server: Fix SpiceWorker-CRITICAL **: red_worker.c:10968:red_push_monitors_config: condition `monitors_config != NULL' failed

During my dynamic monitor support testing today, I hit the following assert
in red_worker.c:
"red_push_monitors_config: condition `monitors_config != NULL' failed"

This is caused by the following scenario:
1) Guest causes handle_dev_monitors_config_async() to be called
2) handle_dev_monitors_config_async() calls worker_update_monitors_config()
3) handle_dev_monitors_config_async() pushes worker->monitors_config, this
   takes a ref on the current monitors_config
4) Guest causes handle_dev_monitors_config_async() to be called *again*
5) handle_dev_monitors_config_async() calls worker_update_monitors_config()
6) worker_update_monitors_config() does a decref on worker->monitors_config,
   releasing the workers reference, this monitor_config from step 2 is
   not yet free-ed though as the pipe-item still holds a ref
7) worker_update_monitors_config() creates a new monitors_config with an
   initial ref-count of 1 and stores that in worker->monitors_config
8) The pipe-item of the *first* monitors_config is send, upon completion
   a decref is done on the monitors_config, and monitors_config_decref not
   only frees the monitor_config, but *also* sets worker->monitors_config
   to NULL, even though worker->monitors_config no longer refers to the
   monitor_config being freed, it refers to the 2nd monitor_config!
9) The client which was connected when this all happened disconnects
10) A new client connects, leading to the assert:
    at red_worker.c:9519
    num_common_caps=1, common_caps=0x5555569b6f60, migrate=0,
    stream=<optimized out>, client=<optimized out>, worker=<optimized out>)
    at red_worker.c:10423
    at red_worker.c:11301

Note that red_worker.c:9519 is:
        red_push_monitors_config(dcc);
gdb does not point to the actual line of the assert because the function gets
inlined.

The fix is easy and obvious, don't set worker->monitors_config to NULL in
monitors_config_decref. I'm a bit baffled as to why that code is there in
the first place, the whole point of ref-counting is to not have one single
unique place to store the reference...

This fix should not have any adverse side-effects as the 4 callers of
monitors_config_decref fall into 2 categories:
1) Code which immediately after the decref replaces worker->monitors_config
   with a new monitors_config:
   worker_update_monitors_config()
   set_monitors_config_to_primary()
2) pipe-item freeing code, which should not touch the worker state at all
   to being with

Signed-off-by: Hans de Goede <hdegoede@redhat.com>

1 files changed, 1 insertions, 2 deletions

diff --git a/server/red_worker.c b/server/red_worker.c
index 085e3e6b..de070a69 100644
--- a/server/red_worker.c
+++ b/server/red_worker.c
@@ -1239,8 +1239,7 @@ static void monitors_config_decref(MonitorsConfig *monitors_config)
         return;
     }
 
-    spice_debug("removing worker monitors config");
-    monitors_config->worker->monitors_config = NULL;
+    spice_debug("freeing monitors config");
     free(monitors_config);
 }