Adding a warmup method on snapshots

Which maps to V8's `V8::WarmUpSnapshotDataBlob` (see
http://v8.paulfryzel.com/docs/master/classv8_1_1_v8.html#abee465cc67755cfaacf0297c3093fb47)

Added unit tests on it.

ext/mini_racer_extension/mini_racer_extension.cc

@@ -330,6 +330,27 @@ static VALUE rb_snapshot_load(VALUE self, VALUE str) {
     return Qnil;
 }
 
+static VALUE rb_snapshot_warmup(VALUE self, VALUE str) {
+    SnapshotInfo* snapshot_info;
+    Data_Get_Struct(self, SnapshotInfo, snapshot_info);
+
+    init_v8();
+
+    StartupData cold_startup_data = {snapshot_info->data, snapshot_info->raw_size};
+    StartupData warm_startup_data = V8::WarmUpSnapshotDataBlob(cold_startup_data, RSTRING_PTR(str));
+
+    if (warm_startup_data.data == NULL && warm_startup_data.raw_size == 0) {
+        rb_raise(rb_eSnapshotError, "Could not warm up snapshot, most likely the source is incorrect");
+    } else {
+        delete[] snapshot_info->data;
+
+        snapshot_info->data = warm_startup_data.data;
+        snapshot_info->raw_size = warm_startup_data.raw_size;
+    }
+
+    return Qnil;
+}
+
 static VALUE rb_context_init_with_snapshot(VALUE self, VALUE snapshot) {
     ContextInfo* context_info;
     Data_Get_Struct(self, ContextInfo, context_info);
@@ -718,6 +739,7 @@ extern "C" {
 	rb_define_alloc_func(rb_cExternalFunction, allocate_external_function);
 
 	rb_define_method(rb_cSnapshot, "size", (VALUE(*)(...))&rb_snapshot_size, 0);
+	rb_define_method(rb_cSnapshot, "warmup", (VALUE(*)(...))&rb_snapshot_warmup, 1);
 	rb_define_private_method(rb_cSnapshot, "load", (VALUE(*)(...))&rb_snapshot_load, 1);
     }
 

lib/mini_racer.rb

@@ -116,8 +116,9 @@ module MiniRacer
 
   end
 
+  # `size` and `warmup` public methods are defined in the C class
   class Snapshot
-    def initialize(str)
+    def initialize(str = '')
       # defined in the C class
       load(str)
     end

test/mini_racer_test.rb

@@ -309,4 +309,52 @@ raise FooError, "I like foos"
       MiniRacer::Snapshot.new('var foo = bar;')
     end
   end
+
+  def test_an_empty_snapshot_is_valid
+    MiniRacer::Snapshot.new('')
+    MiniRacer::Snapshot.new
+  end
+
+  def test_snapshots_can_be_warmed_up_with_no_side_effects
+    # shamelessly insipired by https://github.com/v8/v8/blob/5.3.254/test/cctest/test-serialize.cc#L792-L854
+    snapshot_source = <<-JS
+      function f() { return Math.sin(1); }
+      var a = 5;
+    JS
+
+    snapshot = MiniRacer::Snapshot.new(snapshot_source)
+
+    warmump_source = <<-JS
+      Math.tan(1);
+      var a = f();
+      Math.sin = 1;
+    JS
+
+    snapshot.warmup(warmump_source)
+
+    context = MiniRacer::Context.new(snapshot: snapshot)
+
+    assert_equal 5, context.eval("a")
+    assert_equal "function", context.eval("typeof(Math.sin)")
+  end
+
+  def test_invalid_warmup_sources_throw_an_exception
+    assert_raises(MiniRacer::SnapshotError) do
+      MiniRacer::Snapshot.new('Math.sin = 1;').warmup('var a = Math.sin(1);')
+    end
+  end
+
+  def test_warming_up_with_invalid_source_does_not_affect_the_snapshot_internal_state
+    snapshot = MiniRacer::Snapshot.new('Math.sin = 1;')
+
+    begin
+      snapshot.warmup('var a = Math.sin(1);')
+    rescue
+      # do nothing
+    end
+
+    context = MiniRacer::Context.new(snapshot: snapshot)
+
+    assert_equal 1, context.eval("Math.sin")
+  end
 end