gdb: fix vfork regressions when target-non-stop is off

It was pointed out on the mailing list[1] that after this commit:

  commit b1e0126ec56e099d753c20e91a9f8623aabd6b46
  Date:   Wed Jun 21 14:18:54 2023 +0100

      gdb: don't resume vfork parent while child is still running

the test gdb.base/vfork-follow-parent.exp now has some failures when
run with the native-gdbserver or native-extended-gdbserver boards:

  FAIL: gdb.base/vfork-follow-parent.exp: resolution_method=schedule-multiple: continue to end of inferior 2 (timeout)
  FAIL: gdb.base/vfork-follow-parent.exp: resolution_method=schedule-multiple: inferior 1 (timeout)
  FAIL: gdb.base/vfork-follow-parent.exp: resolution_method=schedule-multiple: print unblock_parent = 1 (timeout)
  FAIL: gdb.base/vfork-follow-parent.exp: resolution_method=schedule-multiple: continue to break_parent (timeout)

The reason that these failures don't show up when run on the standard
unix board is that the test is only run in the default operating mode,
so for Linux this will be all-stop on top of non-stop.

If we adjust the test script so that it runs in the default mode and
with target-non-stop turned off, then we see the same failures on the
unix board.  This commit includes this change.

The way that the test is written means that it is not (currently)
possible to turn on non-stop mode and have the test still work, so
this commit does not do that.

I have also updated the test script so that the vfork child performs
an exec as well as the current exit.  Exec and exit are the two ways
in which a vfork child can release the vfork parent, so testing both
of these cases is useful I think.

In this test the inferior performs a vfork and the vfork-child
immediately exits.  The vfork-parent will wait for the vfork-child and
then blocks waiting for gdb.  Once gdb has released the vfork-parent,
the vfork-parent also exits.

In the test that fails, GDB sets 'detach-on-fork off' and then runs to
the vfork.  At this point the test tries to just "continue", but this
fails as the vfork-parent is still selected, and the parent can't
continue until the vfork-child completes.  As the vfork-child is
stopped by GDB the parent will never stop once resumed, so GDB refuses
to resume it.

The test script then sets 'schedule-multiple on' and once again
continues.  This time GDB, in theory, resumes both the parent and the
child, the parent will be held blocked by the kernel, but the child
will run until it exits, and which point GDB stops again, this time
with inferior 2, the newly exited vfork-child, selected.

What happens after this in the test script is irrelevant as far as
this failure is concerned.

To understand why the test started failing we should consider the
behaviour of four different cases:

  1. All-stop-on-non-stop before commit b1e0126ec56e,

  2. All-stop-on-non-stop after commit b1e0126ec56e,

  3. All-stop-on-all-stop before commit b1e0126ec56e, and

  4. All-stop-on-all-stop after commit b1e0126ec56e.

Only case #4 is failing after commit b1e0126ec56e, but I think the
other cases are interesting because, (a) they inform how we might fix
the regression, and (b) it turns out the behaviour of #2 changed too
with the commit, but the change was harmless.

For #1 All-stop-on-non-stop before commit b1e0126ec56e, what happens
is:

  1. GDB calls proceed with the vfork-parent selected, as schedule
     multiple is on user_visible_resume_ptid returns -1 (everything)
     as the resume_ptid (see proceed function),

  2. As this is all-stop-on-non-stop, every thread is resumed
    individually, so GDB tries to resume both the vfork-parent and the
    vfork-child, both of which succeed,

  3. The vfork-parent is held stopped by the kernel,

  4. The vfork-child completes (exits) at which point the GDB sees the
     EXITED event for the vfork-child and the VFORK_DONE event for the
     vfork-parent,

  5. At this point we might take two paths depending on which event
     GDB handles first, if GDB handles the VFORK_DONE first then:

     (a) As GDB is controlling both parent and child the VFORK_DONE is
         ignored (see handle_vfork_done), the vfork-parent will be
	 resumed,

     (b) GDB processes the EXITED event, selects the (now defunct)
         vfork-child, and stops, returning control to the user.

     Alternatively, if GDB selects the EXITED event first then:

     (c) GDB processes the EXITED event, selects the (now defunct)
         vfork-child, and stops, returning control to the user.

     (d) At some future time the user resumes the vfork-parent, at
         which point the VFORK_DONE is reported to GDB, however, GDB
	 is ignoring the VFORK_DONE (see handle_vfork_done), so the
	 parent is resumed.

For case #2, all-stop-on-non-stop after commit b1e0126ec56e, the
important difference is in step (2) above, now, instead of resuming
both the vfork-parent and the vfork-child, only the vfork-child is
resumed.  As such, when we get to step (5), only a single event, the
EXITED event is reported.

GDB handles the EXITED just as in (5)(c), then, later, when the user
resumes the vfork-parent, the VFORKED_DONE is immediately delivered
from the kernel, but this is ignored just as in (5)(d), and so,
though the pattern of when the vfork-parent is resumed changes, the
overall pattern of which events are reported and when, doesn't
actually change.  In fact, by not resuming the vfork-parent, the order
of events (in this test) is now deterministic, which (maybe?) is a
good thing.

