qemu-system debugging broken ([PATCH 03/18] remote.c/all-stop: Implement TARGET_WAITKIND_NO_RESUMED and TARGET_WNOHANG)

[Luis Machado <lgustavo@codesourcery.com>]

Just a heads-up. It looks like this particular commit...

commit 567420d10895611e03d5ee65e6b24c16a69a6e99
Author: Pedro Alves <palves@redhat.com>
Date:   Fri Aug 7 17:23:56 2015 +0100

     remote.c/all-stop: Implement TARGET_WAITKIND_NO_RESUMED and 
TARGET_WNOHANG

     Even though "target remote" supports target-async, the all-stop
     target_wait implementation ignores TARGET_WNOHANG.  If the core
     happens to poll for events and we've already read the stop reply out
     of the serial/socket, remote_wait_as hangs forever instead of
     returning an indication that there are no events to process.  This
     can't happen currently, but later changes will trigger this.

     gdb/ChangeLog:
     2015-08-07  Pedro Alves  <palves@redhat.com>

         * remote.c (remote_wait_as): If not waiting for a stop reply,
         return TARGET_WAITKIND_NO_RESUMED.  If TARGET_WNOHANG is
         requested, don't block waiting forever.

... broke simulator-based debugging. The following is what i get when 
trying to run the gdb.base/break binary on qemu-system for arm eabi:

_ftext () at arm-vector.S:25
25              ldr pc, [pc, #24] @ reset
(gdb) load
Loading section .text, size 0xc01c lma 0x0
Loading section .eh_frame, size 0x48 lma 0xc01c
Loading section .ARM.exidx, size 0x8 lma 0xc064
Loading section .rodata, size 0x398 lma 0xc070
Loading section .data, size 0x8e0 lma 0xc408
Start address 0x40, load size 52452
Transfer rate: 17074 KB/sec, 1748 bytes/write.
(gdb) c
Continuing.
infrun: clear_proceed_status_thread (Thread 1)
infrun: proceed (addr=0xffffffff, signal=GDB_SIGNAL_DEFAULT)
infrun: resume (step=0, signal=GDB_SIGNAL_0), trap_expected=0, current 
thread [Thread 1] at 0x40
infrun: infrun_async(1)
infrun: prepare_to_wait
infrun: target_wait (-1.0.0, status) =
infrun:   -1.0.0 [Thread 0],
infrun:   status->kind = ignore
infrun: TARGET_WAITKIND_IGNORE
infrun: prepare_to_wait
infrun: target_wait (-1.0.0, status) =
infrun:   -1.0.0 [Thread 0],
infrun:   status->kind = ignore
infrun: TARGET_WAITKIND_IGNORE
infrun: prepare_to_wait
infrun: target_wait (-1.0.0, status) =
infrun:   -1.0.0 [Thread 0],
infrun:   status->kind = no-resumed
infrun: TARGET_WAITKIND_NO_RESUMED
infrun: stop_waiting
infrun: clear_step_over_info
No unwaited-for children left.
infrun: infrun_async(0)
(gdb) c
Continuing.
Cannot execute this command while the selected thread is running.
(gdb)
Continuing.
Cannot execute this command while the selected thread is running.

Upon further inspection, it looks like the setting of status->kind to 
TARGET_WAITKIND_NO_RESUMED in remote_wait_as causes GDB to stop waiting 
for events and marks the active thread as running, returning the GDB 
prompt and printing the old familiar message.

Without setting TARGET_WAITKIND_NO_RESUMED, things worked fine and the 
binary runs to completion, like so:

(gdb) load
Loading section .text, size 0xc01c lma 0x0
Loading section .eh_frame, size 0x48 lma 0xc01c
Loading section .ARM.exidx, size 0x8 lma 0xc064
Loading section .rodata, size 0x398 lma 0xc070
Loading section .data, size 0x8e0 lma 0xc408
Start address 0x40, load size 52452
Transfer rate: 17074 KB/sec, 1748 bytes/write.
(gdb) c
Continuing.
infrun: clear_proceed_status_thread (Thread 1)
infrun: proceed (addr=0xffffffff, signal=GDB_SIGNAL_DEFAULT)
infrun: resume (step=0, signal=GDB_SIGNAL_0), trap_expected=0, current 
thread [Thread 1] at 0x40
infrun: infrun_async(1)
infrun: prepare_to_wait
infrun: target_wait (-1.0.0, status) =
infrun:   -1.0.0 [Thread 0],
infrun:   status->kind = ignore
infrun: TARGET_WAITKIND_IGNORE
infrun: prepare_to_wait
infrun: target_wait (-1.0.0, status) =
infrun:   -1.0.0 [Thread 0],
infrun:   status->kind = ignore
infrun: TARGET_WAITKIND_IGNORE
infrun: prepare_to_wait
infrun: target_wait (-1.0.0, status) =
infrun:   -1.0.0 [Thread 0],
infrun:   status->kind = ignore
infrun: TARGET_WAITKIND_IGNORE
infrun: prepare_to_wait
720
infrun: target_wait (-1.0.0, status) =
infrun:   -1.0.0 [Thread 0],
infrun:   status->kind = ignore
infrun: TARGET_WAITKIND_IGNORE
infrun: prepare_to_wait

*** EXIT code 0
infrun: target_wait (-1.0.0, status) =
infrun:   -1.0.0 [Thread 0],
infrun:   status->kind = ignore
infrun: TARGET_WAITKIND_IGNORE
infrun: prepare_to_wait
infrun: target_wait (-1.0.0, status) =
infrun:   -1.0.0 [Thread 0],
infrun:   status->kind = ignore
infrun: TARGET_WAITKIND_IGNORE
infrun: prepare_to_wait
infrun: target_wait (-1.0.0, status) =
infrun:   -1.0.0 [Thread 0],
infrun:   status->kind = ignore
infrun: TARGET_WAITKIND_IGNORE
infrun: prepare_to_wait
infrun: target_wait (-1.0.0, status) =
infrun:   42000.0.0 [Remote target],
infrun:   status->kind = exited, status = 0
infrun: TARGET_WAITKIND_EXITED
[Inferior 1 (Remote target) exited normally]
infrun: stop_waiting
infrun: clear_step_over_info

I don't have a good fix for this at the moment, but it may be related to 
the fact that qemu doesn't report thread lists. It sounds like we 
shouldn't drop to the prompt while we wait for something to happen in 
all-stop mode either.

Both runs above had target-non-stop off.