[Libguestfs] [PATCH libnbd v2 2/5] lib: Implement closure lifetimes.

[Eric Blake]

On 7/24/19 11:54 AM, Richard W.M. Jones wrote:
> Previously closures had a crude flag which tells if they are
> persistent or transient.  Transient closures (flag = false) last for
> the lifetime of the currently called libnbd function.  Persistent
> closures had an indefinite lifetime which could last for as long as
> the handle.  In language bindings handling persistent closures was
> wasteful as we needed to register a "close callback" to free the
> closure when the handle is closed.  But if you had submitted thousands
> of asynchronous commands you would end up registering thousands of
> close callbacks.
> 
> We actually have far more information about the lifetime of closures.
> We know precisely when they will no longer be needed by the library.
> Callers can use this information to free up associated user data
> earlier.  In particular in language bindings we can remove roots /
> decrement reference counts at the right place to free the closure,
> without waiting for the handle to be closed.
> 
> The solution to this is to introduce the concept of a closure
> lifetime.  The callback is called with an extra valid_flag parameter
> which is a bitmap containing LIBNBD_CALLBACK_VALID and/or
> LIBNBD_CALLBACK_FREE.  LIBNBD_CALLBACK_VALID corresponds to a normal
> call of the callback function by the library.  After the library has
> finished with the callback (declaring that this callback will never be
> needed or called again), it is called once more with
> valid_flag == LIBNBD_CALLBACK_FREE.
> 
> (Note it is also possible for the library to call the callback with
> valid_flag == LIBNBD_CALLBACK_VALID|LIBNBD_CALLBACK_FREE, meaning it's the
> last valid call.)
> 

Our mails crossed, and I had questions about the implementation of v1
that aren't addressed here. But I'm also okay if you check this one in
as-is, and I can write a followup patch that shuffles things around per
my v1 comments as part of rebasing my auto-retire patches on top of this.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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