Ensure global inits in Random are static-safe

Summary:
[Folly] Ensure global inits in Random are static-safe by using the machinery of `static` local variables.

The default mutex type used with `once_flag` is `SharedMutex`, but that mutex type is not trivially-destructible. An alternative mutex type might be trivially-destructible, such as `MicroLock`, so an alternative solution could be to use `once_flag` with `MicroLock`.

Reviewed By: ot, luciang

Differential Revision: D19207285

fbshipit-source-id: a7d817cf8521337543e3e27bc0e0e61241714ced

folly/Random.cpp

@@ -21,13 +21,13 @@
 #include <mutex>
 #include <random>
 
+#include <folly/CppAttributes.h>
 #include <folly/File.h>
 #include <folly/FileUtil.h>
 #include <folly/SingletonThreadLocal.h>
 #include <folly/ThreadLocal.h>
 #include <folly/portability/SysTime.h>
 #include <folly/portability/Unistd.h>
-#include <folly/synchronization/CallOnce.h>
 #include <glog/logging.h>
 
 #ifdef _MSC_VER
@@ -40,25 +40,21 @@ namespace {
 
 void readRandomDevice(void* data, size_t size) {
 #ifdef _MSC_VER
-  static folly::once_flag flag;
-  static HCRYPTPROV cryptoProv;
-  folly::call_once(flag, [&] {
+  static auto const cryptoProv = [] {
+    HCRYPTPROV prov;
     if (!CryptAcquireContext(
-            &cryptoProv,
-            nullptr,
-            nullptr,
-            PROV_RSA_FULL,
-            CRYPT_VERIFYCONTEXT)) {
+            &prov, nullptr, nullptr, PROV_RSA_FULL, CRYPT_VERIFYCONTEXT)) {
       if (GetLastError() == NTE_BAD_KEYSET) {
         // Mostly likely cause of this is that no key container
         // exists yet, so try to create one.
         PCHECK(CryptAcquireContext(
-            &cryptoProv, nullptr, nullptr, PROV_RSA_FULL, CRYPT_NEWKEYSET));
+            &prov, nullptr, nullptr, PROV_RSA_FULL, CRYPT_NEWKEYSET));
       } else {
         LOG(FATAL) << "Failed to acquire the default crypto context.";
       }
     }
-  });
+    return prov;
+  }();
   CHECK(size <= std::numeric_limits<DWORD>::max());
   PCHECK(CryptGenRandom(cryptoProv, (DWORD)size, (BYTE*)data));
 #else
@@ -73,7 +69,6 @@ void readRandomDevice(void* data, size_t size) {
 
 class BufferedRandomDevice {
  public:
-  static once_flag flag;
   static constexpr size_t kDefaultBufferSize = 128;
 
   static void notifyNewGlobalEpoch() {
@@ -107,7 +102,6 @@ class BufferedRandomDevice {
   unsigned char* ptr_;
 };
 
-once_flag BufferedRandomDevice::flag;
 std::atomic<size_t> BufferedRandomDevice::globalEpoch_{0};
 struct RandomTag {};
 
@@ -115,7 +109,7 @@ BufferedRandomDevice::BufferedRandomDevice(size_t bufferSize)
     : bufferSize_(bufferSize),
       buffer_(new unsigned char[bufferSize]),
       ptr_(buffer_.get() + bufferSize) { // refill on first use
-  call_once(flag, [] {
+  FOLLY_MAYBE_UNUSED static auto const init = [] {
     detail::AtFork::registerHandler(
         nullptr,
         /*prepare*/ []() { return true; },
@@ -125,7 +119,8 @@ BufferedRandomDevice::BufferedRandomDevice(size_t bufferSize)
           // Ensure child and parent do not share same entropy pool.
           BufferedRandomDevice::notifyNewGlobalEpoch();
         });
-  });
+    return 0;
+  }();
 }
 
 void BufferedRandomDevice::getSlow(unsigned char* data, size_t size) {