features: Add featurebits_or helper to combine two featurebitstrings

We will be doing this when collecting featurebits from the plugins, so make
this a reusable function.

common/features.c

@@ -253,3 +253,25 @@ const char **list_supported_features(const tal_t *ctx)
 
 	return list;
 }
+
+u8 *featurebits_or(const tal_t *ctx, const u8 *f1 TAKES, const u8 *f2 TAKES)
+{
+	size_t l1 = tal_bytelen(f1), l2 = tal_bytelen(f2);
+	size_t lm = l1 > l2 ? l1 : l2;
+	u8 *result = tal_arrz(ctx, u8, lm);
+
+	for (size_t i = 0; i < l1; i++)
+		result[lm - l1 + i] = f1[i];
+
+	for (size_t i = 0; i < l2; i++)
+		result[lm - l2 + i] |= f2[i];
+
+	/* Cleanup the featurebits if we were told to do so. */
+	if (taken(f1))
+		tal_free(f1);
+
+	if (taken(f2))
+		tal_free(f2);
+
+	return result;
+}

common/features.h

@@ -27,6 +27,9 @@ const char **list_supported_features(const tal_t *ctx);
 bool feature_is_set(const u8 *features, size_t bit);
 void set_feature_bit(u8 **ptr, u32 bit);
 
+/* Given two featurebit vectors, combine them by applying a logical OR. */
+u8 *featurebits_or(const tal_t *ctx, const u8 *f1 TAKES, const u8 *f2 TAKES);
+
 /* BOLT #9:
  *
  * Flags are numbered from the least-significant bit, at bit 0 (i.e. 0x1,

common/test/run-features.c

@@ -31,6 +31,26 @@ const void *fromwire_fail(const u8 **cursor UNNEEDED, size_t *max UNNEEDED)
 { fprintf(stderr, "fromwire_fail called!\n"); abort(); }
 /* AUTOGENERATED MOCKS END */
 
+
+static void test_featurebits_or(void)
+{
+	u8 *f1 = tal_arr(tmpctx, u8, 0);
+	u8 *f2 = tal_arr(tmpctx, u8, 0);
+	u8 *control = tal_arr(tmpctx, u8, 0);
+	for (size_t i = 0; i < 100; i += 3) {
+		set_feature_bit(&f1, i);
+		set_feature_bit(&control, i);
+	}
+
+	for (size_t i = 0; i < 10; i += 2) {
+		set_feature_bit(&f2, i);
+		set_feature_bit(&control, i);
+	}
+	u8 *result = featurebits_or(tmpctx, take(f1), f2);
+	assert(
+	    memeq(result, tal_bytelen(result), control, tal_bytelen(control)));
+}
+
 int main(void)
 {
 	u8 *bits, *lf;
@@ -112,7 +132,10 @@ int main(void)
 		}
 	}
 
+	test_featurebits_or();
+
 	wally_cleanup(0);
 	tal_free(tmpctx);
+	take_cleanup();
 	return 0;
 }

connectd/peer_exchange_initmsg.c

@@ -87,10 +87,7 @@ static struct io_plan *peer_init_received(struct io_conn *conn,
 
 	/* The globalfeatures field is now unused, but there was a
 	 * window where it was: combine the two. */
-	for (size_t i = 0; i < tal_bytelen(globalfeatures) * 8; i++) {
+	features = featurebits_or(tmpctx, take(features), globalfeatures);
-		if (feature_is_set(globalfeatures, i))
-			set_feature_bit(&features, i);
-	}
 
 	/* BOLT #1:
 	 *