Enable deterministic random number generation

This patch makes Math.random() behave deterministically when a fixed
random seed is provided. This is done by re-seeding the random number
generator the first time a script requests a random number. Doing this
ensures Math.random() returns the same sequence across page loads and
across iframes.

BUG=chromium:696001

Review-Url: https://codereview.chromium.org/2760393002
Cr-Commit-Position: refs/heads/master@{#44076}

src/runtime/runtime-maths.cc

@@ -41,10 +41,18 @@ RUNTIME_FUNCTION(Runtime_GenerateRandomNumbers) {
     cache = Handle<FixedDoubleArray>::cast(
         isolate->factory()->NewFixedDoubleArray(kCacheSize, TENURED));
     native_context->set_math_random_cache(*cache);
-    // Initialize state if not yet initialized.
-    while (state0 == 0 || state1 == 0) {
-      isolate->random_number_generator()->NextBytes(&state0, sizeof(state0));
-      isolate->random_number_generator()->NextBytes(&state1, sizeof(state1));
+    // Initialize state if not yet initialized. If a fixed random seed was
+    // requested, use it to reset our state the first time a script asks for
+    // random numbers in this context. This ensures the script sees a consistent
+    // sequence.
+    if (FLAG_random_seed != 0) {
+      state0 = FLAG_random_seed;
+      state1 = FLAG_random_seed;
+    } else {
+      while (state0 == 0 || state1 == 0) {
+        isolate->random_number_generator()->NextBytes(&state0, sizeof(state0));
+        isolate->random_number_generator()->NextBytes(&state1, sizeof(state1));
+      }
     }
   }
 

test/cctest/test-api.cc

@@ -26369,3 +26369,28 @@ UNINITIALIZED_TEST(IncreaseHeapLimitForDebugging) {
   }
   isolate->Dispose();
 }
+
+TEST(DeterministicRandomNumberGeneration) {
+  v8::HandleScope scope(CcTest::isolate());
+
+  int previous_seed = v8::internal::FLAG_random_seed;
+  v8::internal::FLAG_random_seed = 1234;
+
+  double first_value;
+  double second_value;
+  {
+    v8::Local<Context> context = Context::New(CcTest::isolate());
+    Context::Scope context_scope(context);
+    v8::Local<Value> result = CompileRun("Math.random();");
+    first_value = result->ToNumber(context).ToLocalChecked()->Value();
+  }
+  {
+    v8::Local<Context> context = Context::New(CcTest::isolate());
+    Context::Scope context_scope(context);
+    v8::Local<Value> result = CompileRun("Math.random();");
+    second_value = result->ToNumber(context).ToLocalChecked()->Value();
+  }
+  CHECK_EQ(first_value, second_value);
+
+  v8::internal::FLAG_random_seed = previous_seed;
+}