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 one, which introduces the setting
object to represent a static variable that 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 generate values on the fly when
the setting is accessed (or set the value in a context dependent way).
This is useful when the source of truth 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, Guild, 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.

2 files changed, 652 insertions, 40 deletions

diff --git a/gdb/cli/cli-decode.c b/gdb/cli/cli-decode.c
index 865e1331c8f..3b08bda6493 100644
--- a/gdb/cli/cli-decode.c
+++ b/gdb/cli/cli-decode.c
@@ -460,6 +460,10 @@ empty_func (const char *args, int from_tty, cmd_list_element *c)
    CLASS is as in add_cmd.
    VAR_TYPE is the kind of thing we are setting.
    VAR is address of the variable being controlled by this command.
+   SET_SETTING_FUNC is a pointer to an optional function callback used to set
+   the setting value.
+   GET_SETTING_FUNC is a pointer to an optional function callback used to get
+   the setting value.
    DOC is the documentation string.  */
 
 template<typename T>
@@ -469,6 +473,8 @@ add_set_or_show_cmd (const char *name,
 		     enum command_class theclass,
 		     var_types var_type,
 		     T *var,
+		     setting_setter_ftype<T> set_setting_func,
+		     setting_getter_ftype<T> get_setting_func,
 		     const char *doc,
 		     struct cmd_list_element **list)
 {
@@ -476,7 +482,14 @@ add_set_or_show_cmd (const char *name,
 
   gdb_assert (type == set_cmd || type == show_cmd);
   c->type = type;
-  c->var.set_p<T> (var_type, var);
+
+  gdb_assert ((var != nullptr)
+	      != (set_setting_func != nullptr && get_setting_func != nullptr));
+  if (var != nullptr)
+    c->var.set_p<T> (var_type, var);
+  else
+    c->var.set_accessors<T> (var_type, set_setting_func, get_setting_func);
+
   /* This needs to be something besides NULL so that this isn't
      treated as a help class.  */
   c->func = empty_func;
@@ -486,9 +499,11 @@ add_set_or_show_cmd (const char *name,
 /* Add element named NAME to both the command SET_LIST and SHOW_LIST.
    CLASS is as in add_cmd.  VAR_TYPE is the kind of thing we are
    setting.  VAR is address of the variable being controlled by this
-   command.  SET_FUNC and SHOW_FUNC are the callback functions (if
-   non-NULL).  SET_DOC, SHOW_DOC and HELP_DOC are the documentation
-   strings.
+   command.  If nullptr is given as VAR, then both SET_SETTING_FUNC and
+   GET_SETTING_FUNC must be provided. SET_SETTING_FUNC and GET_SETTING_FUNC are
+   callbacks used to access and modify the underlying property, whatever its
+   storage is.  SET_FUNC and SHOW_FUNC are the callback functions (if non-NULL).
+   SET_DOC, SHOW_DOC and HELP_DOC are the documentation strings.
 
    Return the newly created set and show commands.  */
 
@@ -499,6 +514,8 @@ add_setshow_cmd_full (const char *name,
 		      var_types var_type, T *var,
 		      const char *set_doc, const char *show_doc,
 		      const char *help_doc,
+		      setting_setter_ftype<T> set_setting_func,
+		      setting_getter_ftype<T> get_setting_func,
 		      cmd_func_ftype *set_func,
 		      show_value_ftype *show_func,
 		      struct cmd_list_element **set_list,
@@ -520,6 +537,7 @@ add_setshow_cmd_full (const char *name,
       full_show_doc = xstrdup (show_doc);
     }
   set = add_set_or_show_cmd<T> (name, set_cmd, theclass, var_type, var,
+				set_setting_func, get_setting_func,
 				full_set_doc, set_list);
   set->doc_allocated = 1;
 
