When writing the Windows dbgeng driver for Dexter, I couldn't work out why it would either launch a process and leave it free running, or if I started the process suspended, never do anything with it. The result was a hack to create and attach processes manually. This has been flaking out on Reids Windows buildbot, and clearly wasn't a good solution.
Digging into this, it turns out that the "normal" cdb / windbg behaviour of breaking whenever we attach to a process is not the default: it has to be explicitly requested from the debug engine. This patch does so (by setting DEBUG_ENGOPT_INITIAL_BREAK in the engine options), after which we can simply call "CreateProcessAndAttach2" and everything automagically works.
No test for this behaviour: everything was just broken before.