Add set_repeat_arguments function

The "x", "list", and "show commands" commands have special repetition
behavior: repeating the command doesn't re-run it with the same
arguments

This is currently implemented by modifying the passed-in argument; but
that won't work properly with const arguments (and seems pretty
obscure besides).

This patch adds a new "set_repeat_arguments" function and changes the
relevant places to call it.

gdb/ChangeLog
2017-11-07  Tom Tromey  <tom@tromey.com>

	* printcmd.c (x_command): Call set_repeat_arguments.
	* cli/cli-cmds.c (list_command): Call set_repeat_arguments.
	* top.c (repeat_arguments): New global.
	(set_repeat_arguments): New function.
	(execute_command): Handle repeat_arguments.
	(show_commands): Calls set_repeat_arguments.
	* command.h (set_repeat_arguments): Declare.

gdb/ChangeLog

@@ -1,3 +1,13 @@
+2017-11-07  Tom Tromey  <tom@tromey.com>
+
+	* printcmd.c (x_command): Call set_repeat_arguments.
+	* cli/cli-cmds.c (list_command): Call set_repeat_arguments.
+	* top.c (repeat_arguments): New global.
+	(set_repeat_arguments): New function.
+	(execute_command): Handle repeat_arguments.
+	(show_commands): Calls set_repeat_arguments.
+	* command.h (set_repeat_arguments): Declare.
+
 2017-11-07  Tom Tromey  <tom@tromey.com>
 
 	* stack.c (backtrace_command): Use std::string.

gdb/cli/cli-cmds.c

@@ -1082,7 +1082,7 @@ list_command (char *arg, int from_tty)
      turn it into the no-arg variant.  */
 
   if (from_tty)
-    *arg = 0;
+    set_repeat_arguments ("");
 
   if (dummy_beg && sal_end.symtab == 0)
     error (_("No default source file yet.  Do \"help list\"."));

gdb/command.h

@@ -445,6 +445,11 @@ extern void dont_repeat (void);
 
 extern scoped_restore_tmpl<int> prevent_dont_repeat (void);
 
+/* Set the arguments that will be passed if the current command is
+   repeated.  Note that the passed-in string must be a constant.  */
+
+extern void set_repeat_arguments (const char *args);
+
 /* Used to mark commands that don't do anything.  If we just leave the
    function field NULL, the command is interpreted as a help topic, or
    as a class of commands.  */

gdb/printcmd.c

@@ -1646,7 +1646,7 @@ x_command (char *exp, int from_tty)
          repeated with Newline.  But don't clobber a user-defined
          command's definition.  */
       if (from_tty)
-	*exp = 0;
+	set_repeat_arguments ("");
       val = evaluate_expression (expr.get ());
       if (TYPE_IS_REFERENCE (value_type (val)))
 	val = coerce_ref (val);

gdb/top.c

@@ -522,6 +522,19 @@ maybe_wait_sync_command_done (int was_sync)
     wait_sync_command_done ();
 }
 
+/* If not NULL, the arguments that should be passed if the current
+   command is repeated.  */
+
+static const char *repeat_arguments;
+
+/* See command.h.  */
+
+void
+set_repeat_arguments (const char *args)
+{
+  repeat_arguments = args;
+}
+
 /* Execute the line P as a command, in the current user context.
    Pass FROM_TTY as second argument to the defining function.  */
 
@@ -564,6 +577,10 @@ execute_command (char *p, int from_tty)
       c = lookup_cmd (&cmd, cmdlist, "", 0, 1);
       p = (char *) cmd;
 
+      scoped_restore save_repeat_args
+	= make_scoped_restore (&repeat_arguments, nullptr);
+      char *args_pointer = p;
+
       /* Pass null arg rather than an empty one.  */
       arg = *p ? p : 0;
 
@@ -612,6 +629,11 @@ execute_command (char *p, int from_tty)
       /* If this command has been post-hooked, run the hook last.  */
       execute_cmd_post_hook (c);
 
+      if (repeat_arguments != NULL)
+	{
+	  gdb_assert (strlen (args_pointer) >= strlen (repeat_arguments));
+	  strcpy (args_pointer, repeat_arguments);
+	}
     }
 
   check_frame_language_change ();
@@ -1667,7 +1689,7 @@ dont_repeat_command (char *ignored, int from_tty)
 /* Number of commands to print in each call to show_commands.  */
 #define Hist_print 10
 void
-show_commands (char *args, int from_tty)
+show_commands (const char *args, int from_tty)
 {
   /* Index for history commands.  Relative to history_base.  */
   int offset;
@@ -1721,10 +1743,7 @@ show_commands (char *args, int from_tty)
      "show commands +" does.  This is unnecessary if arg is null,
      because "show commands +" is not useful after "show commands".  */
   if (from_tty && args)
-    {
+    set_repeat_arguments ("+");
-      args[0] = '+';
-      args[1] = '\0';
-    }
 }
 
 /* Update the size of our command history file to HISTORY_SIZE.

gdb/value.c

@@ -2003,10 +2003,7 @@ show_values (char *num_exp, int from_tty)
      "show values +".  If num_exp is null, this is unnecessary, since
      "show values +" is not useful after "show values".  */
   if (from_tty && num_exp)
-    {
+    set_repeat_arguments ("+");
-      num_exp[0] = '+';
-      num_exp[1] = '\0';
-    }
 }
 
 enum internalvar_kind