Melodic support complicated by new rosbag dependencies

[dhood]

rosbag has added support for encryption in melodic, which brings in a python dependency on Crypto, which @mikaelarguedas noticed was causing issues building the bridge on bionic, even if the Crypto dependency is installed in the virtualenv on CI:

--- stderr: ros1_bridge
Traceback (most recent call last):
  File "bin/ros1_bridge_generate_factories", line 11, in <module>
    from ros1_bridge import generate_cpp
  File "/home/rosbuild/ci_scripts/ws/src/ros2/ros1_bridge/ros1_bridge/__init__.py", line 82, in <module>
    import rosmsg  # noqa
  File "/opt/ros/melodic/lib/python2.7/dist-packages/rosmsg/__init__.py", line 54, in <module>
    import rosbag
  File "/opt/ros/melodic/lib/python2.7/dist-packages/rosbag/__init__.py", line 33, in <module>
    from .bag import Bag, Compression, ROSBagException, ROSBagFormatException, ROSBagUnindexedException
  File "/opt/ros/melodic/lib/python2.7/dist-packages/rosbag/bag.py", line 53, in <module>
    from Crypto import Random
ModuleNotFoundError: No module named 'Crypto'
make[2]: *** [generated/get_factory.cpp] Error 1

This is complicated in the case of the bridge because we need to import rosmsg dependencies using python2:

ros1_bridge/ros1_bridge/__init__.py

Line 31 in ca30494

# import catkin_pkg and rospkg which are required by rosmsg

and rosbag is a rosmsg dependency.

I have gotten around the Crypto importing issue by importing Crypto using the same workaround as for other dependencies, but it just gets one step closer:

--- stderr: ros1_bridge
Traceback (most recent call last):
  File "bin/ros1_bridge_generate_factories", line 11, in <module>
    from ros1_bridge import generate_cpp
  File "/home/rosbuild/ci_scripts/ws/src/ros2/ros1_bridge/ros1_bridge/__init__.py", line 100, in <module>
    import rosmsg  # noqa
  File "/opt/ros/melodic/lib/python2.7/dist-packages/rosmsg/__init__.py", line 54, in <module>
    import rosbag
  File "/opt/ros/melodic/lib/python2.7/dist-packages/rosbag/__init__.py", line 33, in <module>
    from .bag import Bag, Compression, ROSBagException, ROSBagFormatException, ROSBagUnindexedException
  File "/opt/ros/melodic/lib/python2.7/dist-packages/rosbag/bag.py", line 53, in <module>
    from Crypto import Random
  File "/usr/lib/python2.7/dist-packages/Crypto/Random/__init__.py", line 28, in <module>
    from Crypto.Random import OSRNG
  File "/usr/lib/python2.7/dist-packages/Crypto/Random/OSRNG/__init__.py", line 32, in <module>
    from Crypto.Random.OSRNG.posix import new
  File "/usr/lib/python2.7/dist-packages/Crypto/Random/OSRNG/posix.py", line 66
    except IOError, e:
                  ^
SyntaxError: invalid syntax
make[2]: *** [generated/get_factory.cpp] Error 1

I can keep working around this, but wanted to ask @dirk-thomas if it is possible to instead not import rosbag globally upon import of rosmsg? or to only import Crypto locally inside rosbag, instead? I think then we wouldn't have to modify any import code in the bridge to work around it.

[dirk-thomas]

if it is possible to instead not import rosbag globally upon import of rosmsg?

The import of rosbag could be moved into a local scope in the rosmsg module. That should be the easiest and most straight forward work around.

[mikaelarguedas]

ros/ros_comm#1424 moves the rosbag import to a local scope