docs: static SLF4JServiceProvider getProvider() {
          if (INITIALIZATION_STATE == UNINITIALIZED) {
              synchronized (LoggerFactory.class) {
                  if (INITIALIZATION_STATE == UNINITIALIZED) {
                      INITIALIZATION_STATE = ONGOING_INITIALIZATION;
                      performInitialization();
                  }
              }
          }
          switch (INITIALIZATION_STATE) {
          case SUCCESSFUL_INITIALIZATION:
              return PROVIDER;
          case NOP_FALLBACK_INITIALIZATION:
              return NOP_FALLBACK_FACTORY;
          case FAILED_INITIALIZATION:
              throw new IllegalStateException(UNSUCCESSFUL_INIT_MSG);
          case ONGOING_INITIALIZATION:
              // support re-entrant behavior.
              // See also http://jira.qos.ch/browse/SLF4J-97
              return SUBST_PROVIDER;
          }
          throw new IllegalStateException("Unreachable code");
      }