Re: [Libguestfs] simplify debugging of guestfsd

On Fri, Sep 21, 2012 at 02:44:52PM +0200, Olaf Hering wrote: Stepping back a bit, we have a long-open bug about collecting core dumps from the appliance: https://bugzilla.redhat.com/show_bug.cgi?id=619334 The problem with collecting core dumps is how you get them out of the appliance, given that it is short-lived and disappears on exit. We looked at various methods including writing them to a block device or squirting it down a virtio-serial connection, but none of them are very satisfactory. strace output can do through the usual console logging mechanism. gdb seems intractable to me if you want to use gdb interactively. Just printing stack traces can go via the console logging mechanism. Note there's very limited development tools in the appliance (no debuginfo), and I'd be cautious about how much stuff gdb will pull in via dependencies. That's why we looked at core dumps. Adding options to every tool seems to be a lot of work. The guestfs handle lets you set several useful properties on it using explicit calls (eg. guestfs_set_attach_method). However we don't expose all of these properties in every tool. Instead we allow the properties to take default values when the handle is created from environment variables (eg. $LIBGUESTFS_ATTACH_METHOD), and this means that every tool doesn't need to support every property. The '-v' and '-x' options on most tools are convenient, but are arguably superfluous since people could set LIBGUESTFS_DEBUG=1 and LIBGUESTFS_TRACE=1 and get almost precisely the same effect. So environment variables are the way to go. Another complication is that guestfs_create doesn't have a rich way to return errors, and thus if the environment variables cannot be parsed things start to go wrong. Currently it spits error messages out to stderr in this case, which is very bad behaviour. With the benefit of hindsight, splitting handle creation and environment parsing into two calls would have been better: g = guestfs_create (); if (!g) error; if (guestfs_set_environment (g) == -1) { /* with the handle created, richer error methods are now available */ } But we cannot change that now without breaking clients. Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones virt-top is 'top' for virtual machines. Tiny program with many powerful monitoring features, net stats, disk stats, logging, etc. http://et.redhat.com/~rjones/virt-top