Re: [PATCH v2 3/4] block: move block exports to libblockdev

[Eric Blake <eblake@redhat.com>]

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On 9/29/20 7:55 AM, Stefan Hajnoczi wrote:
> Block exports are used by softmmu, qemu-storage-daemon, and qemu-nbd.
> They are not used by other programs and are not otherwise needed in
> libblock.
> 
> Undo the recent move of blockdev-nbd.c from blockdev_ss into block_ss.
> Since bdrv_close_all() (libblock) calls blk_exp_close_all()
> (libblockdev) a stub function is required..
> 
> Make qemu-ndb.c use signal handling utility functions instead of
> duplicating the code. This helps because os-posix.c is in libblockdev
> and it depends on a qemu_system_killed() symbol that qemu-nbd.c lacks.
> Once we use the signal handling utility functions we also end up
> providing the necessary symbol.

Hmm. I just stumbled on a long-standing bug in qemu-nbd - it installs a
SIGTERM handler, but not a SIGINT or SIGHUP handler.  This matters in
the following sequence:

qemu-nbd -f qcow2 -B bitmap image   # Ctrl-C
qemu-nbd -f qcow2 -B bitmap image

because the first instance dies with SIGINT but there is no handler
installed, qemu-nbd does not release the bitmap from being marked
in-use, and the second instance then fails with:

qemu-nbd: Bitmap 'b0' is inconsistent and cannot be used

And to my surprise, while I was trying to find the root cause to fixing
the bug I just found, I noticed that your patch happens to fix that...

> +++ b/qemu-nbd.c

> @@ -581,20 +586,12 @@ int main(int argc, char **argv)
>      const char *pid_file_name = NULL;
>      BlockExportOptions *export_opts;
>  
> +    os_setup_early_signal_handling();
> +
>  #if HAVE_NBD_DEVICE
> -    /* The client thread uses SIGTERM to interrupt the server.  A signal
> -     * handler ensures that "qemu-nbd -v -c" exits with a nice status code.
> -     */
> -    struct sigaction sa_sigterm;
> -    memset(&sa_sigterm, 0, sizeof(sa_sigterm));
> -    sa_sigterm.sa_handler = termsig_handler;
> -    sigaction(SIGTERM, &sa_sigterm, NULL);
> +    os_setup_signal_handling();

...by installing a SIGINT handler.

Is HAVE_NBD_DEVICE really the best gate for this code, or is it really
whether we are compiling for mingw?  At any rate, you may want to add a
link to https://bugzilla.redhat.com/show_bug.cgi?id=1883608 in the
commit message, and/or separate the bug fix out into a separate commit.

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


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