Handle zero-length multi-part upload

src/main/java/org/gaul/s3proxy/S3ErrorCode.java

@@ -50,6 +50,9 @@ enum S3ErrorCode {
             " more information about Regions, see How to Select" +
             " a Region for Your Buckets."),
     INVALID_REQUEST(HttpServletResponse.SC_BAD_REQUEST, "Bad Request"),
+    MALFORMED_X_M_L(HttpServletResponse.SC_BAD_REQUEST,
+            "The XML you provided was not well-formed or did not validate" +
+            " against our published schema."),
     METHOD_NOT_ALLOWED(HttpServletResponse.SC_METHOD_NOT_ALLOWED,
             "Method Not Allowed"),
     MISSING_CONTENT_LENGTH(HttpServletResponse.SC_LENGTH_REQUIRED,

src/main/java/org/gaul/s3proxy/S3ProxyHandler.java

@@ -1363,6 +1363,13 @@ final class S3ProxyHandler extends AbstractHandler {
                 totalContentLength += contentLength;
             }
 
+            if (partNames.isEmpty()) {
+                // Amazon requires at least one part
+                sendSimpleErrorResponse(response,
+                        S3ErrorCode.MALFORMED_X_M_L);
+                return;
+            }
+
             BlobMetadata blobMetadata = blobStore.blobMetadata(
                     containerName, uploadId);
             ContentMetadata contentMetadata =