[Libguestfs] [libnbd PATCH] docs: Mention that nbd_close is not thread-safe

[Richard W.M. Jones]

On Wed, Jul 24, 2019 at 03:14:28PM -0500, Eric Blake wrote:
> Closing the handle in one thread while another thread is locked causes
> undefined behavior (as closing does not obtain a lock, and can cause
> use-after-free or other nasty problems to the thread that does own the
> lock).  However, it is not sensible to try and obtain a lock in
> nbd_close, as POSIX says that it is also undefined for any other
> thread to wait on a mutex that has already been destroyed.  Therefore,
> we don't need to change our code, but merely remind users that
> nbd_close is not safe until all other uses of the handle have ceased.
> ---
> 
>  generator/generator | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/generator/generator b/generator/generator
> index 896ad2a..bdd8fd7 100755
> --- a/generator/generator
> +++ b/generator/generator
> @@ -3532,7 +3532,8 @@ for how to get further details of the error.
>  Closes the handle and frees any associated resources.  The final
>  status of any command that has not been retired (whether by
>  C<nbd_aio_command_completed> or by a low-level completion callback
> -returning C<1>) is lost.
> +returning C<1>) is lost.  This function is not safe to call while
> +any other thread is still using any C<nbd_*> API on the same handle.
> 
>  =head1 GETTING THE LATEST ERROR MESSAGE IN THE THREAD

ACK.

Yes it's not safe to call nbd_close until all other uses of
the same handle from any other thread are over.

Rich.

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