core: automatically add udev dependency for units using RootImage=

We use udev to wait for /dev/loopX devices to be fully proped hence we
need an implicit ordering dependency on it, for RootImage= to work
reliably in early boot, too.

Fixes: #14972

man/systemd.exec.xml

@@ -139,6 +139,9 @@
         <varname>PrivateDevices=</varname> below, as it may change the setting of
         <varname>DevicePolicy=</varname>.</para>
 
+        <para>Units making use of <varname>RootImage=</varname> automatically gain an
+        <varname>After=</varname> dependency on <filename>systemd-udevd.service</filename>.</para>
+
         <xi:include href="system-only.xml" xpointer="singular"/></listitem>
       </varlistentry>
 

src/basic/special.h

@@ -82,6 +82,7 @@
 #define SPECIAL_QUOTAON_SERVICE "quotaon.service"
 #define SPECIAL_REMOUNT_FS_SERVICE "systemd-remount-fs.service"
 #define SPECIAL_VOLATILE_ROOT_SERVICE "systemd-volatile-root.service"
+#define SPECIAL_UDEVD_SERVICE "systemd-udevd.service"
 
 /* Services systemd relies on */
 #define SPECIAL_DBUS_SERVICE "dbus.service"

src/core/unit.c

@@ -1079,6 +1079,15 @@ int unit_add_exec_dependencies(Unit *u, ExecContext *c) {
                         return r;
         }
 
+        if (c->root_image) {
+                /* We need to wait for /dev/loopX to appear when doing RootImage=, hence let's add an
+                 * implicit dependency on udev */
+
+                r = unit_add_dependency_by_name(u, UNIT_AFTER, SPECIAL_UDEVD_SERVICE, true, UNIT_DEPENDENCY_FILE);
+                if (r < 0)
+                        return r;
+        }
+
         if (!IN_SET(c->std_output,
                     EXEC_OUTPUT_JOURNAL, EXEC_OUTPUT_JOURNAL_AND_CONSOLE,
                     EXEC_OUTPUT_KMSG, EXEC_OUTPUT_KMSG_AND_CONSOLE,