Demote package override error to a warning

[cottsay]

Overriding packages in underlay workspaces appears to be more common than originally believed, so it's a good idea to let the dust settle as a warning rather than breaking builds at this point.

[codecov]

Codecov Report

Merging #466 (a165214) into master (29303fb) will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master     #466   +/-   ##
=======================================
  Coverage   80.81%   80.81%           
=======================================
  Files          59       59           
  Lines        3518     3518           
  Branches      668      668           
=======================================
  Hits         2843     2843           
  Misses        630      630           
  Partials       45       45           

Impacted Files	Coverage Δ
colcon_core/verb/build.py	0.00% <0.00%> (ø)

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[cottsay]

Example invocation:

$ . /home/cottsay/colcon/install/setup.bash
$ colcon build --merge --packages-select colcon-core
[0.758s] colcon.colcon_core.verb WARNING Some selected packages are already built in one or more underlay workspaces:
	'colcon-core' is in: /home/cottsay/colcon/install
If a package in a merged underlay workspace is overridden and it installs headers, then all packages in the overlay must sort their include directories by workspace order. Failure to do so may result in build failures or undefined behavior at run time.
If the overridden package is used by another package in any underlay, then the overriding package in the overlay must be API and ABI compatible or undefined behavior at run time may occur.

If you understand the risks and want to override a package anyways, add the following to the command line:
	--allow-overriding colcon-core

This will be promoted to an error in a future release of colcon-core.
Starting >>> colcon-core
Finished <<< colcon-core [0.76s]          

Summary: 1 package finished [1.15s]

[sloretz]

LGTM while we investigate the fallout

[clalancette]

Besides the fact that this should fix our infrastructure, I think that in general it is a good idea to make this a "soft" transition. That's mostly to see what everyone's reaction to this is. By making it a warning, we can gauge how many people complain about it and then revisit making it a hard error later on.

Additionally, I think it might be a good idea to put a page on https://docs.ros.org colcon docs explaining exactly how overlays can go wrong, and possible workarounds. Then we can link to that page from this warning.

@sloretz What do you think?

[clalancette]

Works for me.

[sloretz]

Additionally, I think it might be a good idea to put a page on https://docs.ros.org colcon docs explaining exactly how overlays can go wrong, and possible workarounds.

Yeah, documentation on about overlays can go wrong definitely needs to be updated.