Test whether created sockets are select()able

src/compat.h

@@ -92,4 +92,12 @@ typedef u_int SOCKET;
 size_t strnlen( const char *start, size_t max_len);
 #endif // HAVE_DECL_STRNLEN
 
+bool static inline IsSelectableSocket(SOCKET s) {
+#ifdef WIN32
+    return true;
+#else
+    return (s >= 0 && s < FD_SETSIZE);
+#endif
+}
+
 #endif // BITCOIN_COMPAT_H