gdb: Have setter and getter callbacks for settings

The main motivation behind this improvement is to help the
implementation of a patch Simon Marchi is preparing to fix a bug when
MI or Python try to access parameters that are inferior dependent (see
PR/28085).

This commit extends the previous ones, which introduces the setting
object to represent a static variable whose value can be set or shown
with the appropriate commands.  This patch proposes that a setting can
either contain a pointer to a static variable holding a setting, or
pointers to a pair of setter and getter callback functions.

The callbacks functions can be used to retrieve or change the value with
custom logic.  This is useful when the source of truth for a given
setting is not contained in the variable pointed to by the setting
instance.

Given that the callback function call is hidden within the setting
abstraction introduced earlier, none of the sites accessing the setting
needs to be updated.  The registered getter or setter is used whatever
the way to access it is (through MI, Python, Guile, the "with" command
and the $_gdb_setting / $_gdb_setting_str convenience functions).

All the add_setshow_*_cmd are given a new overload that will accept the
pair of function pointers (set / get functions) instead of the pointer
to a global variable.

Tested on GNU/Linux x86_64 with no regression observed.

Change-Id: Ieb81fef57550632ff66e6aa85f637372a226be8c
Co-authored-by: Simon Marchi <simon.marchi@polymtl.ca>

1 files changed, 179 insertions, 15 deletions

diff --git a/gdb/command.h b/gdb/command.h
index e6e6ec8..2abfbc4 100644
--- a/gdb/command.h
+++ b/gdb/command.h
@@ -181,6 +181,20 @@ inline bool var_type_uses<const char *> (var_types t)
   return t == var_enum;
 }
 
+/* Function signature for a callback used to get a value from a setting.  */
+
+template<typename T>
+using setting_getter_ftype = const T &(*) ();
+
+/* Function signature for a callback used to set a value to a setting.  */
+
+template<typename T>
+using setting_setter_ftype = void (*) (const T &);
+
+/* Generic/type-erased function pointer.  */
+
+using erased_func = void (*) ();
+
 /* Interface for getting and setting a setting's value.
 
    The underlying data can be of any VAR_TYPES type.  */
@@ -204,14 +218,30 @@ struct setting
   struct erased_args
   {
     void *var;
+    erased_func setter;
+    erased_func getter;
   };
 
   template<typename T>
-  static erased_args erase_args (var_types var_type, T *var)
+  static erased_args erase_args (var_types var_type,
+				 T *var,
+				 setting_setter_ftype<T> set_setting_func,
+				 setting_getter_ftype<T> get_setting_func)
   {
     gdb_assert (var_type_uses<T> (var_type));
-
-    return {var};
+  /* The getter and the setter must be both provided or both omitted.  */
+    gdb_assert
+      ((set_setting_func == nullptr) == (get_setting_func == nullptr));
+
+  /* The caller must provide a pointer to a variable or get/set functions, but
+     not both.  */
+    gdb_assert ((set_setting_func == nullptr) != (var == nullptr));
+
+    return {
+	var,
+	reinterpret_cast<erased_func> (set_setting_func),
+	reinterpret_cast<erased_func> (get_setting_func)
+    };
   }
 
   /* Create a setting backed by pre-validated type-erased args.
@@ -219,16 +249,37 @@ struct setting
      (see VAR_TYPE_USES).  */
   setting (var_types var_type, const erased_args &args)
     : m_var_type (var_type),
-      m_var (args.var)
+      m_var (args.var),
+      m_getter (args.getter),
+      m_setter (args.setter)
   {
   }
 
-  /* Access the type of the current setting.  */
-  var_types type () const
+  /* Create a setting backed by setter and getter functions.
+
+     Type T must match the var type VAR_TYPE (see VAR_TYPE_USES).  */
+  template<typename T>
+  setting (var_types var_type,
+	   setting_setter_ftype<T> setter,
+	   setting_getter_ftype<T> getter)
+    : m_var_type (var_type)
   {
-    return m_var_type;
+    gdb_assert (var_type_uses<T> (var_type));
+
+    /* Getters and setters are cast to and from the arbitrary `void (*) ()`
+       function pointer type.  Make sure that the two types are really of the
+       same size.  */
+    gdb_static_assert (sizeof (m_getter) == sizeof (getter));
+    gdb_static_assert (sizeof (m_setter) == sizeof (setter));
+
+    m_getter = reinterpret_cast<erased_func> (getter);
+    m_setter = reinterpret_cast<erased_func> (setter);
   }
 
+  /* Access the type of the current setting.  */
+  var_types type () const
+  { return m_var_type; }
+
   /* Return the current value.
 
      The template parameter T is the type of the variable used to store the
@@ -239,13 +290,23 @@ struct setting
     gdb_assert (var_type_uses<T> (m_var_type));
     gdb_assert (m_var != nullptr);
 
-    return *static_cast<const T *> (m_var);
+    if (m_var == nullptr)
+      {
+	gdb_assert (m_getter != nullptr);
+	auto getter = reinterpret_cast<setting_getter_ftype<T>> (m_getter);
+	return getter ();
+      }
+    else
+      return *static_cast<const T *> (m_var);
   }
 
   /* Sets the value of the setting to V.
 