If we now consider case #3, all-stop-on-all-stop before commit
b1e0126ec56e, then what happens is:

  1. GDB calls proceed with the vfork-parent selected, as schedule
     multiple is on user_visible_resume_ptid returns -1 (everything)
     as the resume_ptid (see proceed function),

  2. As this is all-stop-on-all-stop, the resume is passed down to the
     linux-nat target, the vfork-parent is the event thread, while the
     vfork-child is a sibling of the event thread,

  3. In linux_nat_target::resume, GDB calls linux_nat_resume_callback
     for all threads, this causes the vfork-child to be resumed.  Then
     in linux_nat_target::resume, the event thread, the vfork-parent,
     is also resumed.

  4. The vfork-parent is held stopped by the kernel,

  5. The vfork-child completes (exits) at which point the GDB sees the
     EXITED event for the vfork-child and the VFORK_DONE event for the
     vfork-parent,

  6. We are now in a situation identical to step (5) as for
     all-stop-on-non-stop above, GDB selects one of the events to
     handle, and whichever we select the user sees the correct
     behaviour.

And so, finally, we can consider #4, all-stop-on-all-stop after commit
b1e0126ec56e, this is the case that started failing.

We start out just like above, in proceed, the resume_ptid is
-1 (resume everything), due to schedule multiple being on.  And just
like above, due to the target being all-stop, we call
proceed_resume_thread_checked just once, for the current thread,
which, remember, is the vfork-parent thread.

The change in commit b1e0126ec56e was to avoid resuming a vfork-parent
thread, read the commit message for the justification for this change.

However, this means that GDB now rejects resuming the vfork-parent in
this case, which means that nothing gets resumed!  Obviously, if
nothing resumes, then nothing will ever stop, and so GDB appears to
hang.

I considered a couple of solutions which, in the end, I didn't go
with, these were:

  1. Move the vfork-parent check out of proceed_resume_thread_checked,
     and place it in proceed, but only on the all-stop-on-non-stop
     path, this should still address the issue seen in b1e0126ec56e,
     but would avoid the issue seen here.  I rejected this just
     because it didn't feel great to split the checks that exist in
     proceed_resume_thread_checked like this,

  2. Extend the condition in proceed_resume_thread_checked by adding a
     target_is_non_stop_p check.  This would have the same effect as
     idea 1, but leaves all the checks in the same place, which I
     think would be better, but this still just didn't feel right to
     me, and so,

What I noticed was that for the all-stop-on-non-stop, after commit
b1e0126ec56e, we only resumed the vfork-child, and this seems fine.
The vfork-parent isn't going to run anyway (the kernel will hold it
back), so if feels like we there's no harm in just waiting for the
child to complete, and then resuming the parent.

So then I started looking at follow_fork, which is called from the top
of proceed.  This function already has the task of switching between
the parent and child based on which the user wishes to follow.  So, I
wondered, could we use this to switch to the vfork-child in the case
that we are attached to both?

Turns out this is pretty simple to do.

Having done that, now the process is for all-stop-on-all-stop after
commit b1e0126ec56e, and with this new fix is:

  1. GDB calls proceed with the vfork-parent selected, but,

  2. In follow_fork, and follow_fork_inferior, GDB switches the
     selected thread to be that of the vfork-child,

  3. Back in proceed user_visible_resume_ptid returns -1 (everything)
     as the resume_ptid still, but now,

  4. When GDB calls proceed_resume_thread_checked, the vfork-child is
     the current selected thread, this is not a vfork-parent, and so
     GDB allows the proceed to continue to the linux-nat target,

  5. In linux_nat_target::resume, GDB calls linux_nat_resume_callback
     for all threads, this does not resume the vfork-parent (because
     it is a vfork-parent), and then the vfork-child is resumed as
     this is the event thread,

At this point we are back in the same situation as for
all-stop-on-non-stop after commit b1e0126ec56e, that is, the
vfork-child is resumed, while the vfork-parent is held stopped by
GDB.

Eventually the vfork-child will exit or exec, at which point the
vfork-parent will be resumed.

[1] https://inbox.sourceware.org/gdb-patches/3e1e1db0-13d9-dd32-b4bb-051149ae6e76@simark.ca/

1 files changed, 5 insertions, 3 deletions

diff --git a/gdb/infrun.c b/gdb/infrun.c
index 8286026..72852e6 100644
--- a/gdb/infrun.c
+++ b/gdb/infrun.c
@@ -713,7 +713,7 @@ holding the child stopped.  Try \"set detach-on-fork\" or \
 	 (do not restore the parent as the current inferior).  */
       gdb::optional<scoped_restore_current_thread> maybe_restore;
 
-      if (!follow_child)
+      if (!follow_child && !sched_multi)
 	maybe_restore.emplace ();
 
       switch_to_thread (*child_inf->threads ().begin ());
@@ -3400,8 +3400,10 @@ proceed (CORE_ADDR addr, enum gdb_signal siggnal)
   struct gdbarch *gdbarch;
   CORE_ADDR pc;
 
-  /* If we're stopped at a fork/vfork, follow the branch set by the
-     "set follow-fork-mode" command; otherwise, we'll just proceed
+  /* If we're stopped at a fork/vfork, switch to either the parent or child
+     thread as defined by the "set follow-fork-mode" command, or, if both
+     the parent and child are controlled by GDB, and schedule-multiple is
+     on, follow the child.  If none of the above apply then we just proceed
      resuming the current thread.  */
   if (!follow_fork ())
     {