[libvirt] [PATCHv2 11/14] event: don't allow mix of old- and new-style registration

John Ferlan jferlan at redhat.com
Tue Jan 7 16:23:26 UTC 2014

On 01/06/2014 05:27 PM, Eric Blake wrote:
> Consider these two calls, in either order:
> id1 = virConnectDomainEventRegisterAny(conn, NULL,
> virConnectDomainEventRegister(conn, callback, NULL, NULL);
> Right now, the second call fails, because under the hood, the
> old-style function registration is tightly coupled to the
> new style lifecycle eventID, and the two calls both try
> to register the same global eventID callback representation.
> We've alreay documented that users should avoid old-style
> registration and deregistration, so anyone heeding the advice
> won't run into this situation.  But it would be even nicer if
> we pretend the two interfaces are completely separate, and
> disallow any cross-linking.  That is, a call to old-style
> deregister should never remove a new-style callback even if it
> is the same function pointer, and a call to new-style callback
> using only callbackIDs obtained legitimately should never
> remove an old-style callback (of course, since our callback
> IDs are sequential, and there is still coupling under the
> hood, you can easily guess the callbackID of an old style
> registration and use new-style deregistration to nuke it - but
> that starts to be blatantly bad coding on your part rather
> than a surprising result on what looks like reasonable
> stand-alone API).
> With this patch, you can now register a global lifecycle event
> handler twice, by using both old and new APIs; if such an event
> occurs, your callback will be entered twice.  But that is not a
> problem in practice, since it is already possible to use the
> new API to register both a global and per-domain event handler
> using the same function, which will likewise fire your callback
> twice for that domain.  Duplicates are still prevented when
> using the same API with same parameters twice (old-style twice,
> new-style global twice, or new-style per-domain with same domain
> twice), and things are still bounded (it is not possible to
> register a single function pointer more than N+2 times per event
> id, where N is the number of domains available on the connection).
> Besides, it has always been possible to register as many
> separate function pointers on the same event id as desired,
> through either old or new style API, where the bound there is
> the physical limitation of writing a program with enough
> distinct function pointers.
> * src/conf/object_event.c (_virObjectEventCallback): Add field.
> (virObjectEventCallbackLookup): Add argument.
> (virObjectEventCallbackListAddID, virObjectEventStateCallbackID):
> Adjust callers.
> Signed-off-by: Eric Blake <eblake at redhat.com>
> ---
>  src/conf/object_event.c | 21 ++++++++++++++++-----
>  1 file changed, 16 insertions(+), 5 deletions(-)

ACK the pair (10 & 11)


It's a tangled web you weave...

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