Writes, or more specifically commands that can write, should call checkIfCanWriteOrBlock in a loop as described in migrating_tenant_access_blocker.h:

  template <typename F>
  auto migrationConflictRetry(OperationContext* opCtx, const Database* db, F&& f) {
      while (true) {
          try {
              return f();
          } catch (const MigrationConflictException&) {
              MigratingTenantAccessBlocker::get(db).checkIfCanWriteOrBlock(opCtx);
          }
      }
  }

The loop should be above any database or collection locks.

The loop might look slightly different (e.g., not take a 'F' argument and instead wrap some existing code) depending on the implementation.

This ticket should also test that all writes block if the "start blocking" write has occurred.