sr_module_change_subscribe URGE locks for Ext SHM

in sr_module_change_subscribe(), the change sub Ext SHM lock should be
acquired in a writer preferred manner (WRITE_URGE).

Adding a change subscr holds the WRITE lock for a very short duration.

But, a change_sub READ lock is held for much longer, because the oper
notify diff needs to be determined correctly based on the subscriptions
present.

So, for modules with frequent writes, subscription can fail due to
starvation.

Author: irfanHaslanded

src/sysrepo.c

@@ -6514,8 +6514,8 @@ sr_module_change_subscribe(sr_session_ctx_t *session, const char *module_name, c
         _sr_subscription_thread_suspend(*subscription);
     }
 
-    /* CHANGE SUB WRITE LOCK - also prevents events from being published */
-    if ((err_info = sr_rwlock(&shm_mod->change_sub[session->ds].lock, SR_SHMEXT_SUB_LOCK_TIMEOUT, SR_LOCK_WRITE,
+    /* CHANGE SUB WRITE URGE LOCK - also prevents events from being published */
+    if ((err_info = sr_rwlock(&shm_mod->change_sub[session->ds].lock, SR_SHMEXT_SUB_LOCK_TIMEOUT, SR_LOCK_WRITE_URGE,
             conn->cid, __func__, NULL, NULL))) {
         goto cleanup;
     }
@@ -6571,8 +6571,8 @@ sr_module_change_subscribe(sr_session_ctx_t *session, const char *module_name, c
     subs_mode = SR_LOCK_NONE;
 
 error1:
-    /* CHANGE SUB WRITE lock - and remove the newly added sub from Ext SHM */
-    if ((tmp_err = sr_rwlock(&shm_mod->change_sub[session->ds].lock, SR_SHMEXT_SUB_LOCK_TIMEOUT, SR_LOCK_WRITE,
+    /* CHANGE SUB WRITE URGE lock - and remove the newly added sub from Ext SHM */
+    if ((tmp_err = sr_rwlock(&shm_mod->change_sub[session->ds].lock, SR_SHMEXT_SUB_LOCK_TIMEOUT, SR_LOCK_WRITE_URGE,
             conn->cid, __func__, NULL, NULL))) {
         sr_errinfo_merge(&err_info, tmp_err);
     }