-     The template parameter T indicates the type of the variable used to
-     store the setting.
+     If we have a user-provided setter, use it to set the setting.  Otherwise
+     copy the value V to the internally referenced buffer.
+
+     The template parameter T indicates the type of the variable used to store
+     the setting.
 
      The var_type of the setting must match T.  */
   template<typename T>
@@ -255,16 +316,32 @@ struct setting
        this instantiation.  */
     gdb_assert (var_type_uses<T> (m_var_type));
 
-    *static_cast<T *> (m_var) = v;
+    if (m_var == nullptr)
+      {
+	gdb_assert (m_setter != nullptr);
+	auto setter = reinterpret_cast<setting_setter_ftype<T>> (m_setter);
+	setter (v);
+      }
+    else
+      *static_cast<T *> (m_var) = v;
   }
 
 private:
-  /* The type of the variable M_VAR is pointing to.  */
+  /* The type of the variable M_VAR is pointing to, or that M_GETTER / M_SETTER
+     get or set.  */
   var_types m_var_type;
 
-  /* Pointer to the enclosed variable.  The type of the variable is encoded
-     in M_VAR_TYPE.  */
-  void *m_var;
+  /* Pointer to the enclosed variable
+
+     Either M_VAR is non-nullptr, or both M_GETTER and M_SETTER are
+     non-nullptr.  */
+  void *m_var = nullptr;
+
+  /* Pointer to a user provided getter.  */
+  erased_func m_getter = nullptr;
+
+  /* Pointer to a user provided setter.  */
+  erased_func m_setter = nullptr;
 };
 
 /* This structure records one command'd definition.  */
@@ -548,72 +625,159 @@ extern set_show_commands add_setshow_enum_cmd
    show_value_ftype *show_func, cmd_list_element **set_list,
    cmd_list_element **show_list);
 
+extern set_show_commands add_setshow_enum_cmd
+  (const char *name, command_class theclass, const char *const *enumlist,
+   const char *set_doc, const char *show_doc,
+   const char *help_doc, setting_setter_ftype<const char *> set_func,
+   setting_getter_ftype<const char *> get_func, show_value_ftype *show_func,
+   cmd_list_element **set_list, cmd_list_element **show_list);
+
 extern set_show_commands add_setshow_auto_boolean_cmd
   (const char *name, command_class theclass, auto_boolean *var,
    const char *set_doc, const char *show_doc, const char *help_doc,
    cmd_func_ftype *set_func, show_value_ftype *show_func,
    cmd_list_element **set_list, cmd_list_element **show_list);
 
+extern set_show_commands add_setshow_auto_boolean_cmd
+  (const char *name, command_class theclass, const char *set_doc,
+   const char *show_doc, const char *help_doc,
+   setting_setter_ftype<enum auto_boolean> set_func,
+   setting_getter_ftype<enum auto_boolean> get_func,
+   show_value_ftype *show_func, cmd_list_element **set_list,
+   cmd_list_element **show_list);
+
 extern set_show_commands add_setshow_boolean_cmd
   (const char *name, command_class theclass, bool *var, const char *set_doc,
    const char *show_doc, const char *help_doc, cmd_func_ftype *set_func,
    show_value_ftype *show_func, cmd_list_element **set_list,
    cmd_list_element **show_list);
 
+extern set_show_commands add_setshow_boolean_cmd
+  (const char *name, command_class theclass, const char *set_doc,
+   const char *show_doc, const char *help_doc,
+   setting_setter_ftype<bool> set_func,
+   setting_getter_ftype<bool> get_func, show_value_ftype *show_func,
+   cmd_list_element **set_list, cmd_list_element **show_list);
+
 extern set_show_commands add_setshow_filename_cmd
   (const char *name, command_class theclass, std::string *var, const char *set_doc,
    const char *show_doc, const char *help_doc, cmd_func_ftype *set_func,
    show_value_ftype *show_func, cmd_list_element **set_list,
    cmd_list_element **show_list);
 
+extern set_show_commands add_setshow_filename_cmd
+  (const char *name, command_class theclass, const char *set_doc,
+   const char *show_doc, const char *help_doc,
+   setting_setter_ftype<std::string> set_func,
+   setting_getter_ftype<std::string> get_func, show_value_ftype *show_func,
+   cmd_list_element **set_list, cmd_list_element **show_list);
+
 extern set_show_commands add_setshow_string_cmd
   (const char *name, command_class theclass, std::string *var, const char *set_doc,
    const char *show_doc, const char *help_doc, cmd_func_ftype *set_func,
    show_value_ftype *show_func, cmd_list_element **set_list,
    cmd_list_element **show_list);
 
