Let SetEntropySource() fail if called after V8::Initialize().

BUG=v8:2905
R=dslomov@chromium.org

Review URL: https://codereview.chromium.org/24357002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@16889 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

include/v8.h

@@ -4540,6 +4540,9 @@ class V8_EXPORT V8 {
   /**
    * Allows the host application to provide a callback which can be used
    * as a source of entropy for random number generators.
+   *
+   * \note Setting an entropy source can only be done very early prior to
+   *   calling Initialize().
    */
   static void SetEntropySource(EntropySource source);
 

src/api.cc

@@ -5175,6 +5175,13 @@ bool v8::V8::Initialize() {
 
 
 void v8::V8::SetEntropySource(EntropySource entropy_source) {
+  // The entropy source must be set before the library is initialized,
+  // as otherwise not all random number generators will pick up the
+  // entropy source and will fall back to weak entropy instead.
+  i::Isolate* isolate = i::Isolate::UncheckedCurrent();
+  ApiCheck(isolate != NULL && isolate->IsInitialized(),
+           "v8::V8::SetEntropySource",
+           "Cannot set entropy source once V8 is initialized.");
   i::RandomNumberGenerator::SetEntropySource(entropy_source);
 }