When the failure handler invokes lldb to create a core it looks something like this:

lldb -o 'attach %p' -o 'process save-core core.%p' -o 'detach' -o 'quit'

However if the attach command fails, the rest of the commands - including 'quit' - are not being run. This means the lldb process is hung.

Adding '--batch' to the command line overcomes this problem.