+extern set_show_commands add_setshow_string_cmd
+  (const char *name, command_class theclass, const char *set_doc,
+   const char *show_doc, const char *help_doc,
+   setting_setter_ftype<std::string> set_func,
+   setting_getter_ftype<std::string> get_func,
+   show_value_ftype *show_func, cmd_list_element **set_list,
+   cmd_list_element **show_list);
+
 extern set_show_commands add_setshow_string_noescape_cmd
   (const char *name, command_class theclass, std::string *var, const char *set_doc,
    const char *show_doc, const char *help_doc, cmd_func_ftype *set_func,
    show_value_ftype *show_func, cmd_list_element **set_list,
    cmd_list_element **show_list);
 
+extern set_show_commands add_setshow_string_noescape_cmd
+  (const char *name, command_class theclass, const char *set_doc,
+   const char *show_doc, const char *help_doc,
+   setting_setter_ftype<std::string> set_func,
+   setting_getter_ftype<std::string> get_func, show_value_ftype *show_func,
+   cmd_list_element **set_list, cmd_list_element **show_list);
+
 extern set_show_commands add_setshow_optional_filename_cmd
   (const char *name, command_class theclass, std::string *var, const char *set_doc,
    const char *show_doc, const char *help_doc, cmd_func_ftype *set_func,
    show_value_ftype *show_func, cmd_list_element **set_list,
    cmd_list_element **show_list);
 
+extern set_show_commands add_setshow_optional_filename_cmd
+  (const char *name, command_class theclass, const char *set_doc,
+   const char *show_doc, const char *help_doc,
+   setting_setter_ftype<std::string> set_func,
+   setting_getter_ftype<std::string> get_func,
+   show_value_ftype *show_func, cmd_list_element **set_list,
+   cmd_list_element **show_list);
+
 extern set_show_commands add_setshow_integer_cmd
   (const char *name, command_class theclass, int *var, const char *set_doc,
    const char *show_doc, const char *help_doc, cmd_func_ftype *set_func,
    show_value_ftype *show_func, cmd_list_element **set_list,
    cmd_list_element **show_list);
 
+extern set_show_commands add_setshow_integer_cmd
+  (const char *name, command_class theclass, const char *set_doc,
+   const char *show_doc, const char *help_doc,
+   setting_setter_ftype<int> set_func,
+   setting_getter_ftype<int> get_func, show_value_ftype *show_func,
+   cmd_list_element **set_list, cmd_list_element **show_list);
+
 extern set_show_commands add_setshow_uinteger_cmd
   (const char *name, command_class theclass, unsigned int *var,
    const char *set_doc, const char *show_doc, const char *help_doc,
    cmd_func_ftype *set_func, show_value_ftype *show_func,
    cmd_list_element **set_list, cmd_list_element **show_list);
 
+extern set_show_commands add_setshow_uinteger_cmd
+  (const char *name, command_class theclass, const char *set_doc,
+   const char *show_doc, const char *help_doc,
+   setting_setter_ftype<unsigned int> set_func,
+   setting_getter_ftype<unsigned int> get_func, show_value_ftype *show_func,
+   cmd_list_element **set_list, cmd_list_element **show_list);
+
 extern set_show_commands add_setshow_zinteger_cmd
   (const char *name, command_class theclass, int *var, const char *set_doc,
    const char *show_doc, const char *help_doc, cmd_func_ftype *set_func,
    show_value_ftype *show_func, cmd_list_element **set_list,
    cmd_list_element **show_list);
 
+extern set_show_commands add_setshow_zinteger_cmd
+  (const char *name, command_class theclass, const char *set_doc,
+   const char *show_doc, const char *help_doc,
+   setting_setter_ftype<int> set_func,
+   setting_getter_ftype<int> get_func, show_value_ftype *show_func,
+   cmd_list_element **set_list, cmd_list_element **show_list);
+
 extern set_show_commands add_setshow_zuinteger_cmd
   (const char *name, command_class theclass, unsigned int *var,
    const char *set_doc, const char *show_doc, const char *help_doc,
    cmd_func_ftype *set_func, show_value_ftype *show_func,
    cmd_list_element **set_list, cmd_list_element **show_list);
 
+extern set_show_commands add_setshow_zuinteger_cmd
+  (const char *name, command_class theclass, const char *set_doc,
+   const char *show_doc, const char *help_doc,
+   setting_setter_ftype<unsigned int> set_func,
+   setting_getter_ftype<unsigned int> get_func, show_value_ftype *show_func,
+   cmd_list_element **set_list, cmd_list_element **show_list);
+
 extern set_show_commands add_setshow_zuinteger_unlimited_cmd
   (const char *name, command_class theclass, int *var, const char *set_doc,
    const char *show_doc, const char *help_doc, cmd_func_ftype *set_func,
    show_value_ftype *show_func, cmd_list_element **set_list,
    cmd_list_element **show_list);
 
+extern set_show_commands add_setshow_zuinteger_unlimited_cmd
+  (const char *name, command_class theclass, const char *set_doc,
+   const char *show_doc, const char *help_doc,
+   setting_setter_ftype<int> set_func, setting_getter_ftype<int> get_func,
+   show_value_ftype *show_func, cmd_list_element **set_list,
+   cmd_list_element **show_list);
+
 /* Do a "show" command for each thing on a command list.  */
 
 extern void cmd_show_list (struct cmd_list_element *, int);