gdbserver: Reinstall software single-step breakpoints in resume_stopped_resumed_lwps

gdbserver: Reinstall software single-step breakpoints in resume_stopped_resumed_lwps

At the moment, while performing a software single-step, gdbserver fails
to reinsert software single-step breakpoints for a LWP when
interrupted by a signal in another thread.  This commit fixes this
problem by reinstalling software single-step breakpoints in
linux_process_target::resume_stopped_resumed_lwps in
gdbserver/linux-low.cc.

This bug was discovered due to a failing assert in maybe_hw_step()
in gdbserver/linux-low.cc.  Looking at the backtrace revealed
that the caller was linux_process_target::resume_stopped_resumed_lwps.
I was uncertain whether the assert should still be valid when called
from that method, so I tried hoisting the assert from maybe_hw_step
to all callers except resume_stopped_resumed_lwps.  But running the
new test case, described below, showed that merely eliminating the
assert for this case was NOT a good fix - a study of the log file for
the test showed that the single-step operation failed to occur.
Instead GDB (via gdbserver) stopped at the next breakpoint that was
hit.

Zhiyong Yan had proposed a fix which resinserted software single-step
breakpoints, albeit at a different location in linux-low.cc.  Testing
revealed that, while running gdb.threads/pending-fork-event-detach,
the executable associated with that test would die due to a SIGTRAP
after the test program was detached.  Examination of the core file(s)
showed that a breakpoint instruction had been left in program memory.
Test results were otherwise very good, so Zhiyong was definitely on
the right track!

This commit causes software single-step breakpoint(s) to be inserted
before the call to maybe_hw_step in resume_stopped_resumed_lwps.  This
will cause 'has_single_step_breakpoints (thread)' to be true, so that
the assert in maybe_hw_step...

      /* GDBserver must insert single-step breakpoint for software
	 single step.  */
      gdb_assert (has_single_step_breakpoints (thread));

...will no longer fail.  And better still, the single-step breakpoints
are reinstalled, so that stepping will actually work, even when
interrupted.

The C code for the test case was loosely adapted from the reproducer
provided in Zhiyong's bug report for this problem.  The .exp file was
copied from next-fork-other-thread.exp and then tweaked slightly.  As
noted in a comment in next-fork-exec-other-thread.exp, I had to remove
"on" from the loop for non-stop as it was failing on all architectures
(including x86-64) that I tested.  I have a feeling that it ought to
work, but this can be investigated separately and (re)enabled once it
works.  I also increased the number of iterations for the loop running
the "next" commands.  I've had some test runs which don't show the bug
until the loop counter exceeded 100 iterations.  The C file for the
new test uses shorter delays than next-fork-other-thread.c though, so
it doesn't take overly long (IMO) to run this new test.

Running the new test on a Raspberry Pi w/ a 32-bit (Arm) kernel and
userland using a gdbserver build without the fix in this commit shows
the following results:

FAIL: gdb.threads/next-fork-exec-other-thread.exp: fork_func=fork: target-non-stop=auto: non-stop=off: displaced-stepping=auto: i=12: next to other line
FAIL: gdb.threads/next-fork-exec-other-thread.exp: fork_func=fork: target-non-stop=auto: non-stop=off: displaced-stepping=on: i=9: next to other line
FAIL: gdb.threads/next-fork-exec-other-thread.exp: fork_func=fork: target-non-stop=auto: non-stop=off: displaced-stepping=off: i=18: next to other line
FAIL: gdb.threads/next-fork-exec-other-thread.exp: fork_func=fork: target-non-stop=off: non-stop=off: displaced-stepping=auto: i=3: next to other line
FAIL: gdb.threads/next-fork-exec-other-thread.exp: fork_func=fork: target-non-stop=off: non-stop=off: displaced-stepping=on: i=11: next to other line
FAIL: gdb.threads/next-fork-exec-other-thread.exp: fork_func=fork: target-non-stop=off: non-stop=off: displaced-stepping=off: i=1: next to other line
FAIL: gdb.threads/next-fork-exec-other-thread.exp: fork_func=vfork: target-non-stop=auto: non-stop=off: displaced-stepping=auto: i=1: next to break here
FAIL: gdb.threads/next-fork-exec-other-thread.exp: fork_func=vfork: target-non-stop=auto: non-stop=off: displaced-stepping=on: i=3: next to break here
FAIL: gdb.threads/next-fork-exec-other-thread.exp: fork_func=vfork: target-non-stop=auto: non-stop=off: displaced-stepping=off: i=1: next to break here
FAIL: gdb.threads/next-fork-exec-other-thread.exp: fork_func=vfork: target-non-stop=on: non-stop=off: displaced-stepping=auto: i=47: next to other line
FAIL: gdb.threads/next-fork-exec-other-thread.exp: fork_func=vfork: target-non-stop=on: non-stop=off: displaced-stepping=on: i=57: next to other line
FAIL: gdb.threads/next-fork-exec-other-thread.exp: fork_func=vfork: target-non-stop=off: non-stop=off: displaced-stepping=auto: i=1: next to break here
FAIL: gdb.threads/next-fork-exec-other-thread.exp: fork_func=vfork: target-non-stop=off: non-stop=off: displaced-stepping=on: i=10: next to break here
FAIL: gdb.threads/next-fork-exec-other-thread.exp: fork_func=vfork: target-non-stop=off: non-stop=off: displaced-stepping=off: i=1: next to break here

		=== gdb Summary ===

 # of unexpected core files	12
 # of expected passes		3011
 # of unexpected failures	14

Each of the 12 core files were caused by the failed assertion in
maybe_hw_step in linux-low.c.  These correspond to 12 of the
unexpected failures.

When the tests are run using a gdbserver build which includes the fix
in this commit, the results are significantly better, but not perfect:

FAIL: gdb.threads/next-fork-exec-other-thread.exp: fork_func=vfork: target-non-stop=on: non-stop=off: displaced-stepping=auto: i=143: next to other line
FAIL: gdb.threads/next-fork-exec-other-thread.exp: fork_func=vfork: target-non-stop=on: non-stop=off: displaced-stepping=on: i=25: next to other line

		=== gdb Summary ===

 # of expected passes		10178
 # of unexpected failures	2

I think that the two remaining failures are due to some different
problem.  They are also racy - I've seen runs with no failures or only
one failure, but never more than two.  Also, those runs were conducted
with the loop count in next-fork-exec-other-thread.exp set to 200.
During his testing of this fix and the new test case, Luis Machado
found that this test was taking a long time and asked about ways to
speed it up.  I then conducted additional tests in which I gradually
reduced the loop count, timing each one, also noting the number of
failures.  With the loop count set to 30, I found that I could still
reliably reproduce the failures that Zhiyong reported (in which, with
the proper settings, core files are created).  But, with the loop
count set to 30, the other failures noted above were much less likely
to show up.  Anyone wishing to investigate those other failures should
set the loop count back up to 200.

Running the new test on x86-64 and aarch64, both native and
native-gdbserver shows no failures.

Also, I see no regressions when running the entire test suite for
armv7l-unknown-linux-gnueabihf (i.e.  the Raspberry Pi w/ 32-bit
kernel+userland) with --target_board=native-gdbserver.  Additionally,
using --target_board=native-gdbserver, I also see no regressions for
the entire test suite for x86-64 and aarch64 running Fedora 38.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30387
Co-Authored-By: Zhiyong Yan <zhiyong.yan@windriver.com>
Tested-By: Zhiyong Yan <zhiyong.yan@windriver.com>
Tested-By: Luis Machado <luis.machado@arm.com>