@@ -527,6 +545,7 @@ add_setshow_cmd_full (const char *name,
     set->func = set_func;
 
   show = add_set_or_show_cmd<T> (name, show_cmd, theclass, var_type, var,
+				 set_setting_func, get_setting_func,
 				 full_show_doc, show_list);
   show->doc_allocated = 1;
   show->show_value_func = show_func;
@@ -558,12 +577,36 @@ add_setshow_enum_cmd (const char *name,
   set_show_commands commands
     =  add_setshow_cmd_full<const char *> (name, theclass, var_enum, var,
 					   set_doc, show_doc, help_doc,
-					   set_func, show_func,
-					   set_list, show_list);
+					   nullptr, nullptr, set_func,
+					   show_func, set_list, show_list);
   commands.set->enums = enumlist;
   return commands;
 }
 
+/* Same as above but using a getter and a setter function instead of a pointer
+   to a global storage buffer.  */
+
+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)
+{
+  auto cmds = add_setshow_cmd_full<const char *> (name, theclass, var_enum,
+						  nullptr, set_doc, show_doc,
+						  help_doc, set_func, get_func,
+						  nullptr, show_func, set_list,
+						  show_list);
+
+  cmds.set->enums = enumlist;
+
+  return cmds;
+}
+
 /* See cli-decode.h.  */
 const char * const auto_boolean_enums[] = { "on", "off", "auto", NULL };
 
@@ -584,17 +627,42 @@ add_setshow_auto_boolean_cmd (const char *name,
 			      struct cmd_list_element **show_list)
 {
   set_show_commands commands
-    = add_setshow_cmd_full<enum auto_boolean> (name, theclass,
-					       var_auto_boolean,var,
-					       set_doc, show_doc, help_doc,
-					       set_func, show_func,
-					       set_list, show_list);
+    = add_setshow_cmd_full<enum auto_boolean> (name, theclass, var_auto_boolean,
+					       var, set_doc, show_doc, help_doc,
+					       nullptr, nullptr, set_func,
+					       show_func, set_list, show_list);
 
   commands.set->enums = auto_boolean_enums;
 
   return commands;
 }
 
+/* Same as above but using a getter and a setter function instead of a pointer
+   to a global storage buffer.  */
+
+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)
+{
+  auto cmds = add_setshow_cmd_full<enum auto_boolean> (name, theclass,
+						       var_auto_boolean,
+						       nullptr, set_doc,
+						       show_doc, help_doc,
+						       set_func, get_func,
+						       nullptr, show_func,
+						       set_list, show_list);
+
+  cmds.set->enums = auto_boolean_enums;
+
+  return cmds;
+}
+
 /* See cli-decode.h.  */
 const char * const boolean_enums[] = { "on", "off", NULL };
 
@@ -616,7 +684,7 @@ add_setshow_boolean_cmd (const char *name, enum command_class theclass, bool *va
   set_show_commands commands
     = add_setshow_cmd_full<bool> (name, theclass, var_boolean, var,
 				  set_doc, show_doc, help_doc,
-				  set_func, show_func,
+				  nullptr, nullptr, set_func, show_func,
 				  set_list, show_list);
 
   commands.set->enums = boolean_enums;
@@ -624,6 +692,29 @@ add_setshow_boolean_cmd (const char *name, enum command_class theclass, bool *va
   return commands;
 }
 
+/* Same as above but using a getter and a setter function instead of a pointer
+   to a global storage buffer.  */
+
+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)
+{
+  auto cmds = add_setshow_cmd_full<bool> (name, theclass, var_boolean, nullptr,
+					  set_doc, show_doc, help_doc,
+					  set_func, get_func, nullptr,
+					  show_func, set_list, show_list);
+
+  cmds.set->enums = boolean_enums;
+
+  return cmds;
+}
+
 /* Add element named NAME to both the set and show command LISTs (the
    list for set/show or some sublist thereof).  */
 
