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DaanDeMeyerDaanDeMeyer
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sd-event: Add exit-on-idle support
Sometimes it's hard to assign responsibility to a specific event source for exiting when there's no more work to be done. So let's add exit-on-idle support where we exit when there are no more event sources.
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‎man/sd_event_set_exit_on_idle.xml‎

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<?xml version='1.0'?>
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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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"http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd">
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<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
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<refentry id="sd_event_set_exit_on_idle" xmlns:xi="http://www.w3.org/2001/XInclude">
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<refentryinfo>
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<title>sd_event_set_exit_on_idle</title>
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<productname>systemd</productname>
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</refentryinfo>
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<refmeta>
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<refentrytitle>sd_event_set_exit_on_idle</refentrytitle>
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<manvolnum>3</manvolnum>
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</refmeta>
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<refnamediv>
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<refname>sd_event_set_exit_on_idle</refname>
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<refname>sd_event_get_exit_on_idle</refname>
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<refpurpose>Enable event loop exit-on-idle support</refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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<funcsynopsis>
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<funcsynopsisinfo>#include &lt;systemd/sd-event.h&gt;</funcsynopsisinfo>
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<funcprototype>
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<funcdef>int <function>sd_event_set_exit_on_idle</function></funcdef>
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<paramdef>sd_event *<parameter>event</parameter></paramdef>
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<paramdef>int b</paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef>int <function>sd_event_get_exit_on_idle</function></funcdef>
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<paramdef>sd_event *<parameter>event</parameter></paramdef>
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</funcprototype>
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</funcsynopsis>
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</refsynopsisdiv>
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<refsect1>
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<title>Description</title>
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<para><function>sd_event_set_exit_on_idle()</function> may be used to
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enable or disable the exit-on-idle support in the
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event loop object specified in the <parameter>event</parameter>
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parameter. If enabled, the event loop will exit with a zero exit code
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there are no more enabled (<constant>SD_EVENT_ON</constant>, <constant>SD_EVENT_ONESHOT</constant>),
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non-exit, non-post event sources.</para>
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<para><function>sd_event_get_exit_on_idle()</function> may be used to
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determine whether exit-on-idle support was previously requested by a
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call to <function>sd_event_set_exit_on_idle()</function> with a true
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<parameter>b</parameter> parameter and successfully enabled.</para>
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</refsect1>
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<refsect1>
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<title>Return Value</title>
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<para>On success, <function>sd_event_set_exit_on_idle()</function> and
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<function>sd_event_get_exit_on_idle()</function> return a non-zero positive integer if the exit-on-idle
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support was successfully enabled. They return zero if the exit-on-idle support was explicitly disabled
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with a false <parameter>b</parameter> parameter. On failure, they return a negative errno-style error
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code.</para>
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<refsect2>
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<title>Errors</title>
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<para>Returned errors may indicate the following problems:</para>
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<variablelist>
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<varlistentry>
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<term><constant>-ECHILD</constant></term>
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<listitem><para>The event loop has been created in a different process, library or module instance.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><constant>-EINVAL</constant></term>
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<listitem><para>The passed event loop object was invalid.</para></listitem>
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</varlistentry>
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</variablelist>
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</refsect2>
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</refsect1>
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<xi:include href="libsystemd-pkgconfig.xml" />
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<refsect1>
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<title>History</title>
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<para><function>sd_event_set_exit_on_idle()</function> and
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<function>sd_event_get_exit_on_idle()</function> were added in version 259.</para>
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</refsect1>
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<refsect1>
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<title>See Also</title>
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<para><simplelist type="inline">
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<member><citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
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<member><citerefentry><refentrytitle>sd-event</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
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<member><citerefentry><refentrytitle>sd_event_new</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
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<member><citerefentry><refentrytitle>sd_event_add_io</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
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<member><citerefentry><refentrytitle>sd_event_add_time</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
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<member><citerefentry><refentrytitle>sd_event_add_signal</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
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<member><citerefentry><refentrytitle>sd_event_add_child</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
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<member><citerefentry><refentrytitle>sd_event_add_inotify</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
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<member><citerefentry><refentrytitle>sd_event_add_defer</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
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<member><citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry></member>
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</simplelist></para>
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</refsect1>
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</refentry>

‎src/libsystemd/libsystemd.sym‎

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@@ -1081,5 +1081,7 @@ global:
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10821082
LIBSYSTEMD_259 {
10831083
global:
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sd_event_set_exit_on_idle;
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sd_event_get_exit_on_idle;
10841086
sd_varlink_is_connected;
10851087
} LIBSYSTEMD_258;

‎src/libsystemd/sd-event/sd-event.c‎

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Original file line numberDiff line numberDiff line change
@@ -157,6 +157,7 @@ struct sd_event {
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bool need_process_child:1;
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bool watchdog:1;
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bool profile_delays:1;
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bool exit_on_idle:1;
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161162
int exit_code;
162163

