[RFAv3,1/4] Implement user defined prefix.

diff --git a/gdb/cli/cli-script.c b/gdb/cli/cli-script.c
index 06cdcd6bfe..066a596fdc 100644
--- a/gdb/cli/cli-script.c
+++ b/gdb/cli/cli-script.c
@@ -29,6 +29,7 @@ 
 #include "cli/cli-cmds.h"
 #include "cli/cli-decode.h"
 #include "cli/cli-script.h"
+#include "cli/cli-style.h"
 
 #include "extension.h"
 #include "interps.h"
@@ -1395,7 +1396,17 @@  do_define_command (const char *comname, int from_tty,
       int q;
 
       if (c->theclass == class_user || c->theclass == class_alias)
-	q = query (_("Redefine command \"%s\"? "), c->name);
+	{
+	  /* if C is a prefix command that was previously defined,
+	     tell the user its subcommands will be kept, and ask
+	     if ok to redefine the command.  */
+	  if (c->prefixlist != nullptr)
+	    q = c->user_commands.get () == nullptr
+	      || query (_("Keeping subcommands of prefix command \"%s\".\n"
+			  "Redefine command \"%s\"? "), c->name, c->name);
+	  else
+	    q = query (_("Redefine command \"%s\"? "), c->name);
+	}
       else
 	q = query (_("Really redefine built-in command \"%s\"? "), c->name);
       if (!q)
@@ -1446,10 +1457,27 @@  do_define_command (const char *comname, int from_tty,
   else
     cmds = *commands;
 
-  newc = add_cmd (comname, class_user, user_defined_command,
-		  (c && c->theclass == class_user)
-		  ? c->doc : xstrdup ("User-defined."), list);
-  newc->user_commands = std::move (cmds);
+  {
+    struct cmd_list_element **c_prefixlist
+      = c == nullptr ? nullptr : c->prefixlist;
+    const char *c_prefixname = c == nullptr ? nullptr : c->prefixname;
+
+    newc = add_cmd (comname, class_user, user_defined_command,
+		    (c != nullptr && c->theclass == class_user)
+		    ? c->doc : xstrdup ("User-defined."), list);
+    newc->user_commands = std::move (cmds);
+
+    /* If we define or re-define a command that was previously defined
+       as a prefix, keep the prefix information.  */
+    if (c_prefixlist != nullptr)
+      {
+	newc->prefixlist = c_prefixlist;
+	newc->prefixname = c_prefixname;
+	/* allow_unknown: see explanation in equivalent logic in
+	   define_prefix_command ().  */
+	newc->allow_unknown = newc->user_commands.get () != nullptr;
+    }
+  }
 
   /* If this new command is a hook, then mark both commands as being
      tied.  */
@@ -1524,6 +1552,54 @@  document_command (const char *comname, int from_tty)
     c->doc = doc;
   }
 }
+
+/* Implementation of the "define-prefix" command.  */
+
+static void
+define_prefix_command (const char *comname, int from_tty)
+{
+  struct cmd_list_element *c, **list;
+  const char *tem;
+  const char *comfull;
+
+  comfull = comname;
+  list = validate_comname (&comname);
+
+  /* Look it up, and verify that we got an exact match.  */
+  tem = comname;
+  c = lookup_cmd (&tem, *list, "", -1, 1);
+  if (c != nullptr && strcmp (comname, c->name) != 0)
+    c = nullptr;
+
+  if (c != nullptr && c->theclass != class_user)
+    error (_("Command \"%s\" is built-in."), comfull);
+
+  if (c != nullptr && c->prefixlist != nullptr)
+    {
+      /* c is already a user defined prefix command.  */
+      return;
+    }
+
+  /* If the command does not exist at all, create it.  */
+  if (c == nullptr)
+    {
+      comname = xstrdup (comname);
+      c = add_cmd (comname, class_user, user_defined_command,
+		   xstrdup ("User-defined."), list);
+    }
+
+  /* Allocate the c->prefixlist, which marks the command as a prefix
+     command.  */
+  c->prefixlist = new struct cmd_list_element*;
+  *(c->prefixlist) = nullptr;
+  c->prefixname = xstrprintf ("%s ", comfull);
+  /* If the prefix command C is not a command, then it must be followed
+     by known subcommands.  Otherwise, if C is also a normal command,
+     it can be followed by C args that must not cause a 'subcommand'
+     not recognised error, and thus we must allow unknown.  */
+  c->allow_unknown = c->user_commands.get () != nullptr;
+}
+
 
