ssl: Only invoke RAND_poll on older versions of OpenSSL

Summary:
Only do RAND_poll on older versions of OpenSSL.

RAND_poll is not required on OpenSSL 1.1.0+, as the RNG is implicitly seeded. On OpenSSL 1.1.1., RAND_poll became aliased to RAND_add. With some custom RNG implementations that did not implement RAND_add, (e.g. python-cryptography's osrandom implementation), this would manifest as a failure.

Reviewed By: ngoyal, yfeldblum

Differential Revision: D16786271

fbshipit-source-id: 4fb7876525006517c3e554d70736d7071bd3b4e3

folly/ssl/Init.cpp

@@ -44,11 +44,16 @@ void initializeOpenSSLLocked() {
     // pointers to global resources.
     throw std::runtime_error("Failed to initialize OpenSSL.");
   }
+
+  // OpenSSL implicitly seeds its RNG since 1.1.0, so this call is unnecessary
+  // and is only here for compatibility with older versions of OpenSSL.
+#if !(FOLLY_OPENSSL_IS_110)
   if (RAND_poll() != 1) {
     // Similarly, if we fail to seed the RNG, future crypto operations
     // may no longer be safe to use; fail fast and hard here.
     throw std::runtime_error("Failed to initialize OpenSSL RNG.");
   }
+#endif
   initialized_ = true;
 }