libstore: make SQLite busy back-off logic portable

Use C++ standard library not Unix functions for sleeping and randomness.

Also change the PRNG seed from `clock()` to `std::random_device` because
there's no good reason not to and this avoids two threads seeding their
PRNG the same.

Upstream-PR: https://github.com/NixOS/nix/pull/10399
Change-Id: I8424e7406366ea8e825d3e93dbb261ce60e3a37f

lix/libstore/sqlite.cc

@@ -5,9 +5,10 @@
 #include "lix/libutil/signals.hh"
 #include "lix/libutil/url.hh"
 
-#include <random>
 #include <sqlite3.h>
 
+#include <random>
+#include <thread>
 
 namespace nix {
 
@@ -284,12 +285,10 @@ void handleSQLiteBusy(const SQLiteBusy & e, time_t & nextWarning)
     /* Sleep for a while since retrying the transaction right away
        is likely to fail again. */
     checkInterrupt();
-    static thread_local std::default_random_engine generator(clock());
+    static thread_local std::default_random_engine generator(std::random_device{}());
     std::uniform_int_distribution<long> uniform_dist(0, 100);
-    struct timespec t;
-    t.tv_sec = 0;
-    t.tv_nsec = uniform_dist(generator) * 1000 * 1000; /* <= 0.1s */
-    nanosleep(&t, 0);
+    /* <= 0.1s */
+    std::this_thread::sleep_for(std::chrono::milliseconds { uniform_dist(generator) });
 }
 
 }