Re: [Libguestfs] [PATCH v2] lib: Use qemu-img info -U option to avoid locking error.

On Friday, 21 September 2018 11:53:52 CEST Richard W.M. Jones wrote: > +/** > + * Test if the qemu-img info command supports the C<-U> option to > + * disable locking. The result is memoized in the handle. > + * > + * Note this option was added in qemu 2.11. We can remove this test > + * when we can assume everyone is using qemu >= 2.11. > + */ > +static int > +qemu_img_supports_U_option (guestfs_h *g) > +{ > + if (g->qemu_img_supports_U_option >= 0) > + return g->qemu_img_supports_U_option; > + > + CLEANUP_CMD_CLOSE struct command *cmd = guestfs_int_new_command (g); > + int r; > + > + guestfs_int_cmd_add_string_unquoted (cmd, > + "qemu-img info --help | " > + "grep -sq -- 'info.*-U'"); This may match something else in future, in case some other command of qemu-img gets another option with "info" in it... TBH this is something we have already, and precisely in the qemu_data struct, which is memoized. One downside is that using it would mean memoizing the qemu help/schema in advance, even if the appliance is not started; so code like `guestfish disk-format foo` will be slower. OTOH, the upside is that there is no need to "reprobe" qemu-img for something that we detect already from the QMP schema.

One possible idea can be to cache the qemu_data struct in the handle, so the direct backend would not get a performance regression (since either one of the info commands already preloaded a qemu_data, or launch would do that).