@@ -640,14 +731,38 @@ add_setshow_filename_cmd (const char *name, enum command_class theclass,
   set_show_commands commands
     = add_setshow_cmd_full<std::string> (name, theclass, var_filename, var,
 					 set_doc, show_doc, help_doc,
-					 set_func, show_func,
-					 set_list, show_list);
+					 nullptr, nullptr, set_func,
+					 show_func, set_list, show_list);
 
   set_cmd_completer (commands.set, filename_completer);
 
   return commands;
 }
 
+/* Same as above but using a getter and a setter function instead of a pointer
+   to a global storage buffer.  */
+
+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)
+{
+  auto cmds = add_setshow_cmd_full<std::string> (name, theclass, var_filename,
+						 nullptr, set_doc, show_doc,
+						 help_doc, set_func, get_func,
+						 nullptr, show_func, set_list,
+						 show_list);
+
+  set_cmd_completer (cmds.set, filename_completer);
+
+  return cmds;
+}
+
 /* Add element named NAME to both the set and show command LISTs (the
    list for set/show or some sublist thereof).  */
 
@@ -663,9 +778,9 @@ add_setshow_string_cmd (const char *name, enum command_class theclass,
 {
   set_show_commands commands
     = add_setshow_cmd_full<std::string> (name, theclass, var_string, var,
-					 set_doc, show_doc, help_doc,
-					 set_func, show_func,
-					 set_list, show_list);
+					set_doc, show_doc, help_doc,
+					nullptr, nullptr, set_func,
+					show_func, set_list, show_list);
 
   /* Disable the default symbol completer.  */
   set_cmd_completer (commands.set, nullptr);
@@ -673,6 +788,31 @@ add_setshow_string_cmd (const char *name, enum command_class theclass,
   return commands;
 }
 
+/* Same as above but using a getter and a setter function instead of a pointer
+   to a global storage buffer.  */
+
+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)
+{
+  auto cmds = add_setshow_cmd_full<std::string> (name, theclass, var_string,
+						 nullptr, set_doc, show_doc,
+						 help_doc, set_func, get_func,
+						 nullptr, show_func, set_list,
+						 show_list);
+
+  /* Disable the default symbol completer.  */
+  set_cmd_completer (cmds.set, nullptr);
+
+  return cmds;
+}
+
 /* Add element named NAME to both the set and show command LISTs (the
    list for set/show or some sublist thereof).  */
 
@@ -689,8 +829,8 @@ add_setshow_string_noescape_cmd (const char *name, enum command_class theclass,
   set_show_commands commands
     = add_setshow_cmd_full<std::string> (name, theclass, var_string_noescape,
 					 var, set_doc, show_doc, help_doc,
-					 set_func, show_func, set_list,
-					 show_list);
+					 nullptr, nullptr, set_func, show_func,
+					 set_list, show_list);
 
   /* Disable the default symbol completer.  */
   set_cmd_completer (commands.set, nullptr);
@@ -698,6 +838,32 @@ add_setshow_string_noescape_cmd (const char *name, enum command_class theclass,
   return commands;
 }
 
+/* Same as above but using a getter and a setter function instead of a pointer
+   to a global storage buffer.  */
+
+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)
+{
+  auto cmds = add_setshow_cmd_full<std::string> (name, theclass,
+						 var_string_noescape, nullptr,
+						 set_doc, show_doc, help_doc,
+						 set_func, get_func,
+						 nullptr, show_func, set_list,
+						 show_list);
+
+  /* Disable the default symbol completer.  */
+  set_cmd_completer (cmds.set, nullptr);
+
+  return cmds;
+}
+
 /* Add element named NAME to both the set and show command LISTs (the
    list for set/show or some sublist thereof).  */
 
