Fix issue 185 - request cannot rewind during retry (#186)

* Ensure the proxied request is type that is rewindable

* Add comment on why we copy request body

* Fix test by dealing with nil cases

handler/proxy_client.go

@@ -18,6 +18,7 @@ package handler
 import (
 	"bytes"
 	"fmt"
+	"io"
 	"io/ioutil"
 	"net/http"
 	"net/http/httputil"
@@ -119,6 +120,14 @@ func chunked(transferEncoding []string) bool {
 	return false
 }
 
+func readDownStreamRequestBody(req *http.Request) ([]byte, error) {
+	if req.Body == nil {
+		return []byte{}, nil
+	}
+	defer req.Body.Close()
+	return io.ReadAll(req.Body)
+}
+
 func (p *ProxyClient) Do(req *http.Request) (*http.Response, error) {
 	proxyURL := *req.URL
 	if p.HostOverride != "" {
@@ -140,7 +149,16 @@ func (p *ProxyClient) Do(req *http.Request) (*http.Response, error) {
 		log.WithField("request", string(initialReqDump)).Debug("Initial request dump:")
 	}
 
-	proxyReq, err := http.NewRequest(req.Method, proxyURL.String(), req.Body)
+	// Save the request body into memory so that it's rewindable during retry.
+	// See https://github.com/awslabs/aws-sigv4-proxy/issues/185
+	// This may increase memory demand, but the demand should be ok for most cases. If there
+	// are cases proven to be very problematic, we can consider adding a flag to disable this.
+	proxyReqBody, err := readDownStreamRequestBody(req)
+	if err != nil {
+		return nil, err
+	}
+
+	proxyReq, err := http.NewRequest(req.Method, proxyURL.String(), bytes.NewReader(proxyReqBody))
 	if err != nil {
 		return nil, err
 	}
@@ -222,15 +240,15 @@ func (p *ProxyClient) Do(req *http.Request) (*http.Response, error) {
 	}
 
 	if (p.LogFailedRequest || log.GetLevel() == log.DebugLevel) && resp.StatusCode >= 400 {
-		b, _ := ioutil.ReadAll(resp.Body)
+		b, _ := io.ReadAll(resp.Body)
 		log.WithField("request", fmt.Sprintf("%s %s", proxyReq.Method, proxyReq.URL)).
 			WithField("status_code", resp.StatusCode).
 			WithField("message", string(b)).
 			Error("error proxying request")
 
 		// Need to "reset" the response body because we consumed the stream above, otherwise caller will
 		// get empty body.
-		resp.Body = ioutil.NopCloser(bytes.NewBuffer(b))
+		resp.Body = io.NopCloser(bytes.NewBuffer(b))
 	}
 
 	return resp, nil