CDRIVER-3978 mix time to avoid duplicate RAND_bytes for the same PIDs (#815)

Author: jmikola

src/libmongoc/src/mongoc/mongoc-rand-openssl.c

@@ -28,6 +28,18 @@
 int
 _mongoc_rand_bytes (uint8_t *buf, int num)
 {
+#if OPENSSL_VERSION_NUMBER < 0x10101000L
+   /* Versions of OpenSSL before 1.1.1 can potentially produce the same random
+    * sequences in processes with the same PID. Rather than attempt to detect
+    * PID changes (useful for parent/child forking but not if PIDs wrap), mix
+    * the current time into the generator's state.
+    * See also: https://wiki.openssl.org/index.php/Random_fork-safety */
+   struct timeval tv;
+
+   bson_gettimeofday (&tv);
+   RAND_add (&tv, sizeof(tv), 0.0);
+#endif
+
    return RAND_bytes (buf, num);
 }