Problem

Subsystems haven't been able to declare dependencies on other scoped subsystems, just global instances. If you try to do, e.g. from the new tests:

# ...
class ScopedDependentSubsystem(Subsystem):
  options_scope = 'scoped-dependent-subsystem'

  @classmethod
  def subsystem_dependencies(cls):
    return super(ScopedDependentSubsystem, cls).subsystem_dependencies() + (
      DummySubsystem.scoped(cls),
    )
# ...

You'll see:

...
>       for dependency in subsystem.subsystem_dependencies():
                     E       AttributeError: 'SubsystemDependency' object has no attribute 'subsystem_dependencies'
                     
                     src/python/pants/subsystem/subsystem.py:112: AttributeError
                     ======= 1 failed, 9 passed in 0.07 seconds =======
...

Solution

The stacktrace helpfully gives the exact issue clearly -- Subsystem.closure() was not using subsystem_dependencies_iter(), and could therefore receive either a Subsystem or a SubsystemDependency. It has been assuming all dependencies were just Subsystems, which I noticed when I tried to declare a dependency on a scoped subsystem.

Using subsystem_dependencies_iter() and extracting the subsystem_cls from the SubsystemDependency object now allows users to declare dependencies on non-globally-scoped subsystems, which is pretty neat.