Use /dev/null in invalidate_fd to avoid infinite loop in OpenSSL

Thanks to Andy Bakun / @thwarted for identifying the issue and
suggesting the /dev/null workaround.

Author: sodabrew

ext/mysql2/client.c

@@ -169,26 +169,30 @@ static void *nogvl_connect(void *ptr) {
 
 #ifndef _WIN32
 /*
- * Redirect clientfd to a dummy socket for mysql_close to
- * write, shutdown, and close on as a no-op.
- * We do this hack because we want to call mysql_close to release
- * memory, but do not want mysql_close to drop connections in the
- * parent if the socket got shared in fork.
+ * Redirect clientfd to /dev/null for mysql_close and SSL_close to write,
+ * shutdown, and close. The hack is needed to prevent shutdown() from breaking
+ * a socket that may be in use by the parent or other processes after fork.
+ *
+ * /dev/null is used to absorb writes; previously a dummy socket was used, but
+ * it could not abosrb writes and caused openssl to go into an infinite loop.
+ *
  * Returns Qtrue or Qfalse (success or failure)
+ *
+ * Note: if this function is needed on Windows, use "nul" instead of "/dev/null"
  */
 static VALUE invalidate_fd(int clientfd)
 {
 #ifdef SOCK_CLOEXEC
   /* Atomically set CLOEXEC on the new FD in case another thread forks */
-  int sockfd = socket(AF_UNIX, SOCK_STREAM | SOCK_CLOEXEC, 0);
+  int sockfd = open("/dev/null", O_RDWR | O_CLOEXEC);
   if (sockfd < 0) {
     /* Maybe SOCK_CLOEXEC is defined but not available on this kernel */
-    int sockfd = socket(AF_UNIX, SOCK_STREAM, 0);
+    int sockfd = open("/dev/null", O_RDWR);
     fcntl(sockfd, F_SETFD, FD_CLOEXEC);
   }
 #else
   /* Well we don't have SOCK_CLOEXEC, so just set FD_CLOEXEC quickly */
-  int sockfd = socket(AF_UNIX, SOCK_STREAM, 0);
+  int sockfd = open("/dev/null", O_RDWR);
   fcntl(sockfd, F_SETFD, FD_CLOEXEC);
 #endif