github.com/aws/aws-lambda-go v1.13.3: lambda.NewHandler

Clean-room public code sample — generation instructions

Write a brand-new, minimal, self-contained code sample in this clean-room directory.
Do not copy, paraphrase, or reference any existing project source. Work only from this spec.

A csx.json manifest scaffold already exists. Do not recreate it from memory. Preserve its case.goal, packages and symbols; fill its empty case.contract with exact assertions and correct its environment, commands and verifierAdapter for the files you generate.

Goal: verify lambda.NewHandler in pkg:golang/github.com/aws/aws-lambda-go@v1.13.3
Kind: HOW

Use EXACTLY these public packages and versions:
  - pkg:golang/github.com/aws/aws-lambda-go@v1.13.3
Demonstrate these symbols/APIs:
  - lambda.NewHandler

Rules:
  - One focused purpose; the smallest project that proves the goal.
  - Include a contract test (test/contract.*) that runs OFFLINE and exits 0 exactly when the goal behavior works.
  - Pin every dependency with a lockfile so resolution is reproducible.
  - No secrets, credentials, or tokens. No real URLs (only example.com or localhost). No absolute paths.
  - No personal names, emails, company names, or project identifiers of any kind.
  - No binaries and no generated output (node_modules, dist, target, venv, .git, .env).
  - Keep it under 200 files and 256KB packed.

{"case":{"caseId":"case:sha256:917063ca21ad2196d7ddd8c45a56f420119c9aadbd755f89e946193d989b792c","contract":["lambda.NewHandler wraps a func(context.Context, InputEvent) (OutputResponse, error) into a lambda.Handler whose Invoke method unmarshals the JSON payload and marshals the returned struct to JSON bytes","Invoke propagates errors returned by the wrapped handler function","lambda.NewHandler returns an errorHandler whose Invoke method fails with a validation error when given a function with a non-error single return value or nil"],"goal":"verify lambda.NewHandler in pkg:golang/github.com/aws/aws-lambda-go@v1.13.3","kind":"HOW","packages":["pkg:golang/github.com/aws/aws-lambda-go@v1.13.3"],"schemaVersion":1,"symbols":["lambda.NewHandler"]},"contractCommand":["go","run","./test"],"environment":{"arch":"x64","distro":"ubuntu","ecosystem":"golang","libc":"glibc","libcVersion":"2.39","os":"linux","osVersionBucket":"24","packageManager":"go","schemaVersion":1},"license":"MIT-0","packages":["pkg:golang/github.com/aws/aws-lambda-go@v1.13.3"],"schemaVersion":1,"subject":"pkg:golang/github.com/aws/aws-lambda-go@v1.13.3","symbols":["lambda.NewHandler"],"verifierAdapter":"golang@1"}

module example.com/sample

go 1.21

require github.com/aws/aws-lambda-go v1.13.3

github.com/BurntSushi/toml v0.3.1/go.mod h1:xHWCNGjB5oqiDr8zfno3MHue2Ht5sIBksp03qcyfWMU=
github.com/aws/aws-lambda-go v1.13.3 h1:SuCy7H3NLyp+1Mrfp+m80jcbi9KYWAs9/BXwppwRDzY=
github.com/aws/aws-lambda-go v1.13.3/go.mod h1:4UKl9IzQMoD+QF79YdCuzCwp8VbmG4VAQwij/eHl5CU=
github.com/cpuguy83/go-md2man/v2 v2.0.0-20190314233015-f79a8a8ca69d/go.mod h1:maD7wRr/U5Z6m/iR4s+kqSMx2CaBsrgA7czyZG/E6dU=
github.com/davecgh/go-spew v1.1.0 h1:ZDRjVQ15GmhC3fiQ8ni8+OwkZQO4DARzQgrnXU1Liz8=
github.com/davecgh/go-spew v1.1.0/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
github.com/russross/blackfriday/v2 v2.0.1/go.mod h1:+Rmxgy9KzJVeS9/2gXHxylqXiyQDYRxCVz55jmeOWTM=
github.com/shurcooL/sanitized_anchor_name v1.0.0/go.mod h1:1NzhyTcUVG4SuEtjjoZeVRXNmyL/1OwPU0+IJeTBvfc=
github.com/stretchr/objx v0.1.0/go.mod h1:HFkY916IF+rwdDfMAkV7OtwuqBVzrE8GR6GFx+wExME=
github.com/stretchr/testify v1.4.0 h1:2E4SXV/wtOkTonXsotYi4li6zVWxYlZuYNCXe9XRJyk=
github.com/stretchr/testify v1.4.0/go.mod h1:j7eGeouHqKxXV5pUuKE4zz7dFj8WfuZ+81PSLYec5m4=
github.com/urfave/cli v1.22.1/go.mod h1:Gos4lmkARVdJ6EkW0WaNv/tZAAMe9V7XWyB60NtXRu0=
gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
gopkg.in/yaml.v2 v2.2.2 h1:ZCJp+EgiOT7lHqUV2J862kp8Qj64Jo6az82+3Td9dZw=
gopkg.in/yaml.v2 v2.2.2/go.mod h1:hI93XBmqTisBFMUTm0b8Fm+jr3Dg1NNxqwp+5A1VGuI=

