rtc: set clock to mtime of driftfile when RTC preinit fails

When the RTC preinit function fails, set the system clock to the time of
the last modification of the driftfile if it's in the future. This makes
the -s option somewhat useful on systems where RTC is not supported or
missing.

This is similar to the functionality implemented in the fake-hwclock
script.

chrony.texi.in

@@ -1018,18 +1018,20 @@ Support for real-time clocks is limited at present - the criteria are
 described in the section on the @code{rtcfile} directive (@pxref{rtcfile
 directive}).
 
-If @code{chronyd} cannot support the real time clock on your computer,
-this option cannot be used and a warning message will be logged to the
-syslog.
-
 If used in conjunction with the `-r' flag, @code{chronyd} will attempt
 to preserve the old samples after setting the system clock from the real
-time clock.  This can be used to allow @code{chronyd} to perform long
+time clock (RTC).  This can be used to allow @code{chronyd} to perform long
 term averaging of the gain or loss rate across system reboots, and is
 useful for dial-up systems that are shut down when not in use.  For this
 to work well, it relies on @code{chronyd} having been able to determine
-accurate statistics for the difference between the real time clock and
+accurate statistics for the difference between the RTC and
 system clock last time the computer was on.
+
+If @code{chronyd} doesn't support the RTC on your computer or there is no RTC
+installed, the system clock will be set with this option to the time of the
+last modification of the drift file (specified by the @code{driftfile}
+directive) to restore the system time at which @code{chronyd} was previously
+stopped.
 @item -u <user>
 This option sets the name of the user to which will @code{chronyd} switch to
 drop root privileges if compiled with Linux capabilities support (default

chronyd.8.in

@@ -88,19 +88,20 @@ Support for real-time clocks is limited at present - the criteria
 are described in the section on the \fIrtcfile\fR directive in the
 documentation supplied with the distribution.
 
-If \fBchronyd\fR cannot support the real time clock on your computer,
-this option cannot be used and a warning message will be logged to
-the syslog.
-
 If used in conjunction with the \fB-r\fR flag, \fBchronyd\fR will attempt
 to preserve the old samples after setting the system clock from
-the real time clock.  This can be used to allow \fBchronyd\fR to
+the real time clock (RTC).  This can be used to allow \fBchronyd\fR to
 perform long term averaging of the gain or loss rate across system
 reboots, and is useful for dial-up systems that are shut down when
 not in use.  For this to work well, it relies on \fBchronyd\fR having
 been able to determine accurate statistics for the difference
-between the real time clock and system clock last time the
-computer was on.
+between the RTC and system clock last time the computer was on.
+
+If \fBchronyd\fR doesn't support the RTC on your computer or there is no RTC
+installed, the system clock will be set with this option to the time of the
+last modification of the drift file (specified by the \fIdriftfile\fR
+directive) to restore the system time at which \fBchronyd\fR was previously
+stopped.
 .TP
 \fB\-u\fR \fIuser\fR
 This option sets the name of the user to which will \fBchronyd\fR switch to

rtc.c

@@ -28,6 +28,7 @@
 #include "sysincl.h"
 
 #include "rtc.h"
+#include "local.h"
 #include "logging.h"
 #include "conf.h"
 
@@ -71,6 +72,33 @@ static struct {
 #endif 
 };
      
+/* ================================================== */
+/* Set the system clock to the time of last modification of driftfile
+   if it's in the future */
+
+static void
+fallback_time_init(void)
+{
+  struct timeval now;
+  struct stat buf;
+  char *drift_file;
+
+  drift_file = CNF_GetDriftFile();
+  if (!drift_file)
+    return;
+
+  if (stat(drift_file, &buf))
+    return;
+
+  LCL_ReadCookedTime(&now, NULL);
+
+  if (now.tv_sec < buf.st_mtime) {
+    LCL_ApplyStepOffset(now.tv_sec - buf.st_mtime);
+    LOG(LOGS_INFO, LOGF_Rtc,
+        "System clock set from driftfile %s", drift_file);
+  }
+}
+
 /* ================================================== */
 
 void
@@ -79,9 +107,13 @@ RTC_Initialise(int initial_set)
   char *file_name;
 
   /* Do an initial read of the RTC and set the system time to it.  This
-     is analogous to what /sbin/hwclock -s would do on Linux. */
-  if (initial_set && driver.time_pre_init) {
-    (driver.time_pre_init)();
+     is analogous to what /sbin/hwclock -s would do on Linux.  If that fails
+     or RTC is not supported, set the clock to the time of the last
+     modification of driftfile, so we at least get closer to the truth. */
+  if (initial_set) {
+    if (!driver.time_pre_init || !driver.time_pre_init()) {
+      fallback_time_init();
+    }
   }
 
   driver_initialised = 0;