Re: [libvirt] [Qemu-devel] [PATCH v2 2/4] qapi: Add passfd QMP command

On 06/13/2012 04:47 PM, Eric Blake wrote: > On 06/13/2012 02:25 PM, Corey Bryant wrote: > >>> Also, getfd automatically closes a fd if an existing fdname is passed >>> again. >>> I don't think this is a good behavior, I think pass-fd should fail >>> instead >>> (note that we can't fix getfd though). >>> >> >> I agree. It makes sense to fail rather than blindly closing the >> existing fd. It can be closed explicitly with closefd if the user wants >> it closed. > > Hmm - what happens if I do 'pass-fd name', learn that qemu is using fd > 42, then do 'getfd name'? I silently wipe out fd 42 and replace it with > the new fd passed in by getfd. Which means my use of /dev/fd/42 will > now be broken. > > Obviously that means that 'getfd' should NOT be used by any application > using 'pass-fd', and that libvirt should NOT be reusing names (I think > the latter is already true). But I agree that for back-compat we can't > get rid of the current (evil) semantics of a duplicated 'getfd'. Yes, users need to be careful and understand how the commands work. I don't think it's a hard rule that 'getfd' can't be used by an application that uses 'pass-fd'. If it were, we could put the fds on separate lists: struct Monitor { ... QLIST_HEAD(,mon_fd_t) fds; + QLIST_HEAD(,mon_fd_t) pass_fds; };

But I don't think this is necessary, so I'll plan on documenting them well.