main: add optional support for sd_notify via libsystemd

If libsystemd is available, and it's not disabled via --without-libsystemd,
link to it and call sd_notify() before and after the main loop.

This allows systemd to track the exact moment when all the setup is completed
and the unit is considered activated (with Type=notify), in a race-free manner.

configure

@@ -140,6 +140,7 @@ For better control, use the options below.
   --with-rtcdevice=PATH  Specify default path to RTC device [/dev/rtc]
   --with-sendmail=PATH   Path to sendmail binary [/usr/lib/sendmail]
   --enable-debug         Enable debugging support
+  --without-libsystemd   Don't use libsystemd even if it is available
 
 Fine tuning of the installation directories:
   --sysconfdir=DIR       chrony.conf location [/etc]
@@ -259,6 +260,7 @@ default_hwclockfile=""
 default_pidfile="/var/run/chrony/chronyd.pid"
 default_rtcdevice="/dev/rtc"
 mail_program="/usr/lib/sendmail"
+try_libsystemd=1
 
 for option
 do
@@ -395,6 +397,9 @@ do
     --without-gnutls )
       try_gnutls=0
     ;;
+    --without-libsystemd )
+      try_libsystemd=0
+    ;;
     --host-system=* )
       OPERATINGSYSTEM=`echo $option | sed -e 's/^.*=//;'`
     ;;
@@ -589,6 +594,7 @@ if ! test_executable "pkg-config" $PKG_CONFIG --version; then
   try_nettle=0
   try_nss=0
   try_gnutls=0
+  try_libsystemd=0
 fi
 
 if test_code '64-bit time_t' 'time.h' '' '' '
@@ -1045,6 +1051,19 @@ if [ $feat_ntp = "1" ] && [ $feat_nts = "1" ] && [ $try_gnutls = "1" ]; then
   fi
 fi
 
+if [ $try_libsystemd = "1" ]; then
+  test_cflags="`pkg_config --cflags libsystemd`"
+  test_link="`pkg_config --libs libsystemd`"
+  if test_code 'libsystemd' 'systemd/sd-daemon.h' \
+    "$test_cflags" "$test_link" \
+    'sd_notify(0, "READY=1");'
+  then
+    LIBS="$LIBS $test_link"
+    MYCPPFLAGS="$MYCPPFLAGS $test_cflags"
+    add_def FEAT_LIBSYSTEMD
+  fi
+fi
+
 if [ $use_pthread = "1" ]; then
   MYCFLAGS="$MYCFLAGS -pthread"
 fi
@@ -1114,7 +1133,7 @@ add_def MAIL_PROGRAM "\"$mail_program\""
 
 common_features="`get_features SECHASH IPV6 DEBUG`"
 chronyc_features="`get_features READLINE`"
-chronyd_features="`get_features CMDMON NTP REFCLOCK RTC PRIVDROP SCFILTER SIGND ASYNCDNS NTS`"
+chronyd_features="`get_features CMDMON NTP REFCLOCK RTC PRIVDROP SCFILTER SIGND ASYNCDNS NTS LIBSYSTEMD`"
 add_def CHRONYC_FEATURES "\"$chronyc_features $common_features\""
 add_def CHRONYD_FEATURES "\"$chronyd_features $common_features\""
 echo "Features : $chronyd_features $chronyc_features $common_features"

doc/installation.adoc

@@ -35,6 +35,7 @@ to enable optional features:
 * timepps.h header: PPS reference clock
 * Asciidoctor: documentation in HTML format
 * Bash: test suite
+* libsystemd: signal readiness to systemd via `sd_notify()` (`LIBSYSTEMD`)
 
 The following programs are needed when building `chrony` from the git
 repository instead of a released tar file:
@@ -104,6 +105,15 @@ If development files for the editline library are available,
 `chronyc` will be built with line editing support. If you don't want this,
 specify the `--disable-readline` flag to `configure`.
 
+If development files for the libsystemd library are available, `chronyd` will
+be built with support for signaling readiness to systemd via `sd_notify()`.
+When this is enabled, `chronyd` in a systemd unit file can be called  with the
+`-n` option to avoid forking in the background, and with `pidfile /` set in the
+`chronyd` configuration file to avoid writing/checking the PID file. This will
+let systemd do the full lifecycle tracking and management of `chronyd`, without
+race conditions. If you don't want to enable the support, specify the
+`--without-libsystemd` flag to `configure`.
+
 If a `timepps.h` header is available (e.g. from the
 http://linuxpps.org[LinuxPPS project]), `chronyd` will be built with PPS API
 reference clock driver. If the header is installed in a location that isn't

main.c

@@ -59,6 +59,10 @@
 #include "tempcomp.h"
 #include "util.h"
 
+#ifdef FEAT_LIBSYSTEMD
+#include <systemd/sd-daemon.h>
+#endif
+
 /* ================================================== */
 
 /* Set when the initialisation chain has been completed.  Prevents finalisation
@@ -698,10 +702,20 @@ int main
     post_init_rtc_hook(NULL);
   }
 
+#ifdef FEAT_LIBSYSTEMD
+  if (nofork)
+    sd_notify(0, "READY=1");
+#endif
+
   /* The program normally runs under control of the main loop in
      the scheduler. */
   SCH_MainLoop();
 
+#ifdef FEAT_LIBSYSTEMD
+  if (nofork)
+    sd_notify(0, "STOPPING=1");
+#endif
+
   LOG(LOGS_INFO, "chronyd exiting");
 
   MAI_CleanupAndExit();