labath wrote:

It can happen through timing, but also in single-threaded code as well. A 
thread could reach-but-not execute a breakpoint insn due to some activity on 
the previous insn. E.g. it could be single-stepping over it (so the stop reason 
is "i've completed a step", not "I've hit a breakpoint"), or (on x86, where 
watchpoint hits are reported after the fact), the previous insn could have 
triggered a watchpoint.

None of this really matters for this patch though. The problem here was that 
the lldb-server implementation has assuming that the stop reason of a 
software-single-stepping thread *must* be "i've completed a step" , but that 
won't be the case if the instruction you are stepping over is actually a 
breakpoint insn -- in that case, the reason will be "i've hit a breakpoint 
(other than the temporary bkpt i've set to single-step)". It just so happens 
that without that Jason's patch, lldb would never actually try to step over a 
breakpoint insn it knew of, so we'd never hit this problem.

https://github.com/llvm/llvm-project/pull/108504
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