Re: [PATCH v2] Prevent vhost-user-blk-test hang

["eblake@redhat.com" <eblake@redhat.com>]

On Mon, Sep 06, 2021 at 01:25:20PM +0000, Raphael Norwitz wrote:
> In the vhost-user-blk-test, as of now there is nothing stoping
> vhost-user-blk in QEMU writing to the socket right after forking off the
> storage daemon before it has a chance to come up properly, leaving the
> test hanging forever. This intermittently hanging test has caused QEMU
> automation failures reported multiple times on the mailing list [1].
> 
> This change makes the storage-daemon notify the vhost-user-blk-test
> that it is fully initialized and ready to handle client connections by
> creating a pidfile on initialiation. This ensures that the storage-daemon
> backend won't miss vhost-user messages and thereby resolves the hang.
> 
> [1] https://lore.kernel.org/qemu-devel/CAFEAcA8kYpz9LiPNxnWJAPSjc=nv532bEdyfynaBeMeohqBp3A@mail.gmail.com/
> 
> Signed-off-by: Raphael Norwitz <raphael.norwitz@nutanix.com>
> ---
>  tests/qtest/vhost-user-blk-test.c | 33 ++++++++++++++++++++++++++++++-
>  1 file changed, 32 insertions(+), 1 deletion(-)
> 
> diff --git a/tests/qtest/vhost-user-blk-test.c b/tests/qtest/vhost-user-blk-test.c
> index 6f108a1b62..78140e6f28 100644
> --- a/tests/qtest/vhost-user-blk-test.c
> +++ b/tests/qtest/vhost-user-blk-test.c
> @@ -24,6 +24,9 @@
>  #define TEST_IMAGE_SIZE         (64 * 1024 * 1024)
>  #define QVIRTIO_BLK_TIMEOUT_US  (30 * 1000 * 1000)
>  #define PCI_SLOT_HP             0x06
> +#define PIDFILE_RETRIES         5
> +
> +const char *pidfile_format = "/tmp/daemon-%d";

Why is this not static?  In fact...

>  
>  typedef struct {
>      pid_t pid;
> @@ -885,7 +888,8 @@ static void start_vhost_user_blk(GString *cmd_line, int vus_instances,
>                                   int num_queues)
>  {
>      const char *vhost_user_blk_bin = qtest_qemu_storage_daemon_binary();
> -    int i;
> +    int i, err, retries;
> +    char *daemon_pidfile_path;
>      gchar *img_path;
>      GString *storage_daemon_command = g_string_new(NULL);
>      QemuStorageDaemonState *qsd;
> @@ -898,6 +902,12 @@ static void start_vhost_user_blk(GString *cmd_line, int vus_instances,
>              " -object memory-backend-memfd,id=mem,size=256M,share=on "
>              " -M memory-backend=mem -m 256M ");
>  
> +    err = asprintf(&daemon_pidfile_path, pidfile_format, getpid());

...action at a distance makes it harder for gcc to warn about bad
formats.  I'd just inline "/tmp/daemon-%d" here in the asprintf call,
and drop pidfile_format altogether.

Why are we using bare asprintf instead of glib's g_strdup_printf?

> +    if (err == -1) {
> +        fprintf(stderr, "Failed to format storage-daemon pidfile name %m");

%m in printf is a glibc-ism; not portable to non-Linux.  Do we care?

> +        abort();

Rather than directly abort, since this is a glib test runner, is there
a glib function we should be using?  For example, using
g_assert_cmpint((err, !=, -1)?

> +    }
> +
>      for (i = 0; i < vus_instances; i++) {
>          int fd;
>          char *sock_path = create_listen_socket(&fd);
> @@ -914,6 +924,9 @@ static void start_vhost_user_blk(GString *cmd_line, int vus_instances,
>                                 i + 1, sock_path);
>      }
>  
> +    g_string_append_printf(storage_daemon_command, "--pidfile %s",

Missing a space on the tail end if there are more arguments to append
to the command line.

> +                           daemon_pidfile_path);
> +
>      g_test_message("starting vhost-user backend: %s",
>                     storage_daemon_command->str);
>      pid_t pid = fork();
> @@ -930,7 +943,25 @@ static void start_vhost_user_blk(GString *cmd_line, int vus_instances,
>          execlp("/bin/sh", "sh", "-c", storage_daemon_command->str, NULL);
>          exit(1);
>      }
> +
> +    /*
> +     * Ensure the storage-daemon has come up properly before allowing the
> +     * test to proceed.
> +     */
> +    retries = 0;
> +    while (access(daemon_pidfile_path, F_OK) != 0) {
> +        if (retries > PIDFILE_RETRIES) {
> +            fprintf(stderr, "The storage-daemon failed to come up after %d "
> +                    "seconds - killing the test", PIDFILE_RETRIES);
> +            abort();

Again, would some form of g_assert*() be better than bare abort?

> +        }
> +
> +        retries++;
> +        usleep(1000);

We're inconsistent on whether qtest files use bare usleep or g_usleep.

> +    }
> +
>      g_string_free(storage_daemon_command, true);
> +    free(daemon_pidfile_path);

Do you want to unlink() the file (if it exists)?

>  
>      qsd = g_new(QemuStorageDaemonState, 1);
>      qsd->pid = pid;
> -- 
> 2.20.1
> 

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org