Re: [Libguestfs] [nbdkit PATCH 1/5] api: Add .default_export

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. Overloading .list_exports in order to determine a canonical export name at .open time is awkward; the two uses (answering NBD_OPT_LIST for a full list, vs. remapping a client's "" into a canonical name during .open) are orthogonal enough to warrant separate plugin callbacks. This will also make it easier to express the notion of no default export (connecting to "" is an error) at the same time as listing other exports. Another consideration is that when tls=1, the choice of export to expose pre-TLS vs. post-TLS may differ, but without a call to .open yet, our just-added nbdkit_is_tls() does not fit our preferred lifecycle, so this has to be a parameter to the new .default_export. We will alter the signature of .list_exports soon; in the meantime, the bool default_only parameter is now ignored.

+++ b/server/plugins.c

@@ -282,15 +283,29 @@ static int plugin_list_exports (struct backend *b, int readonly, int default_only, struct nbdkit_exports *exports) { - GET_CONN; struct backend_plugin *p = container_of (b, struct backend_plugin, backend); - if (!p->plugin.list_exports) - return nbdkit_add_export (exports, "", NULL); + if (!p->plugin.list_exports) { + const char *def = backend_default_export (b, readonly); + if (def == NULL) + return 0; + return nbdkit_add_export (exports, def, NULL); + }