FaultError in stack_trace() about drgn HOT 3 OPEN

Actually I don't think it'll take much refactoring. Let's just make drgn_get_initial_registers() (via all of the arch-specific callbacks that implement it) return a NULL struct drgn_register_state * if there are no initial registers. Then each callback can decide what constitutes a hard error vs. a lack of a stack. I.e.,

struct drgn_register_state *regs;
err = drgn_get_initial_registers(..., &regs);
if (err)
    // handle hard error.
if (!regs)
    // return empty stack trace
// Handle common case

from drgn.

I like your option (a) because in general, when stack unwinding hits a FaultError, it stops and returns what it got so far; this would be an extension of that to the first frame. I don't love the idea of checking task.stack directly though, since I'm also not confident about task lifetimes (and it could be racy). I think it'd be better to catch the FaultError/DRGN_ERROR_FAULT at the source (in this case probably linux_kernel_get_initial_registers_x86_64), although that might require some refactoring.

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Sounds good to me, I'll take a look at what sort of refactor it would be.

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