gdb: fix assert when quitting GDB while a thread is stepping

gdb: fix assert when quitting GDB while a thread is stepping

This commit addresses one of the issues identified in PR gdb/28275.

Bug gdb/28275 identifies a number of situations in which this assert:

  Assertion `!proc_target->commit_resumed_state' failed.

could be triggered.  There's actually a number of similar places where
this assert is found in GDB, the two of interest in gdb/28275 are in
target_wait and target_stop.

In one of the comments:

  https://sourceware.org/bugzilla/show_bug.cgi?id=28275#c1

steps to trigger the assertion within target_stop were identified when
using a modified version of the gdb.threads/detach-step-over.exp test
script.

In the gdb.threads/detach-step-over.exp test, we attach to a
multi-threaded inferior, and continue the inferior in asynchronous
(background) mode.  Each thread is continuously hitting a conditional
breakpoint where the condition is always false.  While the inferior is
running we detach.  The goal is that we detach while GDB is performing a
step-over for the conditional breakpoint in at least one thread.

While detaching, if a step-over is in progress, then GDB has to
complete the step over before we can detach.  This involves calling
target_stop and target_wait (see prepare_for_detach).

As far as gdb/28275 is concerned, the interesting part here, is the
the process_stratum_target::commit_resumed_state variable must be
false when target_stop and target_wait are called.

This is currently ensured because, in detach_command (infrun.c), we
create an instance of scoped_disable_commit_resumed, this ensures that
when target_detach is called, ::commit_resumed_state will be false.

The modification to the test that I propose here, and which exposed
the bug, is that, instead of using "detach" to detach from the
inferior, we instead use "quit".  Quitting GDB after attaching to an
inferior will cause GDB to first detach, and then exit.

When we quit GDB we end up calling target_detach via a different code
path, the stack looks like:

  #0 target_detach
  #1 kill_or_detach
  #2 quit_force
  #3 quit_command

Along this path there is no scoped_disable_commit_resumed created.
::commit_resumed_state can be true when we reach prepare_for_detach,
which calls target_wait and target_stop, so the assertion will trigger.

In this commit, I propose fixing this by adding the creation of a
scoped_disable_commit_resumed into target_detach.  This will ensure
that ::commit_resumed_state is false when calling prepare_for_detach
from within target_detach.

I did consider placing the scoped_disable_commit_resumed in
prepare_for_detach, however, placing it in target_detach ensures that
the target's commit_resumed_state flag is left to false if the detached
inferior was the last one for that target.  It's the same rationale as
for patch "gdb: disable commit resumed in target_kill" that comes later
in this series, but for detach instead of kill.

detach_command still includes a scoped_disable_commit_resumed too, but I
think it is still relevant to cover the resumption at the end of the
function.

Co-Authored-By: Simon Marchi <simon.marchi@efficios.com>
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=28275
Change-Id: Ie128f7aba6ef0e018859275eca372e6ea738e96f