features: don't assume we'll always only advertize optional features.

Generalize things a bit so OPTIONAL_FEATURE() and COMPULSORY_FEATURE()
work with either odd or even features, then explicitly use OPTIONAL_FEATURE
in our internal feature array.

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

common/features.c

@@ -4,12 +4,12 @@
 #include <wire/peer_wire.h>
 
 static const u32 our_localfeatures[] = {
-	LOCAL_DATA_LOSS_PROTECT,
-	LOCAL_INITIAL_ROUTING_SYNC,
-	LOCAL_UPFRONT_SHUTDOWN_SCRIPT,
-	LOCAL_GOSSIP_QUERIES,
+	OPTIONAL_FEATURE(LOCAL_DATA_LOSS_PROTECT),
+	OPTIONAL_FEATURE(LOCAL_INITIAL_ROUTING_SYNC),
+	OPTIONAL_FEATURE(LOCAL_UPFRONT_SHUTDOWN_SCRIPT),
+	OPTIONAL_FEATURE(LOCAL_GOSSIP_QUERIES),
 #if EXPERIMENTAL_FEATURES
-	LOCAL_EXTENDED_GOSSIP_QUERIES
+	OPTIONAL_FEATURE(LOCAL_GOSSIP_QUERIES_EX)
 #endif
 };
 
@@ -40,13 +40,12 @@ static bool test_bit(const u8 *features, size_t byte, unsigned int bit)
 	return features[tal_count(features) - 1 - byte] & (1 << (bit % 8));
 }
 
-/* We don't insist on anything, it's all optional. */
 static u8 *mkfeatures(const tal_t *ctx, const u32 *arr, size_t n)
 {
 	u8 *f = tal_arr(ctx, u8, 0);
 
 	for (size_t i = 0; i < n; i++)
-		set_bit(&f, OPTIONAL_FEATURE(arr[i]));
+		set_bit(&f, arr[i]);
 	return f;
 }
 
@@ -74,9 +73,7 @@ static bool feature_set(const u8 *features, size_t bit)
 
 bool feature_offered(const u8 *features, size_t f)
 {
-	assert(f % 2 == 0);
-
-	return feature_set(features, f)
+	return feature_set(features, COMPULSORY_FEATURE(f))
 		|| feature_set(features, OPTIONAL_FEATURE(f));
 }
 
@@ -85,7 +82,8 @@ static bool feature_supported(int feature_bit,
 			      size_t num_supported)
 {
 	for (size_t i = 0; i < num_supported; i++) {
-		if (supported[i] == feature_bit)
+		if (OPTIONAL_FEATURE(supported[i])
+		    == OPTIONAL_FEATURE(feature_bit))
 			return true;
 	}
 	return false;

common/features.h

@@ -18,8 +18,16 @@ bool feature_offered(const u8 *features, size_t f);
 bool local_feature_negotiated(const u8 *lfeatures, size_t f);
 bool global_feature_negotiated(const u8 *gfeatures, size_t f);
 
-#define COMPULSORY_FEATURE(x)	(x)
-#define OPTIONAL_FEATURE(x)	((x)+1)
+/* BOLT #9:
+ *
+ * Flags are numbered from the least-significant bit, at bit 0 (i.e. 0x1,
+ * an _even_ bit). They are generally assigned in pairs so that features
+ * can be introduced as optional (_odd_ bits) and later upgraded to be compulsory
+ * (_even_ bits), which will be refused by outdated nodes:
+ * see [BOLT #1: The `init` Message](01-messaging.md#the-init-message).
+ */
+#define COMPULSORY_FEATURE(x)	((x) & 0xFFFFFFFE)
+#define OPTIONAL_FEATURE(x)	((x) | 1)
 
 /* BOLT #9:
  *
@@ -35,8 +43,12 @@ bool global_feature_negotiated(const u8 *gfeatures, size_t f);
 #define LOCAL_INITIAL_ROUTING_SYNC		2
 #define LOCAL_UPFRONT_SHUTDOWN_SCRIPT		4
 #define LOCAL_GOSSIP_QUERIES			6
-/* FIXME: Get bolt reference! */
-#define LOCAL_EXTENDED_GOSSIP_QUERIES		10
+
+/* BOLT-927c96daab2338b716708a57cd75c84a2d169e0e #9:
+ * | Bits  | Name                            |...
+ * | 10/11 | `gossip_queries_ex`             |...
+ */
+#define LOCAL_GOSSIP_QUERIES_EX			10
 
 /* BOLT #9:
  *