Fix potential race condition in 'download_checkmd5.py'

[czalidis]

When trying to download multiple files in the same directory, using catkin_download_test_data, there is a race condition (if make has been called with -jN, where N>1) because 2 or more threads are trying to create the same directory.

I have created a simple package to reproduce that issue. The only thing that package does, is to declare 2 calls to catkin_download_test_data in the CMakeLists.txt, where the DESTINATION is the same directory.

The error I get is the following:

#### Running command: "make tests -j8 -l8" in "/home/chris/catkin_ws/build"
####
Scanning dependencies of target test_download_data_32e.pcap
Scanning dependencies of target test_download_data_test.pgm
[100%] [100%] Generating /home/chris/catkin_ws/devel/share/test_download_data/tests/32e.pcap
Generating /home/chris/catkin_ws/devel/share/test_download_data/tests/test.pgm
Downloading http://download.ros.org/data/maps/test.pgm to /home/chris/catkin_ws/devel/share/test_download_data/tests/test.pgm...Traceback (most recent call last):
  File "/home/chris/catkin_ws/src/catkin/cmake/test/download_checkmd5.py", line 177, in <module>
    sys.exit(main())
  File "/home/chris/catkin_ws/src/catkin/cmake/test/download_checkmd5.py", line 151, in main
    download_md5(uri, args.dest)
  File "/home/chris/catkin_ws/src/catkin/cmake/test/download_checkmd5.py", line 91, in download_md5
    os.makedirs(dirname)
  File "/usr/lib/python2.7/os.py", line 157, in makedirs
    mkdir(name, mode)
OSError: [Errno 17] File exists: '/home/chris/catkin_ws/devel/share/test_download_data/tests'
make[3]: *** [/home/chris/catkin_ws/devel/share/test_download_data/tests/32e.pcap] Error 1
make[2]: *** [test_download_data/CMakeFiles/test_download_data_32e.pcap.dir/all] Error 2
make[2]: *** Waiting for unfinished jobs....
 done.
Checking md5sum on /home/chris/catkin_ws/devel/share/test_download_data/tests/test.pgm
[100%] Built target test_download_data_test.pgm
make[1]: *** [CMakeFiles/tests.dir/rule] Error 2
make: *** [tests] Error 2

You can reproduce the same error, cloning that package and running catkin_make tests.

This pull request is supposed to resolve that issue.

[dirk-thomas]

Please use Python 3 compatible syntax:

except OSError as e:

[wjwwood]

Other than the comments that looks reasonable, +1. Thanks!

[dirk-thomas]

Please move the import to the top of the file (between __future__ and os).

[dirk-thomas]

Also please comment on the ticket when committing changes. Otherwise we don't get a notification.

[czalidis]

Ok, sorry for the inconvenience. I have looked into the comments and modified my commit accordingly.

[dirk-thomas]

Np. Thank you for the patch.