FreeBSD: Fix 'Couldn't get registers: Device busy' error (PR gdb/23077)

FreeBSD: Fix 'Couldn't get registers: Device busy' error (PR gdb/23077)

As Rajendra SY reported at
<https://sourceware.org/ml/gdb-patches/2018-04/msg00399.html>, several
attach-related tests are failing on FreeBSD.  The "attach" command
errors with "Couldn't get registers: Device busy".

When the "attach" command is executed, it calls target_attach ->
inf_ptrace_attach, which just does the ptrace(PT_ATTACH), it does not
wait for the child to stop with SIGSTOP.  Afterwards, the command is
complete and we go back to the event loop.  The event loop wakes up
and we end up in target_wait -> fbsd_wait, and handle the SIGSTOP
stop.

At the end of execute_command, though, before going back to the event
loop, we check if the frame language changed via
check_frame_language_change().  That reads the current PC, which is
what leads to the registers read that fails.

The problem is that we fail to mark the attached-to thread as
executing between the initial attach, and the subsequent target_wait.
Until we see the thread stop with SIGSTOP, we shouldn't try to read
registers off of it.  I guess there may a timing issue here - if
you're "lucky", the thread may stop before gdb reads its registers,
masking the problem.

With that fixed, check_frame_language_change() becomes a nop until the
thread is marked not-executing again, after target_wait is called and
we go through handle_inferior_event -> normal_stop.

We haven't seen the problem on Linux because there, the target_attach
implementation waits for the thread to stop before returning.  Still,
that's supposedly hidden from the core, since the Linux target, like
most targets, is a '!to_attach_no_wait' target.

This fixes:
 FAIL: gdb.base/attach.exp: attach1, after setting file
 FAIL: gdb.base/attach.exp: attach2, with no file
 FAIL: gdb.base/attach.exp: load file manually, after attach2 (re-read) (got interactive prompt)
 FAIL: gdb.base/attach.exp: attach when process' a.out not in cwd

 FAIL: gdb.base/dprintf-detach.exp: bai=on ds=gdb dd=on: re-attach to inferior
 FAIL: gdb.base/dprintf-detach.exp: bai=on ds=gdb dd=off: re-attach to inferior
 FAIL: gdb.base/dprintf-detach.exp: bai=on ds=call dd=on: re-attach to inferior
 FAIL: gdb.base/dprintf-detach.exp: bai=on ds=call dd=off: re-attach to inferior
 FAIL: gdb.base/dprintf-detach.exp: bai=on ds=agent dd=on: re-attach to inferior
 FAIL: gdb.base/dprintf-detach.exp: bai=on ds=agent dd=off: re-attach to inferior
 FAIL: gdb.base/dprintf-detach.exp: bai=off ds=gdb dd=on: re-attach to inferior
 FAIL: gdb.base/dprintf-detach.exp: bai=off ds=gdb dd=off: re-attach to inferior
 FAIL: gdb.base/dprintf-detach.exp: bai=off ds=call dd=on: re-attach to inferior
 FAIL: gdb.base/dprintf-detach.exp: bai=off ds=call dd=off: re-attach to inferior
 FAIL: gdb.base/dprintf-detach.exp: bai=off ds=agent dd=on: re-attach to inferior
 FAIL: gdb.base/dprintf-detach.exp: bai=off ds=agent dd=off: re-attach to inferior

gdb/ChangeLog:
2018-04-21  Pedro Alves  <palves@redhat.com>
	    Rajendra SY  <rajendra.sy@gmail.com>

	* inf-ptrace.c (inf_ptrace_attach): Mark the thread as executing.
	* remote.c (extended_remote_attach): In all-stop mode, mark the
	thread as executing.