[Libguestfs] [nbdkit PATCH v2] plugin: add and use nbdkit_realpath

[Eric Blake]

On 02/14/2018 10:53 AM, Pino Toscano wrote:
> Introduce a new helper function to resolve a path name, calling
> nbdkit_error on failure: other than doing what nbdkit_absolute_path
> does, it also checks that the file exists (and thus avoids errors later
> on).  To help distinguish it from nbdkit_absolute_path, improve the
> documentation of the latter.
> 
> Apply it where an existing path is required, both in nbdkit itself and
> in plugins.
> 
> Related to: https://bugzilla.redhat.com/show_bug.cgi?id=1527334
> ---

> +++ b/src/utils.c
> @@ -228,3 +228,22 @@ nbdkit_read_password (const char *value, char **password)
>   
>     return 0;
>   }
> +
> +char *
> +nbdkit_realpath (const char *path)
> +{
> +  char *ret;
> +
> +  if (path == NULL || *path == '\0') {
> +    nbdkit_error ("cannot resolve a null or empty path");
> +    return NULL;
> +  }
> +
> +  ret = realpath (path, NULL);

Wait. Does this even work?  Remember, in nbdkit_absolute_path(), we are 
prepending get_current_dir_name() (why the GNU spelling, instead of 
getcwd(NULL, 0), since either way is a GNU extension?).  But in main.c, 
we call an unconditional chdir("/") as part of fork_into_background, 
which has the annoying result that when using 'nbdkit -f' you honor your 
current $PWD, but when running in the background, relative filenames are 
now parsed against /.  Shouldn't we fix it so that nbdkit saves off 
getcwd() up front, prior to fork_into_background, and then both 
nbdkit_absolute_path() and nbdkit_realpath() convert any relative names 
into something relative to the ORIGINAL working directory, regardless of 
foreground or background operation?

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org