SERVER-43641 upgrade random.h

Respecify PseudoRandom and SecureRandom as template instances of
a `mongo::RandomBase<Urbg>` (Urbg is a UniformRandomBitGenerator).
They will only vary in which algorithm they use for their source
bits, and should otherwise support the same exact operations (e.g.
`nextCanonicalDouble`).

Fix range and stats errors in the implementations of those
RandomBase methods, and specify them in terms of the vetted
`<random>` facilities.

Test uniformity of nextInt32(max), which uses an inappropriate
( x % max) operation.  Verify that refactor fixes this issue.

Just keep a shared urandom file descriptor open.

SecureRandom add fill, remove create, fix callers

Obsoletes SERVER-43643 Re: SecureRandom 8kiB buffering

1 files changed, 41 insertions, 6 deletions

diff --git a/src/mongo/platform/random_test.cpp b/src/mongo/platform/random_test.cpp
index ee82a89490f..c3271f8b926 100644
--- a/src/mongo/platform/random_test.cpp
+++ b/src/mongo/platform/random_test.cpp
@@ -134,13 +134,13 @@ TEST(RandomTest, NextCanonicalDistinctValues) {
 }
 
 /**
- * Test that nextCanonicalDouble() always returns values between 0 and 1.
+ * Test that nextCanonicalDouble() is at least very likely to return values in [0,1).
  */
 TEST(RandomTest, NextCanonicalWithinRange) {
     PseudoRandom prng(10);
-    for (int i = 0; i < 100; i++) {
+    for (size_t i = 0; i < 1'000'000; ++i) {
         double next = prng.nextCanonicalDouble();
-        ASSERT_LTE(0.0, next);
+        ASSERT_GTE(next, 0.0);
         ASSERT_LT(next, 1.0);
     }
 }
@@ -211,12 +211,47 @@ TEST(RandomTest, NextInt64InRange) {
     }
 }
 
+/**
+ * Test uniformity of nextInt32(max)
+ */
+TEST(RandomTest, NextInt32Uniformity) {
+    PseudoRandom prng(10);
+    /* Break the range into sections. */
+    /* Check that all sections get roughly equal # of hits */
+    constexpr int32_t kMax = (int32_t{3} << 29) - 1;
+    constexpr size_t kBuckets = 64;
+    constexpr size_t kNIter = 1'000'000;
+    constexpr double mu = kNIter / kBuckets;
+    constexpr double muSqInv = 1. / (mu * mu);
+    std::vector<size_t> hist(kBuckets);
+    for (size_t i = 0; i < kNIter; ++i) {
+        auto next = prng.nextInt32(kMax);
+        ASSERT_GTE(next, 0);
+        ASSERT_LTE(next, kMax);
+        ++hist[double(next) * kBuckets / (kMax + 1)];
+    }
+    if (kDebugBuild) {
+        for (size_t i = 0; i < hist.size(); ++i) {
+            double dev = std::pow(std::pow((hist[i] - mu) / mu, 2), .5);
+            unittest::log() << format(FMT_STRING("  [{:4}] count:{:4}, dev:{:6f}, {}"),
+                                      i,
+                                      hist[i],
+                                      dev,
+                                      std::string(hist[i] / 256, '*'));
+        }
+    }
+    for (size_t i = 0; i < hist.size(); ++i) {
+        double dev = std::pow(std::pow(hist[i] - mu, 2) * muSqInv, .5);
+        ASSERT_LT(dev, 0.1) << format(FMT_STRING("hist[{}]={}, mu={}"), i, hist[i], mu);
+    }
+}
+
 TEST(RandomTest, Secure1) {
-    auto a = SecureRandom::create();
-    auto b = SecureRandom::create();
+    auto a = SecureRandom();
+    auto b = SecureRandom();
 
     for (int i = 0; i < 100; i++) {
-        ASSERT_NOT_EQUALS(a->nextInt64(), b->nextInt64());
+        ASSERT_NOT_EQUALS(a.nextInt64(), b.nextInt64());
     }
 }
 }  // namespace mongo