Update the database of only a single ROS distribution

[mikaelarguedas]

feature request / question: would it be possible to specify to rosdep we want to update the database of a single ROS distribution?

Rationale: Often rosdep is slow to run and the more active ROS (1 and 2) distributions the longer it takes to run. Ignoring the EOL distros helped a great bit but it seems like performace would be even better with a single ROS distribution to process.

maybe it could be done if the rosdistro argument is explicitly used rosdep update --rosdistro kinetic, or maybe a different flag --only-selected-rosdistro?
Also it would override the eol flag compared to currently even if you specify explicitly an eol distro it doesnt get updated if it's eol

rosdep update --rosdistro indigo
...
Skip end-of-life distro "indigo"
...

[ruffsl]

Would it make sense for rosdep to default to the ROS_DISTRO env if the arg is unset, same as the --rosdistro arg in rosdep install? If both env and arg are unset, then the default behavior could be the same as is today for rosdep update.

[mikaelarguedas]

Would it make sense for rosdep to default to the ROS_DISTRO env if the arg is unset, same as the --rosdistro arg in rosdep install?

Yeas I was going back and forth on that as it is not uncommon to have a given distro sourced by default but you want to update all the database.
e.g. I have melodic sourced in my bashrc and want to get dependencies to build my dshing workspace. I would usually just do

rosdep update
rosdep install --rosdistro dashing ...

And in that case I wont have the latest state of dashing packages in the db to perform the install.

So my preference would be to not change the behavior unless an argument is passed.
--rosdistro seems a sensible choice but maybe maintainers would prefer introducing a new one for existing behavior to stay unchanged

[mikaelarguedas]

@nuclearsandwich @wjwwood gentle ping. Do you have any feedback on this?

[wjwwood]

Makes sense to me. 👍 for a pull request for this feature.

So my preference would be to not change the behavior unless an argument is passed.

I agree.