gdb/python: print name of unwinder that claimed frame in debug message

If we have multiple registered unwinders, this will helps identify which
unwinder was chosen and make it easier to track down potential problems.
Unwinders have a mandatory name argument, which we can use in the
message.

First, make gdb._execute_unwinders return a tuple containing the name,
in addition to the UnwindInfo.  Then, make pyuw_sniffer include the name
in the debug message.

I moved the debug message earlier.  I think it's good to print it as
early as possible, so that we see it in case an assert is hit in the
loop below, for example.

gdb/ChangeLog:

	* python/lib/gdb/__init__.py (_execute_unwinders): Return tuple
	with name of chosen unwinder.
	* python/py-unwind.c (pyuw_sniffer): Print name of chosen
	unwinder in debug message.

Change-Id: Id603545b44a97df2a39dd1872fe1f38ad5059f03

gdb/ChangeLog

@@ -1,3 +1,10 @@
+2021-06-22  Simon Marchi  <simon.marchi@polymtl.ca>
+
+	* python/lib/gdb/__init__.py (_execute_unwinders): Return tuple
+	with name of chosen unwinder.
+	* python/py-unwind.c (pyuw_sniffer): Print name of chosen
+	unwinder in debug message.
+
 2021-06-22  Andreas Schwab  <schwab@suse.de>
 
 	PR symtab/27999

gdb/python/lib/gdb/__init__.py

@@ -90,27 +90,33 @@ def _execute_unwinders(pending_frame):
 
     Arguments:
         pending_frame: gdb.PendingFrame instance.
+
     Returns:
-        gdb.UnwindInfo instance or None.
+        Tuple with:
+
+	  [0] gdb.UnwindInfo instance
+	  [1] Name of unwinder that claimed the frame (type `str`)
+
+	or None, if no unwinder has claimed the frame.
     """
     for objfile in objfiles():
         for unwinder in objfile.frame_unwinders:
             if unwinder.enabled:
                 unwind_info = unwinder(pending_frame)
                 if unwind_info is not None:
-                    return unwind_info
+                    return (unwind_info, unwinder.name)
 
     for unwinder in current_progspace().frame_unwinders:
         if unwinder.enabled:
             unwind_info = unwinder(pending_frame)
             if unwind_info is not None:
-                return unwind_info
+                return (unwind_info, unwinder.name)
 
     for unwinder in frame_unwinders:
         if unwinder.enabled:
             unwind_info = unwinder(pending_frame)
             if unwind_info is not None:
-                return unwind_info
+                return (unwind_info, unwinder.name)
 
     return None
 

gdb/python/py-unwind.c

@@ -556,33 +556,56 @@ pyuw_sniffer (const struct frame_unwind *self, struct frame_info *this_frame,
     }
   gdbpy_ref<> pyo_execute (PyObject_GetAttrString (gdb_python_module,
 						   "_execute_unwinders"));
-  if (pyo_execute == NULL)
+  if (pyo_execute == nullptr)
     {
       gdbpy_print_stack ();
       return 0;
     }
 
-  gdbpy_ref<> pyo_unwind_info
+  /* A (gdb.UnwindInfo, str) tuple, or None.  */
+  gdbpy_ref<> pyo_execute_ret
     (PyObject_CallFunctionObjArgs (pyo_execute.get (),
 				   pyo_pending_frame.get (), NULL));
-  if (pyo_unwind_info == NULL)
+  if (pyo_execute_ret == nullptr)
     {
       /* If the unwinder is cancelled due to a Ctrl-C, then propagate
 	 the Ctrl-C as a GDB exception instead of swallowing it.  */
       gdbpy_print_stack_or_quit ();
       return 0;
     }
-  if (pyo_unwind_info == Py_None)
+  if (pyo_execute_ret == Py_None)
     return 0;
 
+  /* Verify the return value of _execute_unwinders is a tuple of size 2.  */
+  gdb_assert (PyTuple_Check (pyo_execute_ret.get ()));
+  gdb_assert (PyTuple_GET_SIZE (pyo_execute_ret.get ()) == 2);
+
+  if (pyuw_debug)
+    {
+      PyObject *pyo_unwinder_name = PyTuple_GET_ITEM (pyo_execute_ret.get (), 1);
+      gdb::unique_xmalloc_ptr<char> name
+	= python_string_to_host_string (pyo_unwinder_name);
+
+      /* This could happen if the user passed something else than a string
+	 as the unwinder's name.  */
+      if (name == nullptr)
+	{
+	  gdbpy_print_stack ();
+	  name = make_unique_xstrdup ("<failed to get unwinder name>");
+	}
+
+      pyuw_debug_printf ("frame claimed by unwinder %s", name.get ());
+    }
+
   /* Received UnwindInfo, cache data.  */
-  if (PyObject_IsInstance (pyo_unwind_info.get (),
+  PyObject *pyo_unwind_info = PyTuple_GET_ITEM (pyo_execute_ret.get (), 0);
+  if (PyObject_IsInstance (pyo_unwind_info,
 			   (PyObject *) &unwind_info_object_type) <= 0)
     error (_("A Unwinder should return gdb.UnwindInfo instance."));
 
   {
     unwind_info_object *unwind_info =
-      (unwind_info_object *) pyo_unwind_info.get ();
+      (unwind_info_object *) pyo_unwind_info;
     int reg_count = unwind_info->saved_regs->size ();
 
     cached_frame
@@ -613,7 +636,6 @@ pyuw_sniffer (const struct frame_unwind *self, struct frame_info *this_frame,
   }
 
   *cache_ptr = cached_frame;
-  pyuw_debug_printf ("frame claimed");
   return 1;
 }