reading from a handler's returned reader

[bmoffatt]

Issue #, if available:

Description of changes:

Errors that occur when reading a handler's response io.Reader can now be reported as trailers to the Runtime API. Supported client SDKs will be able to observe the error returned.

https://docs.aws.amazon.com/lambda/latest/dg/runtimes-custom.html#runtimes-custom-response-streaming

Here's what a client invoking the Lambda looks like with this:

package main

import (
	"context"
	"encoding/base64"
	"encoding/json"
	"flag"
	"fmt"
	"log"
	"os"

	"github.com/aws/aws-sdk-go-v2/config"
	"github.com/aws/aws-sdk-go-v2/service/lambda"
	"github.com/aws/aws-sdk-go-v2/service/lambda/types"
)

func main() {
	functionName := flag.String("function-name", "", "function name to invoke")
	flag.Parse()
	if *functionName == "" {
		flag.PrintDefaults()
		os.Exit(1)
	}
	cfg, _ := config.LoadDefaultConfig(context.TODO())
	svc := lambda.NewFromConfig(cfg)
	res, err := svc.InvokeWithResponseStream(context.Background(), &lambda.InvokeWithResponseStreamInput{
		FunctionName: functionName,
		LogType:      types.LogTypeTail,
	})
	if err != nil {
		log.Fatal(err)
	}
	stream := res.GetStream()
	for event := range stream.Events() {
		switch event := event.(type) {
		case *types.InvokeWithResponseStreamResponseEventMemberPayloadChunk:
			fmt.Printf("got chunk: %s\n", string(event.Value.Payload))
		case *types.InvokeWithResponseStreamResponseEventMemberInvokeComplete:
			if event.Value.ErrorCode != nil {
				fmt.Printf("Error Code: %s\n", *event.Value.ErrorCode)
				var details any
				json.Unmarshal([]byte(*event.Value.ErrorDetails), &details)
				encoder := json.NewEncoder(os.Stdout)
				encoder.SetIndent("", "  ")
				encoder.Encode(details)
			}
			fmt.Printf("stream complete! ErrorValue: %v\n", event.Value.ErrorDetails)
			if event.Value.LogResult != nil {
				logs, _ := base64.StdEncoding.DecodeString(*event.Value.LogResult)
				fmt.Printf("%s\n", string(logs))
			}
		}
	}
}

output looks like:

go run tools/invoke/main.go -function-name sappyappy-HelloWorldFunction-2Mix3E2Tb1ZM
got chunk: Hello World!

got chunk: Hello Again!

Error Code: errorString
{
  "errorMessage": "hello errr",
  "errorType": "errorString"
}
START RequestId: bacb2136-7704-4d4c-a7aa-ec1985ed86c6 Version: $LATEST
END RequestId: bacb2136-7704-4d4c-a7aa-ec1985ed86c6
REPORT RequestId: bacb2136-7704-4d4c-a7aa-ec1985ed86c6	Duration: 2001.91 ms	Billed Duration: 2002 ms	Memory Size: 128 MB	Max Memory Used: 20 MB

And the function code I invoked was:

package main

import (
	"errors"
	"io"
	"strings"
	"time"

	"github.com/aws/aws-lambda-go/lambda"
)

func main() {
	lambda.Start(func() (any, error) {
		r, w := io.Pipe()
		go func() {
			time.Sleep(1 * time.Second)
			io.Copy(w, strings.NewReader("Hello World!\n"))
			time.Sleep(1 * time.Second)
			io.Copy(w, strings.NewReader("Hello Again!\n"))
			w.CloseWithError(errors.New("hello errr"))
		}()
		return r, nil
	})
}

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[codecov-commenter]

Codecov Report

Merging #495 (34fce13) into main (caace58) will decrease coverage by 0.02%.
The diff coverage is 71.42%.

@@            Coverage Diff             @@
##             main     #495      +/-   ##
==========================================
- Coverage   71.18%   71.17%   -0.02%     
==========================================
  Files          21       21              
  Lines        1253     1273      +20     
==========================================
+ Hits          892      906      +14     
- Misses        295      300       +5     
- Partials       66       67       +1     

Impacted Files	Coverage Δ
lambda/runtime_api_client.go	72.41% <71.42%> (-0.73%)	⬇️

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