[Libguestfs] [PATCH libnbd v3 1/6] lib/opt: Don't use NULL for an empty StringList

[Eric Blake]

On Wed, Sep 28, 2022 at 06:25:34PM +0100, Richard W.M. Jones wrote:
> StringList parameters (char ** in C) will be marked with
> __attribute__((nonnull)).  To pass an empty list you have to use a
> list containing a single NULL element, not a NULL pointer.
> 
> nbd_internal_set_querylist has also been adjusted.

Hmm.  I intentionally WANT to pass NULL to the
nbd_internal_set_querylist function (as a way to explicitly copy the
defaults), while wanting to forbid NULL to the public
nbd_opt_list_meta_context_queries() API.  I'm not sure this patch does
what you think...

> 
> Fixes: commit e33762a86cd5f064e5ef841520baa5fe7361d2c2
> ---
>  generator/states-newstyle-opt-meta-context.c |  4 +++-
>  lib/opt.c                                    | 16 ++++++++++++----
>  lib/utils.c                                  |  4 ++--
>  3 files changed, 17 insertions(+), 7 deletions(-)
> 
> diff --git a/generator/states-newstyle-opt-meta-context.c b/generator/states-newstyle-opt-meta-context.c
> index 46fee15be1..a60aa66f3b 100644
> --- a/generator/states-newstyle-opt-meta-context.c
> +++ b/generator/states-newstyle-opt-meta-context.c
> @@ -65,12 +65,14 @@  NEWSTYLE.OPT_META_CONTEXT.START:
>      }
>    }
>    if (opt != h->opt_current) {
> +    char *empty[] = { NULL };
> +
>      if (!h->request_meta || !h->structured_replies ||
>          h->request_meta_contexts.len == 0) {
>        SET_NEXT_STATE (%^OPT_GO.START);
>        return 0;
>      }
> -    if (nbd_internal_set_querylist (h, NULL) == -1) {
> +    if (nbd_internal_set_querylist (h, empty) == -1) {

By passing an explicit list instead of requesting that we reuse
h->request_meta_contexts, we have broken the set of queries that
nbd_opt_go() will utilize (that's an API that does not have a
StringList parameter, so we do want to reuse our default global list
instead of slamming it to an explicit empty list).  That's probably
why you saw test failures.

>        SET_NEXT_STATE (%.DEAD);
>        return 0;
>      }
> diff --git a/lib/opt.c b/lib/opt.c
> index 1b18920fdb..e323d7b1b0 100644
> --- a/lib/opt.c
> +++ b/lib/opt.c
> @@ -224,7 +224,9 @@ int
>  nbd_unlocked_opt_list_meta_context (struct nbd_handle *h,
>                                      nbd_context_callback *context)
>  {
> -  return nbd_unlocked_opt_list_meta_context_queries (h, NULL, context);
> +  char *empty[] = { NULL };
> +
> +  return nbd_unlocked_opt_list_meta_context_queries (h, empty, context);

Also broken for the same reason.  I'm okay if we force our internal
attributes of nbd_internal_API to match the attributes of nbd_API, but
for that to work, we may need to introduce:

static int
nbd_unlocked_opt_meta_context_queries_helper (struct nbd_handle *h, uint16_t option,
                                              string_vector *queries /* may be NULL */,
                                              nbd_context_callback *context)

and then have our various existing nbd_internal_API calls (for
NBD_OPT_SET/LIST_META_CONTEXT) call in to the helper function, rather
than trying to make one of the internal API calls blindly manage the
work for all the other APIs by accepting a NULL list parameter
different than the restriction on the public API.

I can respin this patch along those lines, if it would be easier to
see in code than in prose.

> +++ b/lib/utils.c
> @@ -100,7 +100,7 @@ nbd_internal_set_querylist (struct nbd_handle *h, char **queries)
>  {
>    size_t i;
>  
> -  if (queries) {
> +  if (queries[0] != NULL /* non-empty list */) {
>      if (nbd_internal_copy_string_list (&h->querylist, queries) == -1) {
>        set_error (errno, "realloc");
>        return -1;
> @@ -109,7 +109,7 @@ nbd_internal_set_querylist (struct nbd_handle *h, char **queries)
>      assert (h->querylist.len > 0);
>      string_vector_remove (&h->querylist, h->querylist.len - 1);
>    }
> -  else {
> +  else /* empty list */ {
>      string_vector_reset (&h->querylist);
>      for (i = 0; i < h->request_meta_contexts.len; ++i) {
>        char *copy = strdup (h->request_meta_contexts.ptr[i]);

This is not what I intended.  For this function, I really DID want
NULL to mean "list not present, use the global list with
nbd_internal_copy_string_list), and non-NULL (including when the list
is empty because queries[0] is NULL) to mean "use this explicit list,
regardless of its length".

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