fix s3 requests sent to outpost arn missing x-amz-content-sha256 header

[aajtodd]

Example code of testing an outpost endpoint. NOTE: You don't actually have to have an outpost endpoint setup, just plugin your own account and a dummy value for the access point (e.g. my-test-access-point).

package main

import (
    "github.com/aws/aws-sdk-go/aws"
    "github.com/aws/aws-sdk-go/aws/session"
    "github.com/aws/aws-sdk-go/service/s3"
    "fmt"
)

func main() {
    sess, err := session.NewSession(&aws.Config{
        Region: aws.String("us-east-1"),
        LogLevel: aws.LogLevel(aws.LogDebugWithHTTPBody),
   },)

    svc := s3.New(sess)
    bucket := "arn:aws:s3-outposts:us-east-1:<account>:outpost/ec2/accesspoint/<my-access-point>"
    resp, err := svc.ListObjectsV2(&s3.ListObjectsV2Input{Bucket: aws.String(bucket)})
    fmt.Printf(" response %v error %v \n", resp, err)
}

output contains

Missing required header for this request: x-amz-content-sha256

The resolved endpoint is an outpost ARN which customizes the signing name to be s3-outposts instead of s3. Requests to S3 must include the x-amz-content-sha256 header but the the v4 signer does not special case s3-outposts to include the header.

This change does affect the actual s3outpost service requests as it shares the same signing name. I've tested that requests with and without the header have the same responses and that it's valid and safe to send this for s3outposts as well.

Go v2 doesn't have this issue since middleware is registered on the operation to add this particular header for S3 requests.