From d77d5a9399e393734fe8c8a5ad085036775854a7 Mon Sep 17 00:00:00 2001
From: Lennart Poettering <lennart@poettering.net>
Date: Tue, 17 Jul 2018 12:01:26 +0200
Subject: [PATCH] man: document the new Type=exec type
And while we are at it, let's rearrange and extend the Type=
documentation a bit. Let's make it an itemized list, and let's add a
paragraph explaining which type best to use.
(cherry picked from commit 79905a246d645d21633f09f564b3672d5085a85c)
Resolves: #1683334
---
man/systemd-run.xml | 10 +++
man/systemd.service.xml | 158 ++++++++++++++++++++++------------------
2 files changed, 97 insertions(+), 71 deletions(-)
diff --git a/man/systemd-run.xml b/man/systemd-run.xml
index 1c254afae3..a134b2c0dc 100644
--- a/man/systemd-run.xml
+++ b/man/systemd-run.xml
@@ -83,6 +83,16 @@
<replaceable>COMMAND</replaceable> may be omitted. In this case, <command>systemd-run</command> creates only a
<filename>.path</filename>, <filename>.socket</filename>, or <filename>.timer</filename> unit that triggers the
specified unit.</para>
+
+ <para>By default, services created with <command>systemd-run</command> default to the <option>simple</option> type,
+ see the description of <varname>Type=</varname> in
+ <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry> for
+ details. Note that when this type is used the service manager (and thus the <command>systemd-run</command> command)
+ considers service start-up successful as soon as the <function>fork()</function> for the main service process
+ succeeded, i.e. before the <function>execve()</function> is invoked, and thus even if the specified command cannot
+ be started. Consider using the <option>exec</option> service type (i.e. <option>--property=Type=exec</option>) to
+ ensure that <command>systemd-run</command> returns successfully only if the specified command line has been
+ successfully started.</para>
</refsect1>
<refsect1>
diff --git a/man/systemd.service.xml b/man/systemd.service.xml
index add54524ce..315b80e704 100644
--- a/man/systemd.service.xml
+++ b/man/systemd.service.xml
@@ -153,77 +153,93 @@
<varlistentry>
<term><varname>Type=</varname></term>
- <listitem><para>Configures the process start-up type for this
- service unit. One of
- <option>simple</option>,
- <option>forking</option>,
- <option>oneshot</option>,
- <option>dbus</option>,
- <option>notify</option> or
- <option>idle</option>.</para>
-
- <para>If set to <option>simple</option> (the default if
- neither <varname>Type=</varname> nor
- <varname>BusName=</varname>, but <varname>ExecStart=</varname>
- are specified), it is expected that the process configured
- with <varname>ExecStart=</varname> is the main process of the
- service. In this mode, if the process offers functionality to
- other processes on the system, its communication channels
- should be installed before the daemon is started up (e.g.
- sockets set up by systemd, via socket activation), as systemd
- will immediately proceed starting follow-up units.</para>
-
- <para>If set to <option>forking</option>, it is expected that
- the process configured with <varname>ExecStart=</varname> will
- call <function>fork()</function> as part of its start-up. The
- parent process is expected to exit when start-up is complete
- and all communication channels are set up. The child continues
- to run as the main daemon process. This is the behavior of
- traditional UNIX daemons. If this setting is used, it is
- recommended to also use the <varname>PIDFile=</varname>
- option, so that systemd can identify the main process of the
- daemon. systemd will proceed with starting follow-up units as
- soon as the parent process exits.</para>
-
- <para>Behavior of <option>oneshot</option> is similar to
- <option>simple</option>; however, it is expected that the
- process has to exit before systemd starts follow-up units.
- <varname>RemainAfterExit=</varname> is particularly useful for
- this type of service. This is the implied default if neither
- <varname>Type=</varname> nor <varname>ExecStart=</varname> are
- specified.</para>
-
- <para>Behavior of <option>dbus</option> is similar to
- <option>simple</option>; however, it is expected that the
- daemon acquires a name on the D-Bus bus, as configured by
- <varname>BusName=</varname>. systemd will proceed with
- starting follow-up units after the D-Bus bus name has been
- acquired. Service units with this option configured implicitly
- gain dependencies on the <filename>dbus.socket</filename>
- unit. This type is the default if <varname>BusName=</varname>
- is specified.</para>
-
- <para>Behavior of <option>notify</option> is similar to
- <option>simple</option>; however, it is expected that the
- daemon sends a notification message via
- <citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry>
- or an equivalent call when it has finished starting up.
