[syncd] Unlock vendor api lock if enabling diag shell (#571)

syncd/VendorSai.cpp

@@ -190,7 +190,7 @@ sai_status_t VendorSai::set(
  _In_ sai_object_id_t objectId,
  _In_ const sai_attribute_t *attr)
 {
- MUTEX();
+ std::unique_lock<std::mutex> _lock(m_apimutex);
  SWSS_LOG_ENTER();
  VENDOR_CHECK_API_INITIALIZED();
 
@@ -222,6 +222,13 @@ sai_status_t VendorSai::set(
 
  sai_object_meta_key_t mk = { .objecttype = objectType, .objectkey = { .key = { .object_id = objectId } } };
 
+ if (objectType == SAI_OBJECT_TYPE_SWITCH && attr && attr->id == SAI_SWITCH_ATTR_SWITCH_SHELL_ENABLE)
+ {
+ // in case of diagnostic shell, this vendor api can be blocking, so
+ // release lock here to not cause deadlock for other events in syncd
+ _lock.unlock();
+ }
+
  return info->set(&mk, attr);
 }