splice: first pass on splice_out command

Added splice_out RPC command and a simple test of it as well.

ChangeLog-Added: splice_out command added for doing a splice out more easily.

bitcoin/test/run-tx-bitcoin_tx_2of2_input_witness_weight.c

@@ -100,6 +100,14 @@ bool psbt_finalize(struct wally_psbt *psbt UNNEEDED)
 struct amount_sat psbt_input_get_amount(const struct wally_psbt *psbt UNNEEDED,
 					size_t in UNNEEDED)
 { fprintf(stderr, "psbt_input_get_amount called!\n"); abort(); }
+/* Generated stub for psbt_input_get_weight */
+size_t psbt_input_get_weight(const struct wally_psbt *psbt UNNEEDED,
+					size_t in UNNEEDED)
+{ fprintf(stderr, "psbt_input_get_weight called!\n"); abort(); }
+/* Generated stub for psbt_output_get_weight */
+size_t psbt_output_get_weight(const struct wally_psbt *psbt UNNEEDED,
+					size_t out UNNEEDED)
+{ fprintf(stderr, "psbt_output_get_weight called!\n"); abort(); }
 /* Generated stub for psbt_input_set_wit_utxo */
 void psbt_input_set_wit_utxo(struct wally_psbt *psbt UNNEEDED, size_t in UNNEEDED,
 			     const u8 *scriptPubkey UNNEEDED, struct amount_sat amt UNNEEDED)

bitcoin/tx.c

@@ -483,6 +483,20 @@ u8 *linearize_wtx(const tal_t *ctx, const struct wally_tx *wtx)
 	return arr;
 }
 
+size_t wally_psbt_weight(const struct wally_psbt *psbt)
+{
+	size_t weight = bitcoin_tx_core_weight(psbt->num_inputs,
+					       psbt->num_outputs);
+
+	for (size_t i = 0; i < psbt->num_inputs; i++)
+		weight += psbt_input_get_weight(psbt, i);
+
+	for (size_t i = 0; i < psbt->num_outputs; i++)
+		weight += psbt_output_get_weight(psbt, i);
+
+	return weight;
+}
+
 size_t wally_tx_weight(const struct wally_tx *wtx)
 {
 	size_t weight;

bitcoin/tx.h

@@ -61,6 +61,7 @@ u8 *linearize_wtx(const tal_t *ctx, const struct wally_tx *wtx);
 /* Get weight of tx in Sipa; assumes it will have witnesses! */
 size_t bitcoin_tx_weight(const struct bitcoin_tx *tx);
 size_t wally_tx_weight(const struct wally_tx *wtx);
+size_t wally_psbt_weight(const struct wally_psbt *psbt);
 
 /* Allocate a tx: you just need to fill in inputs and outputs (they're
  * zeroed with inputs' sequence_number set to FFFFFFFF) */

contrib/pyln-client/pyln/client/lightning.py

@@ -1212,6 +1212,19 @@ def openchannel_abort(self, channel_id):
         }
         return self.call("openchannel_abort", payload)
 
+    def splice_out(self, chan_id, amount, feerate_per_kw=None, force_feerate=None, initialpsbt=None, locktime=None, sign_first=None):
+        """ Initiate a splice """
+        payload = {
+            "channel_id": chan_id,
+            "amount": amount,
+            "feerate_per_kw": feerate_per_kw,
+            "force_feerate": force_feerate,
+            "initialpsbt": initialpsbt,
+            "locktime": locktime,
+            "sign_first": sign_first,
+        }
+        return self.call("splice_out", payload)
+
     def splice_init(self, chan_id, amount, initialpsbt=None, feerate_per_kw=None):
         """ Initiate a splice """
         payload = {

doc/schemas/splice_out.request.json

@@ -0,0 +1,40 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "type": "object",
+  "additionalProperties": false,
+  "required": [
+    "channel_id",
+    "amount"
+  ],
+  "added": "v23.11",
+  "properties": {
+    "channel_id": {
+      "type": "string",
+      "description": "the channel id of the channel to take funds from"
+    },
+    "amount": {
+      "type": "msat",
+      "description": "a positive amount of satoshis to be taken from the channel"
+    },
+    "initialpsbt": {
+      "type": "string",
+      "description": "the (optional) base 64 encoded PSBT to begin with. If not specified, one will be generated automatically"
+    },
+    "feerate_per_kw": {
+      "type": "u32",
+      "description": "the miner fee we will pay from our channel funds. It is calculated by `feerate_per_kw` * our_bytes_in_splice_tx / 1000"
+    },
+    "force_feerate": {
+      "type": "boolean",
+      "description": "by default splices will fail if the fee provided looks too high. This is to protect against accidentally setting your fee higher than intended. Set `force_feerate` to true to skip this saftey check"
+    },
+    "locktime": {
+      "type": "u32",
+      "description": "the locktime to use if initialpsbt is not specified."
+    },
+    "sign_first": {
+      "type": "bool",
+      "description": "offer to our peer to sign the splice first."
+    }
+  }
+}

doc/schemas/splice_out.schema.json

@@ -0,0 +1,20 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "type": "object",
+  "additionalProperties": false,
+  "required": [
+    "tx",
+    "txid"
+  ],
+  "added": "v23.11",
+  "properties": {
+    "tx": {
+      "type": "hex",
+      "description": "The hex representation of the final transaction that is published"
+    },
+    "txid": {
+      "type": "txid",
+      "description": "The txid is of the final transaction"
+    }
+  }
+}