sys: add null driver

Add a new clock driver that doesn't actually try to adjust the clock.
It allows chronyd to run without the capability to adjust/set the system
clock, e.g. in some containers. It can be enabled by the -x option.

sys_null.c

@@ -0,0 +1,140 @@
+/*
+  chronyd/chronyc - Programs for keeping computer clocks accurate.
+
+ **********************************************************************
+ * Copyright (C) Miroslav Lichvar  2017
+ * 
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of version 2 of the GNU General Public License as
+ * published by the Free Software Foundation.
+ * 
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ * 
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+ * 
+ **********************************************************************
+
+  =======================================================================
+
+  Null clock driver for operation with no clock control.
+  */
+
+#include "config.h"
+
+#include "sysincl.h"
+
+#include "sys_null.h"
+
+#include "local.h"
+#include "localp.h"
+#include "logging.h"
+#include "util.h"
+
+/* Current frequency offset of the system clock (in ppm) */
+static double freq;
+
+/* Offset of the system clock at the last update */
+static double offset_register;
+
+/* Time of the last update */
+static struct timespec last_update;
+
+/* Minimum interval between updates when frequency is constant */
+#define MIN_UPDATE_INTERVAL 1000.0
+
+/* ================================================== */
+
+static void
+update_offset(void)
+{
+  struct timespec now;
+  double duration;
+
+  LCL_ReadRawTime(&now);
+  duration = UTI_DiffTimespecsToDouble(&now, &last_update);
+  offset_register += 1.0e-6 * freq * duration;
+  last_update = now;
+
+  DEBUG_LOG("System clock offset=%e freq=%f", offset_register, freq);
+}
+
+/* ================================================== */
+
+static double
+read_frequency(void)
+{
+  return freq;
+}
+
+/* ================================================== */
+
+static double
+set_frequency(double freq_ppm)
+{
+  update_offset();
+  freq = freq_ppm;
+
+  return freq;
+}
+
+/* ================================================== */
+
+static void
+accrue_offset(double offset, double corr_rate)
+{
+  offset_register += offset;
+}
+
+/* ================================================== */
+
+static int
+apply_step_offset(double offset)
+{
+  return 0;
+}
+
+/* ================================================== */
+
+static void
+offset_convert(struct timespec *raw, double *corr, double *err)
+{
+  double duration;
+
+  duration = UTI_DiffTimespecsToDouble(raw, &last_update);
+
+  if (duration > MIN_UPDATE_INTERVAL) {
+    update_offset();
+    duration = 0.0;
+  }
+
+  *corr = -1.0e-6 * freq * duration - offset_register;
+
+  if (err)
+    *err = 0.0;
+}
+
+/* ================================================== */
+
+void
+SYS_Null_Initialise(void)
+{
+  offset_register = 0.0;
+  LCL_ReadRawTime(&last_update);
+
+  lcl_RegisterSystemDrivers(read_frequency, set_frequency, accrue_offset,
+                            apply_step_offset, offset_convert, NULL, NULL);
+
+  LOG(LOGS_INFO, "Disabled control of system clock");
+}
+
+/* ================================================== */
+
+void
+SYS_Null_Finalise(void)
+{
+}