GDB/MI: crash printing "_task" (Ada) argument

In GDB/MI mode, trying to print the arguments of the frame corresponding
to the body of a task ("-stack-list-arguments 1") causes the debugger to
crash.

This is because the compiler adds an implicit argument to that task body
called "_task". mi/mi-cmd-stack.c:list_args_or_locals, which is
responsible for printing the value of our arguments, finds that our
"_task" symbol is an argument, and thus tries to fing the non-argument
equivalent:

              if (SYMBOL_IS_ARGUMENT (sym))
                sym2 = lookup_symbol (SYMBOL_NATURAL_NAME (sym),
                                      block, VAR_DOMAIN,
                                      (int *) NULL);

Unfortunately, it tries using the natural name, which doesn't always
work for Ada parameters, in particular those who are internally-
generated. In our case, The "_task" parameter's natural name is
"<_task>", and that symbol does not exist.  So sym2 is NULL, thus
causing the crash a little later on when trying to dereference it.
We should be using the symbol linkage name in this case, the same
way iterate_over_block_arg_vars already does.

gdb/ChangeLog:

        * mi/mi-cmd-stack.c (list_args_or_locals): For argument symbols,
        use SYMBOL_LINKAGE_NAME to find the corresponding non-argument
        symbol.  Add assertion that sym2 is never NULL.

gdb/testsuite/ChangeLog:

        * gdb.ada/mi_task_arg: New testcase.

gdb/ChangeLog

@@ -1,3 +1,9 @@
+2012-02-03  Joel Brobecker  <brobecker@adacore.com>
+
+	* mi/mi-cmd-stack.c (list_args_or_locals): For argument symbols,
+	use SYMBOL_LINKAGE_NAME to find the corresponding non-argument
+	symbol.  Add assertion that sym2 is never NULL.
+
 2012-02-02  Doug Evans  <dje@google.com>
 
 	* blockframe.c (find_pc_partial_function_gnu_ifunc): Change type of

gdb/mi/mi-cmd-stack.c

@@ -389,11 +389,12 @@ list_args_or_locals (enum what_to_list what, enum print_values values,
 	      struct frame_arg arg, entryarg;
 
 	      if (SYMBOL_IS_ARGUMENT (sym))
-		sym2 = lookup_symbol (SYMBOL_NATURAL_NAME (sym),
+		sym2 = lookup_symbol (SYMBOL_LINKAGE_NAME (sym),
 				      block, VAR_DOMAIN,
 				      (int *) NULL);
 	      else
 		sym2 = sym;
+	      gdb_assert (sym2 != NULL);
 
 	      memset (&arg, 0, sizeof (arg));
 	      arg.sym = sym2;

gdb/testsuite/ChangeLog

@@ -1,3 +1,7 @@
+2012-02-03  Joel Brobecker  <brobecker@adacore.com>
+
+	* gdb.ada/mi_task_arg: New testcase.
+
 2012-02-02  Pedro Alves  <palves@redhat.com>
 
 	* gdb.reverse/until-precsave.exp: Also put "record save" under the

gdb/testsuite/gdb.ada/mi_task_arg.exp

@@ -0,0 +1,51 @@
+# Copyright 2011-2012 Free Software Foundation, Inc.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+load_lib "ada.exp"
+
+set testdir "mi_task_arg"
+set testfile "${testdir}/task_switch"
+set srcfile ${srcdir}/${subdir}/${testfile}.adb
+set binfile ${objdir}/${subdir}/${testfile}
+
+file mkdir ${objdir}/${subdir}/${testdir}
+if {[gdb_compile_ada "${srcfile}" "${binfile}" executable [list debug additional_flags=-gnata ]] != "" } {
+  return -1
+}
+
+load_lib mi-support.exp
+set MIFLAGS "-i=mi"
+
+gdb_exit
+if [mi_gdb_start] {
+    continue
+}
+
+mi_delete_breakpoints
+mi_gdb_reinitialize_dir $srcdir/$subdir
+mi_gdb_load ${binfile}
+
+if ![mi_runto "task_switch.break_me"] then {
+   fail "Cannot run to main, testcase aborted"
+   return 0
+}
+
+# Verify that "-stack-list-arguments" does not cause the debugger to
+# crash when printing the arguments of frame 1 (due to the internally-
+# generated argument "_task").
+mi_gdb_test "-stack-list-arguments 1" \
+            "\\^done,stack-args=\\\[frame=\{level=\"0\",args=\\\[\\\]\},frame=\{level=\"1\",args=\\\[\{name=\"<_task>\",value=\"$hex\"\}\\\]\},frame=\{level=\"2\",args=\\\[\\\]\}.*" \
+            "-stack-list-arguments 1"
+

gdb/testsuite/gdb.ada/mi_task_arg/task_switch.adb

@@ -0,0 +1,70 @@
+--  Copyright 2011-2012 Free Software Foundation, Inc.
+--
+--  This program is free software; you can redistribute it and/or modify
+--  it under the terms of the GNU General Public License as published by
+--  the Free Software Foundation; either version 3 of the License, or
+--  (at your option) any later version.
+--
+--  This program is distributed in the hope that it will be useful,
+--  but WITHOUT ANY WARRANTY; without even the implied warranty of
+--  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+--  GNU General Public License for more details.
+--
+--  You should have received a copy of the GNU General Public License
+--  along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+procedure Task_Switch is
+
+   -------------------
+   -- Declaractions --
+   -------------------
+
+   task type Callee is
+      entry Finito;
+   end Callee;
+   type Callee_Ptr is access Callee;
+
+   task type Caller is
+   end Caller;
+   type Caller_Ptr is access Caller;
+
+   procedure Break_Me;
+
+   My_Caller : Caller_Ptr;
+   My_Callee : Callee_Ptr;
+
+   ------------
+   -- Bodies --
+   ------------
+
+   task body Callee is
+   begin
+      --  Just wait until we are told to terminate this task.
+      --  This is just to maintain this task alive.
+      accept Finito do
+         null;
+      end Finito;
+   end Callee;
+
+   task body Caller is
+   begin
+      Break_Me;
+      My_Callee.Finito;
+   end Caller;
+
+   procedure Break_Me is
+   begin
+      null;
+   end Break_Me;
+
+begin
+
+   --  Make sure to create the Callee task first... And then give it
+   --  enough time to complete its activation phase before we start
+   --  the Caller task.
+   My_Callee := new Callee;
+   delay 0.1;
+
+   My_Caller := new Caller;
+
+end Task_Switch;