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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



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