Inspired by but not completely related to: https://bugzilla.redhat.com/show_bug.cgi?id=664558 "RFE: Allow to set log callback in Ruby bindings" How can we make callbacks useful? First off what are we talking about? There are several callbacks that can be registered through the C API: (a) error callback http://libguestfs.org/guestfs.3.html#guestfs_set_error_handler (b) out of memory callback http://libguestfs.org/guestfs.3.html#guestfs_set_out_of_memory_handler (c) log messages from the daemon http://libguestfs.org/guestfs.3.html#guestfs_set_log_message_callback (d) appliance quits http://libguestfs.org/guestfs.3.html#guestfs_set_subprocess_quit_callback (e) appliance launched http://libguestfs.org/guestfs.3.html#guestfs_set_launch_done_callback (f) handle closed http://libguestfs.org/guestfs.3.html#guestfs_set_close_callback (g) progress notification http://libguestfs.org/guestfs.3.html#guestfs_set_progress_callback (a) is not exposed to language bindings because it is required in order to implement error handling and exceptions. (b) is very specialized and is not likely to be used outside specific C programs. The ones I'm interested in here are (c), (d), (e), (f), (g) -- generic events. There are three problems that I'm finding: (1) You can't register more than one callback for an event. The handle holds one callback pointer for each event type so when you register a second callback it overwrites the first. (2) We don't generate callback bindings, so many language bindings lack coverage. This is the basis of Marek's RFE BZ above. (3) There's no way to capture and redirect debug messages and trace messages (see https://bugzilla.redhat.com/show_bug.cgi?id=664558#c2). It would be useful to do this from GUI programs which usually have their own preferred method to log things. I think we need a more generic mechanism for (c)-(f) which solves problems (1)-(3). Here's my proposed more usable solution: (i) Each event has a single class, one of: { LOG, VERBOSE, TRACE, SUBPROCESS_QUIT, LAUNCH_DONE, CLOSE, PROGRESS } (ii) Give each event a level (an integer). (iii) Each event can optionally have some content. For log messages this is a string with length, for progress messages it is a struct of numbers. Callers can register to receive events in a bitmask class and at a set of levels by doing: int /* event_handle */ guestfs_set_event_callback (guestfs_h *g, guestfs_event_callback cb, uint64_t event_class_bitmask, int min_level, void *opaque); void guestfs_delete_event_callback (guestfs_h *g, int event_handle); 'guestfs_set_event_callback' would register a callback for all event classes in 'event_class_bitmask' at level >= 'min_level'. This would return an integer which can be used to delete this callback, by calling 'guestfs_delete_event_callback'. Callbacks remain until the handle is closed or the callback is deleted, and multiple overlapping callbacks can exist. The callback function is the same for every class of event. LOG|VERBOSE|TRACE events would contain a string. PROGRESS events would contain an array of numbers. typedef void (*guestfs_event_callback) (guestfs_h *g, void *opaque, uint64_t event_class, int level, int event_handle, char *buf, size_t buf_len, uint64_t *array, size_t array_len); [The string/array hack is very ugly, but I cannot think of a better way at the moment ...] We should export the same API through all the language bindings to satisfy condition (2). On the library side we should change verbose message so that instead of: if (g->verbose) fprintf (stderr, "..."); we should have: debug ("..."); where the debug message is converted to a verbose level event if g->verbose is set, otherwise it is discarded. If no handler has been registered for debug messages, they should go to stderr as they do now, otherwise they should go to handlers only. A similar thing should happen for trace messages. Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones libguestfs lets you edit virtual machines. Supports shell scripting, bindings from many languages. http://et.redhat.com/~rjones/libguestfs/ See what it can do: http://et.redhat.com/~rjones/libguestfs/recipes.html