Equal md5-sums for different message definitions

[airballking]

We came across the following effect: For certain message definitions which differ the md5-sums are equal! I created a short example here: airballking/ros_test_pkgs@f2d3daf

Basically, I have a message definition md5_generation_issue/Test.msg which looks like this:

std_msgs/Int32 field
# std_msgs/Int32[] field
# std_msgs/Int32[1] field
# std_msgs/Int32[124] field

No matter which of the lines is uncommented (as long as it is only one), the md5-sum will always be 8b31b5679e76c808441b573f78ce35d1.

Is this a bug or a feature?

[dirk-thomas]

Thank you for reporting this.
That is actually the current behavior of the md5sum calculation.
It only considers the "bare" type of fields but not the information if it is an array and what size the array has (

genmsg/src/genmsg/gentools.py

Line 76 in bc17cb2

msg_type = bare_msg_type(type_)

).

I can't tell if that was an oversight in the first implementation or if there was any reason to do it (which I can't see any).

Anyway it is impossible at this point to change the behavior to differentiate those messages since that would essentially change the md5sum of most of the existing ROS messages.
But we will consider this for future development (aka ROS 2) since those different messages should indeed have different md5sums.

Therefore I will close this as "wontfix".

[vorzawk]

@dirk-thomas I am facing this same issue currently. Is there a workaround for this issue?

[dirk-thomas]

@vorzawk sorry, I am not the maintainer of this package anymore.