fuzz-tests: Add a test for `calculate_our_funding()`

diff --git a/tests/fuzz/fuzz-funder-policy.c b/tests/fuzz/fuzz-funder-policy.c
new file mode 100644
index 00000000..a3f80aaa
--- /dev/null
+++ b/tests/fuzz/fuzz-funder-policy.c
@@ -0,0 +1,167 @@
+#include "config.h"
+#include <ccan/array_size/array_size.h>
+#include <common/setup.h>
+#include <wire/wire.h>
+#include <stdio.h>
+#include <tests/fuzz/libfuzz.h>
+
+#include "../../plugins/funder_policy.c"
+
+/* AUTOGENERATED MOCKS START */
+/* Generated stub for json_add_string */
+void json_add_string(struct json_stream *js UNNEEDED,
+		     const char *fieldname UNNEEDED,
+		     const char *str TAKES UNNEEDED)
+{ fprintf(stderr, "json_add_string called!\n"); abort(); }
+/* AUTOGENERATED MOCKS END */
+
+#define MAX_SATS ((u64)WALLY_SATOSHI_PER_BTC * WALLY_BTC_MAX)
+
+struct test_case {
+	struct amount_sat their_funds;
+	struct amount_sat available_funds;
+	struct amount_sat *our_last_funds;
+	struct amount_sat max_channel_size;
+	struct amount_sat lease_request;
+	struct funder_policy policy;
+};
+
+static struct amount_sat fromwire_amount_sat_bounded(const u8 **cursor, size_t *max)
+{
+	struct amount_sat amt = fromwire_amount_sat(cursor, max);
+	amt.satoshis %= (MAX_SATS + 1);
+	return amt;
+}
+
+static struct test_case *new_test_case(const tal_t *ctx, const u8 **cursor, size_t *max)
+{
+	struct test_case *tcase = tal(ctx, struct test_case);
+	tcase->their_funds = fromwire_amount_sat_bounded(cursor, max);
+	tcase->available_funds = fromwire_amount_sat_bounded(cursor, max);
+
+	/* Read flag for our_last_funds */
+	u8 flag = fromwire_u8(cursor, max);
+
+	/* Handle our_last_funds conditionally */
+	struct amount_sat *our_last_funds_val = tal(ctx, struct amount_sat);
+	if (flag)
+	{
+		*our_last_funds_val = fromwire_amount_sat_bounded(cursor, max);
+		tcase->our_last_funds = our_last_funds_val;
+	}
+	else
+		tcase->our_last_funds = NULL;
+
+	tcase->max_channel_size = fromwire_amount_sat_bounded(cursor, max);
+	tcase->lease_request = fromwire_amount_sat_bounded(cursor, max);
+
+	tcase->policy.opt = (enum funder_opt)(fromwire_u8(cursor, max) % 3);
+	tcase->policy.mod = fromwire_u64(cursor, max);
+	switch (tcase->policy.opt)
+	{
+	case MATCH:
+		tcase->policy.mod %= 201;
+		break;
+	case AVAILABLE:
+		tcase->policy.mod %= 101;
+		break;
+	case FIXED:
+		tcase->policy.mod %= MAX_SATS + 1;
+		break;
+	default:
+		assert(false && "invalid policy");
+	}
+	tcase->policy.min_their_funding = fromwire_amount_sat_bounded(cursor, max);
+	tcase->policy.max_their_funding = fromwire_amount_sat_bounded(cursor, max);
+
+	if (amount_sat_greater(tcase->policy.min_their_funding, tcase->policy.max_their_funding)) {
+		struct amount_sat tmp = tcase->policy.min_their_funding;
+		tcase->policy.min_their_funding = tcase->policy.max_their_funding;
+		tcase->policy.max_their_funding = tmp;
+	}
+
+	tcase->policy.per_channel_max = fromwire_amount_sat_bounded(cursor, max);
+	tcase->policy.per_channel_min = fromwire_amount_sat_bounded(cursor, max);
+
+	if (amount_sat_greater(tcase->policy.per_channel_min, tcase->policy.per_channel_max)) {
+		struct amount_sat tmp = tcase->policy.per_channel_min;
+		tcase->policy.per_channel_min = tcase->policy.per_channel_max;
+		tcase->policy.per_channel_max = tmp;
+	}
+
+	tcase->policy.fuzz_factor = fromwire_u8(cursor, max) % 101;
+	tcase->policy.reserve_tank = fromwire_amount_sat_bounded(cursor, max);
+	tcase->policy.fund_probability = fromwire_u8(cursor, max) % 101;
+	tcase->policy.leases_only = fromwire_u8(cursor, max) & 1;
+
+	return tcase;
+}
+
+void init(int *argc, char ***argv)
+{}
+
+void run(const u8 *data, size_t size)
+{
+	struct test_case *tcase = new_test_case(tmpctx, &data, &size);
+
+	struct node_id id;
+	memset(&id, 1, sizeof(id));
+	const char *err;
+	struct amount_sat our_funds;
+
+	/* Call the function under test */
+	err = calculate_our_funding(&tcase->policy, id,
+				tcase->their_funds,
+				tcase->our_last_funds,
+				tcase->available_funds,
+				tcase->max_channel_size,
+				tcase->lease_request,
+				&our_funds);
+
+	/* Validate invariants */
+	if (!err)
+	{
+		/* Check total doesn't exceed max_channel_size */
+		struct amount_sat total;
+		if (!amount_sat_add(&total, tcase->their_funds, our_funds)) {
+			fprintf(stderr, "Overflow in total channel capacity\n");
+			abort();
+		}
+		if (amount_sat_greater(total, tcase->max_channel_size)) {
+			fprintf(stderr, "Total channel capacity %"PRIu64" exceeds size %"PRIu64"\n",
+				total.satoshis, tcase->max_channel_size.satoshis);
+			abort();
+		}
+
+		/* Check our_funds is within per-channel limits */
+		if (amount_sat_less(our_funds, tcase->policy.per_channel_min) &&
+		   !amount_sat_is_zero(our_funds)) {
+			fprintf(stderr, "our_funds %"PRIu64" < per_channel_min %"PRIu64"\n",
+				our_funds.satoshis, tcase->policy.per_channel_min.satoshis);
+			abort();
+		}
+		if (amount_sat_greater(our_funds, tcase->policy.per_channel_max)) {
+			fprintf(stderr, "our_funds %"PRIu64" > per_max_channel_size %"PRIu64"\n",
+				our_funds.satoshis, tcase->policy.per_channel_max.satoshis);
+			abort();
+		}
+	}
+
+	/* Check available funds constraint */
+	struct amount_sat available_minus_reserve;
+	if (amount_sat_sub(&available_minus_reserve, tcase->available_funds, tcase->policy.reserve_tank)) {
+		if (amount_sat_greater(our_funds, available_minus_reserve)) {
+			fprintf(stderr, "our_funds %"PRIu64" > available %"PRIu64" - reserve %"PRIu64"\n",
+				our_funds.satoshis, tcase->available_funds.satoshis,
+				tcase->policy.reserve_tank.satoshis);
+			abort();
+		}
+	} else if (!amount_sat_eq(our_funds, AMOUNT_SAT(0))) {
+		fprintf(stderr, "Reserve %"PRIu64" >= available %"PRIu64" but our_funds %"PRIu64" != 0\n",
+			tcase->policy.reserve_tank.satoshis, tcase->available_funds.satoshis,
+			our_funds.satoshis);
+		abort();
+	}
+
+	clean_tmpctx();
+}