Add perf counter data to GetStrongRandBytes state in scheduler

Cherry-picked from: 888cce5

src/random.cpp

@@ -16,6 +16,8 @@
 
 #include <stdlib.h>
 #include <limits>
+#include <chrono>
+#include <thread>
 
 #ifndef WIN32
 #include <sys/time.h>
@@ -129,6 +131,23 @@ void GetRandBytes(unsigned char* buf, int num)
     }
 }
 
+static void AddDataToRng(void* data, size_t len);
+
+void RandAddSeedSleep()
+{
+    int64_t nPerfCounter1 = GetPerformanceCounter();
+    std::this_thread::sleep_for(std::chrono::milliseconds(1));
+    int64_t nPerfCounter2 = GetPerformanceCounter();
+
+    // Combine with and update state
+    AddDataToRng(&nPerfCounter1, sizeof(nPerfCounter1));
+    AddDataToRng(&nPerfCounter2, sizeof(nPerfCounter2));
+
+    memory_cleanse(&nPerfCounter1, sizeof(nPerfCounter1));
+    memory_cleanse(&nPerfCounter2, sizeof(nPerfCounter2));
+}
+
+
 static std::mutex cs_rng_state;
 static unsigned char rng_state[32] = {0};
 static uint64_t rng_counter = 0;

src/random.h

@@ -21,6 +21,13 @@ uint64_t GetRand(uint64_t nMax);
 int GetRandInt(int nMax);
 uint256 GetRandHash();
 
+/**
+ * Add a little bit of randomness to the output of GetStrongRangBytes.
+ * This sleeps for a millisecond, so should only be called when there is
+ * no other work to be done.
+ */
+void RandAddSeedSleep();
+
 /**
  * Function to gather random data from multiple sources, failing whenever any
  * of those source fail to provide a result.

src/scheduler.cpp

@@ -5,6 +5,7 @@
 
 #include "scheduler.h"
 
+#include "random.h"
 #include "reverselock.h"
 
 #include <assert.h>
@@ -38,6 +39,11 @@ void CScheduler::serviceQueue()
     // is called.
     while (!shouldStop()) {
         try {
+            if (!shouldStop() && taskQueue.empty()) {
+                reverse_lock<boost::unique_lock<boost::mutex> > rlock(lock);
+                // Use this chance to get a tiny bit more entropy
+                RandAddSeedSleep();
+            }
             while (!shouldStop() && taskQueue.empty()) {
                 // Wait until there is something to do.
                 newTaskScheduled.wait(lock);