start-api ignores isBase64Encoded: true

Description

When my function returns a base64 encoded body with isBase64Encoded: true, sam local start-api returns a response that is base64 encoded.

Steps to reproduce

Write a function

package main

import (
	"context"
	"encoding/base64"
	"net/http"

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

func handle(ctx context.Context, r events.APIGatewayProxyRequest) (events.APIGatewayProxyResponse, error) {
	return events.APIGatewayProxyResponse{
		StatusCode:      http.StatusOK,
		Body:            base64.StdEncoding.EncodeToString([]byte("hello")),
		IsBase64Encoded: true,
	}, nil
}

func main() {
	lambda.Start(handle)
}

Define template

AWSTemplateFormatVersion: '2010-09-09'
Transform: AWS::Serverless-2016-10-31

Resources:
  Function:
    Type: AWS::Serverless::Function
    Properties:
      CodeUri: build
      FunctionName: test
      Handler: main
      Runtime: go1.x
      Timeout: 3
      Events:
        Root:
          Type: Api
          Properties:
            Path: /
            Method: ANY
        CatchAll:
          Type: Api
          Properties:
            Path: /{proxy+}
            Method: ANY

Build function

GOOS=linux go build -o build/main .

Start sam

sam local start-api -t template.yml --skip-pull-image

Get the result

curl http://127.0.0.1:3000

Observed result

aGVsbG8=

Expected result

hello

Additional environment details (Ex: Windows, Mac, Amazon Linux etc)

1. OS: Mac
2. sam --version: SAM CLI, version 0.43.0

–debug output

Telemetry endpoint configured to be https://xxx
local start-api command is called
No Parameters detected in the template
2 resources found in the template
Found Serverless function with name='Function' and CodeUri='build'
No Parameters detected in the template
2 resources found in the template
Found '2' API Events in Serverless function with name 'Function'
Detected Inline Swagger definition
Lambda function integration not found in Swagger document at path='/' method='x-amazon-apigateway-any-method'
Lambda function integration not found in Swagger document at path='/{proxy+}' method='x-amazon-apigateway-any-method'
Found '0' APIs in resource 'ServerlessRestApi'
Removed duplicates from '0' Explicit APIs and '2' Implicit APIs to produce '2' APIs
2 APIs found in the template
Mounting Function at http://127.0.0.1:3000/ [DELETE, GET, HEAD, OPTIONS, PATCH, POST, PUT]
Mounting Function at http://127.0.0.1:3000/{proxy+} [DELETE, GET, HEAD, OPTIONS, PATCH, POST, PUT]
You can now browse to the above endpoints to invoke your functions. You do not need to restart/reload SAM CLI while working on your functions, changes will be reflected instantly/automatically. You only need to restart SAM CLI if you update your AWS SAM template
Localhost server is starting up. Multi-threading = True
2020-02-27 17:25:58  * Running on http://127.0.0.1:3000/ (Press CTRL+C to quit)
Constructed String representation of Event to invoke Lambda. Event: {"httpMethod": "GET", "body": null, "resource": "/", "requestContext": {"resourceId": "123456", "apiId": "1234567890", "resourcePath": "/", "httpMethod": "GET", "requestId": "c6af9ac6-7b61-11e6-9a41-93e8deadbeef", "accountId": "123456789012", "stage": "Prod", "identity": {"apiKey": null, "userArn": null, "cognitoAuthenticationType": null, "caller": null, "userAgent": "Custom User Agent String", "user": null, "cognitoIdentityPoolId": null, "cognitoAuthenticationProvider": null, "sourceIp": "127.0.0.1", "accountId": null}, "extendedRequestId": null, "path": "/"}, "queryStringParameters": null, "multiValueQueryStringParameters": null, "headers": {"Host": "127.0.0.1:3000", "User-Agent": "curl/7.54.0", "Accept": "*/*", "X-Forwarded-Proto": "http", "X-Forwarded-Port": "3000"}, "multiValueHeaders": {"Host": ["127.0.0.1:3000"], "User-Agent": ["curl/7.54.0"], "Accept": ["*/*"], "X-Forwarded-Proto": ["http"], "X-Forwarded-Port": ["3000"]}, "pathParameters": null, "stageVariables": null, "path": "/", "isBase64Encoded": false}
Found one Lambda function with name 'Function'
Invoking main (go1.x)
No environment variables found for function 'Function'
Environment variables overrides data is standard format
Loading AWS credentials from session with profile 'None'
Resolving code path. Cwd=/path, CodeUri=build
Resolved absolute path to code is /path/build
Code /path/build is not a zip/jar file
Skipping building an image since no layers were defined
Requested to skip pulling images ...

Mounting /path/build as /var/task:ro,delegated inside runtime container
Starting a timer for 3 seconds for function 'Function'
START RequestId: ba25691e-7a74-1f8f-bd6a-15527b27c20f Version: $LATEST
END RequestId: ba25691e-7a74-1f8f-bd6a-15527b27c20f
REPORT RequestId: ba25691e-7a74-1f8f-bd6a-15527b27c20f  Init Duration: 211.99 ms        Duration: 6.47 ms       Billed Duration: 100 ms   Memory Size: 128 MB     Max Memory Used: 22 MB
No Content-Type given. Defaulting to 'application/json'.
2020-02-27 17:26:05 127.0.0.1 - - [27/Feb/2020 17:26:05] "GET / HTTP/1.1" 200 -

[shadow-light]

Looks like this has been fixed in https://github.com/aws/aws-sam-cli/pull/2941

[stschwark]

I ran into the same issue and my impression also is that SAM fails to find the returned Content-Type in the list of BinaryMediaTypes which is only available / required for AWS::Serverless::Api but not for AWS::Serverless::HttpApi. Would it be correct to conclude that sam local cannot currently be used to test endpoints that return base64 encoded binary content?