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

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.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 qemu-nbd.c                | 21 +++++++++------------
 stubs/blk-exp-close-all.c |  7 +++++++
 block/export/meson.build  |  4 ++--
 meson.build               |  4 ++--
 nbd/meson.build           |  2 ++
 stubs/meson.build         |  1 +
 6 files changed, 23 insertions(+), 16 deletions(-)
 create mode 100644 stubs/blk-exp-close-all.c

On 29/09/20 14:55, 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.
> 
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
>  qemu-nbd.c                | 21 +++++++++------------
>  stubs/blk-exp-close-all.c |  7 +++++++
>  block/export/meson.build  |  4 ++--
>  meson.build               |  4 ++--
>  nbd/meson.build           |  2 ++
>  stubs/meson.build         |  1 +
>  6 files changed, 23 insertions(+), 16 deletions(-)
>  create mode 100644 stubs/blk-exp-close-all.c
> 
> diff --git a/qemu-nbd.c b/qemu-nbd.c
> index 6d7ac7490f..06774ca615 100644
> --- a/qemu-nbd.c
> +++ b/qemu-nbd.c
> @@ -25,6 +25,7 @@
>  #include "qapi/error.h"
>  #include "qemu/cutils.h"
>  #include "sysemu/block-backend.h"
> +#include "sysemu/runstate.h" /* for qemu_system_killed() prototype */
>  #include "block/block_int.h"
>  #include "block/nbd.h"
>  #include "qemu/main-loop.h"
> @@ -155,7 +156,11 @@ QEMU_COPYRIGHT "\n"
>  }
>  
>  #if HAVE_NBD_DEVICE
> -static void termsig_handler(int signum)
> +/*
> + * The client thread uses SIGTERM to interrupt the server.  A signal
> + * handler ensures that "qemu-nbd -v -c" exits with a nice status code.
> + */
> +void qemu_system_killed(int signum, pid_t pid)
>  {
>      atomic_cmpxchg(&state, RUNNING, TERMINATE);
>      qemu_notify_event();
> @@ -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();
>  #endif /* HAVE_NBD_DEVICE */
>  
> -#ifdef CONFIG_POSIX
> -    signal(SIGPIPE, SIG_IGN);
> -#endif
> -
>      socket_init();
>      error_init(argv[0]);
>      module_call_init(MODULE_INIT_TRACE);
> diff --git a/stubs/blk-exp-close-all.c b/stubs/blk-exp-close-all.c
> new file mode 100644
> index 0000000000..1c71316763
> --- /dev/null
> +++ b/stubs/blk-exp-close-all.c
> @@ -0,0 +1,7 @@
> +#include "qemu/osdep.h"
> +#include "block/export.h"
> +
> +/* Only used in programs that support block exports (libblockdev.fa) */
> +void blk_exp_close_all(void)
> +{
> +}
> diff --git a/block/export/meson.build b/block/export/meson.build
> index 469a7aa0f5..a2772a0dce 100644
> --- a/block/export/meson.build
> +++ b/block/export/meson.build
> @@ -1,2 +1,2 @@
> -block_ss.add(files('export.c'))
> -block_ss.add(when: 'CONFIG_VHOST_USER', if_true: files('vhost-user-blk-server.c'))
> +blockdev_ss.add(files('export.c'))
> +blockdev_ss.add(when: 'CONFIG_VHOST_USER', if_true: files('vhost-user-blk-server.c'))
> diff --git a/meson.build b/meson.build
> index 18d689b423..0e9528adab 100644
> --- a/meson.build
> +++ b/meson.build
> @@ -835,7 +835,6 @@ subdir('dump')
>  
>  block_ss.add(files(
>    'block.c',
> -  'blockdev-nbd.c',
>    'blockjob.c',
>    'job.c',
>    'qemu-io-cmds.c',
> @@ -848,6 +847,7 @@ subdir('block')
>  
>  blockdev_ss.add(files(
>    'blockdev.c',
> +  'blockdev-nbd.c',
>    'iothread.c',
>    'job-qmp.c',
>  ))
> @@ -1171,7 +1171,7 @@ if have_tools
>    qemu_io = executable('qemu-io', files('qemu-io.c'),
>               dependencies: [block, qemuutil], install: true)
>    qemu_nbd = executable('qemu-nbd', files('qemu-nbd.c'),
> -               dependencies: [block, qemuutil], install: true)
> +               dependencies: [blockdev, qemuutil], install: true)
>  
>    subdir('storage-daemon')
>    subdir('contrib/rdmacm-mux')
> diff --git a/nbd/meson.build b/nbd/meson.build
> index 0c00a776d3..2baaa36948 100644
> --- a/nbd/meson.build
> +++ b/nbd/meson.build
> @@ -1,5 +1,7 @@
>  block_ss.add(files(
>    'client.c',
>    'common.c',
> +))
> +blockdev_ss.add(files(
>    'server.c',
>  ))
> diff --git a/stubs/meson.build b/stubs/meson.build
> index e0b322bc28..0fdcf93c09 100644
> --- a/stubs/meson.build
> +++ b/stubs/meson.build
> @@ -1,6 +1,7 @@
>  stub_ss.add(files('arch_type.c'))
>  stub_ss.add(files('bdrv-next-monitor-owned.c'))
>  stub_ss.add(files('blk-commit-all.c'))
> +stub_ss.add(files('blk-exp-close-all.c'))
>  stub_ss.add(files('blockdev-close-all-bdrv-states.c'))
>  stub_ss.add(files('change-state-handler.c'))
>  stub_ss.add(files('clock-warp.c'))
> 

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>

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

nbd

> 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.
> 
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---

Reviewed-by: Eric Blake <eblake@redhat.com>

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.

On Tue, Sep 29, 2020 at 12:36:10PM -0500, Eric Blake wrote:
> 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.

Thanks for letting me know about the bug report. The link can be
added to the commit description when merging (or if I resend).

Stefan