core: introduce new KillMode=mixed which sends SIGTERM only to the main process, but SIGKILL to all daemon processes

This should fix some race with terminating systemd --user, where the
system systemd instance might race against the user systemd instance
when sending SIGTERM.

man/systemd.kill.xml

@@ -44,39 +44,44 @@
 
         <refnamediv>
                 <refname>systemd.kill</refname>
-                <refpurpose>Kill environment configuration</refpurpose>
+                <refpurpose>Process killing procedure
+                configuration</refpurpose>
         </refnamediv>
 
         <refsynopsisdiv>
                 <para><filename><replaceable>service</replaceable>.service</filename>,
                 <filename><replaceable>socket</replaceable>.socket</filename>,
                 <filename><replaceable>mount</replaceable>.mount</filename>,
-                <filename><replaceable>swap</replaceable>.swap</filename></para>
+                <filename><replaceable>swap</replaceable>.swap</filename>,
+                <filename><replaceable>scope</replaceable>.scope</filename></para>
         </refsynopsisdiv>
 
         <refsect1>
                 <title>Description</title>
 
                 <para>Unit configuration files for services, sockets,
-                mount points and swap devices share a subset of
+                mount points, swap devices and scopes share a subset
-                configuration options which define the process killing
+                of configuration options which define the
-                parameters of spawned processes.</para>
+                killing procedure of processes belonging to the unit.</para>
 
                 <para>This man page lists the configuration options
-                shared by these four unit types. See
+                shared by these five unit types. See
                 <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>
-                for the common options of all unit configuration
+                for the common options shared by all unit
-                files, and
+                configuration files, and
                 <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
                 <citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
-                <citerefentry><refentrytitle>systemd.swap</refentrytitle><manvolnum>5</manvolnum></citerefentry>
+                <citerefentry><refentrytitle>systemd.swap</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
-                and
                 <citerefentry><refentrytitle>systemd.mount</refentrytitle><manvolnum>5</manvolnum></citerefentry>
-                for more information on the specific unit
+                and
-                configuration files. The execution specific
+                <citerefentry><refentrytitle>systemd.scope</refentrytitle><manvolnum>5</manvolnum></citerefentry>
+                for more information on the configuration file options
+                specific to each unit type.</para>
+
+                <para>The kill procedure
                 configuration options are configured in the [Service],
-                [Socket], [Mount], or [Swap] section, depending on the unit
+                [Socket], [Mount] or [Swap] section, depending on the
-                type.</para>
+                unit type.</para>
         </refsect1>
 
         <refsect1>
@@ -87,32 +92,40 @@
                         <varlistentry>
                                 <term><varname>KillMode=</varname></term>
                                 <listitem><para>Specifies how
-                                processes of this service shall be
+                                processes of this unit shall be
                                 killed. One of
                                 <option>control-group</option>,
                                 <option>process</option>,
+                                <option>mixed</option>,
                                 <option>none</option>.</para>
 
                                 <para>If set to
                                 <option>control-group</option>, all
                                 remaining processes in the control
-                                group of this unit will be terminated
+                                group of this unit will be killed on
-                                on unit stop (for services: after the
+                                unit stop (for services: after the
                                 stop command is executed, as
                                 configured with
                                 <varname>ExecStop=</varname>). If set
                                 to <option>process</option>, only the
                                 main process itself is killed. If set
-                                to <option>none</option>, no process is
+                                to <option>mixed</option> the
+                                <constant>SIGTERM</constant> signal
+                                (see below) is sent to the main
+                                process while the subsequent
+                                <constant>SIGKILL</constant> signal
+                                (see below) is sent to all remaining
+                                processes of the unit's control
+                                group. If set to
+                                <option>none</option>, no process is
                                 killed. In this case only the stop
-                                command will be executed on unit
+                                command will be executed on unit stop,
-                                stop, but no process be killed
+                                but no process be killed
                                 otherwise. Processes remaining alive
                                 after stop are left in their control
                                 group and the control group continues
                                 to exist after stop unless it is
-                                empty. Defaults to
+                                empty.</para>
-                                <option>control-group</option>.</para>
 
                                 <para>Processes will first be
                                 terminated via
@@ -133,14 +146,24 @@
                                 option). See
                                 <citerefentry><refentrytitle>kill</refentrytitle><manvolnum>2</manvolnum></citerefentry>
                                 for more
-                                information.</para></listitem>
+                                information.</para>
+
+                                <para>Defaults to
+                                <option>control-group</option>.</para></listitem>
                         </varlistentry>
 
                         <varlistentry>
                                 <term><varname>KillSignal=</varname></term>
                                 <listitem><para>Specifies which signal
-                                to use when killing a
+                                to use when killing a service. This
-                                service. Defaults to <constant>SIGTERM</constant>.
+                                controls the signal that is sent as
+                                first step of shutting down a unit
+                                (see above), and is usually followed
+                                by <constant>SIGKILL</constant> (see
+                                above and below). For a list of valid
+                                signals, see
+                                <citerefentry><refentrytitle>signal</refentrytitle><manvolnum>7</manvolnum></citerefentry>. Defaults
+                                to <constant>SIGTERM</constant>.
                                 </para></listitem>
                         </varlistentry>
 
@@ -184,7 +207,9 @@
                           <citerefentry><refentrytitle>systemd.swap</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
                           <citerefentry><refentrytitle>systemd.mount</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
                           <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
-                          <citerefentry><refentrytitle>systemd.directives</refentrytitle><manvolnum>7</manvolnum></citerefentry>
+                          <citerefentry><refentrytitle>systemd.directives</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
+                          <citerefentry><refentrytitle>kill</refentrytitle><manvolnum>2</manvolnum></citerefentry>,
+                          <citerefentry><refentrytitle>signal</refentrytitle><manvolnum>7</manvolnum></citerefentry>
                   </para>
         </refsect1>
 

src/core/kill.c

@@ -52,6 +52,7 @@ void kill_context_dump(KillContext *c, FILE *f, const char *prefix) {
 static const char* const kill_mode_table[_KILL_MODE_MAX] = {
         [KILL_CONTROL_GROUP] = "control-group",
         [KILL_PROCESS] = "process",
+        [KILL_MIXED] = "mixed",
         [KILL_NONE] = "none"
 };
 

src/core/kill.h

@@ -32,6 +32,7 @@ typedef enum KillMode {
         /* The kill mode is a property of a unit. */
         KILL_CONTROL_GROUP = 0,
         KILL_PROCESS,
+        KILL_MIXED,
         KILL_NONE,
         _KILL_MODE_MAX,
         _KILL_MODE_INVALID = -1

src/core/unit.c

@@ -2966,7 +2966,7 @@ int unit_kill_context(
                 }
         }
 
-        if (c->kill_mode == KILL_CONTROL_GROUP && u->cgroup_path) {
+        if ((c->kill_mode == KILL_CONTROL_GROUP || (c->kill_mode == KILL_MIXED && sigkill)) && u->cgroup_path) {
                 _cleanup_set_free_ Set *pid_set = NULL;
 
                 /* Exclude the main/control pids from being killed via the cgroup */
@@ -2980,6 +2980,7 @@ int unit_kill_context(
                                 log_warning_unit(u->id, "Failed to kill control group: %s", strerror(-r));
                 } else if (r > 0) {
                         wait_for_exit = true;
+
                         if (c->send_sighup) {
                                 set_free(pid_set);
 

units/user@.service.in

@@ -15,3 +15,4 @@ PAMName=systemd-user
 Type=notify
 ExecStart=-@rootlibexecdir@/systemd --user
 Slice=user-%i.slice
+KillMode=mixed