extmod/modurandom: Support urandom.seed() without an argument.

If a port provides MICROPY_PY_URANDOM_SEED_INIT_FUNC as a source of
randomness then this will be used when urandom.seed() is called without
an argument (or with None as the argument) to seed the pRNG.

Other related changes in this commit:
- mod_urandom___init__ is changed to call seed() without arguments, instead
  of explicitly passing in the result of MICROPY_PY_URANDOM_SEED_INIT_FUNC.
- mod_urandom___init__ will only ever seed the pRNG once (before it could
  seed it again if imported by, eg, random and then urandom).
- The Yasmarang state is moved to the BSS for builds where the state is
  guaranteed to be initialised on import of the (u)random module.

Signed-off-by: Damien George <damien@micropython.org>

extmod/modurandom.c

@@ -31,15 +31,29 @@
 
 #if MICROPY_PY_URANDOM
 
+// Work out if the seed will be set on import or not.
+#if MICROPY_MODULE_BUILTIN_INIT && defined(MICROPY_PY_URANDOM_SEED_INIT_FUNC)
+#define SEED_ON_IMPORT (1)
+#else
+#define SEED_ON_IMPORT (0)
+#endif
+
 // Yasmarang random number generator
 // by Ilya Levin
 // http://www.literatecode.com/yasmarang
 // Public Domain
 
 #if !MICROPY_ENABLE_DYNRUNTIME
+#if SEED_ON_IMPORT
+// If the state is seeded on import then keep these variables in the BSS.
+STATIC uint32_t yasmarang_pad, yasmarang_n, yasmarang_d;
+STATIC uint8_t yasmarang_dat;
+#else
+// Without seed-on-import these variables must be initialised via the data section.
 STATIC uint32_t yasmarang_pad = 0xeda4baba, yasmarang_n = 69, yasmarang_d = 233;
 STATIC uint8_t yasmarang_dat = 0;
 #endif
+#endif
 
 STATIC uint32_t yasmarang(void) {
     yasmarang_pad += yasmarang_dat + yasmarang_d * yasmarang_n;
@@ -83,15 +97,24 @@ STATIC mp_obj_t mod_urandom_getrandbits(mp_obj_t num_in) {
 }
 STATIC MP_DEFINE_CONST_FUN_OBJ_1(mod_urandom_getrandbits_obj, mod_urandom_getrandbits);
 
-STATIC mp_obj_t mod_urandom_seed(mp_obj_t seed_in) {
-    mp_uint_t seed = mp_obj_get_int_truncated(seed_in);
+STATIC mp_obj_t mod_urandom_seed(size_t n_args, const mp_obj_t *args) {
+    mp_uint_t seed;
+    if (n_args == 0 || args[0] == mp_const_none) {
+        #ifdef MICROPY_PY_URANDOM_SEED_INIT_FUNC
+        seed = MICROPY_PY_URANDOM_SEED_INIT_FUNC;
+        #else
+        mp_raise_ValueError(MP_ERROR_TEXT("no default seed"));
+        #endif
+    } else {
+        seed = mp_obj_get_int_truncated(args[0]);
+    }
     yasmarang_pad = seed;
     yasmarang_n = 69;
     yasmarang_d = 233;
     yasmarang_dat = 0;
     return mp_const_none;
 }
-STATIC MP_DEFINE_CONST_FUN_OBJ_1(mod_urandom_seed_obj, mod_urandom_seed);
+STATIC MP_DEFINE_CONST_FUN_OBJ_VAR_BETWEEN(mod_urandom_seed_obj, 0, 1, mod_urandom_seed);
 
 #if MICROPY_PY_URANDOM_EXTRA_FUNCS
 
@@ -189,9 +212,15 @@ STATIC MP_DEFINE_CONST_FUN_OBJ_2(mod_urandom_uniform_obj, mod_urandom_uniform);
 
 #endif // MICROPY_PY_URANDOM_EXTRA_FUNCS
 
-#ifdef MICROPY_PY_URANDOM_SEED_INIT_FUNC
+#if SEED_ON_IMPORT
 STATIC mp_obj_t mod_urandom___init__() {
-    mod_urandom_seed(MP_OBJ_NEW_SMALL_INT(MICROPY_PY_URANDOM_SEED_INIT_FUNC));
+    // This module may be imported by more than one name so need to ensure
+    // that it's only ever seeded once.
+    static bool seeded = false;
+    if (!seeded) {
+        seeded = true;
+        mod_urandom_seed(0, NULL);
+    }
     return mp_const_none;
 }
 STATIC MP_DEFINE_CONST_FUN_OBJ_0(mod_urandom___init___obj, mod_urandom___init__);
@@ -200,7 +229,7 @@ STATIC MP_DEFINE_CONST_FUN_OBJ_0(mod_urandom___init___obj, mod_urandom___init__)
 #if !MICROPY_ENABLE_DYNRUNTIME
 STATIC const mp_rom_map_elem_t mp_module_urandom_globals_table[] = {
     { MP_ROM_QSTR(MP_QSTR___name__), MP_ROM_QSTR(MP_QSTR_urandom) },
-    #ifdef MICROPY_PY_URANDOM_SEED_INIT_FUNC
+    #if SEED_ON_IMPORT
     { MP_ROM_QSTR(MP_QSTR___init__), MP_ROM_PTR(&mod_urandom___init___obj) },
     #endif
     { MP_ROM_QSTR(MP_QSTR_getrandbits), MP_ROM_PTR(&mod_urandom_getrandbits_obj) },

tests/extmod/urandom_seed_default.py

@@ -0,0 +1,30 @@
+# test urandom.seed() without any arguments
+
+try:
+    import urandom as random
+except ImportError:
+    try:
+        import random
+    except ImportError:
+        print("SKIP")
+        raise SystemExit
+
+try:
+    random.seed()
+except ValueError:
+    # no default seed on this platform
+    print("SKIP")
+    raise SystemExit
+
+
+def rng_seq():
+    return [random.getrandbits(16) for _ in range(10)]
+
+
+# seed with default and check that doesn't produce the same RNG sequence
+random.seed()
+seq = rng_seq()
+random.seed()
+print(seq == rng_seq())
+random.seed(None)
+print(seq == rng_seq())