From 3d63a68dd065c6bed8dbebfb2e48d6b9db2888a8 Mon Sep 17 00:00:00 2001
From: ridiculousfish <corydoras@ridiculousfish.com>
Date: Sun, 28 Apr 2019 15:13:30 -0700
Subject: [PATCH] Make builtin_random thread safe

---
 src/builtin_random.cpp | 25 +++++++++++++++----------
 1 file changed, 15 insertions(+), 10 deletions(-)

diff --git a/src/builtin_random.cpp b/src/builtin_random.cpp
index 617082e51..e20f22718 100644
--- a/src/builtin_random.cpp
+++ b/src/builtin_random.cpp
@@ -15,6 +15,17 @@
 #include "io.h"
 #include "wutil.h"  // IWYU pragma: keep
 
+/// \return a random-seeded engine.
+static std::minstd_rand get_seeded_engine() {
+    std::minstd_rand engine;
+    // seed engine with 2*32 bits of random data
+    // for the 64 bits of internal state of minstd_rand
+    std::random_device rd;
+    std::seed_seq seed{rd(), rd()};
+    engine.seed(seed);
+    return engine;
+}
+
 /// The random builtin generates random numbers.
 int builtin_random(parser_t &parser, io_streams_t &streams, wchar_t **argv) {
     wchar_t *cmd = argv[0];
@@ -30,16 +41,10 @@ int builtin_random(parser_t &parser, io_streams_t &streams, wchar_t **argv) {
         return STATUS_CMD_OK;
     }
 
-    static bool seeded = false;
-    static std::minstd_rand engine;
-    if (!seeded) {
-        // seed engine with 2*32 bits of random data
-        // for the 64 bits of internal state of minstd_rand
-        std::random_device rd;
-        std::seed_seq seed{rd(), rd()};
-        engine.seed(seed);
-        seeded = true;
-    }
+    // We have a single engine which we lazily seed. Lock it here.
+    static owning_lock<std::minstd_rand> s_engine{get_seeded_engine()};
+    auto engine_lock = s_engine.acquire();
+    std::minstd_rand &engine = *engine_lock;
 
     int arg_count = argc - optind;
     long long start, end;