mfc: Add a `commitment_feerate` optional parameter to multifundchannel

Technically there *are* two feerates that we need to know:
  - the feerate to use for the funding transaction, and
  - the feerate to tell our peer to use for our commitment txs/htlc txs

As written, `multifundchannel` uses the same feerate for both. This
optional parameter will allow us to differentiate between the two, which
will be exceedingly handy for anchor output worlds. ;)

FIXME: test this

Changelog-Added: JSON API: `multifundchannel` has a new optional argument, 'commitment_feerate', which can be used to differentiate between the funding feerate and the channel's initial commitment feerate

doc/lightning-multifundchannel.7

@@ -3,7 +3,7 @@
 lightning-multifundchannel - Command for establishing many lightning channels
 .SH SYNOPSIS
 
-\fBmultifundchannel\fR \fIdestinations\fR [\fIfeerate\fR] [\fIminconf\fR] [\fIutxos\fR] [\fIminchannels\fR]
+\fBmultifundchannel\fR \fIdestinations\fR [\fIfeerate\fR] [\fIminconf\fR] [\fIutxos\fR] [\fIminchannels\fR] [\fIcommitment_feerate\fR]
 
 .SH DESCRIPTION
 
@@ -65,8 +65,9 @@ There must be at least one entry in \fIdestinations\fR;
 it cannot be an empty array\.
 
 
-\fIfeerate\fR is an optional feerate used for the opening transaction and as
-initial feerate for commitment and HTLC transactions\. It can be one of
+\fIfeerate\fR is an optional feerate used for the opening transaction and, if
+\fIcommitment_feerate\fR is not set, as the initial feerate for
+commitment and HTLC transactions\. It can be one of
 the strings \fIurgent\fR (aim for next block), \fInormal\fR (next 4 blocks or
 so) or \fIslow\fR (next 100 blocks or so) to use lightningd’s internal
 estimates: \fInormal\fR is the default\.
@@ -91,6 +92,10 @@ this many peers remain (must not be zero)\.
 The \fBmultifundchannel\fR command will only fail if too many peers fail
 the funding process\.
 
+
+\fIcommitment_feerate\fR is the initial feerate for commitment and HTLC
+transactions\. See \fIfeerate\fR for valid values\.
+
 .SH RETURN VALUE
 
 On success, the \fItx\fR and \fItxid\fR of the signed and broadcsted funding
@@ -172,4 +177,4 @@ ZmnSCPxj \fI<ZmnSCPxj@protonmail.com\fR> is mainly responsible\.
 
 Main web site: \fIhttps://github.com/ElementsProject/lightning\fR
 
-\" SHA256STAMP:70be8ab919df4a69f18103dcf3d1067e4b1d7466eb69ca474ffae3ea1f8d0ae1
+\" SHA256STAMP:45015c53c73dc4d84850c84cb753d62739288365746b47f0403fb67e5f65d582

doc/lightning-multifundchannel.7.md

@@ -4,7 +4,7 @@ lightning-multifundchannel -- Command for establishing many lightning channels
 SYNOPSIS
 --------
 
-**multifundchannel** *destinations* \[*feerate*\] \[*minconf*\] \[*utxos*\] \[*minchannels*\]
+**multifundchannel** *destinations* \[*feerate*\] \[*minconf*\] \[*utxos*\] \[*minchannels*\] \[*commitment_feerate*\]
 
 DESCRIPTION
 -----------
@@ -55,8 +55,9 @@ Readiness is indicated by **listpeers** reporting a *state* of
 There must be at least one entry in *destinations*;
 it cannot be an empty array.
 
-*feerate* is an optional feerate used for the opening transaction and as
-initial feerate for commitment and HTLC transactions. It can be one of
+*feerate* is an optional feerate used for the opening transaction and, if
+*commitment_feerate* is not set, as the initial feerate for
+commitment and HTLC transactions. It can be one of
 the strings *urgent* (aim for next block), *normal* (next 4 blocks or
 so) or *slow* (next 100 blocks or so) to use lightningd’s internal
 estimates: *normal* is the default.
@@ -77,6 +78,9 @@ this many peers remain (must not be zero).
 The **multifundchannel** command will only fail if too many peers fail
 the funding process.
 
+*commitment_feerate* is the initial feerate for commitment and HTLC
+transactions. See *feerate* for valid values.
+
 RETURN VALUE
 ------------
 

plugins/spender/multifundchannel.c

@@ -147,8 +147,17 @@ struct multifundchannel_command {
 	*/
 	size_t pending;
 
-	/* The feerate desired by the user.  */
+	/* The feerate desired by the user.
+	 * If cmtmt_feerate_str is present, will only be used
+	 * for the funding transaction. */
 	const char *feerate_str;
+
+	/* The feerate desired by the user for
+	 * the channel commitment and HTLC txs.
+	 * If not provided, defaults to the feerate_str
+	 * value. */
+	const char *cmtmt_feerate_str;
+
 	/* The minimum number of confirmations for owned
 	UTXOs to be selected.
 	*/
@@ -1164,7 +1173,9 @@ fundchannel_start_dest(struct multifundchannel_destination *dest)
 	json_add_string(req->js, "amount",
 			fmt_amount_sat(tmpctx, &dest->amount));
 
-	if (mfc->feerate_str)
+	if (mfc->cmtmt_feerate_str)
+		json_add_string(req->js, "feerate", mfc->cmtmt_feerate_str);
+	else if (mfc->feerate_str)
 		json_add_string(req->js, "feerate", mfc->feerate_str);
 	json_add_bool(req->js, "announce", dest->announce);
 	json_add_string(req->js, "push_msat",
@@ -1988,7 +1999,7 @@ json_multifundchannel(struct command *cmd,
 		      const jsmntok_t *params)
 {
 	struct multifundchannel_destination *dests;
-	const char *feerate_str;
+	const char *feerate_str, *cmtmt_feerate_str;
 	u32 *minconf;
 	const jsmntok_t *utxos_tok;
 	u32 *minchannels;
@@ -2001,6 +2012,7 @@ json_multifundchannel(struct command *cmd,
 		   p_opt_def("minconf", param_number, &minconf, 1),
 		   p_opt("utxos", param_tok, &utxos_tok),
 		   p_opt("minchannels", param_positive_number, &minchannels),
+		   p_opt("commitment_feerate", param_string, &cmtmt_feerate_str),
 		   NULL))
 		return command_param_failed();
 
@@ -2017,6 +2029,7 @@ json_multifundchannel(struct command *cmd,
 		mfc->destinations[i].mfc = mfc;
 
 	mfc->feerate_str = feerate_str;
+	mfc->cmtmt_feerate_str = cmtmt_feerate_str;
 	mfc->minconf = *minconf;
 	if (utxos_tok)
 		mfc->utxos_str = tal_strndup(mfc, json_tok_full(buf, utxos_tok),