- systemd will proceed with starting follow-up units after this
- notification message has been sent. If this option is used,
- <varname>NotifyAccess=</varname> (see below) should be set to
- open access to the notification socket provided by systemd. If
- <varname>NotifyAccess=</varname> is missing or set to
- <option>none</option>, it will be forcibly set to
- <option>main</option>. Note that currently
- <varname>Type=</varname><option>notify</option> will not work
- if used in combination with
- <varname>PrivateNetwork=</varname><option>yes</option>.</para>
-
- <para>Behavior of <option>idle</option> is very similar to <option>simple</option>; however, actual execution
- of the service program is delayed until all active jobs are dispatched. This may be used to avoid interleaving
- of output of shell services with the status output on the console. Note that this type is useful only to
- improve console output, it is not useful as a general unit ordering tool, and the effect of this service type
- is subject to a 5s time-out, after which the service program is invoked anyway.</para>
+ <listitem>
+ <para>Configures the process start-up type for this service unit. One of <option>simple</option>,
+ <option>exec</option>, <option>forking</option>, <option>oneshot</option>, <option>dbus</option>,
+ <option>notify</option> or <option>idle</option>:</para>
+
+ <itemizedlist>
+ <listitem><para>If set to <option>simple</option> (the default if <varname>ExecStart=</varname> is
+ specified but neither <varname>Type=</varname> nor <varname>BusName=</varname> are), the service manager
+ will consider the unit started immediately after the main service process has been forked off. It is
+ expected that the process configured with <varname>ExecStart=</varname> is the main process of the
+ service. In this mode, if the process offers functionality to other processes on the system, its
+ communication channels should be installed before the service is started up (e.g. sockets set up by
+ systemd, via socket activation), as the service manager will immediately proceed starting follow-up units,
+ right after creating the main service process, and before executing the service's binary. Note that this
+ means <command>systemctl start</command> command lines for <option>simple</option> services will report
+ success even if the service's binary cannot be invoked successfully (for example because the selected
+ <varname>User=</varname> doesn't exist, or the service binary is missing).</para></listitem>
+
+ <listitem><para>The <option>exec</option> type is similar to <option>simple</option>, but the service
+ manager will consider the unit started immediately after the main service binary has been executed. The service
+ manager will delay starting of follow-up units until that point. (Or in other words:
+ <option>simple</option> proceeds with further jobs right after <function>fork()</function> returns, while
+ <option>exec</option> will not proceed before both <function>fork()</function> and
+ <function>execve()</function> in the service process succeeded.) Note that this means <command>systemctl
+ start</command> command lines for <option>exec</option> services will report failure when the service's
+ binary cannot be invoked successfully (for example because the selected <varname>User=</varname> doesn't
+ exist, or the service binary is missing).</para></listitem>
+
+ <listitem><para>If set to <option>forking</option>, it is expected that the process configured with
+ <varname>ExecStart=</varname> will call <function>fork()</function> as part of its start-up. The parent
+ process is expected to exit when start-up is complete and all communication channels are set up. The child
+ continues to run as the main service process, and the service manager will consider the unit started when
+ the parent process exits. This is the behavior of traditional UNIX services. If this setting is used, it is
+ recommended to also use the <varname>PIDFile=</varname> option, so that systemd can reliably identify the
+ main process of the service. systemd will proceed with starting follow-up units as soon as the parent
+ process exits.</para></listitem>
+
+ <listitem><para>Behavior of <option>oneshot</option> is similar to <option>simple</option>; however, the
+ service manager will consider the unit started after the main process exits. It will then start follow-up
+ units. <varname>RemainAfterExit=</varname> is particularly useful for this type of
+ service. <varname>Type=</varname><option>oneshot</option> is the implied default if neither
+ <varname>Type=</varname> nor <varname>ExecStart=</varname> are specified.</para></listitem>
+
+ <listitem><para>Behavior of <option>dbus</option> is similar to <option>simple</option>; however, it is
+ expected that the service acquires a name on the D-Bus bus, as configured by
+ <varname>BusName=</varname>. systemd will proceed with starting follow-up units after the D-Bus bus name
+ has been acquired. Service units with this option configured implicitly gain dependencies on the
+ <filename>dbus.socket</filename> unit. This type is the default if <varname>BusName=</varname> is
+ specified.</para></listitem>
+
+ <listitem><para>Behavior of <option>notify</option> is similar to <option>exec</option>; however, it is
+ expected that the service sends a notification message via
+ <citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry> or an
+ equivalent call when it has finished starting up. systemd will proceed with starting follow-up units after
+ this notification message has been sent. If this option is used, <varname>NotifyAccess=</varname> (see
+ below) should be set to open access to the notification socket provided by systemd. If
+ <varname>NotifyAccess=</varname> is missing or set to <option>none</option>, it will be forcibly set to
+ <option>main</option>. Note that currently <varname>Type=</varname><option>notify</option> will not work if
+ used in combination with <varname>PrivateNetwork=</varname><option>yes</option>.</para></listitem>
+
+ <listitem><para>Behavior of <option>idle</option> is very similar to <option>simple</option>; however,
+ actual execution of the service program is delayed until all active jobs are dispatched. This may be used
+ to avoid interleaving of output of shell services with the status output on the console. Note that this
+ type is useful only to improve console output, it is not useful as a general unit ordering tool, and the
+ effect of this service type is subject to a 5s time-out, after which the service program is invoked
+ anyway.</para></listitem>
+ </itemizedlist>
+
+ <para>It is generally recommended to use <varname>Type=</varname><option>simple</option> for long-running
+ services whenever possible, as it is the simplest and fastest option. However, as this service type won't
+ propagate service start-up failures and doesn't allow ordering of other units against completion of
+ initialization of the service (which for example is useful if clients need to connect to the service through
+ some form of IPC, and the IPC channel is only established by the service itself — in contrast to doing this
+ ahead of time through socket or bus activation or similar), it might not be sufficient for many cases. If so,
+ <option>notify</option> or <option>dbus</option> (the latter only in case the service provides a D-Bus
+ interface) are the preferred options as they allow service program code to precisely schedule when to
+ consider the service started up successfully and when to proceed with follow-up units. The
+ <option>notify</option> service type requires explicit support in the service codebase (as
+ <function>sd_notify()</function> or an equivalent API needs to be invoked by the service at the appropriate
+ time) — if it's not supported, then <option>forking</option> is an alternative: it supports the traditional
+ UNIX service start-up protocol. Finally, <option>exec</option> might be an option for cases where it is
+ enough to ensure the service binary is invoked, and where the service binary itself executes no or little
+ initialization on its own (and its initialization is unlikely to fail). Note that using any type other than
+ <option>simple</option> possibly delays the boot process, as the service manager needs to wait for service
+ initialization to complete. It is hence recommended not to needlessly use any types other than
+ <option>simple</option>. (Also note it is generally not recommended to use <option>idle</option> or
+ <option>oneshot</option> for long-running services.)</para>
</listitem>
</varlistentry>