golang AWS-SDK (github.com/aws/aws-sdk-go)でEC2IAMRole権限でアクセスする

s3.go

package main

import (
    "net/http"
    "time"

    "github.com/aws/aws-sdk-go/aws"
    "github.com/aws/aws-sdk-go/aws/credentials"
    "github.com/aws/aws-sdk-go/aws/credentials/ec2rolecreds"
    "github.com/aws/aws-sdk-go/aws/ec2metadata"
    "github.com/aws/aws-sdk-go/aws/session"
    "github.com/aws/aws-sdk-go/service/s3"
)

// AwsConfig get config
func AwsConfig() *aws.Config {
    ec2m := ec2metadata.New(session.New(), &aws.Config{
        HTTPClient: &http.Client{Timeout: 10 * time.Second},
    })
    cr := credentials.NewCredentials(&ec2rolecreds.EC2RoleProvider{
        Client: ec2m,
    })
    return &aws.Config{
        Region:      aws.String("ap-northeast-1"),
        Credentials: cr,
    }
}

func main() {
    s3c := s3.New(session.New(), AwsConfig())
    ...
}

credentialsファイルが有ればそっちを見て、なければIAMRoleを使うみたいなのを作りました
https://github.com/ushios/awsconfig