fix: use portable C++ RNG

I used "A Tour of C++, Third Edition" (ISBN-10 0136816487) as inspiration for the Random class.
This replaces all occurrences of `rand()` and `srand()`.

src/libmain/shared.cc

@@ -177,11 +177,6 @@ void initNix(bool loadConfig)
     /* Initialise the PRNG. */
     struct timeval tv;
     gettimeofday(&tv, 0);
-#ifndef _WIN32
-    srandom(tv.tv_usec);
-#endif
-    srand(tv.tv_usec);
-
 
 }
 

src/libstore/build/derivation-goal.cc

@@ -17,6 +17,7 @@
 #include "topo-sort.hh"
 #include "callback.hh"
 #include "local-store.hh" // TODO remove, along with remaining downcasts
+#include "random.hh"
 
 #include <regex>
 #include <queue>
@@ -803,7 +804,7 @@ void replaceValidPath(const Path & storePath, const Path & tmpPath)
        tmpPath (the replacement), so we have to move it out of the
        way first.  We'd better not be interrupted here, because if
        we're repairing (say) Glibc, we end up with a broken system. */
-    Path oldPath = fmt("%1%.old-%2%-%3%", storePath, getpid(), rand());
+    Path oldPath = fmt("%1%.old-%2%-%3%", storePath, getpid(), Random{}());
     if (pathExists(storePath))
         movePath(storePath, oldPath);
 

src/libstore/indirect-root-store.cc

@@ -1,4 +1,5 @@
 #include "indirect-root-store.hh"
+#include "random.hh"
 
 namespace nix {
 
@@ -8,7 +9,7 @@ void IndirectRootStore::makeSymlink(const Path & link, const Path & target)
     createDirs(dirOf(link));
 
     /* Create the new symlink. */
-    Path tempLink = fmt("%1%.tmp-%2%-%3%", link, getpid(), rand());
+    Path tempLink = fmt("%1%.tmp-%2%-%3%", link, getpid(), Random{}());
     createSymlink(target, tempLink);
 
     /* Atomically replace the old one. */

src/libstore/optimise-store.cc

@@ -3,6 +3,7 @@
 #include "signals.hh"
 #include "posix-fs-canonicalise.hh"
 #include "posix-source-accessor.hh"
+#include "random.hh"
 
 #include <cstdlib>
 #include <cstring>
@@ -222,7 +223,7 @@ void LocalStore::optimisePath_(Activity * act, OptimiseStats & stats,
        its timestamp back to 0. */
     MakeReadOnly makeReadOnly(mustToggle ? dirOfPath : "");
 
-    std::filesystem::path tempLink = fmt("%1%/.tmp-link-%2%-%3%", realStoreDir, getpid(), rand());
+    std::filesystem::path tempLink = fmt("%1%/.tmp-link-%2%-%3%", realStoreDir, getpid(), Random{}());
 
     try {
         std::filesystem::create_hard_link(linkPath, tempLink);

src/libstore/sqlite.cc

@@ -3,6 +3,7 @@
 #include "util.hh"
 #include "url.hh"
 #include "signals.hh"
+#include "random.hh"
 
 #include <sqlite3.h>
 
@@ -258,7 +259,7 @@ void handleSQLiteBusy(const SQLiteBusy & e, time_t & nextWarning)
        is likely to fail again. */
     checkInterrupt();
     /* <= 0.1s */
-    std::this_thread::sleep_for(std::chrono::milliseconds { rand() % 100 });
+    std::this_thread::sleep_for(std::chrono::milliseconds { Random{0, 100}() });
 }
 
 }

src/libutil/meson.build

@@ -248,6 +248,7 @@ headers = [config_h] + files(
   'util.hh',
   'variant-wrapper.hh',
   'xml-writer.hh',
+  'random.hh'
 )
 
 if host_machine.system() == 'linux'

src/libutil/random.hh

@@ -0,0 +1,28 @@
+#pragma once
+
+#include <random>
+#include <limits>
+
+namespace nix {
+
+// Inspired by the book "A Tour of C++, Third Edition" (ISBN-10 0136816487)
+class Random
+{
+public:
+    Random(int low = std::numeric_limits<int>::min(), int high = std::numeric_limits<int>::max())
+        : re(std::random_device{}())
+        , dist(low, high){};
+    int operator()()
+    {
+        return dist(re);
+    }
+    void seed(int s)
+    {
+        re.seed(s);
+    }
+private:
+    std::default_random_engine re;
+    std::uniform_int_distribution<int> dist;
+};
+
+} // namespace nix