Re: [PATCH 02/11] gdb: introduce intrusive_list, make thread_info use it

[Simon Marchi via Gdb-patches <gdb-patches@sourceware.org>]

> Or can we define a pretty printer on the `struct thread_info *` type,
> such that when a thread_info pointer is printed, we can print some extra
> information next to the pointer value?  Like:
> 
>   (gdb) print tp
>   $1 = (thread_info *) 0x61700002c000 { id=1.1, ptid=1000.1000.0, state=THREAD_RUNNING }
> 
> So presumably, when printing the list, that would give:
> 
>   $1 = intrusive list of thread_info = {
>     0x61700002c000 {id = 1.1, ptid = 1000.1000.0, state = THREAD_RUNNING},
>     0x617000069080 {id = 1.3, ptid = 1000.1002.0, state = THREAD_STOPPED},
>     0x617000069400 {id = 1.5, ptid = 1000.3672.0, state = THREAD_STOPPED}
>   }
> 
> But I don't even know if that is possible.  I'll give it a try and report back.

That kind of works:

    (top-gdb) p current_inferior_.m_obj .thread_list .m_front
    $5 = (struct thread_info *) 0x61700003b180 {id = 1.1, ptid = 2243033.2243033.0}
    (top-gdb) p current_inferior_.m_obj .thread_list
    $6 = intrusive list of struct thread_info = {(struct thread_info *) 0x61700003b180 {id = 1.1, ptid = 2243033.2243033.0}, (struct thread_info *) 0x61700003b500 {id = 1.2, ptid = 2243033.2243036.0}, (struct thread_info *) 0x61700003b880 {id = 1.3, ptid = 2243033.2243037.0}, (struct thread_info *) 0x61700003bc00 {id = 1.4, ptid = 2243033.2243038.0}}
    (top-gdb) set print array-indexes
    (top-gdb) p current_inferior_.m_obj .thread_list
    $7 = intrusive list of struct thread_info = {[0] = (struct thread_info *) 0x61700003b180 {id = 1.1, ptid = 2243033.2243033.0}, [1] = (struct thread_info *) 0x61700003b500 {id = 1.2, ptid = 2243033.2243036.0}, [2] = (struct thread_info *) 0x61700003b880 {id = 1.3, ptid = 2243033.2243037.0}, [3] = (struct thread_info *) 0x61700003bc00 {id = 1.4, ptid = 2243033.2243038.0}}

It looks like the output of pretty-printers with display_hint 'array'
isn't affected by "set print pretty", so it's always displayed on one
line, that's strange.

What I don't like about this solution is that I have to replicate the
'(struct thread_info *) 0x12345' part of the display, that is normally
displayed by GDB for a pointer:

    class ThreadInfoPointerPrinter:
	def __init__(self, val):
	    self._val = val

	def to_string(self):
	    ptid = self._val['ptid']
	    inf_num = self._val['inf']['num']
	    per_inf_num = self._val['per_inf_num']

	    return '(struct thread_info *) {} {{id = {}.{}, ptid = {}}}'.format(hex(int(self._val)), inf_num, per_inf_num, ptid)

It would be nice if there was a way to just append some information
after the default display.

> Perhaps that could be done by pretending intrusive_list is a map, making
> the display_hint method return 'map' instead of 'array':
> 
>   https://sourceware.org/gdb/onlinedocs/gdb/Pretty-Printing-API.html#Pretty-Printing-API

I tried this, it does work:

    $1 = intrusive list of struct thread_info = {
      [0] = 0x61700003b180,
      [1] = 0x61700003b500,
      [2] = 0x61700003b880,
      [3] = 0x61700003bc00
    }

However, I see this as going around / abusing our pretty-printing API.
If one doesn't want to see the indices, it becomes impossible (AFAIK).
If you want array-indexes, why not just set array-indexes on?

It is the same when printing an std::vector, you don't have indices:

  $1 = std::vector of length 10, capacity 16 = {0x55555556aeb0, 0x55555556aef0, 0x55555556aed0, 0x55555556af10, 0x55555556af60, 0x55555556afd0, 0x55555556aff0, 0x55555556b010, 0x55555556b030, 0x55555556b0e0}

And for a vector, it's even more natural to index with [].  There too,
it becomes useful to enable array-indexes:

    $4 = std::vector of length 10, capacity 16 = {[0] = 0x55555556aeb0, [1] = 0x55555556aef0, [2] = 0x55555556aed0, [3] = 0x55555556af10, [4] = 0x55555556af60, [5] = 0x55555556afd0, [6] = 0x55555556aff0, [7] = 0x55555556b010, [8] = 0x55555556b030, [9] = 0x55555556b0e0}

Simon