gdb: assert that we don't try to get a frame's id while it is computed

I'm dealing these days with a class of bugs that involve trying to get a
certain frame's id while we are in the process of computing it.  In other
words, compute_frame_id being called for a frame, eventually calling
get_frame_id for that same frame.  I don't think this is ever supposed to
happen, as that creates a cyclic dependency.

Usually, these problems cause some failure down the line.  I'm proposing with
this patch to catch them as early as possible, as soon as the situation
described above happens.  I think that helps because the failed assertion will
be closer to the root of the problem.

To do so, the patch changes the frame_info::this_id::p flag from a boolean (is
the frame id computed or not) to a tri-state:

- the frame id is not computed
- the frame id is being computed
- the frame id is computed

Then, we can properly assert that get_frame_id doesn't get called for a frame
whose id is being computed.

To illustrate how that can help, let's imagine we apply the following change to
frame_unwind_got_optimized:

    --- a/gdb/frame-unwind.c
    +++ b/gdb/frame-unwind.c
    @@ -260,8 +260,7 @@ frame_unwind_got_optimized (struct frame_info *frame, int regnum)
       mark_value_bytes_optimized_out (val, 0, TYPE_LENGTH (type));
       VALUE_LVAL (val) = lval_register;
       VALUE_REGNUM (val) = regnum;
    -  VALUE_NEXT_FRAME_ID (val)
    -    = get_frame_id (get_next_frame_sentinel_okay (frame));
    +  VALUE_NEXT_FRAME_ID (val) = get_frame_id (frame);
       return val;
     }

... and run the following command, which leads to a failed assertion (you need
to run the corresponding test to generate the binary first):

    $ ./gdb -q -nx testsuite/outputs/gdb.dwarf2/dw2-undefined-ret-addr/dw2-undefined-ret-addr -ex "b stop_frame" -ex r

Without this patch applied, we catch the issue indirectly, when the top-level
get_frame_id tries to stash the frame:

    /home/smarchi/src/binutils-gdb/gdb/frame.c:593: internal-error: frame_id get_frame_id(frame_info*): Assertion `stashed' failed.

    ...
    #9  0x0000000001af1c3a in internal_error (file=0x1cea060 "/home/smarchi/src/binutils-gdb/gdb/frame.c", line=593, fmt=0x1ce9f80 "%s: Assertion `%s' failed.") at /home/smarchi/src/binutils-gdb/gdbsupport/errors.cc:55
    #10 0x0000000000e9b413 in get_frame_id (fi=0x6210005105e0) at /home/smarchi/src/binutils-gdb/gdb/frame.c:593
    #11 0x0000000000e99e35 in scoped_restore_selected_frame::scoped_restore_selected_frame (this=0x7fff1d8b9760) at /home/smarchi/src/binutils-gdb/gdb/frame.c:308
    #12 0x000000000149a261 in print_frame_args (fp_opts=..., func=0x6210000dd7d0, frame=0x6210005105e0, num=-1, stream=0x60300008a580) at /home/smarchi/src/binutils-gdb/gdb/stack.c:750
    #13 0x000000000149d938 in print_frame (fp_opts=..., frame=0x6210005105e0, print_level=0, print_what=SRC_AND_LOC, print_args=1, sal=...) at /home/smarchi/src/binutils-gdb/gdb/stack.c:1394
    #14 0x000000000149c0c8 in print_frame_info (fp_opts=..., frame=0x6210005105e0, print_level=0, print_what=SRC_AND_LOC, print_args=1, set_current_sal=1) at /home/smarchi/src/binutils-gdb/gdb/stack.c:1119
    #15 0x0000000001498100 in print_stack_frame (frame=0x6210005105e0, print_level=0, print_what=SRC_AND_LOC, set_current_sal=1) at /home/smarchi/src/binutils-gdb/gdb/stack.c:366
    #16 0x00000000010234b7 in print_stop_location (ws=0x7fff1d8ba1f0) at /home/smarchi/src/binutils-gdb/gdb/infrun.c:8366
    #17 0x000000000102362d in print_stop_event (uiout=0x607000018660, displays=true) at /home/smarchi/src/binutils-gdb/gdb/infrun.c:8382
    ...

It freaks out because the frame is already in the stash: it was added by an
inner call to get_frame_id, called indirectly by compute_frame_id.  Debugging
this failure is difficult because we have to backtrack to where this happened.

