sd-daemon,man: ignore missing $WATCHDOG_PID

Systemd 209 started setting $WATCHDOG_PID, and sd-daemon watch was
modified to check for this variable. This means that
sd_watchdog_enabled() stopped working with previous versions of
systemd. But sd-event is a public library and API and we must keep it
working even when a program compiled with a newer version of the
libary is used on a system running an older version of the manager.

getenv() and unsetenv() are fairly expensive calls, so optimize
sd_watchdog_enabled() by not calling them when unnecessary.

man: centralize the description of $WATCHDOG_PID and $WATCHDOG_USEC in
the sd_watchdog_enabled manpage. It is better not to repeat the same
stuff in two places.

man/sd_notify.xml

@@ -192,17 +192,12 @@
                                 <varname>WatchdogSec=</varname> is
                                 enabled for it. See
                                 <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>
-                                for details. It is recommended to send
+                                for information how to enable this
-                                this message if the
+                                functionality and
-                                <varname>$WATCHDOG_PID</varname>
-                                environment variable has been set to
-                                the PID of the service process, in
-                                every half the time interval that is
-                                specified in the
-                                <varname>$WATCHDOG_USEC</varname>
-                                environment variable. See
                                 <citerefentry><refentrytitle>sd_watchdog_enabled</refentrytitle><manvolnum>3</manvolnum></citerefentry>
-                                for details.</para></listitem>
+                                for the details of how the service can
+                                check if the the watchdog is enabled.
+                                </para></listitem>
                         </varlistentry>
                 </variablelist>
 

man/sd_watchdog_enabled.xml

@@ -69,30 +69,37 @@
                 which the manager will act on the service if it did
                 not get such a notification.</para>
 
+                <para>If the <varname>$WATCHDOG_USEC</varname>
+                environment variable is set, and the
+                <varname>$WATCHDOG_PID</varname> variable is unset or
+                set to the PID of the current process, the service
+                manager expects notifications from this process. The
+                manager will usually terminate a service when it does
+                not get a notification message within the specified
+                time after startup and after each previous message. It
+                is recommended that a daemon sends a keep-alive
+                notification message to the service manager every half
+                of the time returned here. Notification messages may
+                be sent with
+                <citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry>
+                with a message string of
+                <literal>WATCHDOG=1</literal>.</para>
+
                 <para>If the <parameter>unset_environment</parameter>
                 parameter is non-zero,
                 <function>sd_watchdog_enabled()</function> will unset
                 the <varname>$WATCHDOG_USEC</varname> and
                 <varname>$WATCHDOG_PID</varname> environment variables
-                before returning (regardless of whether the function call
+                before returning (regardless of whether the function
-                itself succeeded or not). Further calls to
+                call itself succeeded or not). Those variables are no
-                <function>sd_watchdog_enabled()</function> will then
+                longer inherited by child processes. Further calls to
-                return with zero, but the variable is no longer
+                <function>sd_watchdog_enabled()</function> will also
-                inherited by child processes.</para>
+                return with zero.</para>
 
                 <para>If the <parameter>usec</parameter> parameter is
                 non-NULL, <function>sd_watchdog_enabled()</function>
-                will return the timeout in µs for the watchdog
+                will write the timeout in µs for the watchdog
-                logic. The service manager will usually terminate a
+                logic to it.</para>
-                service when it did not get a notification message
-                within the specified time after startup and after each
-                previous message. It is recommended that a daemon
-                sends a keep-alive notification message to the service
-                manager every half of the time returned
-                here. Notification messages may be sent with
-                <citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry>
-                with a message string of
-                <literal>WATCHDOG=1</literal>.</para>
 
                 <para>To enable service supervision with the watchdog
                 logic, use <varname>WatchdogSec=</varname> in service
@@ -126,7 +133,6 @@
                 of the current process, under the assumption that in
                 that case, the variables were set for a different
                 process further up the process tree.</para>
-
         </refsect1>
 
         <refsect1>
@@ -156,6 +162,19 @@
                 </variablelist>
         </refsect1>
 
+        <refsect1>
+                <title>History</title>
+
+                <para>The watchdog functionality and the
+                <varname>$WATCHDOG_USEC</varname> variable were
+                added in systemd-41.</para>
+
+                <para><function>sd_watchdog_enabled()</function>
+                function was added in systemd-209. Since that version
+                the <varname>$WATCHDOG_PID</varname> variable is also
+                set.</para>
+        </refsect1>
+
         <refsect1>
                 <title>See Also</title>
                 <para>

src/libsystemd/sd-daemon/sd-daemon.c

@@ -491,34 +491,15 @@ _public_ int sd_booted(void) {
 }
 
 _public_ int sd_watchdog_enabled(int unset_environment, uint64_t *usec) {
-        const char *e;
+        const char *s, *p = ""; /* p is set to dummy value to do unsetting */
         uint64_t u;
-        pid_t pid;
+        int r = 0;
-        int r;
 
-        e = getenv("WATCHDOG_PID");
+        s = getenv("WATCHDOG_USEC");
-        if (!e) {
+        if (!s)
-                r = 0;
-                goto finish;
-        }
-
-        r = parse_pid(e, &pid);
-        if (r < 0)
                 goto finish;
 
-        /* Is this for us? */
+        r = safe_atou64(s, &u);
-        if (getpid() != pid) {
-                r = 0;
-                goto finish;
-        }
-
-        e = getenv("WATCHDOG_USEC");
-        if (!e) {
-                r = -EINVAL;
-                goto finish;
-        }
-
-        r = safe_atou64(e, &u);
         if (r < 0)
                 goto finish;
         if (u <= 0) {
@@ -526,16 +507,31 @@ _public_ int sd_watchdog_enabled(int unset_environment, uint64_t *usec) {
                 goto finish;
         }
 
+        p = getenv("WATCHDOG_PID");
+        if (p) {
+                pid_t pid;
+
+                r = parse_pid(p, &pid);
+                if (r < 0)
+                        goto finish;
+
+                /* Is this for us? */
+                if (getpid() != pid) {
+                        r = 0;
+                        goto finish;
+                }
+        }
+
         if (usec)
                 *usec = u;
 
         r = 1;
 
 finish:
-        if (unset_environment) {
+        if (unset_environment && s)
-                unsetenv("WATCHDOG_PID");
                 unsetenv("WATCHDOG_USEC");
-        }
+        if (unset_environment && p)
+                unsetenv("WATCHDOG_PID");
 
         return r;
 }