Method EnsureAllocated

cpp/src/arrow/compute/exec.cc:546–553

  }

  Status EnsureAllocated() {
    if (bitmap_preallocated_) {
      return Status::OK();
    }
    ARROW_ASSIGN_OR_RAISE(output_->buffers[0], ctx_->AllocateBitmap(output_->length));
    bitmap_ = output_->buffers[0]->mutable_data();
    return Status::OK();
  }

  Status AllNullShortCircuit() {
    // OK, the output should be all null