Fix up the watchdog NTP driftfile management.

Previously, if the drift file didn't exist at boot time, drift was not tracked.
Now we check at each report time instead, since ntpd can create the file and
that might not happen for a hour or more after boot.

clientside/tmcc/common/watchdog

@@ -141,6 +141,7 @@ my %immediate = (
 #
 sub setintervals($);
 sub sendisalive($);
+sub readdrift();
 sub ntpdrift($);
 sub runcvsup($);
 sub sendrusage($);
@@ -277,13 +278,8 @@ if (JAILED() || GENVNODE() || PLAB()) {
 #
 # For sending back ntpdrift.
 #
-if (-e "/etc/ntp.drift") {
-    $driftfile = "/etc/ntp.drift";
-} elsif (-e "/etc/ntp/drift") {
-    $driftfile = "/etc/ntp/drift";
-} elsif (-e "/var/lib/ntp/drift") {
-    $driftfile = "/var/lib/ntp/drift";
-}
+$lastdrift = readdrift();
+logmsg("Initial NTP drift: $lastdrift\n");
 
 #
 # Location of dhcpd.conf
@@ -294,14 +290,6 @@ if (-e "/etc/dhcpd.conf") {
 	$dhcpdconf = "/usr/local/etc/dhcpd.conf";
 }
 
-#
-# Initial drift value, we only update if it changes
-#
-if (defined($driftfile)) {
-    $lastdrift = `cat $driftfile`;
-    chomp($lastdrift);
-}
-
 #
 # Retry state for failed tmcc calls
 #
@@ -667,6 +655,48 @@ sub setintervals($)
 	if ($iv{check});
 }
 
+#
+# Read the current value of the driftfile.
+# Returns bogus value "NONE" if no driftfile.
+#
+sub readdrift()
+{
+    #
+    # If we haven't yet, locate the driftfile.
+    # We do this on every drift report because the driftfile
+    # may not exist initially in the booted OS image.
+    #
+    if (!defined($driftfile)) {
+	if (-e "/etc/ntp.drift") {
+	    $driftfile = "/etc/ntp.drift";
+	} elsif (-e "/etc/ntp/drift") {
+	    $driftfile = "/etc/ntp/drift";
+	} elsif (-e "/var/lib/ntp/drift") {
+	    $driftfile = "/var/lib/ntp/drift";
+	} elsif (-e "/var/lib/ntp/ntp.drift") {
+	    $driftfile = "/var/lib/ntp/ntp.drift";
+	}
+	logmsg("NTP drift file is $driftfile\n")
+	    if (defined($driftfile));
+    }
+
+    #
+    # Read the current driftfile value.
+    #
+    if (defined($driftfile)) {
+	$drift = `cat $driftfile`;
+	chomp($drift);
+	if ($drift eq "") {
+	    $drift = 0.0;
+	}
+    } else {
+	logmsg("WARNING: no NTP drift file found; no drift update done.\n");
+	$drift = "NONE";
+    }
+
+    return $drift;
+}
+
 sub ntpdrift($)
 {
     my ($curtime) = @_;
@@ -681,7 +711,7 @@ sub ntpdrift($)
 
     logmsg("ntpdrift: reporting NTP drift\n", 1);
 
-    my $drift = `cat $driftfile`;
+    my $drift = readdrift();
     chomp($drift);
 
     if ($drift ne $lastdrift && $drift =~ /^([-\d\.]*)$/) {