Fix for writes from Apache Spark. (#4074)

- Due to usage of amazon SDK, spark expects md5sum of empty string to be
  returned when it does PUT on a directory.
- The fix returns md5sum of a empty string for the above mentioned case.
- This fixes the issue of Apache Spark not being able to write into Minio.

cmd/object-api-common.go

@@ -34,6 +34,9 @@ const (
 
 	// Buckets meta prefix.
 	bucketMetaPrefix = "buckets"
+
+	// Md5Sum of empty string.
+	emptyStrMd5Sum = "d41d8cd98f00b204e9800998ecf8427e"
 )
 
 // Global object layer mutex, used for safely updating object layer.
@@ -67,6 +70,10 @@ func dirObjectInfo(bucket, object string, size int64, metadata map[string]string
 	// return success.
 	md5Sum := metadata["md5Sum"]
 	delete(metadata, "md5Sum")
+	if md5Sum == "" {
+		md5Sum = emptyStrMd5Sum
+	}
+
 	return ObjectInfo{
 		Bucket:      bucket,
 		Name:        object,

cmd/server_test.go

@@ -1099,6 +1099,26 @@ func (s *TestSuiteCommon) TestPutObject(c *C) {
 	// asserted the contents of the fetched object with the expected result.
 	c.Assert(true, Equals, bytes.Equal(buffer2.Bytes(), []byte("hello world")))
 
+	// Test the response when object name ends with a slash.
+	// This is a special case with size as '0' and object ends with
+	// a slash separator, we treat it like a valid operation and
+	// return success.
+	// The response Etag headers should contain Md5Sum of empty string.
+	objectName = "objectwith/"
+	// create HTTP request for object upload.
+	request, err = newTestSignedRequest("PUT", getPutObjectURL(s.endPoint, bucketName, objectName),
+		0, nil, s.accessKey, s.secretKey, s.signer)
+	if s.signer == signerV2 {
+		c.Assert(err, IsNil)
+		err = signRequestV2(request, s.accessKey, s.secretKey)
+	}
+	c.Assert(err, IsNil)
+	// execute the HTTP request for object upload.
+	response, err = client.Do(request)
+	c.Assert(err, IsNil)
+	c.Assert(response.StatusCode, Equals, http.StatusOK)
+	// The response Etag header should contain Md5sum of an empty string.
+	c.Assert(response.Header.Get("Etag"), Equals, "\""+emptyStrMd5Sum+"\"")
 }
 
 // TestListBuckets - Make request for listing of all buckets.