[Libguestfs] [PATCH libnbd proposal] api: Add semi-private function for freeing persistent data.

[Eric Blake]

On 8/11/19 8:03 AM, Richard W.M. Jones wrote:
> This adds a C-only semi-private function for freeing various types of
> persistent data passed to libnbd.
> 
> There are some similarities with nbd_add_close_callback which we
> removed in commit 7f191b150b52ed50098976309a6af883d245fc56.
> ---
>  generator/generator | 46 +++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 46 insertions(+)
> 
> diff --git a/generator/generator b/generator/generator
> index 55c4dfc..0ea6b61 100755
> --- a/generator/generator
> +++ b/generator/generator
> @@ -3284,6 +3284,7 @@ let generate_lib_libnbd_syms () =
>        pr "LIBNBD_%d.%d {\n" major minor;
>        pr "  global:\n";
>        if (major, minor) = (1, 0) then (
> +        pr "    nbd_add_free_callback;\n";
>          pr "    nbd_create;\n";
>          pr "    nbd_close;\n";
>          pr "    nbd_get_errno;\n";
> @@ -3581,6 +3582,12 @@ let generate_include_libnbd_h () =
>    pr "extern int nbd_get_errno (void);\n";
>    pr "#define LIBNBD_HAVE_NBD_GET_ERRNO 1\n";
>    pr "\n";
> +  pr "typedef void (*nbd_free_callback) (void *ptr);\n";
> +  pr "extern int nbd_add_free_callback (struct nbd_handle *h,\n";
> +  pr "                                  nbd_free_callback cb,\n";
> +  pr "                                  void *ptr);\n";
> +  pr "#define LIBNBD_HAVE_NBD_ADD_FREE_CALLBACK 1\n";
> +  pr "\n";

Would this change the signature of callbacks? With this in place, you
no longer need the VALID|FREE parameter, but can defer the free action
to an added free callback.

How would you actually track when to call the free callback? I guess
each time a pointer lifetime ends (basically, when retiring a completed
command) we check each pointer grabbed by that command and compare it
against the list of registered pointers for any free callbacks to use?
How does that scale, if the user can register an arbitrary number of
pointers because they queue up a large number of requests before
starting the poll loop?

Is the existing idea of a VALID|FREE parameter not sufficient?  If we
can convince the C bindings for python nbd.aio_pread to increase the
refcount of buf, and then install a C completion handler (whether the
python user called nbd.aio_pread with no python completer, or whether
they called nbd.aio_pread_complete) that does the decref when called
with FREE, then that would do the same job, even without the need for
nbd_add_free_callback, wouldn't it?  And there's no issue of scanning
through a list of pointers with a free callback associated with them
(storing registered pointers with some sort of hash may eventually get
to amortized O(1) lookup, but that seems like a lot of overhead compared
to the fact that we already have O(1) access to when to call the
completion callback with the FREE flag).

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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