Fix GDB crash after Quit thrown from unwinder sniffer

I ran into a GDB crash in gdb.base/bp-cmds-continue-ctrl-c.exp in my
multi-target branch, which turns out exposed a bug that exists in
master too.

That testcase has a breakpoint with a "continue" command associated.
Then the breakpoint is constantly being hit.  At the same time, the
testcase is continualy interrupting the program with Ctrl-C, and
re-resuming it, in a loop.

Running that testcase manually under Valgrind, after a few sequences
of 'Ctrl-C' + 'continue', I got:

 Breakpoint 1, Quit
 (gdb) ==21270== Invalid read of size 8
 ==21270==    at 0x4D8185: pyuw_this_id(frame_info*, void**, frame_id*) (py-unwind.c:461)
 ==21270==    by 0x6D426A: compute_frame_id(frame_info*) (frame.c:505)
 ==21270==    by 0x6D43B7: get_frame_id(frame_info*) (frame.c:537)
 ==21270==    by 0x84F3B8: scoped_restore_current_thread::scoped_restore_current_thread() (thread.c:1678)
 ==21270==    by 0x718E3D: fetch_inferior_event(void*) (infrun.c:4076)
 ==21270==    by 0x7067C9: inferior_event_handler(inferior_event_type, void*) (inf-loop.c:43)
 ==21270==    by 0x45BEF9: handle_target_event(int, void*) (linux-nat.c:4419)
 ==21270==    by 0x6C4255: handle_file_event(file_handler*, int) (event-loop.c:733)
 ==21270==    by 0x6C47F8: gdb_wait_for_event(int) (event-loop.c:859)
 ==21270==    by 0x6C3666: gdb_do_one_event() (event-loop.c:322)
 ==21270==    by 0x6C3712: start_event_loop() (event-loop.c:371)
 ==21270==    by 0x746801: captured_command_loop() (main.c:329)
 ==21270==  Address 0x0 is not stack'd, malloc'd or (recently) free'd
 ==21270==
 ==21270==
 ==21270== Process terminating with default action of signal 11 (SIGSEGV): dumping core
 ==21270==  Access not within mapped region at address 0x0
 ==21270==    at 0x4D8185: pyuw_this_id(frame_info*, void**, frame_id*) (py-unwind.c:461)
 ==21270==    by 0x6D426A: compute_frame_id(frame_info*) (frame.c:505)
 ==21270==    by 0x6D43B7: get_frame_id(frame_info*) (frame.c:537)
 ==21270==    by 0x84F3B8: scoped_restore_current_thread::scoped_restore_current_thread() (thread.c:1678)
 ==21270==    by 0x718E3D: fetch_inferior_event(void*) (infrun.c:4076)
 ==21270==    by 0x7067C9: inferior_event_handler(inferior_event_type, void*) (inf-loop.c:43)
 ==21270==    by 0x45BEF9: handle_target_event(int, void*) (linux-nat.c:4419)
 ==21270==    by 0x6C4255: handle_file_event(file_handler*, int) (event-loop.c:733)
 ==21270==    by 0x6C47F8: gdb_wait_for_event(int) (event-loop.c:859)
 ==21270==    by 0x6C3666: gdb_do_one_event() (event-loop.c:322)
 ==21270==    by 0x6C3712: start_event_loop() (event-loop.c:371)
 ==21270==    by 0x746801: captured_command_loop() (main.c:329)
 ==21270==  If you believe this happened as a result of a stack
 ==21270==  overflow in your program's main thread (unlikely but
 ==21270==  possible), you can try to increase the size of the
 ==21270==  main thread stack using the --main-stacksize= flag.
 ==21270==  The main thread stack size used in this run was 8388608.
 ==21270==

Above, when we get to compute_frame_id, fi->unwind is non-NULL,
meaning, we found an unwinder, in this case the Python unwinder, but
somehow, fi->prologue_cache is left NULL.  pyuw_this_id then crashes
because it assumes fi->prologue_cache is non-NULL:

  static void
  pyuw_this_id (struct frame_info *this_frame, void **cache_ptr,
		struct frame_id *this_id)
  {
    *this_id = ((cached_frame_info *) *cache_ptr)->frame_id;
                                      ^^^^^^^^^^

'*cache_ptr' here is 'fi->prologue_cache'.

There's a quit() call in pyuw_sniffer that I believe is the one that
sometimes triggers the crash above.  The crash can be reproduced
easily with this hack to force a quit out of the python unwinder:

 --- a/gdb/python/py-unwind.c
 +++ b/gdb/python/py-unwind.c
 @@ -497,6 +497,8 @@ pyuw_sniffer (const struct frame_unwind *self, struct frame_info *this_frame,
    struct gdbarch *gdbarch = (struct gdbarch *) (self->unwind_data);
    cached_frame_info *cached_frame;

 +  quit ();
 +
    gdbpy_enter enter_py (gdbarch, current_language);

    TRACE_PY_UNWIND (3, "%s (SP=%s, PC=%s)\n", __FUNCTION__,

After that quit is thrown, any subsequent operation that involves
unwinding results in GDB crashing with SIGSEGV like above.

The problem is that this commit:

  commit 30a9c02feff56bd58a276c2a7262f364baa558ac
  CommitDate: Sun Oct 8 23:16:42 2017 -0600
  Subject: Remove cleanup from frame_prepare_for_sniffer

missed that we need to call frame_cleanup_after_sniffer before
rethrowing the exception too.

Without the fix, the "bt" added to
gdb.base/bp-cmds-continue-ctrl-c.exp in this commit makes GDB crash:

  Running src/gdb/testsuite/gdb.base/bp-cmds-continue-ctrl-c.exp ...
  ERROR: Process no longer exists

gdb/ChangeLog:
2018-02-14  Pedro Alves  <palves@redhat.com>

	* frame-unwind.c (frame_unwind_try_unwinder): Always call
	frame_cleanup_after_sniffer on exception.

gdb/testsuite/ChangeLog:
2018-02-14  Pedro Alves  <palves@redhat.com>

	* gdb.base/bp-cmds-continue-ctrl-c.exp (do_test): Test "bt" after
	getting a "Quit".

1 files changed, 2 insertions, 1 deletions

diff --git a/gdb/frame-unwind.c b/gdb/frame-unwind.c
index 66a28ae..e6e6353 100644
--- a/gdb/frame-unwind.c
+++ b/gdb/frame-unwind.c
@@ -110,13 +110,14 @@ frame_unwind_try_unwinder (struct frame_info *this_frame, void **this_cache,
       /* Catch all exceptions, caused by either interrupt or error.
 	 Reset *THIS_CACHE.  */
       *this_cache = NULL;
+      frame_cleanup_after_sniffer (this_frame);
+
       if (ex.error == NOT_AVAILABLE_ERROR)
 	{
 	  /* This usually means that not even the PC is available,
 	     thus most unwinders aren't able to determine if they're
 	     the best fit.  Keep trying.  Fallback prologue unwinders
 	     should always accept the frame.  */
-	  frame_cleanup_after_sniffer (this_frame);
 	  return 0;
 	}
       throw_exception (ex);