Update readme and docstrings, minor refactors.

simples3.go

@@ -7,6 +7,8 @@ import (
 	"bytes"
 	"crypto/hmac"
 	"crypto/sha256"
+	"encoding/json"
+	"errors"
 	"fmt"
 	"io"
 	"io/ioutil"
@@ -16,6 +18,10 @@ import (
 	"time"
 )
 
+const (
+	securityCredentialsURL = "http://169.254.169.254/latest/meta-data/iam/security-credentials/"
+)
+
 // S3 provides a wrapper around your S3 credentials.
 type S3 struct {
 	AccessKey string
@@ -40,6 +46,18 @@ type DeleteInput struct {
 	ObjectKey string
 }
 
+// IAMResponse is used by NewUsingIAM to auto
+// detect the credentials
+type IAMResponse struct {
+	Code            string `json:"Code"`
+	LastUpdated     string `json:"LastUpdated"`
+	Type            string `json:"Type"`
+	AccessKeyID     string `json:"AccessKeyId"`
+	SecretAccessKey string `json:"SecretAccessKey"`
+	Token           string `json:"Token"`
+	Expiration      string `json:"Expiration"`
+}
+
 // New returns an instance of S3.
 func New(region, accessKey, secretKey string) *S3 {
 	return &S3{
@@ -51,6 +69,53 @@ func New(region, accessKey, secretKey string) *S3 {
 	}
 }
 
+// NewUsingIAM automatically generates an Instance of S3
+// using instance metatdata.
+func NewUsingIAM(region string) (*S3, error) {
+
+	// Get the IAM role
+	resp, err := http.Get(securityCredentialsURL)
+	if err != nil {
+		return nil, err
+	}
+	defer resp.Body.Close()
+	if resp.StatusCode != 200 {
+		return nil, errors.New(http.StatusText(resp.StatusCode))
+	}
+
+	role, err := ioutil.ReadAll(resp.Body)
+	if err != nil {
+		return nil, err
+	}
+
+	resp, err = http.Get(securityCredentialsURL + "/" + string(role))
+	if err != nil {
+		return nil, err
+	}
+	defer resp.Body.Close()
+	if resp.StatusCode != 200 {
+		return nil, errors.New(http.StatusText(resp.StatusCode))
+	}
+
+	var jsonResp IAMResponse
+	jsonString, err := ioutil.ReadAll(resp.Body)
+	if err != nil {
+		return nil, err
+	}
+
+	if err := json.Unmarshal(jsonString, &jsonResp); err != nil {
+		return nil, err
+	}
+
+	return &S3{
+		Region:    region,
+		AccessKey: jsonResp.AccessKeyID,
+		SecretKey: jsonResp.SecretAccessKey,
+
+		URIFormat: "https://s3.%s.amazonaws.com/%s",
+	}, nil
+}
+
 func detectFileSize(body multipart.File) int64 {
 	switch r := body.(type) {
 	case io.Seeker:
@@ -133,7 +198,7 @@ func (s3 *S3) FileUpload(u UploadInput) error {
 	return nil
 }
 
-// FileDelete makes a POST call with the file written as multipart
+// FileDelete makes a DELETE call with the file written as multipart
 // and on successful upload, checks for 204 No Content.
 func (s3 *S3) FileDelete(u DeleteInput) error {
 	url := fmt.Sprintf(