Re: [PATCH 10/16] fast-import: use skip_prefix for parsing input

> Fast-import does a lot of parsing of commands and
> dispatching to sub-functions. For example, given "option
> foo", we might recognize "option " using starts_with, and
> then hand it off to parse_option() to do the rest.
>
> However, we do not let parse_option know that we have parsed
> the first part already. It gets the full buffer, and has to
> skip past the uninteresting bits. Some functions simply add
> a magic constant:
>
>   char *option = command_buf.buf + 7;
>
> Others use strlen:
>
>   char *option = command_buf.buf + strlen("option ");
>
> And others use strchr:
>
>   char *option = strchr(command_buf.buf, ' ') + 1;
>
> All of these are brittle and easy to get wrong (especially
> given that the starts_with call and the code that assumes
> the presence of the prefix are far apart). Instead, we can
> use skip_prefix, and just pass each handler a pointer to its
> arguments.
>
> Signed-off-by: Jeff King <peff@peff.net>
> ---
> diff --git a/fast-import.c b/fast-import.c
> index a3ffe30..5f17adb 100644
> --- a/fast-import.c
> +++ b/fast-import.c
> @@ -2713,20 +2706,22 @@ static void parse_new_commit(void)
>
>         /* file_change* */
>         while (command_buf.len > 0) {
> -               if (starts_with(command_buf.buf, "M "))
> -                       file_change_m(b);
> -               else if (starts_with(command_buf.buf, "D "))
> -                       file_change_d(b);
> -               else if (starts_with(command_buf.buf, "R "))
> -                       file_change_cr(b, 1);
> -               else if (starts_with(command_buf.buf, "C "))
> -                       file_change_cr(b, 0);
> -               else if (starts_with(command_buf.buf, "N "))
> -                       note_change_n(b, &prev_fanout);
> +               const char *v;

> +               if (skip_prefix(command_buf.buf, "M ", &v))
> +                       file_change_m(v, b);
> +               else if (skip_prefix(command_buf.buf, "D ", &v))
> +                       file_change_d(v, b);
> +               else if (skip_prefix(command_buf.buf, "R ", &v))
> +                       file_change_cr(v, b, 1);
> +               else if (skip_prefix(command_buf.buf, "C ", &v))
> +                       file_change_cr(v, b, 0);
> +               else if (skip_prefix(command_buf.buf, "N ", &v))
> +                       note_change_n(v, b, &prev_fanout);
>                 else if (!strcmp("deleteall", command_buf.buf))
>                         file_change_deleteall(b);
> -               else if (starts_with(command_buf.buf, "ls "))
> -                       parse_ls(b);
> +               else if (skip_prefix(command_buf.buf, "ls ", &v))
> +                       parse_ls(v, b);
>                 else {
>                         unread_command_buf = 1;
>                         break;