gdb: make string-like set show commands use std::string variable

String-like settings (var_string, var_filename, var_optional_filename,
var_string_noescape) currently take a pointer to a `char *` storage
variable (typically global) that holds the setting's value.  I'd like to
"mordernize" this by changing them to use an std::string for storage.

An obvious reason is that string operations on std::string are often
easier to write than with C strings.  And they avoid having to do any
manual memory management.

Another interesting reason is that, with `char *`, nullptr and an empty
string often both have the same meaning of "no value".  String settings
are initially nullptr (unless initialized otherwise).  But when doing
"set foo" (where `foo` is a string setting), the setting now points to
an empty string.  For example, solib_search_path is nullptr at startup,
but points to an empty string after doing "set solib-search-path".  This
leads to some code that needs to check for both to check for "no value".
Or some code that converts back and forth between NULL and "" when
getting or setting the value.  I find this very error-prone, because it
is very easy to forget one or the other.  With std::string, we at least
know that the variable is not "NULL".  There is only one way of
representing an empty string setting, that is with an empty string.

I was wondering whether the distinction between NULL and "" would be
important for some setting, but it doesn't seem so.  If that ever
happens, it would be more C++-y and self-descriptive to use
optional<string> anyway.

Actually, there's one spot where this distinction mattered, it's in
init_history, for the test gdb.base/gdbinit-history.exp.  init_history
sets the history filename to the default ".gdb_history" if it sees that
the setting was never set - if history_filename is nullptr.  If
history_filename is an empty string, it means the setting was explicitly
cleared, so it leaves it as-is.  With the change to std::string, this
distinction doesn't exist anymore.  This can be fixed by moving the code
that chooses a good default value for history_filename to
_initialize_top.  This is ran before -ex commands are processed, so an
-ex command can then clear that value if needed (what
gdb.base/gdbinit-history.exp tests).

Another small improvement, in my opinion is that we can now easily
give string parameters initial values, by simply initializing the global
variables, instead of xstrdup-ing it in the _initialize function.

In Python and Guile, when registering a string-like parameter, we
allocate (with new) an std::string that is owned by the param_smob (in
Guile) and the parmpy_object (in Python) objects.

This patch started by changing all relevant add_setshow_* commands to
take an `std::string *` instead of a `char **` and fixing everything
that failed to build.  That includes of course all string setting
variable and their uses.

string_option_def now uses an std::string also, because there's a
connection between options and settings (see
add_setshow_cmds_for_options).

The add_path function in source.c is really complex and twisted, I'd
rather not try to change it to work on an std::string right now.
Instead, I added an overload that copies the std:string to a `char *`
and back.  This means more copying, but this is not used in a hot path
at all, so I think it is acceptable.

Change-Id: I92c50a1bdd8307141cdbacb388248e4e4fc08c93
Co-authored-by: Lancelot SIX <lsix@lancelotsix.com>

1 files changed, 35 insertions, 27 deletions

diff --git a/gdb/guile/scm-param.c b/gdb/guile/scm-param.c
index 0ae368a..17746da 100644
--- a/gdb/guile/scm-param.c
+++ b/gdb/guile/scm-param.c
@@ -44,7 +44,7 @@ union pascm_variable
   unsigned int uintval;
 
   /* Hold a string, for the various string types.  */
-  char *stringval;
+  std::string *stringval;
 
   /* Hold a string, for enums.  */
   const char *cstringval;
@@ -57,10 +57,7 @@ union pascm_variable
    2) Call register-parameter! to add the parameter to gdb.
    It is done this way so that the constructor, make-parameter, doesn't have
    any side-effects.  This means that the smob needs to store everything
-   that was passed to make-parameter.
-
-   N.B. There is no free function for this smob.
-   All objects pointed to by this smob must live in GC space.  */
+   that was passed to make-parameter.  */
 
 struct param_smob
 {
@@ -120,7 +117,6 @@ struct param_smob
 static setting
 make_setting (param_smob *s)
 {
-
   if (var_type_uses<bool> (s->type))
     return setting (s->type, &s->value.boolval);
   else if (var_type_uses<int> (s->type))
@@ -129,8 +125,8 @@ make_setting (param_smob *s)
     return setting (s->type, &s->value.autoboolval);
   else if (var_type_uses<unsigned int> (s->type))
     return setting (s->type, &s->value.uintval);
-  else if (var_type_uses<char *> (s->type))
-    return setting (s->type, &s->value.stringval);
+  else if (var_type_uses<std::string> (s->type))
+    return setting (s->type, s->value.stringval);
   else if (var_type_uses<const char *> (s->type))
     return setting (s->type, &s->value.cstringval);
   else
@@ -432,14 +428,14 @@ add_setshow_generic (enum var_types param_type, enum command_class cmd_class,
 