{
  "schemaVersion": 1,
  "goal": "verify lambda.NewHandler in pkg:golang/github.com/aws/aws-lambda-go@v1.13.3",
  "kind": "HOW",
  "packages": [
    "pkg:golang/github.com/aws/aws-lambda-go@v1.13.3"
  ],
  "symbols": [
    "lambda.NewHandler"
  ]
}

package main

import (
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"os"
	"strings"

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

type InputEvent struct {
	Name  string `json:"name"`
	Count int    `json:"count"`
}

type OutputResponse struct {
	Greeting string `json:"greeting"`
	Doubled  int    `json:"doubled"`
}

func main() {
	ctx := context.Background()

	// 1. Valid handler taking (context.Context, InputEvent) and returning (OutputResponse, error)
	validHandler := lambda.NewHandler(func(ctx context.Context, req InputEvent) (OutputResponse, error) {
		if req.Count < 0 {
			return OutputResponse{}, errors.New("negative count")
		}
		return OutputResponse{
			Greeting: "hello " + req.Name,
			Doubled:  req.Count * 2,
		}, nil
	})

	outBytes, err := validHandler.Invoke(ctx, []byte(`{"name":"world","count":21}`))
	if err != nil {
		fmt.Fprintf(os.Stderr, "expected no error on valid invocation, got: %v\n", err)
		os.Exit(1)
	}

	var parsed OutputResponse
	if err := json.Unmarshal(outBytes, &parsed); err != nil {
		fmt.Fprintf(os.Stderr, "failed to unmarshal output JSON: %v\n", err)
		os.Exit(1)
	}
	if parsed.Greeting != "hello world" || parsed.Doubled != 42 {
		fmt.Fprintf(os.Stderr, "unexpected response: %+v\n", parsed)
		os.Exit(1)
	}

	// 2. Handler error propagation
	_, err = validHandler.Invoke(ctx, []byte(`{"name":"world","count":-1}`))
	if err == nil || err.Error() != "negative count" {
		fmt.Fprintf(os.Stderr, "expected handler error 'negative count', got: %v\n", err)
		os.Exit(1)
	}

	// 3. Invalid signature validation (single return value that is not error)
	invalidSigHandler := lambda.NewHandler(func() string {
		return "not an error"
	})
	_, err = invalidSigHandler.Invoke(ctx, []byte(`{}`))
	if err == nil || !strings.Contains(err.Error(), "does not implement error") {
		fmt.Fprintf(os.Stderr, "expected signature validation error, got: %v\n", err)
		os.Exit(1)
	}

	// 4. Nil handler validation
	nilHandler := lambda.NewHandler(nil)
	_, err = nilHandler.Invoke(ctx, nil)
	if err == nil || err.Error() != "handler is nil" {
		fmt.Fprintf(os.Stderr, "expected nil handler error, got: %v\n", err)
		os.Exit(1)
	}
}