problem debugging ravenscar programs if runtime is stripped

Trying to debug a program using a stripped version of the ravenscar
runtime, we can get the following error:

    (gdb) cont
    Continuing.
    Cannot find Ada_Task_Control_Block type. Aborting

This is because the ravenscar-thread layer makes the assumption that
the runtime is built the way we expect it, meaning that the Ada tasking
units we rely on for Ada tasking debugging, are built with debugging
information, and that this debug information has not been stripped from
the runtime.

When this assumption is not true, resuming such a program can trigger
the error above, which then leads GDB a little confused. For instance,
we can see things like:

     (gdb) bt
     Target is executing.

This patch fixes the issue by disabling the ravenscar thread layer
if we detect that the runtime is missing some of the debugging info
we need in order to support Ada task debugging. This is the best
we can do, as the ravenscar-thread layer actually depends on the
ada-tasks layer to implement thread debugging.

gdb/ChangeLog:

        * ada-lang.h (ada_get_tcb_types_info): Add declaration.
        * ada-tasks.c (ada_get_tcb_types_info): Renames get_tcb_types_info.
        Make non-static.  Change return type to char *.  Adjust code
        accordingly.  Rewrite the function's documentation.
        (read_atcb): Adjust call to get_tcb_types_info accordingly.
        * ravenscar-thread.c (ravenscar_inferior_created): Check that
        we have enough debugging information in the runtime to support
        Ada task debugging before we enable the ravenscar-thread layer.

gdb/ChangeLog

@@ -1,3 +1,14 @@
+2017-11-21  Joel Brobecker  <brobecker@adacore.com>
+
+	* ada-lang.h (ada_get_tcb_types_info): Add declaration.
+	* ada-tasks.c (ada_get_tcb_types_info): Renames get_tcb_types_info.
+	Make non-static.  Change return type to char *.  Adjust code
+	accordingly.  Rewrite the function's documentation.
+	(read_atcb): Adjust call to get_tcb_types_info accordingly.
+	* ravenscar-thread.c (ravenscar_inferior_created): Check that
+	we have enough debugging information in the runtime to support
+	Ada task debugging before we enable the ravenscar-thread layer.
+
 2017-11-21  Joel Brobecker  <brobecker@adacore.com>
 
 	* ada-lang.h (ada_get_task_info_from_ptid): Add declaration.

gdb/ada-lang.h

@@ -409,6 +409,8 @@ typedef void (ada_task_list_iterator_ftype) (struct ada_task_info *task);
 extern void iterate_over_live_ada_tasks
   (ada_task_list_iterator_ftype *iterator);
 
+extern const char *ada_get_tcb_types_info (void);
+
 extern int ada_build_task_list (void);
 
 extern void print_ada_task_info (struct ui_out *uiout,

gdb/ada-tasks.c

@@ -466,18 +466,17 @@ read_fat_string_value (char *dest, struct value *val, int max_len)
   dest[len] = '\0';
 }
 
-/* Get from the debugging information the type description of all types
-   related to the Ada Task Control Block that will be needed in order to
-   read the list of known tasks in the Ada runtime.  Also return the
-   associated ATCB_FIELDNOS.
+/* Get, from the debugging information, the type description of all types
+   related to the Ada Task Control Block that are needed in order to
+   read the list of known tasks in the Ada runtime.  If all of the info
+   needed to do so is found, then save that info in the module's per-
+   program-space data, and return NULL.  Otherwise, if any information
+   cannot be found, leave the per-program-space data untouched, and
+   return an error message explaining what was missing (that error
+   message does NOT need to be deallocated).  */
 
-   Error handling:  Any data missing from the debugging info will cause
-   an error to be raised, and none of the return values to be set.
-   Users of this function can depend on the fact that all or none of the
-   return values will be set.  */
-
-static void
-get_tcb_types_info (void)
+const char *
+ada_get_tcb_types_info (void)
 {
   struct type *type;
   struct type *common_type;
@@ -518,7 +517,7 @@ get_tcb_types_info (void)
 					    NULL).symbol;
 
       if (atcb_sym == NULL || atcb_sym->type == NULL)
-        error (_("Cannot find Ada_Task_Control_Block type. Aborting"));
+        return _("Cannot find Ada_Task_Control_Block type");
 
       type = atcb_sym->type;
     }
@@ -531,11 +530,11 @@ get_tcb_types_info (void)
     }
 
   if (common_atcb_sym == NULL || common_atcb_sym->type == NULL)
-    error (_("Cannot find Common_ATCB type. Aborting"));
+    return _("Cannot find Common_ATCB type");
   if (private_data_sym == NULL || private_data_sym->type == NULL)
-    error (_("Cannot find Private_Data type. Aborting"));
+    return _("Cannot find Private_Data type");
   if (entry_call_record_sym == NULL || entry_call_record_sym->type == NULL)
-    error (_("Cannot find Entry_Call_Record type. Aborting"));
+    return _("Cannot find Entry_Call_Record type");
 
   /* Get the type for Ada_Task_Control_Block.Common.  */
   common_type = common_atcb_sym->type;
@@ -583,6 +582,7 @@ get_tcb_types_info (void)
   pspace_data->atcb_ll_type = ll_type;
   pspace_data->atcb_call_type = call_type;
   pspace_data->atcb_fieldno = fieldnos;
+  return NULL;
 }
 
 /* Build the PTID of the task from its COMMON_VALUE, which is the "Common"
@@ -630,7 +630,12 @@ read_atcb (CORE_ADDR task_id, struct ada_task_info *task_info)
     = get_ada_tasks_pspace_data (current_program_space);
 
   if (!pspace_data->initialized_p)
-    get_tcb_types_info ();
+    {
+      const char *err_msg = ada_get_tcb_types_info ();
+
+      if (err_msg != NULL)
+	error (_("%s. Aborting"), err_msg);
+    }
 
   tcb_value = value_from_contents_and_address (pspace_data->atcb_type,
 					       NULL, task_id);

gdb/ravenscar-thread.c

@@ -519,12 +519,20 @@ ravenscar_core_of_thread (struct target_ops *ops, ptid_t ptid)
 static void
 ravenscar_inferior_created (struct target_ops *target, int from_tty)
 {
+  const char *err_msg;
 
   if (!ravenscar_task_support
       || gdbarch_ravenscar_ops (target_gdbarch ()) == NULL
       || !has_ravenscar_runtime ())
     return;
 
+  err_msg = ada_get_tcb_types_info ();
+  if (err_msg != NULL)
+    {
+      warning (_("%s. Task/thread support disabled.\n"), err_msg);
+      return;
+    }
+
   ravenscar_update_inferior_ptid ();
   push_target (&ravenscar_ops);
 }