With the patch applied, we catch the issue earlier, here:

    /home/smarchi/src/binutils-gdb/gdb/frame.c:601: internal-error: frame_id get_frame_id(frame_info*): Assertion `fi->this_id.p != frame_id_status::COMPUTING' failed

    ...
    #9  0x0000000001af22bc in internal_error (file=0x1cea6e0 "/home/smarchi/src/binutils-gdb/gdb/frame.c", line=601, fmt=0x1cea600 "%s: Assertion `%s' failed.") at /home/smarchi/src/binutils-gdb/gdbsupport/errors.cc:55
    #10 0x0000000000e9b7e3 in get_frame_id (fi=0x62100050dde0) at /home/smarchi/src/binutils-gdb/gdb/frame.c:601
    #11 0x0000000000e989b3 in frame_unwind_got_optimized (frame=0x62100050dde0, regnum=16) at /home/smarchi/src/binutils-gdb/gdb/frame-unwind.c:264
    #12 0x0000000000cbe386 in dwarf2_frame_prev_register (this_frame=0x62100050dde0, this_cache=0x62100050ddf8, regnum=16) at /home/smarchi/src/binutils-gdb/gdb/dwarf2/frame.c:1267
    #13 0x0000000000e9f569 in frame_unwind_register_value (next_frame=0x62100050dde0, regnum=16) at /home/smarchi/src/binutils-gdb/gdb/frame.c:1266
    #14 0x0000000000e9eaab in frame_register_unwind (next_frame=0x62100050dde0, regnum=16, optimizedp=0x7ffca814ade0, unavailablep=0x7ffca814adf0, lvalp=0x7ffca814ae10, addrp=0x7ffca814ae20, realnump=0x7ffca814ae00, bufferp=0x7ffca814aec0 "") at /home/smarchi/src/binutils-gdb/gdb/frame.c:1169
    #15 0x0000000000e9f233 in frame_unwind_register (next_frame=0x62100050dde0, regnum=16, buf=0x7ffca814aec0 "") at /home/smarchi/src/binutils-gdb/gdb/frame.c:1225
    #16 0x0000000000f84262 in i386_unwind_pc (gdbarch=0x6210000eed10, next_frame=0x62100050dde0) at /home/smarchi/src/binutils-gdb/gdb/i386-tdep.c:1969
    #17 0x0000000000ec95dd in gdbarch_unwind_pc (gdbarch=0x6210000eed10, next_frame=0x62100050dde0) at /home/smarchi/src/binutils-gdb/gdb/gdbarch.c:3062
    #18 0x0000000000cb5e9d in dwarf2_tailcall_sniffer_first (this_frame=0x62100050dde0, tailcall_cachep=0x62100050dee0, entry_cfa_sp_offsetp=0x7ffca814b160) at /home/smarchi/src/binutils-gdb/gdb/dwarf2/frame-tailcall.c:387
    #19 0x0000000000cbdd38 in dwarf2_frame_cache (this_frame=0x62100050dde0, this_cache=0x62100050ddf8) at /home/smarchi/src/binutils-gdb/gdb/dwarf2/frame.c:1198
    #20 0x0000000000cbe026 in dwarf2_frame_this_id (this_frame=0x62100050dde0, this_cache=0x62100050ddf8, this_id=0x62100050de40) at /home/smarchi/src/binutils-gdb/gdb/dwarf2/frame.c:1226
    #21 0x0000000000e9b447 in compute_frame_id (fi=0x62100050dde0) at /home/smarchi/src/binutils-gdb/gdb/frame.c:580
    #22 0x0000000000e9b89e in get_frame_id (fi=0x62100050dde0) at /home/smarchi/src/binutils-gdb/gdb/frame.c:613
    #23 0x0000000000e99e35 in scoped_restore_selected_frame::scoped_restore_selected_frame (this=0x7ffca814b610) at /home/smarchi/src/binutils-gdb/gdb/frame.c:315
    #24 0x000000000149a8e3 in print_frame_args (fp_opts=..., func=0x6210000dd7d0, frame=0x62100050dde0, num=-1, stream=0x60300008a520) at /home/smarchi/src/binutils-gdb/gdb/stack.c:750
    #25 0x000000000149dfba in print_frame (fp_opts=..., frame=0x62100050dde0, print_level=0, print_what=SRC_AND_LOC, print_args=1, sal=...) at /home/smarchi/src/binutils-gdb/gdb/stack.c:1394
    #26 0x000000000149c74a in print_frame_info (fp_opts=..., frame=0x62100050dde0, print_level=0, print_what=SRC_AND_LOC, print_args=1, set_current_sal=1) at /home/smarchi/src/binutils-gdb/gdb/stack.c:1119
    #27 0x0000000001498782 in print_stack_frame (frame=0x62100050dde0, print_level=0, print_what=SRC_AND_LOC, set_current_sal=1) at /home/smarchi/src/binutils-gdb/gdb/stack.c:366
    #28 0x0000000001023b39 in print_stop_location (ws=0x7ffca814c0a0) at /home/smarchi/src/binutils-gdb/gdb/infrun.c:8366
    #29 0x0000000001023caf in print_stop_event (uiout=0x607000018660, displays=true) at /home/smarchi/src/binutils-gdb/gdb/infrun.c:8382
    ...

