Why is htlc_weight added twice when calculating fee buffer?

[ShookLyngs]

If our node is the initiator of an ANCHORS channel with someone, when calculating the Bandwidth() of our node to determine whether we can add one more HTLC to the commitment transaction and prevent a series of problems, based on BOLT02, we will need to recalculate the commit_fee by doubling the current fee_per_kw.

So the estimated calculation for our case, if I'm not misunderstanding, should be:

commit_fee = fee_per_kw * 2 * (commit_weight + htlc_weight) / 1000

However, in the current implementation, the htlc_weight is added twice during the calculation:

1. In LightningChannel.availableCommitmentBalance()
2. In CalcFeeBuffer()

Did I misunderstand? Or is this some kind of bug?

[ShookLyngs]

SHOULD NOT offer amount_msat if, after adding that HTLC to its commitment transaction, its remaining balance doesn't allow it to pay the commitment transaction fee when receiving or sending a future additional non-dust HTLC while maintaining its channel reserve. It is recommended that this "fee spike buffer" can handle twice the current feerate_per_kw to ensure predictability between implementations.

Okay, it seems the htlc_weight * 2 rule is written in the BOLT 02.