@@ -714,14 +880,38 @@ add_setshow_optional_filename_cmd (const char *name, enum command_class theclass
   set_show_commands commands
     = add_setshow_cmd_full<std::string> (name, theclass, var_optional_filename,
 					 var, set_doc, show_doc, help_doc,
-					 set_func, show_func, set_list,
-					 show_list);
-
+					 nullptr, nullptr, set_func, show_func,
+					 set_list, show_list);
+		
   set_cmd_completer (commands.set, filename_completer);
 
   return commands;
 }
 
+/* Same as above but using a getter and a setter function instead of a pointer
+   to a global storage buffer.  */
+
+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)
+{
+  auto cmds =
+    add_setshow_cmd_full<std::string> (name, theclass, var_optional_filename,
+				       nullptr, set_doc, show_doc, help_doc,
+				       set_func, get_func, nullptr, show_func,
+				       set_list,show_list);
+
+  set_cmd_completer (cmds.set, filename_completer);
+
+  return cmds;
+}
+
 /* Completes on literal "unlimited".  Used by integer commands that
    support a special "unlimited" value.  */
 
@@ -756,15 +946,39 @@ add_setshow_integer_cmd (const char *name, enum command_class theclass,
 			 struct cmd_list_element **show_list)
 {
   set_show_commands commands
-    = add_setshow_cmd_full<int> (name, theclass, var_integer, var, set_doc,
-				 show_doc, help_doc, set_func, show_func,
-				 set_list, show_list);
+    = add_setshow_cmd_full<int> (name, theclass, var_integer, var,
+				 set_doc, show_doc, help_doc,
+				 nullptr, nullptr, set_func,
+				 show_func, set_list, show_list);
 
   set_cmd_completer (commands.set, integer_unlimited_completer);
 
   return commands;
 }
 
+/* Same as above but using a getter and a setter function instead of a pointer
+   to a global storage buffer.  */
+
+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)
+{
+  auto cmds = add_setshow_cmd_full<int> (name, theclass, var_integer, nullptr,
+					 set_doc, show_doc, help_doc, set_func,
+					 get_func, nullptr, show_func, set_list,
+					 show_list);
+
+  set_cmd_completer (cmds.set, integer_unlimited_completer);
+
+  return cmds;
+}
+
 /* Add element named NAME to both the set and show command LISTs (the
    list for set/show or some sublist thereof).  CLASS is as in
    add_cmd.  VAR is address of the variable which will contain the
@@ -783,14 +997,38 @@ add_setshow_uinteger_cmd (const char *name, enum command_class theclass,
   set_show_commands commands
     = add_setshow_cmd_full<unsigned int> (name, theclass, var_uinteger, var,
 					  set_doc, show_doc, help_doc,
-					  set_func, show_func,
-					  set_list, show_list);
+					  nullptr, nullptr, set_func,
+					  show_func, set_list, show_list);
 
   set_cmd_completer (commands.set, integer_unlimited_completer);
 
   return commands;
 }
 
+/* Same as above but using a getter and a setter function instead of a pointer
+   to a global storage buffer.  */
+
+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)
+{
+  auto cmds = add_setshow_cmd_full<unsigned int> (name, theclass, var_uinteger,
+						  nullptr, set_doc, show_doc,
+						  help_doc, set_func, get_func,
+						  nullptr, show_func, set_list,
+						  show_list);
+
+  set_cmd_completer (cmds.set, integer_unlimited_completer);
+
+  return cmds;
+}
+
 /* Add element named NAME to both the set and show command LISTs (the
    list for set/show or some sublist thereof).  CLASS is as in
    add_cmd.  VAR is address of the variable which will contain the
@@ -808,8 +1046,27 @@ add_setshow_zinteger_cmd (const char *name, enum command_class theclass,
 {
   return add_setshow_cmd_full<int> (name, theclass, var_zinteger, var,
 				    set_doc, show_doc, help_doc,
-				    set_func, show_func,
-				    set_list, show_list);
+				    nullptr, nullptr, set_func,
+				    show_func, set_list, show_list);
+}
+
+/* Same as above but using a getter and a setter function instead of a pointer
+   to a global storage buffer.  */
+
+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)
+{
+  return add_setshow_cmd_full<int> (name, theclass, var_zinteger, nullptr,
+				    set_doc, show_doc, help_doc, set_func,
+				    get_func, nullptr, show_func, set_list,
+				    show_list);
 }
 
