Buildfarm: melodic, debian stretch: gazebo unmet dependency

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Hi,

I'm trying to release a package for melodic. My prerelease builds for ubuntu bionic all work fine, but they are failing for debian stretch.

I'm apparently running into an issue with the gazebo dependencies in debian stretch. My package depends on gazebo, gazebo_ros, and gazebo_plugins. It looks like the gazebo rosdep key resolves to gazebo7 on debian stretch. However, I get the error below when it tries to install gazebo7.

I've not been able to figure out a path forward for resolving this. Any suggestions or ideas?

Thanks!

Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 gazebo7 : Depends: libgazebo7 (= 7.3.1+dfsg-3) but it is not going to be installed
           Depends: libsdformat4 but it is not going to be installed
           Depends: gazebo7-common (= 7.3.1+dfsg-3) but it is not going to be installed
           Recommends: gazebo7-plugin-base but it is not going to be installed
 ros-melodic-gazebo-plugins : Depends: ros-melodic-gazebo-dev but it is not going to be installed
 ros-melodic-gazebo-ros : Depends: ros-melodic-gazebo-dev but it is not going to be installed
E: Unable to correct problems, you have held broken packages.
Invocation failed without any known error condition, printing all lines to debug known error detection:
  1 'Reading package lists...'
  2 'Building dependency tree...'
  3 'Reading state information...'
  4 'Starting pkgProblemResolver with broken count: 3'
  5 'Starting 2 pkgProblemResolver with broken count: 3'
  6 'Investigating (0) gazebo7:amd64 < none -> 7.3.1+dfsg-3 @un puN Ib >'
  7 'Broken gazebo7:amd64 Depends on libgazebo7:amd64 < none | 7.3.1+dfsg-3 @un uH > (= 7.3.1+dfsg-3)'
  8 '  Considering libgazebo7:amd64 0 as a solution to gazebo7:amd64 9998'
  9 '  Re-Instated libsdformat4:amd64'
  10 '  Re-Instated libgazebo7:amd64'
  11 'Broken gazebo7:amd64 Depends on gazebo7-common:amd64 < none | 7.3.1+dfsg-3 @un uH > (= 7.3.1+dfsg-3)'
  12 '  Considering gazebo7-common:amd64 -1 as a solution to gazebo7:amd64 9998'
  13 '  Re-Instated gazebo7-common:amd64'
  14 'Investigating (0) libsdformat6:amd64 < none -> 6.0.0-1~stretch @un uN Ib >'
  15 'Broken libsdformat6:amd64 Breaks on libsdformat4:amd64 < none -> 4.2.0-1 @un uN >'
  16 '  Considering libsdformat4:amd64 1 as a solution to libsdformat6:amd64 8'
  17 '  Added libsdformat4:amd64 to the remove list'
  18 '  Fixing libsdformat6:amd64 via keep of libsdformat4:amd64'
  19 'Investigating (0) libgazebo9:amd64 < none -> 9.0.0-2~stretch @un uN Ib >'
  20 'Broken libgazebo9:amd64 Conflicts on libgazebo7:amd64 < none -> 7.3.1+dfsg-3 @un uN Ib >'
  21 '  Considering libgazebo7:amd64 0 as a solution to libgazebo9:amd64 3'
  22 '  Added libgazebo7:amd64 to the remove list'
  23 '  Fixing libgazebo9:amd64 via keep of libgazebo7:amd64'
  24 'Investigating (0) gazebo9-common:amd64 < none -> 9.0.0-2 ...