Now, we can clearly see that get_frame_id for frame `fi=0x62100050dde0` gets
called while compute_frame_id is active for that frame.  The backtrace is more
helpful to identify the root of the problem.

gdb/ChangeLog:

	* frame.c (enum class frame_id_status): New.
	(struct frame_info) <this_id::p>: Change type to frame_id_status.
	(fprintf_frame): Update.
	(compute_frame_id): Set frame id status to "computing" on entry.
	Set it back to "not_computed" on failure and to "computed" on
	success.
	(get_frame_id): Assert the frame id is not being computed.
	(create_sentinel_frame): Use frame_id_status::COMPUTED.
	(create_new_frame): Likewise.
	(frame_cleanup_after_sniffer): Update assert.

Change-Id: I5f1a25fafe045f756bd75f358892720b30ed20c9

gdb/ChangeLog

@@ -1,3 +1,16 @@
+2020-08-22  Simon Marchi  <simon.marchi@efficios.com>
+
+	* frame.c (enum class frame_id_status): New.
+	(struct frame_info) <this_id::p>: Change type to frame_id_status.
+	(fprintf_frame): Update.
+	(compute_frame_id): Set frame id status to "computing" on entry.
+	Set it back to "not_computed" on failure and to "computed" on
+	success.
+	(get_frame_id): Assert the frame id is not being computed.
+	(create_sentinel_frame): Use frame_id_status::COMPUTED.
+	(create_new_frame): Likewise.
+	(frame_cleanup_after_sniffer): Update assert.
+
 2020-08-20  Simon Marchi  <simon.marchi@polymtl.ca>
 
 	* regcache.c (pid_ptid_regcache_map): New type.

gdb/frame.c

@@ -87,6 +87,18 @@ enum cached_copy_status
   CC_UNAVAILABLE
 };
 
+enum class frame_id_status
+{
+  /* Frame id is not computed.  */
+  NOT_COMPUTED = 0,
+
+  /* Frame id is being computed (compute_frame_id is active).  */
+  COMPUTING,
+
+  /* Frame id has been computed.  */
+  COMPUTED,
+};
+
 /* We keep a cache of stack frames, each of which is a "struct
    frame_info".  The innermost one gets allocated (in
    wait_for_inferior) each time the inferior stops; sentinel_frame
@@ -149,7 +161,7 @@ struct frame_info
   /* This frame's ID.  */
   struct
   {
-    bool p;
+    frame_id_status p;
     struct frame_id value;
   } this_id;
 
@@ -439,21 +451,25 @@ fprint_frame (struct ui_file *file, struct frame_info *fi)
       fprintf_unfiltered (file, "<NULL frame>");
       return;
     }
+
   fprintf_unfiltered (file, "{");
   fprintf_unfiltered (file, "level=%d", fi->level);
   fprintf_unfiltered (file, ",");
+
   fprintf_unfiltered (file, "type=");
   if (fi->unwind != NULL)
     fprint_frame_type (file, fi->unwind->type);
   else
     fprintf_unfiltered (file, "<unknown>");
   fprintf_unfiltered (file, ",");
+
   fprintf_unfiltered (file, "unwind=");
   if (fi->unwind != NULL)
     gdb_print_host_address (fi->unwind, file);
   else
     fprintf_unfiltered (file, "<unknown>");
   fprintf_unfiltered (file, ",");
+
   fprintf_unfiltered (file, "pc=");
   if (fi->next == NULL || fi->next->prev_pc.status == CC_UNKNOWN)
     fprintf_unfiltered (file, "<unknown>");
@@ -468,12 +484,16 @@ fprint_frame (struct ui_file *file, struct frame_info *fi)
   else if (fi->next->prev_pc.status == CC_UNAVAILABLE)
     val_print_unavailable (file);
   fprintf_unfiltered (file, ",");
+
   fprintf_unfiltered (file, "id=");
-  if (fi->this_id.p)
+  if (fi->this_id.p == frame_id_status::NOT_COMPUTED)
-    fprint_frame_id (file, fi->this_id.value);
+    fprintf_unfiltered (file, "<not computed>");
+  else if (fi->this_id.p == frame_id_status::COMPUTING)
+    fprintf_unfiltered (file, "<computing>");
   else
-    fprintf_unfiltered (file, "<unknown>");
+    fprint_frame_id (file, fi->this_id.value);
   fprintf_unfiltered (file, ",");
+
   fprintf_unfiltered (file, "func=");
   if (fi->next != NULL && fi->next->prev_func.status == CC_VALUE)
     fprintf_unfiltered (file, "%s", hex_string (fi->next->prev_func.addr));
@@ -544,20 +564,32 @@ skip_tailcall_frames (struct frame_info *frame)
 static void
 compute_frame_id (struct frame_info *fi)
 {
-  gdb_assert (!fi->this_id.p);
+  gdb_assert (fi->this_id.p == frame_id_status::NOT_COMPUTED);
+
+  unsigned int entry_generation = get_frame_cache_generation ();
+
+  try
+    {
+      /* Mark this frame's id as "being computed.  */
+      fi->this_id.p = frame_id_status::COMPUTING;
 
