Re: [PATCH v3 7/8] update-ref: support multiple simultaneous updates

[Brad King <brad.king@kitware.com>]

On 09/02/2013 01:48 PM, Brad King wrote:
> +	/* Parse the argument: */
> +	strbuf_reset(arg);
> +	if (*next == '"') {
> +		if (unquote_c_style(arg, next, &next))
> +			die("badly quoted argument: %s", next);
> +		return next;
> +	}
> +	while (*next && !isspace(*next))
> +		strbuf_addch(arg, *next++);
> +	return next;

This quoting proposal was written in response to $gmane/233479:

On 08/30/2013 06:51 PM, Junio C Hamano wrote:
> When we need to deal with arbitrary strings (like pathnames), other
> parts of the system usually give the user two interfaces, --stdin
> with and without -z, and the strings are C-quoted when run without
> the -z option, and terminated with NUL when run with the -z option.

1. Do we want to allow arbitrary non-space characters in unquoted
arguments (while loop above) or reserve some syntax for future use?

2. Thinking about how the -z variation might work, I ran:

$ git grep '\[0\] == '"'"'"' -- '*.c'
builtin/check-attr.c:           if (line_termination && buf.buf[0] == '"') {
builtin/check-ignore.c:         if (line_termination && buf.buf[0] == '"') {
builtin/checkout-index.c:                       if (line_termination && buf.buf[0] == '"') {
builtin/hash-object.c:          if (buf.buf[0] == '"') {
builtin/mktree.c:       if (line_termination && path[0] == '"') {
builtin/update-index.c:         if (line_termination && path_name[0] == '"') {
builtin/update-index.c:                 if (line_termination && buf.buf[0] == '"') {

All of these support quoting only in the non-z mode (the hash-object.c
line follows a getline using hard-coded '\n').  However, they are
all in cases looking for one value on a line or at the end of a line
so their -z option allows NUL-terminated lines containing LF.

What distinguishes the "update-ref --stdin" case is that we want to
represent multiple arguments on one line, each allowing arbitrary
characters or an empty string.  From a brief search a couple places
I found that do something related are:

* apply: Read multiple paths from a diff header, using unquote_c_style
  for quoted paths and separated by spaces.  There is no -z input mode.

* config: Output keyword=value\n becomes keyword\nvalue\0 in -z mode.
  This works because the first piece (keyword) cannot have a LF
  and there is at most one value so all LFs belong to it.

* quote.c: sq_dequote_to_argv handles single quotes like a shell
  would but allows only one space between arguments.  No -z mode.
  This is similar to my v2 proposal.

If we use unquote_c_style and spaces to divide LF-terminated lines,
how shall we divide arguments on NUL-terminated lines?

Thanks,
-Brad