uuid: Break code to read /dev/urandom into a new module.

This code is useful for seeding other random number generators, so we might
as well make it a separate source file.

lib/entropy.c

@@ -0,0 +1,65 @@
+/* Copyright (c) 2008, 2009, 2010 Nicira Networks
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at:
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include <config.h>
+
+#include "entropy.h"
+
+#include <errno.h>
+#include <fcntl.h>
+#include <unistd.h>
+
+#include "socket-util.h"
+#include "vlog.h"
+
+VLOG_DEFINE_THIS_MODULE(entropy)
+
+static const char urandom[] = "/dev/urandom";
+
+/* Initializes 'buffer' with 'n' bytes of high-quality random numbers.  Returns
+ * 0 if successful, otherwise a positive errno value or EOF on error. */
+int
+get_entropy(void *buffer, size_t n)
+{
+    size_t bytes_read;
+    int error;
+    int fd;
+
+    fd = open(urandom, O_RDONLY);
+    if (fd < 0) {
+        VLOG_ERR("%s: open failed (%s)", urandom, strerror(errno));
+        return errno ? errno : EINVAL;
+    }
+
+    error = read_fully(fd, buffer, n, &bytes_read);
+    close(fd);
+
+    if (error) {
+        VLOG_ERR("%s: read error (%s)", urandom,
+                 error == EOF ? "unexpected end of file" : strerror(error));
+    }
+    return error;
+}
+
+/* Initializes 'buffer' with 'n' bytes of high-quality random numbers.  Exits
+ * if an error occurs. */
+void
+get_entropy_or_die(void *buffer, size_t n)
+{
+    int error = get_entropy(buffer, n);
+    if (error) {
+        ovs_fatal(error, "%s: read error", urandom);
+    }
+}