Support /etc/system-update for OSTree systems

This is required when / is immutable and cannot be written at runtime.

Co-authored-by: Richard Hughes <richard@hughsie.com>

man/systemd-system-update-generator.xml

@@ -30,7 +30,7 @@
     <para><filename>systemd-system-update-generator</filename> is a
     generator that automatically redirects the boot process to
     <filename>system-update.target</filename>, if
-    <filename>/system-update</filename> exists. This is required to
+    <filename>/system-update</filename> or <filename>/etc/system-update</filename> exists. This is required to
     implement the logic explained in the
     <citerefentry><refentrytitle>systemd.offline-updates</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
     </para>

man/systemd.offline-updates.xml

@@ -40,18 +40,20 @@
       </listitem>
 
       <listitem>
-        <para>When the user OK'ed the update, the symlink <filename>/system-update</filename> is
+        <para>When the user OK'ed the update, the symlink <filename>/system-update</filename> or
-        created that points to <filename index="false">/var/lib/system-update</filename> (or
+        <filename>/etc/system-update</filename> is created that points to
-        wherever the directory with the upgrade files is located) and the system is rebooted. This
+        <filename index="false">/var/lib/system-update</filename> (or wherever the directory with
-        symlink is in the root directory, since we need to check for it very early at boot, at a
+        the upgrade files is located) and the system is rebooted. This symlink is in the root
-        time where <filename>/var/</filename> is not available yet.</para>
+        directory, since we need to check for it very early at boot, at a time where
+        <filename>/var/</filename> is not available yet.</para>
       </listitem>
 
       <listitem>
         <para>Very early in the new boot
         <citerefentry><refentrytitle>systemd-system-update-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
-        checks whether <filename>/system-update</filename> exists. If so, it (temporarily and for
+        checks whether <filename>/system-update</filename> or
-        this boot only) redirects (i.e. symlinks) <filename>default.target</filename> to
+        <filename>/etc/system-update</filename> exists. If so, it (temporarily and for this boot
+        only) redirects (i.e. symlinks) <filename>default.target</filename> to
         <filename>system-update.target</filename>, a special target that pulls in the base system
         (i.e. <filename>sysinit.target</filename>, so that all file systems are mounted but little
         else) and the system update units.</para>
@@ -68,12 +70,13 @@
 
       <listitem>
         <para>As the first step, an update service should check if the
-        <filename>/system-update</filename> symlink points to the location used by that update
+        <filename>/system-update</filename> or <filename>/etc/system-update</filename> symlink
-        service. In case it does not exist or points to a different location, the service must exit
+        points to the location used by that update service. In case it does not exist or points to a
-        without error. It is possible for multiple update services to be installed, and for multiple
+        different location, the service must exit without error. It is possible for multiple update
-        update services to be launched in parallel, and only the one that corresponds to the tool
+        services to be installed, and for multiple update services to be launched in parallel, and
-        that <emphasis>created</emphasis> the symlink before reboot should perform any actions. It
+        only the one that corresponds to the tool that <emphasis>created</emphasis> the symlink
-        is unsafe to run multiple updates in parallel.</para>
+        before reboot should perform any actions. It is unsafe to run multiple updates in
+        parallel.</para>
       </listitem>
 
       <listitem>
@@ -88,14 +91,16 @@
         <para>The update scripts should exit only after the update is finished. It is expected
         that the service which performs the update will cause the machine to reboot after it
         is done. If the <filename>system-update.target</filename> is successfully reached, i.e.
-        all update services have run, and the <filename>/system-update</filename> symlink still
+        all update services have run, and the <filename>/system-update</filename> or
-        exists, it will be removed and the machine rebooted as a safety measure.</para>
+        <filename>/etc/system-update</filename> symlink still exists, it will be removed and
+        the machine rebooted as a safety measure.</para>
       </listitem>
 