     case var_string:
       commands = add_setshow_string_cmd (cmd_name, cmd_class,
-					 &self->value.stringval, set_doc,
+					 self->value.stringval, set_doc,
 					 show_doc, help_doc, set_func,
 					 show_func, set_list, show_list);
       break;
 
     case var_string_noescape:
       commands = add_setshow_string_noescape_cmd (cmd_name, cmd_class,
-						  &self->value.stringval,
+						  self->value.stringval,
 						  set_doc, show_doc, help_doc,
 						  set_func, show_func, set_list,
 						  show_list);
@@ -448,7 +444,7 @@ add_setshow_generic (enum var_types param_type, enum command_class cmd_class,
 
     case var_optional_filename:
       commands = add_setshow_optional_filename_cmd (cmd_name, cmd_class,
-						    &self->value.stringval,
+						    self->value.stringval,
 						    set_doc, show_doc, help_doc,
 						    set_func, show_func,
 						    set_list, show_list);
@@ -456,7 +452,7 @@ add_setshow_generic (enum var_types param_type, enum command_class cmd_class,
 
     case var_filename:
       commands = add_setshow_filename_cmd (cmd_name, cmd_class,
-					   &self->value.stringval, set_doc,
+					   self->value.stringval, set_doc,
 					   show_doc, help_doc, set_func,
 					   show_func, set_list, show_list);
       break;
@@ -602,14 +598,14 @@ pascm_param_value (const setting &var, int arg_pos, const char *func_name)
     case var_string_noescape:
     case var_optional_filename:
     case var_filename:
-    case var_enum:
       {
-	const char *str;
-	if (var.type () == var_enum)
-	  str = var.get<const char *> ();
-	else
-	  str = var.get<char *> ();
+	const std::string &str = var.get<std::string> ();
+	return gdbscm_scm_from_host_string (str.c_str (), str.length ());
+      }
 
+    case var_enum:
+      {
+	const char *str = var.get<const char *> ();
 	if (str == nullptr)
 	  str = "";
 	return gdbscm_scm_from_host_string (str, strlen (str));
@@ -682,13 +678,7 @@ pascm_set_param_value_x (param_smob *p_smob,
 		       value, arg_pos, func_name,
 		       _("string or #f for non-PARAM_FILENAME parameters"));
       if (gdbscm_is_false (value))
-	{
-	  xfree (var.get<char *> ());
-	  if (var.type () == var_optional_filename)
-	    var.set<char *> (xstrdup (""));
-	  else
-	    var.set<char *> (nullptr);
-	}
+	var.set<std::string> ("");
       else
 	{
 	  SCM exception;
@@ -697,8 +687,7 @@ pascm_set_param_value_x (param_smob *p_smob,
 	    = gdbscm_scm_to_host_string (value, nullptr, &exception);
 	  if (string == nullptr)
 	    gdbscm_throw (exception);
-	  xfree (var.get<char *> ());
-	  var.set<char *> (string.release ());
+	  var.set<std::string> (string.release ());
 	}
       break;
 
@@ -798,6 +787,21 @@ pascm_set_param_value_x (param_smob *p_smob,
       gdb_assert_not_reached ("bad parameter type");
     }
 }
+
+/* Free function for a param_smob.  */
+static size_t
+pascm_free_parameter_smob (SCM self)
+{
+  param_smob *p_smob = (param_smob *) SCM_SMOB_DATA (self);
+
+  if (var_type_uses<std::string> (p_smob->type))
+    {
+      delete p_smob->value.stringval;
+      p_smob->value.stringval = nullptr;
+    }
+
+  return 0;
+}
 
 /* Parameter Scheme functions.  */
 
@@ -954,6 +958,10 @@ gdbscm_make_parameter (SCM name_scm, SCM rest)
   p_smob->set_func = set_func;
   p_smob->show_func = show_func;
 
+  scm_set_smob_free (parameter_smob_tag, pascm_free_parameter_smob);
+  if (var_type_uses<std::string> (p_smob->type))
+    p_smob->value.stringval = new std::string;
+
   if (initial_value_arg_pos > 0)
     {
       if (gdbscm_is_procedure (initial_value_scm))