os-release: define /usr/lib/os-release as fallback for /etc/os-release

The file should have been in /usr/lib/ in the first place, since it
describes the OS container in /usr (and not the configuration in /etc),
hence, let's support os-release files in /usr/lib as fallback if no
version in /etc exists, following the usual override logic.

A prior commit already enabled tmpfiles to create /etc/os-release as a
symlink to /usr/lib/os-release should it be missing, thus providing nice
compatibility with applications only checking in /etc.

While it's probably a good idea if all apps check both locations via a
fallback logic, it is only necessary in the early boot process, as long
as the /etc/os-release symlink has not been restored, in case we boot
with an empty /etc.

3 files changed, 44 insertions, 16 deletions

diff --git a/man/kernel-install.xml b/man/kernel-install.xml
index 3612b7d934..178ad1d2bd 100644
--- a/man/kernel-install.xml
+++ b/man/kernel-install.xml
@@ -95,14 +95,20 @@ along with systemd; If not, see <http://www.gnu.org/licenses/>.
 
           <para>The kernel-install plugin <filename>50-depmod.install</filename> runs depmod for the <replaceable>KERNEL-VERSION</replaceable>.</para>
 
-          <para>The kernel-install plugin <filename>90-loaderentry.install</filename> copies
+          <para>The kernel-install plugin
+          <filename>90-loaderentry.install</filename> copies
           <replaceable>KERNEL-IMAGE</replaceable> to
           <filename>/boot/<replaceable>MACHINE-ID</replaceable>/<replaceable>KERNEL-VERSION</replaceable>/linux</filename>.
-          It also creates a boot loader entry according to the boot loader specification in
+          It also creates a boot loader entry according to the boot
+          loader specification in
           <filename>/boot/loader/entries/<replaceable>MACHINE-ID</replaceable>-<replaceable>KERNEL-VERSION</replaceable>.conf</filename>.
-          The title of the entry is the <replaceable>PRETTY_NAME</replaceable> parameter specified in <filename>/etc/os-release</filename>,
-          or "Linux <replaceable>KERNEL-VERSION</replaceable>", if unset.
-          If the file <filename>initrd</filename> is found next to the
+          The title of the entry is the
+          <replaceable>PRETTY_NAME</replaceable> parameter specified
+          in <filename>/etc/os-release</filename> or
+          <filename>/usr/lib/os-release</filename> (if the former is
+          missing), or "Linux
+          <replaceable>KERNEL-VERSION</replaceable>", if unset.  If
+          the file <filename>initrd</filename> is found next to the
           <filename>linux</filename> file, the initrd will be added to
           the configuration.</para>
         </listitem>
@@ -165,6 +171,7 @@ along with systemd; If not, see <http://www.gnu.org/licenses/>.
       <varlistentry>
         <term>
           <filename>/etc/os-release</filename>
+          <filename>/usr/lib/os-release</filename>
         </term>
           <listitem>
             <para>The content of the file specifies the operating system title <replaceable>PRETTY_NAME</replaceable>.</para>
diff --git a/man/os-release.xml b/man/os-release.xml
index c1dd62f715..11443638e5 100644
--- a/man/os-release.xml
+++ b/man/os-release.xml
@@ -49,13 +49,15 @@
 
         <refsynopsisdiv>
                 <para><filename>/etc/os-release</filename></para>
+                <para><filename>/usr/lib/os-release</filename></para>
         </refsynopsisdiv>
 
         <refsect1>
                 <title>Description</title>
 
-                <para>The <filename>/etc/os-release</filename> file
-                contains operating system identification data.</para>
+                <para>The <filename>/etc/os-release</filename> and
+                <filename>/usr/lib/os-release</filename> files contain
+                operating system identification data.</para>
 
                 <para>The basic file format of
                 <filename>os-release</filename> is a newline-separated
@@ -78,20 +80,38 @@
                 strings. Lines beginning with "#" shall be ignored as
                 comments.</para>
 
-                <para><filename>/etc/os-release</filename> contains
-                data that is defined by the operating system vendor
-                and should not be changed by the administrator.</para>
+                <para>The file <filename>/etc/os-release</filename>
+                takes precedence over
+                <filename>/usr/lib/os-release</filename>. Applications
+                should check for the former, and exclusively use its
+                data if it exists, and only fall back to
+                <filename>/usr/lib/os-release</filename> if it is
+                missing. Applications should not read data from both
+                files at the same
+                time. <filename>/usr/lib/os-release</filename> is the
+                recommended place to store OS release information as
+                part of vendor trees. Frequently
+                <filename>/etc/os-release</filename> is simply a
+                symlink to <filename>/usr/lib/os-release</filename>,
+                to provide compatibility with applications only
+                looking at <filename>/etc</filename>.</para>
+
+                <para><filename>os-release</filename> contains data
+                that is defined by the operating system vendor and
+                should generally not be changed by the
+                administrator.</para>
 
                 <para>As this file only encodes names and identifiers
                 it should not be localized.</para>
 
-                <para>The file <filename>/etc/os-release</filename> might
-                be a symlink to another file, but it is important that
+                <para>The <filename>/etc/os-release</filename> and
+                <filename>/usr/lib/os-release</filename> files might
+                be symlinks to other files, but it is important that
                 the file is available from earliest boot on, and hence
                 must be located on the root file system.</para>
 
                 <para>For a longer rationale for
-                <filename>/etc/os-release</filename> please refer to
+                <filename>os-release</filename> please refer to
                 the <ulink
                 url="http://0pointer.de/blog/projects/os-release">Announcement of <filename>/etc/os-release</filename></ulink>.</para>
         </refsect1>
@@ -100,7 +120,7 @@
                 <title>Options</title>
 
                 <para>The following OS identifications parameters may be set using
-                <filename>/etc/os-release</filename>:</para>
+                <filename>os-release</filename>:</para>
 
                 <variablelist>
 
diff --git a/man/systemd-nspawn.xml b/man/systemd-nspawn.xml
index d6687038af..950558feec 100644
--- a/man/systemd-nspawn.xml
+++ b/man/systemd-nspawn.xml
@@ -136,8 +136,9 @@
 
                 <para>As a safety check
                 <command>systemd-nspawn</command> will verify the
-                existence of <filename>/etc/os-release</filename> in
-                the container tree before starting the container (see
+                existence of <filename>/usr/lib/os-release</filename>
+                or <filename>/etc/os-release</filename> in the
+                container tree before starting the container (see
                 <citerefentry><refentrytitle>os-release</refentrytitle><manvolnum>5</manvolnum></citerefentry>). It
                 might be necessary to add this file to the container
                 tree manually if the OS of the container is too old to