Fix crash with core + TUI + run

Fix crash with core + TUI + run

Ref.: https://bugzilla.redhat.com/show_bug.cgi?id=1765117

A segfault can happen in a specific scenario when using TUI + a
corefile, as explained in the bug mentioned above.  The problem
happens when opening a corefile on GDB:

  $ gdb ./core program

entering TUI (C-x a), and then issuing a "run" command.  GDB segfaults
with the following stack trace:

  (top-gdb) bt
  #0  0x00000000004cd5da in target_ops::shortname (this=0x0) at ../../binutils-gdb/gdb/target.h:449
  #1  0x0000000000ac08fb in target_shortname () at ../../binutils-gdb/gdb/target.h:1323
  #2  0x0000000000ac09ae in tui_locator_window::make_status_line[abi:cxx11]() const (this=0x23e1fa0 <_locator>) at ../../binutils-gdb/gdb/tui/tui-stack.c:86
  #3  0x0000000000ac1043 in tui_locator_window::rerender (this=0x23e1fa0 <_locator>) at ../../binutils-gdb/gdb/tui/tui-stack.c:231
  #4  0x0000000000ac1632 in tui_show_locator_content () at ../../binutils-gdb/gdb/tui/tui-stack.c:369
  #5  0x0000000000ac63b6 in tui_set_key_mode (mode=TUI_COMMAND_MODE) at ../../binutils-gdb/gdb/tui/tui.c:321
  #6  0x0000000000aaf9be in tui_inferior_exit (inf=0x2d446a0) at ../../binutils-gdb/gdb/tui/tui-hooks.c:181
  #7  0x000000000044cddf in std::_Function_handler<void (inferior*), void (*)(inferior*)>::_M_invoke(std::_Any_data const&, inferior*&&) (__functor=..., __args#0=@0x7fffffffd650: 0x2d446a0)
      at /usr/include/c++/9/bits/std_function.h:300
  #8  0x0000000000757db9 in std::function<void (inferior*)>::operator()(inferior*) const (this=0x2cf3168, __args#0=0x2d446a0) at /usr/include/c++/9/bits/std_function.h:690
  #9  0x0000000000757876 in gdb::observers::observable<inferior*>::notify (this=0x23de0c0 <gdb::observers::inferior_exit>, args#0=0x2d446a0)
      at ../../binutils-gdb/gdb/gdbsupport/observable.h:106
  #10 0x000000000075532d in exit_inferior_1 (inftoex=0x2d446a0, silent=1) at ../../binutils-gdb/gdb/inferior.c:191
  #11 0x0000000000755460 in exit_inferior_silent (inf=0x2d446a0) at ../../binutils-gdb/gdb/inferior.c:234
  #12 0x000000000059f47c in core_target::close (this=0x2d68590) at ../../binutils-gdb/gdb/corelow.c:265
  #13 0x0000000000a7688c in target_close (targ=0x2d68590) at ../../binutils-gdb/gdb/target.c:3293
  #14 0x0000000000a63d74 in target_stack::push (this=0x23e1800 <g_target_stack>, t=0x23c38c8 <the_amd64_linux_nat_target>) at ../../binutils-gdb/gdb/target.c:568
  #15 0x0000000000a63dbf in push_target (t=0x23c38c8 <the_amd64_linux_nat_target>) at ../../binutils-gdb/gdb/target.c:583
  #16 0x0000000000748088 in inf_ptrace_target::create_inferior (this=0x23c38c8 <the_amd64_linux_nat_target>, exec_file=0x2d58d30 "/usr/bin/cat", allargs="", env=0x25f12b0, from_tty=1)
      at ../../binutils-gdb/gdb/inf-ptrace.c:128
  #17 0x0000000000795ccb in linux_nat_target::create_inferior (this=0x23c38c8 <the_amd64_linux_nat_target>, exec_file=0x2d58d30 "/usr/bin/cat", allargs="", env=0x25f12b0, from_tty=1)
      at ../../binutils-gdb/gdb/linux-nat.c:1094
  #18 0x000000000074eae9 in run_command_1 (args=0x0, from_tty=1, run_how=RUN_NORMAL) at ../../binutils-gdb/gdb/infcmd.c:639
  ...

The problem happens because 'tui_locator_window::make_status_line'
needs the value of 'target_shortname' in order to update the status
line.  'target_shortname' is a macro which expands to:

  #define	target_shortname	(current_top_target ()->shortname ())

and, in our scenario, 'current_top_target ()' returns NULL, which
obviously causes a segfault.  But why does it return NULL, since,
according to its comment on target.h, it should never do that?

What is happening is that we're being caught in the middle of a
"target switch".  We had the 'core_target' on top, because we were
inspecting a corefile, but when the user decided to invoke "run" GDB
had to actually create the inferior, which ends up detecting that we
have a target already, and tries to close it (from target.c):

  /* See target.h.  */

  void
  target_stack::push (target_ops *t)
  {
    /* If there's already a target at this stratum, remove it.  */
    strata stratum = t->stratum ();

    if (m_stack[stratum] != NULL)
      {
	target_ops *prev = m_stack[stratum];
	m_stack[stratum] = NULL;
	target_close (prev); // <-- here
      }
  ...

When the current target ('core_target') is being closed, it checks for
possible observers registered with it and calls them.  TUI is one of
those observers, it gets called, tries to update the status line, and
GDB crashes.

The real problem is that we are clearing 'm_stack[stratum]', but
forgetting to adjust 'm_top'.  Interestingly, this scenario is covered
in 'target_stack::unpush', but Pedro said he forgot to call it here..
The fix, therefore, is to call '::unpush' if there's a target on the
stack.

This patch has been tested on the Buildbot and no regressions have
been found.  I'm also submitting a testcase for it.

gdb/ChangeLog:
2019-11-18  Sergio Durigan Junior  <sergiodj@redhat.com>
	    Pedro Alves  <palves@redhat.com>

	https://bugzilla.redhat.com/show_bug.cgi?id=1765117
	* target.c (target_stack::push): Call 'unpush' if there's a
	target on top of the stack.

gdb/testsuite/ChangeLog:
2019-11-18  Sergio Durigan Junior  <sergiodj@redhat.com>

	https://bugzilla.redhat.com/show_bug.cgi?id=1765117
	* gdb.tui/corefile-run.exp: New file.

Change-Id: I39e2f8b538c580c8ea5bf1d657ee877e47746c8f