 /* Used to implement source_command.  */
 
@@ -1564,7 +1640,21 @@  void
 show_user_1 (struct cmd_list_element *c, const char *prefix, const char *name,
 	     struct ui_file *stream)
 {
-  struct command_line *cmdlines;
+  if (cli_user_command_p (c))
+    {
+      struct command_line *cmdlines = c->user_commands.get ();
+
+      fprintf_filtered (stream, "User %scommand \"",
+			c->prefixlist == NULL ? "" : "prefix ");
+      fprintf_styled (stream, title_style.style (), "%s%s",
+		      prefix, name);
+      fprintf_filtered (stream, "\":\n");
+      if (cmdlines)
+	{
+	  print_command_lines (current_uiout, cmdlines, 1);
+	  fputs_filtered ("\n", stream);
+	}
+    }
 
   if (c->prefixlist != NULL)
     {
@@ -1573,25 +1663,23 @@  show_user_1 (struct cmd_list_element *c, const char *prefix, const char *name,
       for (c = *c->prefixlist; c != NULL; c = c->next)
 	if (c->theclass == class_user || c->prefixlist != NULL)
 	  show_user_1 (c, prefixname, c->name, gdb_stdout);
-      return;
     }
 
-  cmdlines = c->user_commands.get ();
-  fprintf_filtered (stream, "User command \"%s%s\":\n", prefix, name);
-
-  if (!cmdlines)
-    return;
-  print_command_lines (current_uiout, cmdlines, 1);
-  fputs_filtered ("\n", stream);
 }
 
 void
 _initialize_cli_script (void)
 {
-  add_com ("document", class_support, document_command, _("\
+  struct cmd_list_element *c;
+
+  /* "document", "define" and "define-prefix" use command_completer,
+     as this helps the user to either type the command name and/or
+     its prefixes.  */
+  c = add_com ("document", class_support, document_command, _("\
 Document a user-defined command.\n\
 Give command name as argument.  Give documentation on following lines.\n\
 End with a line of just \"end\"."));
+  set_cmd_completer (c, command_completer);
   define_cmd_element = add_com ("define", class_support, define_command, _("\
 Define a new command name.  Command name is argument.\n\
 Definition appears on following lines, one command per line.\n\
@@ -1600,6 +1688,14 @@  Use the \"document\" command to give documentation for the new command.\n\
 Commands defined in this way may accept an unlimited number of arguments\n\
 accessed via $arg0 .. $argN.  $argc tells how many arguments have\n\
 been passed."));
+  set_cmd_completer (define_cmd_element, command_completer);
+  c = add_com ("define-prefix", class_support, define_prefix_command,
+	   _("\
+Define or mark a command as a user-defined prefix command.\n\
+User defined prefix commands can be used as prefix commands for\n\
+other user defined commands.\n\
+If the command already exists, it is changed to a prefix command."));
+  set_cmd_completer (c, command_completer);
 
   while_cmd_element = add_com ("while", class_support, while_command, _("\
 Execute nested commands WHILE the conditional expression is non zero.\n\
diff --git a/gdb/top.c b/gdb/top.c
index 2953eac819..e8ed3b2747 100644
--- a/gdb/top.c
+++ b/gdb/top.c
@@ -627,6 +627,18 @@  execute_command (const char *p, int from_tty)
       /* c->user_commands would be NULL in the case of a python command.  */
       if (c->theclass == class_user && c->user_commands)
 	execute_user_command (c, arg);
+      else if (c->theclass == class_user
+	       && c->prefixlist && !c->allow_unknown)
+	/* If this is a user defined prefix that does not allow unknown
+	   (in other words, C is a prefix command and not a command
+	   that can be followed by its args), report the list of
+	   subcommands.  */
+	{
+	  printf_unfiltered
+	    ("\"%.*s\" must be followed by the name of a subcommand.\n",
+	     (int) strlen (c->prefixname) - 1, c->prefixname);
+	  help_list (*c->prefixlist, c->prefixname, all_commands, gdb_stdout);
+	}
       else if (c->type == set_cmd)
 	do_set_command (arg, from_tty, c);
       else if (c->type == show_cmd)