Change extension language pretty-printers to use value API

This changes the extension language pretty-printers to use the value
API.

Note that new functions were needed, for both Guile and Python.
Currently both languages always wrap values by removing the values
from the value chain.  This makes sense to avoid strange behavior with
watchpoints, and to avoid excessive memory use.  However, when
printing, it's important to leave the passed-in value untouched, in
case pretty-printing does nothing -- that way the caller can still
access it.

gdb/ChangeLog
2020-03-13  Tom Tromey  <tom@tromey.com>

	* valprint.c (do_val_print): Update.
	* python/python-internal.h (gdbpy_apply_val_pretty_printer): Take
	a struct value.
	(value_to_value_object_no_release): Declare.
	* python/py-value.c (value_to_value_object_no_release): New
	function.
	* python/py-prettyprint.c (gdbpy_apply_val_pretty_printer): Take a
	struct value.
	* guile/scm-value.c (vlscm_scm_from_value_no_release): New
	function.
	* guile/scm-pretty-print.c (gdbscm_apply_val_pretty_printer): Take
	a struct value.
	* guile/guile-internal.h (vlscm_scm_from_value_no_release):
	Declare.
	(gdbscm_apply_val_pretty_printer): Take a struct value.
	* extension.h (apply_ext_lang_val_pretty_printer): Take a struct
	value.
	* extension.c (apply_ext_lang_val_pretty_printer): Take a struct
	value.
	* extension-priv.h (struct extension_language_ops)
	<apply_val_pretty_printer>: Take a struct value.
	* cp-valprint.c (cp_print_value): Create a struct value.
	(cp_print_value): Update.

gdb/python/py-prettyprint.c

@@ -558,22 +558,20 @@ print_children (PyObject *printer, const char *hint,
 
 enum ext_lang_rc
 gdbpy_apply_val_pretty_printer (const struct extension_language_defn *extlang,
-				struct type *type,
-				LONGEST embedded_offset, CORE_ADDR address,
+				struct value *value,
 				struct ui_file *stream, int recurse,
-				struct value *val,
 				const struct value_print_options *options,
 				const struct language_defn *language)
 {
+  struct type *type = value_type (value);
   struct gdbarch *gdbarch = get_type_arch (type);
-  struct value *value;
   enum string_repr_result print_result;
 
-  if (value_lazy (val))
-    value_fetch_lazy (val);
+  if (value_lazy (value))
+    value_fetch_lazy (value);
 
   /* No pretty-printer support for unavailable values.  */
-  if (!value_bytes_available (val, embedded_offset, TYPE_LENGTH (type)))
+  if (!value_bytes_available (value, 0, TYPE_LENGTH (type)))
     return EXT_LANG_RC_NOP;
 
   if (!gdb_python_initialized)
@@ -581,10 +579,7 @@ gdbpy_apply_val_pretty_printer (const struct extension_language_defn *extlang,
 
   gdbpy_enter enter_py (gdbarch, language);
 
-  /* Instantiate the printer.  */
-  value = value_from_component (val, type, embedded_offset);
-
-  gdbpy_ref<> val_obj (value_to_value_object (value));
+  gdbpy_ref<> val_obj (value_to_value_object_no_release (value));
   if (val_obj == NULL)
     {
       print_stack_unless_memory_error (stream);

gdb/python/py-value.c

@@ -1788,6 +1788,27 @@ value_to_value_object (struct value *val)
   return (PyObject *) val_obj;
 }
 
+/* Returns an object for a value, but without releasing it from the
+   all_values chain.  */
+PyObject *
+value_to_value_object_no_release (struct value *val)
+{
+  value_object *val_obj;
+
+  val_obj = PyObject_New (value_object, &value_object_type);
+  if (val_obj != NULL)
+    {
+      value_incref (val);
+      val_obj->value = val;
+      val_obj->address = NULL;
+      val_obj->type = NULL;
+      val_obj->dynamic_type = NULL;
+      note_value (val_obj);
+    }
+
+  return (PyObject *) val_obj;
+}
+
 /* Returns a borrowed reference to the struct value corresponding to
    the given value object.  */
 struct value *

gdb/python/python-internal.h

@@ -391,10 +391,8 @@ extern int gdbpy_auto_load_enabled (const struct extension_language_defn *);
 
 extern enum ext_lang_rc gdbpy_apply_val_pretty_printer
   (const struct extension_language_defn *,
-   struct type *type,
-   LONGEST embedded_offset, CORE_ADDR address,
+   struct value *value,
    struct ui_file *stream, int recurse,
-   struct value *val,
    const struct value_print_options *options,
    const struct language_defn *language);
 extern enum ext_lang_bt_status gdbpy_apply_frame_filter
@@ -456,6 +454,7 @@ PyObject *symbol_to_symbol_object (struct symbol *sym);
 PyObject *block_to_block_object (const struct block *block,
 				 struct objfile *objfile);
 PyObject *value_to_value_object (struct value *v);
+PyObject *value_to_value_object_no_release (struct value *v);
 PyObject *type_to_type_object (struct type *);
 PyObject *frame_info_to_frame_object (struct frame_info *frame);
 PyObject *symtab_to_linetable_object (PyObject *symtab);