[PR gdb/16238] Add completer for the show user command

The 'show user' command (which shows the definition of non-python/scheme
user defined commands) is currently missing a completer. This is
mentioned in PR 16238.  Having one can improve the user experience.

In this commit I propose an implementation for such completer as well as
the associated tests.

Tested on x86_64 GNU/Linux.

All feedbacks are welcome.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=16238

gdb/cli/cli-cmds.c

@@ -1631,6 +1631,75 @@ show_user (const char *args, int from_tty)
     }
 }
 
+/* Return true if COMMAND or any of its sub-commands is a user defined command.
+   This is a helper function for show_user_completer.  */
+
+static bool
+has_user_subcmd (struct cmd_list_element *command)
+{
+  if (cli_user_command_p (command))
+    return true;
+
+  /* Alias command can yield false positive.  Ignore them as the targeted
+     command should be reachable anyway.  */
+  if (command->is_alias ())
+    return false;
+
+  if (command->is_prefix ())
+    for (struct cmd_list_element *subcommand = *command->subcommands;
+	 subcommand != nullptr;
+	 subcommand = subcommand->next)
+      if (has_user_subcmd (subcommand))
+	return true;
+
+  return false;
+}
+
+/* Implement completer for the 'show user' command.  */
+
+static void
+show_user_completer (cmd_list_element *,
+		     completion_tracker &tracker, const char *text,
+		     const char *word)
+{
+  struct cmd_list_element *cmd_group = cmdlist;
+
+  /* TEXT can contain a chain of commands and subcommands.  Follow the
+     commands chain until we reach the point where the user wants a
+     completion.  */
+  while (word > text)
+    {
+      const char *curr_cmd = text;
+      const char *after = skip_to_space (text);
+      const size_t curr_cmd_len = after - text;
+      text = skip_spaces (after);
+
+      for (struct cmd_list_element *c = cmd_group; c != nullptr; c = c->next)
+	{
+	  if (strlen (c->name) == curr_cmd_len
+	      && strncmp (c->name, curr_cmd, curr_cmd_len) == 0)
+	    {
+	      if (c->subcommands == nullptr)
+		/* We arrived after a command with no child, so nothing more
+		   to complete.  */
+		return;
+
+	      cmd_group = *c->subcommands;
+	      break;
+	    }
+	}
+    }
+
+  const int wordlen = strlen (word);
+  for (struct cmd_list_element *c = cmd_group; c != nullptr; c = c->next)
+    if (has_user_subcmd (c))
+      {
+	if (strncmp (c->name, word, wordlen) == 0)
+	  tracker.add_completion
+	    (gdb::unique_xmalloc_ptr<char> (xstrdup (c->name)));
+      }
+}
+
 /* Search through names of commands and documentations for a certain
    regular expression.  */
 
@@ -2593,10 +2662,11 @@ you must type \"disassemble 'foo.c'::bar\" and not \"disassemble foo.c:bar\"."))
   c = add_com ("make", class_support, make_command, _("\
 Run the ``make'' program using the rest of the line as arguments."));
   set_cmd_completer (c, filename_completer);
-  add_cmd ("user", no_class, show_user, _("\
+  c = add_cmd ("user", no_class, show_user, _("\
 Show definitions of non-python/scheme user defined commands.\n\
 Argument is the name of the user defined command.\n\
 With no argument, show definitions of all user defined commands."), &showlist);
+  set_cmd_completer (c, show_user_completer);
   add_com ("apropos", class_support, apropos_command, _("\
 Search for commands matching a REGEXP.\n\
 Usage: apropos [-v] REGEXP\n\

gdb/testsuite/gdb.base/show-user-completion.exp

@@ -0,0 +1,72 @@
+# Copyright 2021 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# Test tab-completion for the 'show user' command.
+
+load_lib completion-support.exp
+
+# This test does not require any file to be loaded.
+clean_restart
+
+# Define the 'foo' and 'bar' commands so we have something to complete.
+set re [multi_line "Type commands for definition of \"foo\"." \
+		   "End with a line saying just \"end\"." \
+		   ">$" ]
+gdb_test_multiple "define foo" "define user command: foo" {
+    -re $re {
+	gdb_test "print \"foo\"\nend" "" $gdb_test_name
+    }
+}
+
+set re [multi_line "Type commands for definition of \"bar\"." \
+		   "End with a line saying just \"end\"." \
+		   ">$"]
+gdb_test_multiple "define bar" "define user command: bar" {
+    -re  $re {
+	gdb_test "print \"bar\"\nend" "" $gdb_test_name
+    }
+}
+
+# The completer should show both options.
+test_gdb_complete_multiple "show user " "" "" "bar foo"
+
+# If we give the beginning of one of the commands, it should complete it.
+test_gdb_complete_unique "show user f" "show user foo"
+test_gdb_complete_unique "show user b" "show user bar"
+
+# Define a user prefix.
+gdb_test "define-prefix mygroup"
+
+# Add a user defined command in the user defined prefix.
+set re [multi_line  "Type commands for definition of \"mygroup mycommand\"." \
+		    "End with a line saying just \"end\"." \
+		    ">$"]
+set test_name "define user command: mygroup mycommand"
+gdb_test_multiple "define mygroup mycommand" $test_name {
+    -re  $re {
+	gdb_test "print \"42\"\nend" "" $gdb_test_name
+    }
+}
+
+with_test_prefix "with user-prefix" {
+    # We now expect the completion to yield only 3 results.  As the 'mycommand'
+    # is within the 'mygroup' prefix, it should not be reachable without
+    # traversing 'mygroup' first.
+    test_gdb_complete_multiple "show user " "" "" "bar foo mygroup"
+}
+
+# Check that we can complete commands defined under a prefix.
+test_gdb_complete_unique "show user m" "show user mygroup"
+test_gdb_complete_unique "show user mygroup " "show user mygroup mycommand"