Handle pending stops from the Windows kernel

Handle pending stops from the Windows kernel

PR gdb/22992 concerns an assertion failure in gdb when debugging a
certain inferior:

    int finish_step_over(execution_control_state*): Assertion `ecs->event_thread->control.trap_expected' failed.

Initially the investigation centered on the discovery that gdb was not
suspending other threads when attempting to single-step.  This
oversight is corrected in this patch: now, when stepping a thread, gdb
will call SuspendThread on all other threads.

However, the bug persisted even after this change.  In particular,
WaitForDebugEvent could see a stop for a thread that was ostensibly
suspended.  Our theory of what is happening here is that there are
actually simultaneous breakpoint hits, and the Windows kernel queues
the events, causing the second stop to be reported on a suspended
thread.

In Windows 10 or later gdb could use the DBG_REPLY_LATER flag to
ContinueDebugEvent to request that such events be re-reported later.
However, relying on that did not seem advisable, so this patch instead
arranges to queue such "pending" stops, and then to report them later,
once the step has completed.

In the PR, Pedro pointed out that it's best in this scenario to
implement the stopped_by_sw_breakpoint method, so this patch does this
as well.

gdb/ChangeLog
2020-04-08  Tom Tromey  <tromey@adacore.com>

	PR gdb/22992
	* windows-nat.c (current_event): Update comment.
	(last_wait_event, desired_stop_thread_id): New globals.
	(struct pending_stop): New.
	(pending_stops): New global.
	(windows_nat_target) <stopped_by_sw_breakpoint>
	<supports_stopped_by_sw_breakpoint>: New methods.
	(windows_fetch_one_register): Add assertions.  Adjust PC.
	(windows_continue): Handle pending stops.  Suspend other threads
	when stepping.  Use last_wait_event
	(wait_for_debug_event): New function.
	(get_windows_debug_event): Use wait_for_debug_event.  Handle
	pending stops.  Queue spurious stops.
	(windows_nat_target::wait): Set stopped_at_software_breakpoint.
	(windows_nat_target::kill): Use wait_for_debug_event.
	* nat/windows-nat.h (struct windows_thread_info)
	<stopped_at_software_breakpoint>: New field.
	* nat/windows-nat.c (windows_thread_info::resume): Clear
	stopped_at_software_breakpoint.