@@ -4422,6 +4423,28 @@ static int event_memory_pressure_write_list(sd_event *e) {
44224423
return 0;
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}
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4426+
static bool event_loop_idle(sd_event *e) {
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assert(e);
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LIST_FOREACH(sources, s, e->sources) {
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/* Exit sources only trigger on exit, so whether they're enabled or not doesn't matter when
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* we're deciding if the event loop is idle or not. */
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if (s->type == SOURCE_EXIT)
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continue;
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if (s->enabled == SD_EVENT_OFF)
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continue;
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/* Post event sources always need another active event source to become pending. */
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if (s->type == SOURCE_POST && !s->pending)
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continue;
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return false;
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}
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return true;
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}
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44254448
_public_ int sd_event_prepare(sd_event *e) {
44264449
int r;
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@@ -4439,6 +4462,9 @@ _public_ int sd_event_prepare(sd_event *e) {
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/* Make sure that none of the preparation callbacks ends up freeing the event source under our feet */
44404463
PROTECT_EVENT(e);
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4465+
if (!e->exit_requested && e->exit_on_idle && event_loop_idle(e))
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(void) sd_event_exit(e, 0);
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44424468
if (e->exit_requested)
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goto pending;
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@@ -5243,6 +5269,22 @@ _public_ int sd_event_set_signal_exit(sd_event *e, int b) {
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return change;
52445270
}
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5272+
_public_ int sd_event_set_exit_on_idle(sd_event *e, int b) {
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assert_return(e, -EINVAL);
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assert_return(e = event_resolve(e), -ENOPKG);
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assert_return(!event_origin_changed(e), -ECHILD);
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return e->exit_on_idle = b;
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}
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_public_ int sd_event_get_exit_on_idle(sd_event *e) {
5281+
assert_return(e, -EINVAL);
5282+
assert_return(e = event_resolve(e), -ENOPKG);
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assert_return(!event_origin_changed(e), -ECHILD);
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5285+
return e->exit_on_idle;
5286+
}
5287+
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_public_ int sd_event_source_set_memory_pressure_type(sd_event_source *s, const char *ty) {
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_cleanup_free_ char *b = NULL;
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_cleanup_free_ void *w = NULL;

‎src/libsystemd/sd-event/test-event.c‎

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@@ -1065,4 +1065,70 @@ TEST(child_pidfd_wnowait) {
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ASSERT_EQ(si.si_status, 42);
10661066
}
10671067

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static int exit_on_idle_defer_handler(sd_event_source *s, void *userdata) {
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unsigned *c = ASSERT_PTR(userdata);
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/* Should not be reached on third call because the event loop should exit before */
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ASSERT_LT(*c, 2u);
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(*c)++;
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/* Disable ourselves, which should trigger exit-on-idle after the second iteration */
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if (*c == 2)
1078+
sd_event_source_set_enabled(s, SD_EVENT_OFF);
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1080+
return 0;
1081+
}
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static int exit_on_idle_post_handler(sd_event_source *s, void *userdata) {
1084+
unsigned *c = ASSERT_PTR(userdata);
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1086+
/* Should not be reached on third call because the event loop should exit before */
1087+
ASSERT_LT(*c, 2u);
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1089+
(*c)++;
1090+
return 0;
1091+
}
1092+
1093+
static int exit_on_idle_exit_handler(sd_event_source *s, void *userdata) {
1094+
return 0;
1095+
}
1096+
1097+
TEST(exit_on_idle) {
1098+
_cleanup_(sd_event_unrefp) sd_event *e = NULL;
1099+
ASSERT_OK(sd_event_new(&e));
1100+
ASSERT_OK(sd_event_set_exit_on_idle(e, true));
1101+
ASSERT_OK_POSITIVE(sd_event_get_exit_on_idle(e));
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1103+
/* Create a recurring defer event source. */
1104+
_cleanup_(sd_event_source_unrefp) sd_event_source *d = NULL;
1105+
unsigned dc = 0;
1106+
ASSERT_OK(sd_event_add_defer(e, &d, exit_on_idle_defer_handler, &dc));
1107+
ASSERT_OK(sd_event_source_set_enabled(d, SD_EVENT_ON));
1108+
1109+
/* This post event source should not keep the event loop running after the defer source is disabled. */
1110+
_cleanup_(sd_event_source_unrefp) sd_event_source *p = NULL;
1111+
unsigned pc = 0;
1112+
ASSERT_OK(sd_event_add_post(e, &p, exit_on_idle_post_handler, &pc));
1113+
ASSERT_OK(sd_event_source_set_enabled(p, SD_EVENT_ON));
1114+
ASSERT_OK(sd_event_source_set_priority(p, SD_EVENT_PRIORITY_IMPORTANT));
1115+
1116+
/* And neither should this exit event source. */
1117+
ASSERT_OK(sd_event_add_exit(e, NULL, exit_on_idle_exit_handler, NULL));
1118+
1119+
/* Run the event loop - it should exit after we disable the event source */
1120+
ASSERT_OK(sd_event_loop(e));
1121+
ASSERT_EQ(dc, 2u);
1122+
ASSERT_EQ(pc, 2u);
1123+
}
1124+
1125+
TEST(exit_on_idle_no_sources) {
1126+
_cleanup_(sd_event_unrefp) sd_event *e = NULL;
1127+
ASSERT_OK(sd_event_new(&e));
1128+
ASSERT_OK(sd_event_set_exit_on_idle(e, true));
1129+
1130+
/* Running loop with no sources should return immediately with success */
1131+
ASSERT_OK(sd_event_loop(e));
1132+
}
1133+
10681134
DEFINE_TEST_MAIN(LOG_DEBUG);

‎src/systemd/sd-event.h‎

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Original file line numberDiff line numberDiff line change
@@ -116,6 +116,8 @@ int sd_event_set_watchdog(sd_event *e, int b);
116116
int sd_event_get_watchdog(sd_event *e);
117117
int sd_event_get_iteration(sd_event *e, uint64_t *ret);
118118
int sd_event_set_signal_exit(sd_event *e, int b);
119+
int sd_event_set_exit_on_idle(sd_event *e, int b);
120+
int sd_event_get_exit_on_idle(sd_event *e);
119121

120122
sd_event_source* sd_event_source_ref(sd_event_source *s);
121123
sd_event_source* sd_event_source_unref(sd_event_source *s);

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