 set_show_commands
@@ -826,14 +1083,39 @@ add_setshow_zuinteger_unlimited_cmd (const char *name,
 {
   set_show_commands commands
     = add_setshow_cmd_full<int> (name, theclass, var_zuinteger_unlimited, var,
-				 set_doc, show_doc, help_doc, set_func,
-				 show_func, set_list, show_list);
+				 set_doc, show_doc, help_doc, nullptr,
+				 nullptr, set_func, show_func, set_list,
+				 show_list);
 
   set_cmd_completer (commands.set, integer_unlimited_completer);
 
   return commands;
 }
 
+/* Same as above but using a getter and a setter function instead of a pointer
+   to a global storage buffer.  */
+
+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)
+{
+  auto cmds
+    = add_setshow_cmd_full<int> (name, theclass, var_zuinteger_unlimited,
+				 nullptr, set_doc, show_doc, help_doc, set_func,
+				 get_func, nullptr, show_func, set_list,
+				 show_list);
+
+  set_cmd_completer (cmds.set, integer_unlimited_completer);
+
+  return cmds;
+}
+
 /* Add element named NAME to both the set and show command LISTs (the
    list for set/show or some sublist thereof).  CLASS is as in
    add_cmd.  VAR is address of the variable which will contain the
@@ -851,7 +1133,27 @@ add_setshow_zuinteger_cmd (const char *name, enum command_class theclass,
 {
   return add_setshow_cmd_full<unsigned int> (name, theclass, var_zuinteger,
 					     var, set_doc, show_doc, help_doc,
-					     set_func, show_func, set_list,
+					     nullptr, nullptr, set_func,
+					     show_func, set_list, show_list);
+}
+
+/* Same as above but using a getter and a setter function instead of a pointer
+   to a global storage buffer.  */
+
+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)
+{
+  return add_setshow_cmd_full<unsigned int> (name, theclass, var_zuinteger,
+					     nullptr, set_doc, show_doc,
+					     help_doc, set_func, get_func,
+					     nullptr, show_func, set_list,
 					     show_list);
 }
 
