builtin/help: fix leaks in check_git_cmd()

The `check_git_cmd()` function is declared to return a string constant.
And while it sometimes does return a constant, it may also return an
allocated string in two cases:

  - When handling aliases. This case is already marked with `UNLEAK()`
    to work around the leak.

  - When handling unknown commands in case "help.autocorrect" is
    enabled. This one is not marked with `UNLEAK()`.

The function only has a single caller, so let's fix its return type to
be non-constant, consistently return an allocated string and free it at
its callsite to plug the leak.

Signed-off-by: Patrick Steinhardt <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: pks-t

builtin/help.c

@@ -551,12 +551,12 @@ static void show_html_page(const char *page)
 	open_html(page_path.buf);
 }
 
-static const char *check_git_cmd(const char* cmd)
+static char *check_git_cmd(const char *cmd)
 {
 	char *alias;
 
 	if (is_git_command(cmd))
-		return cmd;
+		return xstrdup(cmd);
 
 	alias = alias_lookup(cmd);
 	if (alias) {
@@ -589,14 +589,13 @@ static const char *check_git_cmd(const char* cmd)
 			die(_("bad alias.%s string: %s"), cmd,
 			    split_cmdline_strerror(count));
 		free(argv);
-		UNLEAK(alias);
 		return alias;
 	}
 
 	if (exclude_guides)
 		return help_unknown_cmd(cmd);
 
-	return cmd;
+	return xstrdup(cmd);
 }
 
 static void no_help_format(const char *opt_mode, enum help_format fmt)
@@ -642,6 +641,7 @@ int cmd_help(int argc,
 {
 	int nongit;
 	enum help_format parsed_help_format;
+	char *command = NULL;
 	const char *page;
 
 	argc = parse_options(argc, argv, prefix, builtin_help_options,
@@ -713,9 +713,9 @@ int cmd_help(int argc,
 	if (help_format == HELP_FORMAT_NONE)
 		help_format = parse_help_format(DEFAULT_HELP_FORMAT);
 
-	argv[0] = check_git_cmd(argv[0]);
+	command = check_git_cmd(argv[0]);
 
-	page = cmd_to_page(argv[0]);
+	page = cmd_to_page(command);
 	switch (help_format) {
 	case HELP_FORMAT_NONE:
 	case HELP_FORMAT_MAN:
@@ -729,5 +729,6 @@ int cmd_help(int argc,
 		break;
 	}
 
+	free(command);
 	return 0;
 }