Fix issue #375 error uploading empty artifact

Author: jonasfj

runtime/artifact.go

@@ -153,6 +153,13 @@ func putArtifact(urlStr, mime string, stream ioext.ReadSeekCloser, additionalArt
 		if err != nil {
 			return errors.Wrap(err, "Failed to seek start before uploading stream")
 		}
+		body := ioutil.NopCloser(stream)
+		if contentLength == 0 {
+			// Zero is the default value for ContentLength, so if we want to avoid
+			// using transfer-encoding: chunked, not supported by S3, we have to
+			// specify http.NoBody when content-length is zero
+			body = http.NoBody
+		}
 		req := &http.Request{
 			Method:        "PUT",
 			URL:           u,
@@ -161,13 +168,13 @@ func putArtifact(urlStr, mime string, stream ioext.ReadSeekCloser, additionalArt
 			ProtoMinor:    1,
 			Header:        header,
 			ContentLength: contentLength,
-			Body:          ioutil.NopCloser(stream),
+			Body:          body,
 			GetBody: func() (io.ReadCloser, error) {
 				// In case we have to follow any redirects, which shouldn't happen
 				if _, serr := stream.Seek(0, io.SeekStart); serr != nil {
 					return nil, errors.Wrap(serr, "failed to seek to start of stream")
 				}
-				return ioutil.NopCloser(stream), nil
+				return body, nil
 			},
 		}
 		resp, err := client.Do(req)