diff --git a/gdb/command.h b/gdb/command.h
index 30f91bb1b9b..f7a193334f6 100644
--- a/gdb/command.h
+++ b/gdb/command.h
@@ -181,6 +181,194 @@ 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 = T (*) ();
+
+/* Contains a function to access the value of a setting.  */
+
+union setting_getter
+{
+  setting_getter_ftype<bool> get_bool;
+  setting_getter_ftype<int> get_int;
+  setting_getter_ftype<unsigned int> get_uint;
+  setting_getter_ftype<auto_boolean> get_auto_boolean;
+  setting_getter_ftype<std::string> get_string;
+  setting_getter_ftype<const char *> get_const_string;
+};
+
+
+/* Get the function from GETTERS that is appropriate for type T (const and non
+   const versions).  */
+
+template<typename T>
+inline const setting_getter_ftype<T> &
+get_setting_getter (const setting_getter &getters);
+
+template<typename T>
+inline setting_getter_ftype<T> &
+get_setting_getter (setting_getter &getters);
+
+/* Extract the setting_getter_ftype<bool> field from GETTERS.  */
+template<>
+inline const setting_getter_ftype<bool> &
+get_setting_getter<bool> (const setting_getter &getters)
+{ return getters.get_bool; }
+
+template<>
+inline setting_getter_ftype<bool> &
+get_setting_getter<bool> (setting_getter &getters)
+{ return getters.get_bool; }
+
+/* Extract the setting_getter_ftype<int> field from GETTERS.  */
+template<>
+inline const setting_getter_ftype<int> &
+get_setting_getter<int> (const setting_getter &getters)
+{ return getters.get_int; }
+
+template<>
+inline setting_getter_ftype<int> &
+get_setting_getter<int> (setting_getter &getters)
+{ return getters.get_int; }
+
+/* Extract the setting_getter_ftype<unsigned int> field from GETTERS.  */
+template<>
+inline const setting_getter_ftype<unsigned int> &
+get_setting_getter<unsigned int> (const setting_getter &getters)
+{ return getters.get_uint; }
+
+template<>
+inline setting_getter_ftype<unsigned int> &
+get_setting_getter<unsigned int> (setting_getter &getters)
+{ return getters.get_uint; }
+
+/* Extract the setting_getter_ftype<auto_boolean> field from GETTERS.  */
+template<>
+inline const setting_getter_ftype<auto_boolean> &
+get_setting_getter<auto_boolean> (const setting_getter &getters)
+{ return getters.get_auto_boolean; }
+
+template<>
+inline setting_getter_ftype<auto_boolean> &
+get_setting_getter<auto_boolean> (setting_getter &getters)
+{ return getters.get_auto_boolean; }
+
+/* Extract the setting_getter_ftype<std::string> field from GETTERS. */
+template<>
+inline const setting_getter_ftype<std::string> &
+get_setting_getter<std::string> (const setting_getter &getters)
+{ return getters.get_string; }
+
+template<>
+inline setting_getter_ftype<std::string> &
+get_setting_getter<std::string> (setting_getter &getters)
+{ return getters.get_string; }
+
+/* Extract the setting_getter_ftype<const char *> field from GETTERS. */
+template<>
+inline const setting_getter_ftype<const char *> &
+get_setting_getter<const char *> (const setting_getter &getters)
+{ return getters.get_const_string; }
+
+template<>
+inline setting_getter_ftype<const char *> &
+get_setting_getter<const char *> (setting_getter &getters)
+{ return getters.get_const_string; }
+
+/* Function signature for a callback used to set a value to a setting.  */
+
+template<typename T>
+using setting_setter_ftype = void (*) (T);
+
+/* Contains a function to set a value of a setting.  */
+union setting_setter
+{
+  setting_setter_ftype<bool> set_bool;
+  setting_setter_ftype<int> set_int;
+  setting_setter_ftype<unsigned int> set_uint;
+  setting_setter_ftype<auto_boolean> set_auto_boolean;
+  setting_setter_ftype<std::string> set_string;
+  setting_setter_ftype<const char *> set_const_string;
+};
+
+/* Get the function from SETTERS that is appropriate for type T (const and non
+   const versions).  */
+
+template<typename T>
+inline const setting_setter_ftype<T> &
+get_setting_setter (const setting_setter &setters);
+
+template<typename T>
+inline setting_setter_ftype<T> &
+get_setting_setter (setting_setter &setters);
+
+/* Extract the setting_setter_ftype<bool> field from SETTERS. */
+template<>
+inline const setting_setter_ftype<bool> &
+get_setting_setter<bool> (const setting_setter &setters)
+{ return setters.set_bool; }
+
+template<>
+inline setting_setter_ftype<bool> &
+get_setting_setter<bool> (setting_setter &setters)
+{ return setters.set_bool; }
+
+/* Extract the setting_setter_ftype<int> field from SETTERS. */
+template<>
+inline const setting_setter_ftype<int> &
+get_setting_setter<int> (const setting_setter &setters)
+{ return setters.set_int; }
+
+template<>
+inline setting_setter_ftype<int> &
+get_setting_setter<int> (setting_setter &setters)
+{ return setters.set_int; }
+
+/* Extract the setting_setter_ftype<unsigned int> field from SETTERS. */
+template<>
+inline const setting_setter_ftype<unsigned int> &
+get_setting_setter<unsigned int> (const setting_setter &setters)
+{ return setters.set_uint; }
+
+template<>
+inline setting_setter_ftype<unsigned int> &
+get_setting_setter<unsigned int> (setting_setter &setters)
+{ return setters.set_uint; }
+
+/* Extract the setting_setter_ftype<auto_boolean> field from SETTERS. */
+template<>
+inline const setting_setter_ftype<auto_boolean> &
+get_setting_setter<auto_boolean> (const setting_setter &setters)
+{ return setters.set_auto_boolean; }
+
+template<>
+inline setting_setter_ftype<auto_boolean> &
+get_setting_setter<auto_boolean> (setting_setter &setters)
+{ return setters.set_auto_boolean; }
+
+/* Extract the setting_setter_ftype<std::string> field from SETTERS. */
+template<>
+inline const setting_setter_ftype<std::string> &
+get_setting_setter<std::string> (const setting_setter &setters)
+{ return setters.set_string; }
+
+template<>
+inline setting_setter_ftype<std::string> &
+get_setting_setter<std::string> (setting_setter &setters)
+{ return setters.set_string; }
+
+/* Extract the setting_setter_ftype<const char *> field from SETTERS. */
+template<>
+inline const setting_setter_ftype<const char *> &
+get_setting_setter<const char *> (const setting_setter &setters)
+{ return setters.set_const_string; }
+
+template<>
+inline setting_setter_ftype<const char *> &
+get_setting_setter<const char *> (setting_setter &setters)
+{ return setters.set_const_string; }
+
 /* Abstraction that contains access to data that can be set or shown.
 
