mfc-psbt: mark all of our inputs as "ours", then only sign those

we only want to sign the inputs that we've reserved via utxopsbt or
fundpsbt. we mark them with a flag (reusing the now defunct max-len
flag is fine), then look for inputs with that flag to pass to signonly

common/psbt_open.c

@@ -464,3 +464,20 @@ void psbt_add_serials(struct wally_psbt *psbt, enum tx_role role)
 		psbt_output_set_serial_id(psbt, &psbt->outputs[i], serial_id);
 	}
 }
+
+void psbt_input_mark_ours(const tal_t *ctx,
+			  struct wally_psbt_input *input)
+{
+	u8 *key = psbt_make_key(tmpctx, PSBT_TYPE_INPUT_MARKER, NULL);
+	beint16_t bev = cpu_to_be16(1);
+
+	psbt_input_set_unknown(ctx, input, key, &bev, sizeof(bev));
+}
+
+bool psbt_input_is_ours(const struct wally_psbt_input *input)
+{
+	size_t unused;
+	void *result = psbt_get_lightning(&input->unknowns,
+					  PSBT_TYPE_INPUT_MARKER, &unused);
+	return !(!result);
+}

common/psbt_open.h

@@ -38,7 +38,7 @@ struct psbt_changeset {
 };
 
 #define PSBT_TYPE_SERIAL_ID 0x01
-#define PSBT_TYPE_MAX_WITNESS_LEN 0x02
+#define PSBT_TYPE_INPUT_MARKER 0x02
 
 /* psbt_get_serial_id - Returns the serial_id from an unknowns map
  *
@@ -163,4 +163,15 @@ bool psbt_side_finalized(const struct wally_psbt *psbt,
  * @role - the role we should use to select serial parity
  */
 void psbt_add_serials(struct wally_psbt *psbt, enum tx_role role);
+
+/* psbt_input_mark_ours - Sets the PSBT_TYPE_INPUT_MARKER on this input
+ */
+void psbt_input_mark_ours(const tal_t *ctx,
+			  struct wally_psbt_input *input);
+
+/* psbt_input_is_ours  - Returns true if this psbt input has
+ * 			 the PSBT_TYPE_INPUT_MARKER set on it.
+ */
+bool psbt_input_is_ours(const struct wally_psbt_input *input);
+
 #endif /* LIGHTNING_COMMON_PSBT_OPEN_H */

plugins/spender/multifundchannel.c

@@ -915,6 +915,11 @@ mfc_psbt_acquired(struct multifundchannel_command *mfc)
 	 * for the life of the tx */
 	psbt_add_serials(mfc->psbt, TX_INITIATOR);
 
+	/* We also mark all of our inputs as *ours*, so we
+	 * can easily identify them for `signpsbt`, later */
+	for (size_t i = 0; i < mfc->psbt->num_inputs; i++)
+		psbt_input_mark_ours(mfc->psbt, &mfc->psbt->inputs[i]);
+
 	return perform_channel_start(mfc);
 }
 
@@ -1522,13 +1527,21 @@ perform_signpsbt(struct multifundchannel_command *mfc)
 	plugin_log(mfc->cmd->plugin, LOG_DBG,
 		   "mfc %"PRIu64": signpsbt.", mfc->id);
 
-	/* FIXME: indicate our inputs with signonly */
 	req = jsonrpc_request_start(mfc->cmd->plugin, mfc->cmd,
 				    "signpsbt",
 				    &after_signpsbt,
 				    &mfc_forward_error,
 				    mfc);
 	json_add_psbt(req->js, "psbt", mfc->psbt);
+
+	/* Use input markers to identify which inputs
+	 * are ours, only sign those */
+	json_array_start(req->js, "signonly");
+	for (size_t i = 0; i < mfc->psbt->num_inputs; i++) {
+		if (psbt_input_is_ours(&mfc->psbt->inputs[i]))
+			json_add_num(req->js, NULL, i);
+	}
+	json_array_end(req->js);
 	return send_outreq(mfc->cmd->plugin, req);
 }