The Spice server doesn't like to be started or stopped twice . It
aborts with:

(process:6191): Spice-ERROR **: 19:29:35.912: red-worker.c:623:handle_dev_start: assertion `!worker->running' failed

It's easy to avoid that situation since qemu spice_display_is_running
tracks the server state.

After the commit "spice: do not stop spice if VM is paused", it will
be possible to pause and resume the VM, and this will call
qemu_spice_display_start() twice. The easiest is to add a check for
spice_display_is_running with this patch to avoid the assert.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Tested-by: Victor Toso <victortoso@redhat.com>
---
 ui/spice-core.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/ui/spice-core.c b/ui/spice-core.c
index a40fb2c00d..94b31ce3ad 100644
--- a/ui/spice-core.c
+++ b/ui/spice-core.c
@@ -921,12 +921,20 @@ int qemu_spice_display_add_client(int csock, int skipauth, int tls)
 
 void qemu_spice_display_start(void)
 {
+    if (spice_display_is_running) {
+        return;
+    }
+
     spice_display_is_running = true;
     spice_server_vm_start(spice_server);
 }
 
 void qemu_spice_display_stop(void)
 {
+    if (!spice_display_is_running) {
+        return;
+    }
+
     spice_server_vm_stop(spice_server);
     spice_display_is_running = false;
 }
-- 
2.21.0.rc1