Re: [libvirt] [PATCH] qemu: Report errors from iohelper

On 10/23/2012 03:39 AM, Michal Privoznik wrote: I agree that catching iohelper errors is needed, but I'm worried that you might introduce deadlock with this approach (then again, it may just be theoretical): You aren't doing any reading of the stderr file descriptor until you get to the cleanup label. But suppose that iohelper encounters situations that cause it to write more than PIPE_BUF bytes to stderr, but while still continuing to run. Then, because no one is reading the other end of the pipe, iohelper blocks on writes to stderr instead of proceeding on with the rest of its operations, and thus never gets to a point where it will exit in order to kick libvirtd over to the cleanup label to start reading the stderr pipe in the first place. Would it work to use virCommandSetErrorBuffer() instead, or does that rely on using virCommandRun() to properly populate the buffer via poll(), where we are stuck using virCommandRunAsync() instead? Or, since we are already using virCommandRunAsync(), does that mean we need to figure out how to incorporate a poll() to favorably handle stderr in addition to everything else that iohelper is already handling? Or am I worrying too much, and if iohelper hits any issue where it would be logging to stderr in the first place, it will be exiting before filling up the stderr pipe and thus never deadlocking? At any rate, depending on how that question is answered, this code does indeed look useful. -- Eric Blake eblake(a)redhat.com +1-919-301-3266 Libvirt virtualization library http://libvirt.org