    The underlying data can be of any VAR_TYPES type.  */
@@ -214,10 +402,18 @@ struct base_setting
   template<typename T>
   T get () const
   {
-    return *get_p<T> ();
+    gdb_assert (var_type_uses<T> (this->m_var_type));
+    auto getter = get_setting_getter<T> (this->m_getter);
+
+    if (getter != nullptr)
+      return (*getter) ();
+    else
+      return *get_p<T> ();
   }
 
-  /* Sets the value V to the underlying buffer.
+  /* Sets the value referenced by the setting to V.  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.
@@ -229,29 +425,56 @@ struct base_setting
     /* Check that the current instance is of one of the supported types for
        this instantiation.  */
     gdb_assert (var_type_uses<T> (this->m_var_type));
-    gdb_assert (this->m_var != nullptr);
 
-    *static_cast<T *> (this->m_var) = v;
+    auto setter = get_setting_setter<T> (this->m_setter);
+
+    if (setter != nullptr)
+      (*setter) (v);
+    else
+      {
+	gdb_assert (this->m_var != nullptr);
+	*static_cast<T *> (this->m_var) = v;
+      }
   }
 
-  /* A setting is valid if it contains a non null pointer to a memory buffer.
+  /* Set the user provided setter and getter functions.  */
+  template<typename T>
+  void
+  set_accessors (var_types var_type,
+		 setting_setter_ftype<T> setter,
+		 setting_getter_ftype<T> getter)
+  {
+    this->m_var_type = var_type;
+    get_setting_setter<T> (this->m_setter) = setter;
+    get_setting_getter<T> (this->m_getter) = getter;
+  }
+
+  /* A setting is valid if it contains a non null pointer to a memory buffer
+     or a getter and a setter function.
 
      On an instance which is not valid, any call to get, get_p or set is
      guaranteed to generate a runtime error (using gdb_assert).
    */
   bool valid () const
   {
-    return this->m_var != nullptr;
+    return ((m_getter.get_bool != nullptr && m_setter.set_bool != nullptr)
+	    || m_var != nullptr);
   }
 
 protected:
-  /* The type of the variable M_VAR is pointing to.  If M_VAR is nullptr,
-     M_VAR_TYPE is ignored.  */
+  /* The type of the variable M_VAR is pointing to.  If M_VAR is nullptr or if
+     m_getter and m_setter are nullptr, M_VAR_TYPE is ignored.  */
   var_types m_var_type { var_boolean };
 
   /* Pointer to the enclosed variable.  The type of the variable is encoded
      in M_VAR_TYPE.  Can be nullptr.  */
   void *m_var { nullptr };
+
+  /* Pointer to a user provided getter.  */
+  union setting_getter m_getter { .get_bool = nullptr };
+
+  /* Pointer to a user provided setter.  */
+  union setting_setter m_setter { .set_bool = nullptr };
 };
 
 /* A base_setting with additional methods to set the underlying buffer and
@@ -558,72 +781,159 @@ 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);