psbt: new methods for generating serial_ids for an input/output

we need to do this elsewhere later, pull it out so we can use it

common/psbt_open.c

@@ -7,6 +7,7 @@
 #include <ccan/ccan/endian/endian.h>
 #include <ccan/ccan/mem/mem.h>
 #include <common/channel_id.h>
+#include <common/pseudorand.h>
 #include <common/utils.h>
 #include <wire/peer_wire.h>
 
@@ -441,6 +442,35 @@ int psbt_find_serial_output(struct wally_psbt *psbt, u16 serial_id)
 	return -1;
 }
 
+static u16 get_random_serial(enum tx_role role)
+{
+	return pseudorand(1 << 15) << 1 | role;
+}
+
+u16 psbt_new_input_serial(struct wally_psbt *psbt, enum tx_role role)
+{
+	u16 serial_id;
+
+	while ((serial_id = get_random_serial(role)) &&
+		psbt_find_serial_input(psbt, serial_id) != -1) {
+		/* keep going; */
+	}
+
+	return serial_id;
+}
+
+u16 psbt_new_output_serial(struct wally_psbt *psbt, enum tx_role role)
+{
+	u16 serial_id;
+
+	while ((serial_id = get_random_serial(role)) &&
+		psbt_find_serial_output(psbt, serial_id) != -1) {
+		/* keep going; */
+	}
+
+	return serial_id;
+}
+
 bool psbt_has_required_fields(struct wally_psbt *psbt)
 {
 	u16 serial_id;

common/psbt_open.h

@@ -130,6 +130,24 @@ int psbt_find_serial_input(struct wally_psbt *psbt, u16 serial_id);
  */
 int psbt_find_serial_output(struct wally_psbt *psbt, u16 serial_id);
 
+/* psbt_new_input_serial - Generate a new serial for an input for {role}
+ *
+ * @psbt - psbt to get a new serial for
+ * @role - which tx role to generate the serial for
+ *
+ * Returns a new, unique serial of the correct parity for the specified {role}
+ */
+u16 psbt_new_input_serial(struct wally_psbt *psbt, enum tx_role role);
+
+/* psbt_new_output_serial - Generate a new serial for an output for {role}
+ *
+ * @psbt - psbt to get a new serial for
+ * @role - which tx role to generate the serial for
+ *
+ * Returns a new, unique serial of the correct parity for the specified {role}
+ */
+u16 psbt_new_output_serial(struct wally_psbt *psbt, enum tx_role role);
+
 /* psbt_has_required_fields - Validates psbt field completion
  *
  * Required fields are:

common/test/exp-run-psbt_diff.c

@@ -42,6 +42,9 @@ u64 fromwire_u64(const u8 **cursor UNNEEDED, size_t *max UNNEEDED)
 /* Generated stub for fromwire_u8 */
 u8 fromwire_u8(const u8 **cursor UNNEEDED, size_t *max UNNEEDED)
 { fprintf(stderr, "fromwire_u8 called!\n"); abort(); }
+/* Generated stub for pseudorand */
+uint64_t pseudorand(uint64_t max UNNEEDED)
+{ fprintf(stderr, "pseudorand called!\n"); abort(); }
 /* Generated stub for towire */
 void towire(u8 **pptr UNNEEDED, const void *data UNNEEDED, size_t len UNNEEDED)
 { fprintf(stderr, "towire called!\n"); abort(); }

hsmd/Makefile

@@ -41,7 +41,8 @@ HSMD_COMMON_OBJS :=				\
 	common/version.o
 
 ifeq ($(EXPERIMENTAL_FEATURES),1)
-HSMD_COMMON_OBJS += common/psbt_open.o
+HSMD_COMMON_OBJS += common/psbt_open.o		\
+	common/pseudorand.o
 endif
 
 lightningd/lightning_hsmd: $(HSMD_OBJS) $(HSMD_COMMON_OBJS) $(BITCOIN_OBJS) $(WIRE_OBJS)

lightningd/dual_open_control.c

@@ -359,16 +359,12 @@ static bool psbt_side_contribs_changed(struct wally_psbt *orig,
 static void psbt_add_serials(struct wally_psbt *psbt, enum tx_role role)
 {
 	u16 serial_id;
-	const u64 serial_space = 100000;
 	for (size_t i = 0; i < psbt->num_inputs; i++) {
 		/* Skip ones that already have a serial id */
 		if (psbt_get_serial_id(&psbt->inputs[i].unknowns, &serial_id))
 			continue;
 
-		while ((serial_id = pseudorand(serial_space)) % 2 != role ||
-			psbt_find_serial_input(psbt, serial_id) != -1) {
-			/* keep going; */
-		}
+		serial_id = psbt_new_input_serial(psbt, role);
 		psbt_input_add_serial_id(psbt, &psbt->inputs[i], serial_id);
 	}
 	for (size_t i = 0; i < psbt->num_outputs; i++) {
@@ -376,10 +372,7 @@ static void psbt_add_serials(struct wally_psbt *psbt, enum tx_role role)
 		if (psbt_get_serial_id(&psbt->outputs[i].unknowns, &serial_id))
 			continue;
 
-		while ((serial_id = pseudorand(serial_space)) % 2 != role ||
-			psbt_find_serial_output(psbt, serial_id) != -1) {
-			/* keep going; */
-		}
+		serial_id = psbt_new_output_serial(psbt, role);
 		psbt_output_add_serial_id(psbt, &psbt->outputs[i], serial_id);
 	}
 }

onchaind/Makefile

@@ -64,7 +64,8 @@ ONCHAIND_COMMON_OBJS :=				\
 	common/wallet.o
 
 ifeq ($(EXPERIMENTAL_FEATURES),1)
-ONCHAIND_COMMON_OBJS += common/psbt_open.o
+ONCHAIND_COMMON_OBJS += common/psbt_open.o	\
+	common/pseudorand.o
 endif
 
 lightningd/lightning_onchaind: $(ONCHAIND_OBJS) $(WIRE_ONION_OBJS) $(ONCHAIND_COMMON_OBJS) $(WIRE_OBJS) $(BITCOIN_OBJS) $(HSMD_CLIENT_OBJS)

openingd/Makefile

@@ -86,7 +86,8 @@ OPENINGD_COMMON_OBJS :=				\
 	lightningd/gossip_msg.o
 
 ifeq ($(EXPERIMENTAL_FEATURES),1)
-OPENINGD_COMMON_OBJS += common/psbt_open.o
+OPENINGD_COMMON_OBJS += common/psbt_open.o	\
+	common/pseudorand.o
 endif
 
 lightningd/lightning_openingd: $(OPENINGD_OBJS) $(OPENINGD_COMMON_OBJS) $(WIRE_OBJS) $(BITCOIN_OBJS) $(HSMD_CLIENT_OBJS)

openingd/dualopend.c

@@ -1391,7 +1391,7 @@ static u8 *opener_start(struct state *state, u8 *msg)
 					 scriptpubkey_p2wsh(tmpctx, wscript),
 					 total);
 	/* Add a serial_id for this output */
-	serial_id = 0; /* FIXME: generate new serial */
+	serial_id = psbt_new_input_serial(psbt, TX_INITIATOR);
 	psbt_output_add_serial_id(psbt, funding_out, serial_id);
 
 	/* Add all of our inputs/outputs to the changeset */