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.
You may also want to mention that when using 'getfd' or 'pass-fd', there
are some commands (like migrate) that use the fd:name protocol, and that
a successful use of one of these commands implicitly closes the named
fd; but that all new uses of /dev/fd/nnn leave the fd open and an
explicit closefd must be used to avoid leaking indefinitely-opened fds
in qemu.
Ok, I'll mention this too. Thanks.
--
Regards,
Corey