Fix frame cycle detection

Fix frame cycle detection

The recent commit to make scoped_restore_current_thread's cdtors
exception free regressed gdb.base/eh_return.exp:

  Breakpoint 1, 0x00000000004012bb in eh2 (gdb/frame.c:641: internal-error: frame_id get_frame_id(frame_info*): Assertion `stashed' failed.
  A problem internal to GDB has been detected,
  further debugging may prove unreliable.
  Quit this debugging session? (y or n) FAIL: gdb.base/eh_return.exp: hit breakpoint (GDB internal error)

That testcase uses __builtin_eh_return and, before the regression, the
backtrace at eh2 looked like this:

 (gdb) bt
 #0  0x00000000004006eb in eh2 (p=0x4006ec <continuation>) at src/gdb/testsuite/gdb.base/eh_return.c:54
 Backtrace stopped: previous frame identical to this frame (corrupt stack?)

That "previous frame identical to this frame" is caught by the cycle
detection based on frame id.

The assertion failing is this one:

 638           /* Since this is the first frame in the chain, this should
 639              always succeed.  */
 640           bool stashed = frame_stash_add (fi);
 641           gdb_assert (stashed);

originally added by

  commit f245535cf583ae4ca13b10d47b3c7d3334593ece
  Author:     Pedro Alves <palves@redhat.com>
  AuthorDate: Mon Sep 5 18:41:38 2016 +0100

      Fix PR19927: Avoid unwinder recursion if sniffer uses calls parse_and_eval

The assertion is failing because frame #1's frame id was stashed
before the id of frame #0 is stashed.  The frame id of frame #1 was
stashed here:

 (top-gdb) bt
 #0  frame_stash_add (frame=0x1e24c90) at src/gdb/frame.c:276
 #1  0x0000000000669c1b in get_prev_frame_if_no_cycle (this_frame=0x19f8370) at src/gdb/frame.c:2120
 #2  0x000000000066a339 in get_prev_frame_always_1 (this_frame=0x19f8370) at src/gdb/frame.c:2303
 #3  0x000000000066a360 in get_prev_frame_always (this_frame=0x19f8370) at src/gdb/frame.c:2319
 #4  0x000000000066b56c in get_frame_unwind_stop_reason (frame=0x19f8370) at src/gdb/frame.c:3028
 #5  0x000000000059f929 in dwarf2_frame_cfa (this_frame=0x19f8370) at src/gdb/dwarf2/frame.c:1462
 #6  0x00000000005ce434 in dwarf_evaluate_loc_desc::get_frame_cfa (this=0x7fffffffc070) at src/gdb/dwarf2/loc.c:666
 #7  0x00000000005989a9 in dwarf_expr_context::execute_stack_op (this=0x7fffffffc070, op_ptr=0x1b2a053 "\364\003", op_end=0x1b2a053 "\364\003") at src/gdb/dwarf2/expr.c:1161
 #8  0x0000000000596af6 in dwarf_expr_context::eval (this=0x7fffffffc070, addr=0x1b2a052 "\234\364\003", len=1) at src/gdb/dwarf2/expr.c:303
 #9  0x0000000000597b4e in dwarf_expr_context::execute_stack_op (this=0x7fffffffc070, op_ptr=0x1b2a063 "", op_end=0x1b2a063 "") at src/gdb/dwarf2/expr.c:865
 #10 0x0000000000596af6 in dwarf_expr_context::eval (this=0x7fffffffc070, addr=0x1b2a061 "\221X", len=2) at src/gdb/dwarf2/expr.c:303
 #11 0x00000000005c8b5a in dwarf2_evaluate_loc_desc_full (type=0x1b564d0, frame=0x19f8370, data=0x1b2a061 "\221X", size=2, per_cu=0x1b28760, per_objfile=0x1a84930, subobj_type=0x1b564d0, subobj_byte_offset=0) at src/gdb/dwarf2/loc.c:2260
 #12 0x00000000005c9243 in dwarf2_evaluate_loc_desc (type=0x1b564d0, frame=0x19f8370, data=0x1b2a061 "\221X", size=2, per_cu=0x1b28760, per_objfile=0x1a84930) at src/gdb/dwarf2/loc.c:2444
 #13 0x00000000005cb769 in locexpr_read_variable (symbol=0x1b59840, frame=0x19f8370) at src/gdb/dwarf2/loc.c:3687
 #14 0x0000000000663137 in language_defn::read_var_value (this=0x122ea60 <c_language_defn>, var=0x1b59840, var_block=0x0, frame=0x19f8370) at src/gdb/findvar.c:618
 #15 0x0000000000663c3b in read_var_value (var=0x1b59840, var_block=0x0, frame=0x19f8370) at src/gdb/findvar.c:822
 #16 0x00000000008c7d9f in read_frame_arg (fp_opts=..., sym=0x1b59840, frame=0x19f8370, argp=0x7fffffffc470, entryargp=0x7fffffffc490) at src/gdb/stack.c:542
 #17 0x00000000008c89cd in print_frame_args (fp_opts=..., func=0x1b597c0, frame=0x19f8370, num=-1, stream=0x1aba860) at src/gdb/stack.c:890
 #18 0x00000000008c9bf8 in print_frame (fp_opts=..., frame=0x19f8370, print_level=0, print_what=SRC_AND_LOC, print_args=1, sal=...) at src/gdb/stack.c:1394
 #19 0x00000000008c92b9 in print_frame_info (fp_opts=..., frame=0x19f8370, print_level=0, print_what=SRC_AND_LOC, print_args=1, set_current_sal=1) at src/gdb/stack.c:1119
 #20 0x00000000008c75f0 in print_stack_frame (frame=0x19f8370, print_level=0, print_what=SRC_AND_LOC, set_current_sal=1) at src/gdb/stack.c:366
 #21 0x000000000070250b in print_stop_location (ws=0x7fffffffc9e0) at src/gdb/infrun.c:8110
 #22 0x0000000000702569 in print_stop_event (uiout=0x1a8b9e0, displays=true) at src/gdb/infrun.c:8126
 #23 0x000000000096d04b in tui_on_normal_stop (bs=0x1bcd1c0, print_frame=1) at src/gdb/tui/tui-interp.c:98
 ...