       <listitem>
-        <para>After a reboot, now that the <filename>/system-update</filename> symlink is gone,
+        <para>After a reboot, now that the <filename>/system-update</filename> and
-        the generator won't redirect <filename>default.target</filename> anymore and the system
+        <filename>/etc/system-update</filename> symlink is gone, the generator won't redirect
-        now boots into the default target again.</para>
+        <filename>default.target</filename> anymore and the system now boots into the default
+        target again.</para>
       </listitem>
     </orderedlist>
   </refsect1>
@@ -115,8 +120,9 @@
       </listitem>
 
       <listitem>
-        <para>Make sure to remove the <filename>/system-update</filename> symlink as early as
+        <para>Make sure to remove the <filename>/system-update</filename> and
-        possible in the update script to avoid reboot loops in case the update fails.</para>
+        <filename>/etc/system-update</filename> symlinks as early as possible in the update
+        script to avoid reboot loops in case the update fails.</para>
       </listitem>
 
       <listitem>

man/systemd.special.xml

@@ -753,8 +753,8 @@
           <listitem>
             <para>A special target unit that is used for offline system updates.
             <citerefentry><refentrytitle>systemd-system-update-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
-            will redirect the boot process to this target if <filename>/system-update</filename>
+            will redirect the boot process to this target if <filename>/system-update</filename> or
-            exists. For more information see
+            <filename>/etc/system-update</filename> exists. For more information see
             <citerefentry><refentrytitle>systemd.offline-updates</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
             </para>
 
@@ -764,9 +764,10 @@
             <filename>system-update-pre.target</filename> but not pull it in. Services which want to
             run during system updates only, but before the actual system update is executed should
             order themselves before this unit and pull it in. As a safety measure, if this does not
-            happen, and <filename>/system-update</filename> still exists after
+            happen, and <filename>/system-update</filename> or
+            <filename>/etc/system-update</filename> still exists after
             <filename>system-update.target</filename> is reached,
-            <filename>system-update-cleanup.service</filename> will remove this symlink and reboot
+            <filename>system-update-cleanup.service</filename> will remove the symlinks and reboot
             the machine.</para>
           </listitem>
         </varlistentry>

src/system-update-generator/system-update-generator.c

@@ -6,6 +6,7 @@
 #include "fs-util.h"
 #include "generator.h"
 #include "log.h"
+#include "path-util.h"
 #include "proc-cmdline.h"
 #include "special.h"
 #include "string-util.h"
@@ -18,19 +19,25 @@
 static const char *arg_dest = NULL;
 
 static int generate_symlink(void) {
-        const char *p = NULL;
+        _cleanup_free_ char *j = NULL;
 
-        if (laccess("/system-update", F_OK) < 0) {
+        FOREACH_STRING(p, "/system-update", "/etc/system-update") {
-                if (errno == ENOENT)
+                if (laccess(p, F_OK) >= 0)
-                        return 0;
+                        goto link_found;
 
-                log_error_errno(errno, "Failed to check for system update: %m");
+                if (errno != ENOENT)
-                return -EINVAL;
+                        log_warning_errno(errno, "Failed to check if %s symlink exists, ignoring: %m", p);
         }
 
-        p = strjoina(arg_dest, "/" SPECIAL_DEFAULT_TARGET);
+        return 0;
-        if (symlink(SYSTEM_DATA_UNIT_DIR "/system-update.target", p) < 0)
+
-                return log_error_errno(errno, "Failed to create symlink %s: %m", p);
+link_found:
+        j = path_join(arg_dest, SPECIAL_DEFAULT_TARGET);
+        if (!j)
+                return log_oom();
+
+        if (symlink(SYSTEM_DATA_UNIT_DIR "/system-update.target", j) < 0)
+                return log_error_errno(errno, "Failed to create symlink %s: %m", j);
 
         return 1;
 }

units/system-update-cleanup.service

@@ -29,7 +29,9 @@ SuccessAction=reboot
 # reboot or some other action on its own.
 ConditionPathExists=|/system-update
 ConditionPathIsSymbolicLink=|/system-update
+ConditionPathExists=|/etc/system-update
+ConditionPathIsSymbolicLink=|/etc/system-update
 
 [Service]
 Type=oneshot
-ExecStart=rm -fv /system-update
+ExecStart=rm -fv /system-update /etc/system-update