crypto: fix rand module

vlib/crypto/rand/rand_linux.c.v

@@ -0,0 +1,40 @@
+// Copyright (c) 2019-2020 Alexander Medvednikov. All rights reserved.
+// Use of this source code is governed by an MIT license
+// that can be found in the LICENSE file.
+
+module rand
+
+import math
+
+#include <sys/syscall.h>
+
+const (
+	read_batch_size = 256
+)
+
+pub fn read(bytes_needed int) ?[]byte {	
+	buffer := malloc(bytes_needed)
+	mut bytes_read := 0
+	// getrandom syscall wont block if requesting <= 256 bytes
+	if bytes_needed > read_batch_size {
+		no_batches := int(math.floor(f64(bytes_needed/read_batch_size)))
+		for i:=0; i<no_batches; i++ {
+			if getrandom(read_batch_size, buffer+bytes_read) == -1 {
+				return read_error
+			}
+			bytes_read += read_batch_size
+		}
+	}
+	if getrandom(bytes_needed-bytes_read, buffer+bytes_read) == -1 {
+		return read_error
+	}
+	
+	return c_array_to_bytes_tmp(bytes_needed, buffer)
+}
+
+fn getrandom(bytes_needed int, buffer voidptr) int {
+	if bytes_needed > read_batch_size {
+		panic('getrandom() dont request more than $read_batch_size bytes at once.')
+	}
+	return C.syscall(C.SYS_getrandom, buffer, bytes_needed, 0)
+}