       if (frame_debug)
 	fprintf_unfiltered (gdb_stdlog, "{ compute_frame_id (fi=%d) ",
 			    fi->level);
+
       /* Find the unwinder.  */
       if (fi->unwind == NULL)
 	frame_unwind_find_by_frame (fi, &fi->prologue_cache);
+
       /* Find THIS frame's ID.  */
       /* Default to outermost if no ID is found.  */
       fi->this_id.value = outer_frame_id;
       fi->unwind->this_id (fi, &fi->prologue_cache, &fi->this_id.value);
       gdb_assert (frame_id_p (fi->this_id.value));
-  fi->this_id.p = true;
+
+      /* Mark this frame's id as "computed".  */
+      fi->this_id.p = frame_id_status::COMPUTED;
+
       if (frame_debug)
 	{
 	  fprintf_unfiltered (gdb_stdlog, "-> ");
@@ -565,6 +597,17 @@ compute_frame_id (struct frame_info *fi)
 	  fprintf_unfiltered (gdb_stdlog, " }\n");
 	}
     }
+  catch (const gdb_exception &ex)
+    {
+      /* On error, revert the frame id status to not computed.  If the frame
+         cache generation changed, the frame object doesn't exist anymore, so
+	 don't touch it.  */
+      if (get_frame_cache_generation () == entry_generation)
+	fi->this_id.p = frame_id_status::NOT_COMPUTED;
+
+      throw;
+    }
+}
 
 /* Return a frame uniq ID that can be used to, later, re-find the
    frame.  */
@@ -575,7 +618,11 @@ get_frame_id (struct frame_info *fi)
   if (fi == NULL)
     return null_frame_id;
 
-  if (!fi->this_id.p)
+  /* It's always invalid to try to get a frame's id while it is being
+     computed.  */
+  gdb_assert (fi->this_id.p != frame_id_status::COMPUTING);
+
+  if (fi->this_id.p == frame_id_status::NOT_COMPUTED)
     {
       /* If we haven't computed the frame id yet, then it must be that
 	 this is the current frame.  Compute it now, and stash the
@@ -1577,7 +1624,7 @@ create_sentinel_frame (struct program_space *pspace, struct regcache *regcache)
      (the unwound PC is the same as the pc), so make it so.  */
   frame->next = frame;
   /* The sentinel frame has a special ID.  */
-  frame->this_id.p = true;
+  frame->this_id.p = frame_id_status::COMPUTED;
   frame->this_id.value = sentinel_frame_id;
   if (frame_debug)
     {
@@ -1797,7 +1844,7 @@ create_new_frame (CORE_ADDR addr, CORE_ADDR pc)
      based on the PC.  */
   frame_unwind_find_by_frame (fi, &fi->prologue_cache);
 
-  fi->this_id.p = true;
+  fi->this_id.p = frame_id_status::COMPUTED;
   fi->this_id.value = frame_id_build (addr, pc);
 
   if (frame_debug)
@@ -2908,7 +2955,7 @@ frame_cleanup_after_sniffer (struct frame_info *frame)
   gdb_assert (!frame->prev_p);
 
   /* The sniffer should not check the frame's ID; that's circular.  */
-  gdb_assert (!frame->this_id.p);
+  gdb_assert (frame->this_id.p != frame_id_status::COMPUTED);
 
   /* Clear cached fields dependent on the unwinder.