libxl: fix segfault in libxlReconnectDomain

In case of error, libxlReconnectDomain may call
virDomainObjListRemoveLocked. However it has no local reference on
the domain object, leading to segfault. Get a reference to the domain
object at the start of the function and release it at the end to avoid
problems.

This commit also factorizes code between the error and normal ends.

src/libxl/libxl_driver.c

@@ -373,11 +373,13 @@ libxlReconnectDomain(virDomainObjPtr vm,
     uint8_t *data = NULL;
     virHostdevManagerPtr hostdev_mgr = driver->hostdevMgr;
     unsigned int hostdev_flags = VIR_HOSTDEV_SP_PCI;
+    int ret = -1;
 
 #ifdef LIBXL_HAVE_PVUSB
     hostdev_flags |= VIR_HOSTDEV_SP_USB;
 #endif
 
+    virObjectRef(vm);
     virObjectLock(vm);
 
     libxl_dominfo_init(&d_info);
@@ -385,18 +387,18 @@ libxlReconnectDomain(virDomainObjPtr vm,
     /* Does domain still exist? */
     rc = libxl_domain_info(cfg->ctx, &d_info, vm->def->id);
     if (rc == ERROR_INVAL) {
-        goto out;
+        goto error;
     } else if (rc != 0) {
         VIR_DEBUG("libxl_domain_info failed (code %d), ignoring domain %d",
                   rc, vm->def->id);
-        goto out;
+        goto error;
     }
 
     /* Is this a domain that was under libvirt control? */
     if (libxl_userdata_retrieve(cfg->ctx, vm->def->id,
                                 "libvirt-xml", &data, &len)) {
         VIR_DEBUG("libxl_userdata_retrieve failed, ignoring domain %d", vm->def->id);
-        goto out;
+        goto error;
     }
 
     /* Update domid in case it changed (e.g. reboot) while we were gone? */
@@ -405,7 +407,7 @@ libxlReconnectDomain(virDomainObjPtr vm,
     /* Update hostdev state */
     if (virHostdevUpdateActiveDomainDevices(hostdev_mgr, LIBXL_DRIVER_NAME,
                                             vm->def, hostdev_flags) < 0)
-        goto out;
+        goto error;
 
     if (virAtomicIntInc(&driver->nactive) == 1 && driver->inhibitCallback)
         driver->inhibitCallback(true, driver->inhibitOpaque);
@@ -413,21 +415,25 @@ libxlReconnectDomain(virDomainObjPtr vm,
     /* Enable domain death events */
     libxl_evenable_domain_death(cfg->ctx, vm->def->id, 0, &priv->deathW);
 
+    ret = 0;
+
+ cleanup:
     libxl_dominfo_dispose(&d_info);
     virObjectUnlock(vm);
+    virObjectUnref(vm);
     virObjectUnref(cfg);
-    return 0;
+    return ret;
 
- out:
-    libxl_dominfo_dispose(&d_info);
+ error:
     libxlDomainCleanup(driver, vm);
-    if (!vm->persistent)
+    if (!vm->persistent) {
         virDomainObjListRemoveLocked(driver->domains, vm);
-    else
-        virObjectUnlock(vm);
-    virObjectUnref(cfg);
 
-    return -1;
+        /* virDomainObjListRemoveLocked leaves the object unlocked,
+         * lock it again to factorize more code. */
+        virObjectLock(vm);
+    }
+    goto cleanup;
 }
 
 static void