feat: receivable_msat to listpeers

[m-schmoock]

This adds the receivable_msat amount to the lightning-cli listpeers output. The logic is similar to spendable_msat and applies for the maximum possible amount in one HTLC in that situation.

Adds code, test and manpage - Closes #3555

[niftynei]

ACK 5710421

two tiny nits, otherwise lgtm

[m-schmoock]

From my side, I'm more or less okay with this PR, aside I don't like the way I duplicated code (i.e. commit_txfee_spend/recv) with all just changing the REMOTE / LOCAL or our_config / their_config ... But any approach to unify the blocks lead to a iffy mess.

Also I think there might be an off by one in current (master) implementation, see https://github.com/ElementsProject/lightning/pull/3572/files/7d1f8416e1af8bf9579914db46da6ce5b5cb394a#diff-e6fd9e4a383e2ab7819a6292c1edf70bR2357 for my remark in the testcase.

[cdecker]

From my side, I'm more or less okay with this PR, aside I don't like the way I duplicated code (i.e. commit_txfee_spend/recv) with all just changing the REMOTE / LOCAL or our_config / their_config ... But any approach to unify the blocks lead to a iffy mess.

We might want to extract the LOCAL/REMOTE parameter into the function signature, and then internally use side and !side whenever either is used in the function. This is already what we do in a couple of places, and the enum is such that !LOCAL == REMOTE and vice-versa :-)

Might save us from future divergence of the two copies of the code.

Otherwise we can follow up with cleanups for the off-by-one once we have identified a cause (I'm confident it's not something introduced in this PR). Opened #3595 so it doesn't get lost.

[m-schmoock]

We might want to extract the LOCAL/REMOTE parameter into the function signature, and then internally use side and !side whenever either is used in the function. This is already what we do in a couple of places, and the enum is such that !LOCAL == REMOTE and vice-versa :-)

I'll check if I can come up with something...

[m-schmoock]

@cdecker Is there more I can do? I.e. squash
f9dde34 and 2b4f034 or merge the two spendable receivable blocks with all the similar duplicated comments line peer_control.c 725 following:

lightning/lightningd/peer_control.c

Lines 725 to 784 in 2b4f034

	/* Compute how much we can send via this channel in one payment. */
	if (!amount_msat_sub_sat(&spendable,
	channel->our_msat,
	channel->channel_info.their_config.channel_reserve))
	spendable = AMOUNT_MSAT(0);
	/* Take away any currently-offered HTLCs. */
	subtract_offered_htlcs(channel, &spendable);
	/* If we're funder, subtract txfees we'll need to spend this */
	if (channel->funder == LOCAL) {
	if (!amount_msat_sub_sat(&spendable, spendable,
	commit_txfee(channel, spendable,
	LOCAL)))
	spendable = AMOUNT_MSAT(0);
	}
	/* We can't offer an HTLC less than the other side will accept. */
	if (amount_msat_less(spendable,
	channel->channel_info.their_config.htlc_minimum))
	spendable = AMOUNT_MSAT(0);
	/* We can't offer an HTLC over the max payment threshold either. */
	if (amount_msat_greater(spendable, chainparams->max_payment))
	spendable = chainparams->max_payment;
	/* append spendable to JSON output */
	json_add_amount_msat_compat(response, spendable,
	"spendable_msatoshi", "spendable_msat");
	/* Compute how much we can receive via this channel in one payment */
	if (!amount_sat_sub_msat(&their_msat, channel->funding, channel->our_msat))
	their_msat = AMOUNT_MSAT(0);
	if (!amount_msat_sub_sat(&receivable,
	their_msat,
	channel->our_config.channel_reserve))
	receivable = AMOUNT_MSAT(0);
	/* Take away any currently-offered HTLCs. */
	subtract_received_htlcs(channel, &receivable);
	/* If they're funder, subtract txfees they'll need to spend this */
	if (channel->funder == REMOTE) {
	if (!amount_msat_sub_sat(&receivable, receivable,
	commit_txfee(channel,
	receivable, REMOTE)))
	receivable = AMOUNT_MSAT(0);
	}
	/* They can't offer an HTLC less than what we will accept. */
	if (amount_msat_less(receivable, channel->our_config.htlc_minimum))
	receivable = AMOUNT_MSAT(0);
	/* They can't offer an HTLC over the max payment threshold either. */
	if (amount_msat_greater(receivable, chainparams->max_payment))
	receivable = chainparams->max_payment;
	/* append receivable to JSON output */
	json_add_amount_msat_compat(response, receivable,
	"receivable_msatoshi", "receivable_msat");

Suggestion

	/* Compute how much we can send/receive via this channel in one payment. */
	if (!amount_msat_sub_sat(&spendable, channel->our_msat,
				 channel->channel_info.their_config.channel_reserve))
		spendable = AMOUNT_MSAT(0);
	if (!amount_sat_sub_msat(&their_msat, channel->funding, channel->our_msat))
		their_msat = AMOUNT_MSAT(0);
	if (!amount_msat_sub_sat(&receivable, their_msat, channel->our_config.channel_reserve))
		receivable = AMOUNT_MSAT(0);

	/* Take away any currently-offered HTLCs. */
	subtract_offered_htlcs(channel, &spendable);
	subtract_received_htlcs(channel, &receivable);

	/* Substract the funders txfees */
	if (channel->funder == LOCAL) {
		if (!amount_msat_sub_sat(&spendable, spendable,
					 commit_txfee(channel, spendable, LOCAL)))
			spendable = AMOUNT_MSAT(0);
	} else {
		if (!amount_msat_sub_sat(&receivable, receivable,
					 commit_txfee(channel, receivable, REMOTE)))
			receivable = AMOUNT_MSAT(0);
	}

	/* Can't offer an HTLC less than the other side will accept. */
	if (amount_msat_less(spendable, channel->channel_info.their_config.htlc_minimum))
		spendable = AMOUNT_MSAT(0);
	if (amount_msat_less(receivable, channel->our_config.htlc_minimum))
		receivable = AMOUNT_MSAT(0);

	/* Can't offer an HTLC over the max payment threshold either. */
	if (amount_msat_greater(spendable, chainparams->max_payment))
		spendable = chainparams->max_payment;
	if (amount_msat_greater(receivable, chainparams->max_payment))
		receivable = chainparams->max_payment;

	/* append spendable/receivable to JSON output */
	json_add_amount_msat_compat(response, spendable,
				    "spendable_msatoshi", "spendable_msat");
	json_add_amount_msat_compat(response, receivable,
				    "receivable_msatoshi", "receivable_msat");

[cdecker]

ACK 2b4f034