Before the commit to make scoped_restore_current_thread's cdtors
exception free, scoped_restore_current_thread's dtor would call
get_frame_id on the selected frame, and we use
scoped_restore_current_thread pervasively.  That had the side effect
of stashing the frame id of frame #0 before reaching the path shown in
the backtrace.  I.e., the frame id of frame #0 happened to be stashed
before the frame id of frame #1.  But that was by chance, not by
design.

This commit:

  commit 256ae5dbc73d1348850f86ee77a0dc3b04bc7cc0
  Author:     Kevin Buettner <kevinb@redhat.com>
  AuthorDate: Mon Oct 31 12:47:42 2016 -0700

      Stash frame id of current frame before stashing frame id for previous frame

Fixed a similar problem, by making sure get_prev_frame computes the
frame id of the current frame before unwinding the previous frame, so
that the cycle detection works properly.  That fix misses the scenario
we're now running against, because if you notice, the backtrace above
shows that frame #4 calls get_prev_frame_always, not get_prev_frame.
I.e., nothing is calling get_frame_id on the current frame.

The fix here is to move Kevin's fix down from get_prev_frame to
get_prev_frame_always.  Or actually, a bit further down to
get_prev_frame_always_1 -- note that inline_frame_this_id calls
get_prev_frame_always, so we need to be careful to avoid recursion in
that scenario.

gdb/ChangeLog:

	* frame.c (get_prev_frame): Move get_frame_id call from here ...
	(get_prev_frame_always_1): ... to here.
	* inline-frame.c (inline_frame_this_id): Mention
	get_prev_frame_always_1 in comment.

Change-Id: Id960c98ab2d072c48a436c3eb160cc4b2a5cfd1d