[Libguestfs] [RFC nbdkit PATCH] protocol: Alter .list_exports, add .default_export

[Eric Blake]

On 8/24/20 7:52 AM, Eric Blake wrote:
> I'm about to add an 'exportname' filter, and in the process, I
> noticed a few shortcomings in our API.  Time to fix those before
> the 1.22 release locks our API in stone.  First, .list_exports
> needs to know if it is pre- or post-TLS, as that may affect which
> names are exported.  Next, overloading .list_exports to do both
> NBD_OPT_LIST and mapping "" to a canonical name is proving to be
> awkward; the canonical mapping is only needed during an
> NBD_INFO_NAME response to NBD_OPT_GO, and making .open try to
> grab the entire .list_exports list just to use only the first
> entry (even if the plugin optimized based on the bool to only
> provide one entry) is awkward, compared to just having a
> dedicated function.  Finally, as long as we are going to support
> NBD_INFO_NAME, we can also support NBD_INFO_DESCRIPTION; but
> while we map "" to a canonical name prior to calling .open,
> getting the description makes more sense after the connection
> is established, alongside .get_size.
> ---
> 
> I obviously need to finish the code to go with this, but here's where
> I would like to see the API before we finalize the 1.22 release.
> 

> +++ b/docs/nbdkit-plugin.pod

> +=head2 C<.default_export>
> +
> + const char *default_export (int readonly, int is_tls);

Oh fun.  For some plugins (like ondemand), this is trivial: return a 
compile-time constant string.  But for others (like sh and eval), 
there's a lifetime issue: this callback is used _before_ .open, ergo 
there is no handle struct that it can be associated with.  What's more, 
this is called _after_ .preconnect, which means it is logical to expect 
that the default export name might change over time (consider a plugin 
that advertises the largest file in a directory as its default, but 
where the directory can change _which_ file is largest between when the 
first client connects and when the second client connects).  And the 
string returned by the sh script is in malloc'd memory (by it's very 
nature of coming from the user script, rather than being a compile-time 
constant).  Without a handle to store this string in, we would have a 
memory leak: there is no way to associate this inside the handle's 
struct so that .close can reclaim it, but storing it globally is not 
thread-safe to parallel client connections.  So I'm thinking I need to 
add a helper function:

const char *nbdkit_string_intern (const char *str);

Return a pointer to a copy of str, where nbdkit owns the lifetime of the 
copy (allowing the caller to not have to worry about persisting str 
indefinitely).  If called when there is no client connection (such as 
during .load), the copy will remain valid through .unload; if called in 
the context of a client connection (any callback from .preconnect 
through .close), the copy will remain valid through .close.

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