misc/random: add xoshiro random number implementation

Add xoshiro as a PRNG implementation instead of relying
on srand() and rand() from the C standard library. This,
in particular, lets us avoid platform-defined behavior with
respect to threading.

common/playlist.c

@@ -21,6 +21,7 @@
 #include "common/common.h"
 #include "common/global.h"
 #include "common/msg.h"
+#include "misc/random.h"
 #include "mpv_talloc.h"
 #include "options/path.h"
 
@@ -147,7 +148,7 @@ void playlist_shuffle(struct playlist *pl)
     for (int n = 0; n < pl->num_entries; n++)
         pl->entries[n]->original_index = n;
     for (int n = 0; n < pl->num_entries - 1; n++) {
-        int j = (int)((double)(pl->num_entries - n) * rand() / (RAND_MAX + 1.0));
+        size_t j = (size_t)((pl->num_entries - n) * mp_rand_next_double());
         MPSWAP(struct playlist_entry *, pl->entries[n], pl->entries[n + j]);
     }
     playlist_update_indexes(pl, 0, -1);

libmpv/client.h

@@ -152,8 +152,6 @@ extern "C" {
  * - Using UNIX IPC (off by default) will override the SIGPIPE signal handler,
  *   and set it to SIG_IGN. Some invocations of the "subprocess" command will
  *   also do that.
- * - mpv will reseed the legacy C random number generator by calling srand() at
- *   some random point once.
  * - mpv may start sub processes, so overriding SIGCHLD, or waiting on all PIDs
  *   (such as calling wait()) by the parent process or any other library within
  *   the process must be avoided. libmpv itself only waits for its own PIDs.

meson.build

@@ -134,6 +134,7 @@ sources = files(
     'misc/json.c',
     'misc/natural_sort.c',
     'misc/node.c',
+    'misc/random.c',
     'misc/rendezvous.c',
     'misc/thread_pool.c',
     'misc/thread_tools.c',

misc/random.c

@@ -0,0 +1,75 @@
+/*
+ * Implementation of non-cryptographic pseudo-random number
+ * generator algorithm known as xoshiro.
+ *
+ * This file is part of mpv.
+ *
+ * mpv is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * mpv is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with mpv.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include <stdint.h>
+#include <pthread.h>
+
+#include "random.h"
+
+static uint64_t state[4];
+static pthread_mutex_t state_mutex = PTHREAD_MUTEX_INITIALIZER;
+
+static inline uint64_t rotl_u64(const uint64_t x, const int k)
+{
+    return (x << k) | (x >> (64 - k));
+}
+
+static inline uint64_t splitmix64(uint64_t *const x)
+{
+    uint64_t z = (*x += UINT64_C(0x9e3779b97f4a7c15));
+    z = (z ^ (z >> 30)) * UINT64_C(0xbf58476d1ce4e5b9);
+    z = (z ^ (z >> 27)) * UINT64_C(0x94d049bb133111eb);
+    return z ^ (z >> 31);
+}
+
+void mp_rand_seed(uint64_t seed)
+{
+    pthread_mutex_lock(&state_mutex);
+    state[0] = seed;
+    for (int i = 1; i < 4; i++)
+        state[i] = splitmix64(&seed);
+    pthread_mutex_unlock(&state_mutex);
+}
+
+uint64_t mp_rand_next(void)
+{
+    uint64_t result, t;
+
+    pthread_mutex_lock(&state_mutex);
+
+    result = rotl_u64(state[1] * 5, 7) * 9;
+    t = state[1] << 17;
+
+    state[2] ^= state[0];
+    state[3] ^= state[1];
+    state[1] ^= state[2];
+    state[0] ^= state[3];
+    state[2] ^= t;
+    state[3] = rotl_u64(state[3], 45);
+
+    pthread_mutex_unlock(&state_mutex);
+
+    return result;
+}
+
+double mp_rand_next_double(void)
+{
+    return (mp_rand_next() >> 11) * 0x1.0p-53;
+}

misc/random.h

@@ -0,0 +1,41 @@
+/*
+ * Implementation of non-cryptographic pseudo-random number
+ * generator algorithm known as xoshiro.
+ *
+ * This file is part of mpv.
+ *
+ * mpv is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * mpv is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with mpv.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#pragma once
+
+#include <stdint.h>
+
+/*
+ * Initialize the pseudo-random number generator's state with
+ * the given 64-bit seed.
+ */
+void mp_rand_seed(uint64_t seed);
+
+/*
+ * Return the next 64-bit psuedo-random integer, and update the state
+ * accordingly.
+ */
+uint64_t mp_rand_next(void);
+
+/*
+ * Return a double value in the range of [0.0, 1.0) with uniform
+ * distribution, and update the state accordingly.
+ */
+double mp_rand_next_double(void);

osdep/io.c

@@ -32,6 +32,7 @@
 #include "mpv_talloc.h"
 
 #include "config.h"
+#include "misc/random.h"
 #include "osdep/io.h"
 #include "osdep/terminal.h"
 
@@ -804,7 +805,7 @@ int mp_mkostemps(char *template, int suffixlen, int flags)
     for (size_t fuckshit = 0; fuckshit < UINT32_MAX; fuckshit++) {
         // Using a random value may make it require fewer iterations (even if
         // not truly random; just a counter would be sufficient).
-        size_t fuckmess = rand();
+        size_t fuckmess = mp_rand_next();
         char crap[7] = "";
         snprintf(crap, sizeof(crap), "%06zx", fuckmess);
         memcpy(t, crap, 6);

osdep/timer.c

@@ -25,6 +25,7 @@
 
 #include "common/common.h"
 #include "common/msg.h"
+#include "misc/random.h"
 #include "timer.h"
 
 static uint64_t raw_time_offset;
@@ -33,7 +34,7 @@ static pthread_once_t timer_init_once = PTHREAD_ONCE_INIT;
 static void do_timer_init(void)
 {
     mp_raw_time_init();
-    srand(mp_raw_time_us());
+    mp_rand_seed(mp_raw_time_us());
     raw_time_offset = mp_raw_time_us();
     // Arbitrary additional offset to avoid confusing relative/absolute times.
     // Also,we rule that the timer never returns 0 (so default-initialized

wscript_build.py

@@ -335,6 +335,7 @@ def build(ctx):
         ( "misc/natural_sort.c" ),
         ( "misc/node.c" ),
         ( "misc/rendezvous.c" ),
+        ( "misc/random.c" ),
         ( "misc/thread_pool.c" ),
         ( "misc/thread_tools.c" ),