channeld: extract HTLC trim logic into common.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

channeld/Makefile

@@ -49,6 +49,7 @@ CHANNELD_COMMON_OBJS :=				\
 	common/gen_peer_status_wire.o		\
 	common/gossip_store.o			\
 	common/htlc_state.o			\
+	common/htlc_trim.o			\
 	common/htlc_tx.o			\
 	common/htlc_wire.o			\
 	common/initial_channel.o		\

channeld/commit_tx.c

@@ -2,6 +2,7 @@
 #include <bitcoin/tx.h>
 #include <ccan/endian/endian.h>
 #include <channeld/commit_tx.h>
+#include <common/htlc_trim.h>
 #include <common/htlc_tx.h>
 #include <common/keyset.h>
 #include <common/permute_tx.h>
@@ -16,36 +17,8 @@ static bool trim(const struct htlc *htlc,
 		 struct amount_sat dust_limit,
 		 enum side side)
 {
-	struct amount_sat htlc_fee, htlc_min;
+	return htlc_is_trimmed(htlc_owner(htlc), htlc->amount,
-
+			       feerate_per_kw, dust_limit, side);
-	/* BOLT #3:
-	 *
-	 *   - for every offered HTLC:
-	 *    - if the HTLC amount minus the HTLC-timeout fee would be less than
-	 *    `dust_limit_satoshis` set by the transaction owner:
-	 *      - MUST NOT contain that output.
-	 *    - otherwise:
-	 *      - MUST be generated as specified in
-	 *      [Offered HTLC Outputs](#offered-htlc-outputs).
-	 */
-	if (htlc_owner(htlc) == side)
-		htlc_fee = htlc_timeout_fee(feerate_per_kw);
-	/* BOLT #3:
-	 *
-	 *  - for every received HTLC:
-	 *    - if the HTLC amount minus the HTLC-success fee would be less than
-	 *    `dust_limit_satoshis` set by the transaction owner:
-	 *      - MUST NOT contain that output.
-	 *    - otherwise:
-	 *      - MUST be generated as specified in
-	 */
-	else
-		htlc_fee = htlc_success_fee(feerate_per_kw);
-
-	/* If these overflow, it implies htlc must be less. */
-	if (!amount_sat_add(&htlc_min, dust_limit, htlc_fee))
-		return true;
-	return amount_msat_less_sat(htlc->amount, htlc_min);
 }
 
 size_t commit_tx_num_untrimmed(const struct htlc **htlcs,

channeld/test/Makefile

@@ -11,6 +11,7 @@ CHANNELD_TEST_COMMON_OBJS :=			\
 	common/amount.o				\
 	common/daemon_conn.o			\
 	common/htlc_state.o			\
+	common/htlc_trim.o			\
 	common/htlc_tx.o			\
 	common/initial_commit_tx.o		\
 	common/key_derive.o			\

common/Makefile

@@ -21,6 +21,7 @@ COMMON_SRC_NOGEN :=				\
 	common/gossip_store.c			\
 	common/hash_u5.c			\
 	common/htlc_state.c			\
+	common/htlc_trim.c			\
 	common/htlc_tx.c			\
 	common/htlc_wire.c			\
 	common/initial_channel.c		\

common/htlc_trim.c

@@ -0,0 +1,41 @@
+#include <common/htlc_trim.h>
+#include <common/htlc_tx.h>
+
+/* If this htlc too small to create an output on @side's commitment tx? */
+bool htlc_is_trimmed(enum side htlc_owner,
+		     struct amount_msat htlc_amount,
+		     u32 feerate_per_kw,
+		     struct amount_sat dust_limit,
+		     enum side side)
+{
+	struct amount_sat htlc_fee, htlc_min;
+
+	/* BOLT #3:
+	 *
+	 *   - for every offered HTLC:
+	 *    - if the HTLC amount minus the HTLC-timeout fee would be less than
+	 *    `dust_limit_satoshis` set by the transaction owner:
+	 *      - MUST NOT contain that output.
+	 *    - otherwise:
+	 *      - MUST be generated as specified in
+	 *      [Offered HTLC Outputs](#offered-htlc-outputs).
+	 */
+	if (htlc_owner == side)
+		htlc_fee = htlc_timeout_fee(feerate_per_kw);
+	/* BOLT #3:
+	 *
+	 *  - for every received HTLC:
+	 *    - if the HTLC amount minus the HTLC-success fee would be less than
+	 *    `dust_limit_satoshis` set by the transaction owner:
+	 *      - MUST NOT contain that output.
+	 *    - otherwise:
+	 *      - MUST be generated as specified in
+	 */
+	else
+		htlc_fee = htlc_success_fee(feerate_per_kw);
+
+	/* If these overflow, it implies htlc must be less. */
+	if (!amount_sat_add(&htlc_min, dust_limit, htlc_fee))
+		return true;
+	return amount_msat_less_sat(htlc_amount, htlc_min);
+}

common/htlc_trim.h

@@ -0,0 +1,14 @@
+#ifndef LIGHTNING_COMMON_HTLC_TRIM_H
+#define LIGHTNING_COMMON_HTLC_TRIM_H
+#include "config.h"
+#include <common/amount.h>
+#include <common/htlc.h>
+
+/* If this htlc too small to create an output on @side's commitment tx? */
+bool htlc_is_trimmed(enum side htlc_owner,
+		     struct amount_msat htlc_amount,
+		     u32 feerate_per_kw,
+		     struct amount_sat dust_limit,
+		     enum side side);
+
+#endif /* LIGHTNING_COMMON_HTLC_TRIM_H */

common/htlc_tx.h

@@ -4,9 +4,12 @@
 #include <common/amount.h>
 #include <common/htlc.h>
 
+struct bitcoin_signature;
+struct bitcoin_txid;
 struct keyset;
 struct preimage;
 struct pubkey;
+struct ripemd160;
 
 static inline struct amount_sat htlc_timeout_fee(u32 feerate_per_kw)
 {