fuzz: add a 'to_string' function to libfuzz

We want to use it outside of fuzz-amount

Signed-off-by: Antoine Poinsot <darosior@protonmail.com>

tests/fuzz/fuzz-amount.c

@@ -21,10 +21,7 @@ void run(const uint8_t *data, size_t size)
 
 	/* We should not crash when parsing any string. */
 
-	string = tal_arr(NULL, char, size);
-	for (size_t i = 0; i < size; i++)
-		string[i] = (char) data[i] % (CHAR_MAX + 1);
-
+	string = to_string(NULL, data, size);
 	parse_amount_msat(&msat, string, tal_count(string));
 	parse_amount_sat(&sat, string, tal_count(string));
 	tal_free(string);

tests/fuzz/libfuzz.c

@@ -31,3 +31,13 @@ const uint8_t **get_chunks(const void *ctx, const uint8_t *data,
 
 	return chunks;
 }
+
+char *to_string(const tal_t *ctx, const u8 *data, size_t data_size)
+{
+	char *string = tal_arr(ctx, char, data_size);
+
+	for (size_t i = 0; i < data_size; i++)
+		string[i] = (char) data[i] % (CHAR_MAX + 1);
+
+	return string;
+}

tests/fuzz/libfuzz.h

@@ -1,6 +1,8 @@
 #ifndef LIGHTNING_TESTS_FUZZ_LIBFUZZ_H
 #define LIGHTNING_TESTS_FUZZ_LIBFUZZ_H
 
+#include <ccan/ccan/short_types/short_types.h>
+#include <ccan/ccan/tal/tal.h>
 #include <stddef.h>
 #include <stdint.h>
 
@@ -15,4 +17,7 @@ void run(const uint8_t *data, size_t size);
 const uint8_t **get_chunks(const void *ctx, const uint8_t *data,
 			  size_t data_size, size_t chunk_size);
 
+/* Copy the data as a string. */
+char *to_string(const tal_t *ctx, const u8 *data, size_t data_size);
+
 #endif /* LIGHTNING_TESTS_FUZZ_LIBFUZZ_H */