luzer: introduce oneof() in FuzzedDataProvider

The patch introduces a new FDP method - `oneof()`, it returns
a random element in the specified Lua array:

```
> fdp:oneof({'clickhouse', 'ydb', 'tarantool'})
---
- tarantool
...

>
```

`oneof()` with empty table returns a `nil`.

Author: ligurio

CHANGELOG.md

@@ -15,6 +15,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Examples with tests.
 - Documentation with usecases, API etc.
 - Support command-line options.
+- Method `oneof()` in FuzzedDataProvider.
 
 ### Changed
 

docs/api.md

@@ -68,6 +68,8 @@ The `FuzzedDataProvider` then supports the following functions:
   If there's no input data left, always returns 0.
 - `remaining_bytes()` - returns the number of unconsumed bytes in the fuzzer
   input.
+- `oneof()` - returns a random element in the specified Lua array. With empty
+  table `oneof()` returns a `nil` value.
 
 Examples:
 

luzer/fuzzed_data_provider.cc

@@ -11,6 +11,9 @@ extern "C" {
 #include "lauxlib.h"
 #include "lualib.h"
 #include <float.h>
+
+int table_nkeys(lua_State *L, int idx);
+
 #ifdef __cplusplus
 } /* extern "C" */
 #endif
@@ -208,6 +211,31 @@ luaL_remaining_bytes(lua_State *L)
 	return 1;
 }
 
+/* Returns a random element of the specified array. */
+NO_SANITIZE static int
+luaL_oneof(lua_State *L)
+{
+	lua_userdata_t *lfdp;
+	lfdp = (lua_userdata_t *)luaL_checkudata(L, 1, FDP_LUA_UDATA_NAME);
+	luaL_checktype(L, 2, LUA_TTABLE);
+
+	int len = 0;
+#if LUA_VERSION_NUM == 501
+	len = table_nkeys(L, 2);
+#else
+	len = lua_rawlen(L, 2);
+#endif
+	if (len == 0) {
+		lua_pushnil(L);
+		return 1;
+	}
+	int idx = lfdp->fdp->ConsumeIntegralInRange(1, len);
+	lua_pushinteger(L, idx);
+	lua_gettable(L, -2);
+
+	return 1;
+}
+
 NO_SANITIZE static int close(lua_State *L) {
 	lua_userdata_t *lfdp;
 	lfdp = (lua_userdata_t *)luaL_checkudata(L, 1, FDP_LUA_UDATA_NAME);
@@ -233,6 +261,7 @@ const luaL_Reg methods[] =
 	{ "consume_integers", luaL_consume_integers },
 	{ "consume_probability", luaL_consume_probability },
 	{ "remaining_bytes", luaL_remaining_bytes },
+	{ "oneof", luaL_oneof },
 	{ "__gc", close },
 	{ "__tostring", tostring },
 	{ NULL, NULL }

luzer/luzer.c

@@ -315,7 +315,7 @@ load_custom_mutator_lib(void) {
 }
 
 /* Find amount of fields in the table on the top of the stack. */
-NO_SANITIZE static int
+NO_SANITIZE int
 table_nkeys(lua_State *L, int idx)
 {
 	int len = 0;

luzer/tests/test_unit.lua

@@ -174,6 +174,32 @@ assert(p1 >= 0 and p2 >= 0)
 assert(p1 <= 1 and p2 <= 1)
 assert(p1 ~= p2)
 
+-- luzer.FuzzedDataProvider.oneof()
+fdp = luzer.FuzzedDataProvider("ABCDEFGHIJKLMNOPQRSTUVWXYZ")
+assert(type(fdp.oneof) == "function")
+
+-- Call `oneof()` with no values should raise an error.
+ok, err = pcall(fdp.oneof, fdp)
+assert(ok == false)
+assert(err:match("table expected, got no value"), err)
+
+-- Call `oneof()` with empty table returns a `nil`.
+local n = fdp:oneof({})
+assert(n == nil)
+
+-- Call `oneof()` with numbers.
+local a = 3
+local b = 4
+n = fdp:oneof({a, b})
+assert(type(n) == "number" and (n == a or n == b))
+
+-- Call `oneof()` with strings.
+local str1 = "Python"
+local str2 = "Lua"
+local str = fdp:oneof({str1, str2})
+print(("'%s'"):format(str))
+assert(type(str) == "string" and (str == str1 or str == str2))
+
 local function custom_mutator(data, size, max_size, seed)
     assert(type(data) == "string")
     assert(type(size) == "number")