portable: add PORTABLE_NAME_AND_VERSION= and other metadata to LogsExtraFields=

This is useful to identify log messages with metadata from the images
they run on. Look for ID/VERSION_ID/IMAGE_ID/IMAGE_VERSION/BUILD_ID,
with a SYSEXT_ prefix if we are looking at an extension, and append via
LogExtraFields= as respectively PORTABLE_NAME_AND_VERSION= in case of a
single image. In case of extensions, append as PORTABLE_ROOT_NAME_AND_VERSION=
for the base and one PORTABLE_EXTENSION_AND_VERSION= for each extension.

Example with a base and two extensions, with the unit coming from the
first extension:

[Service]
RootImage=/home/bluca/git/systemd/base.raw
Environment=PORTABLE=app0.raw
BindReadOnlyPaths=/etc/os-release:/run/host/os-release
LogExtraFields=PORTABLE=app0.raw
Environment=PORTABLE_ROOT=base.raw
LogExtraFields=PORTABLE_ROOT=base.raw
LogExtraFields=PORTABLE_ROOT_NAME_AND_VERSION=debian_10

ExtensionImages=/home/bluca/git/systemd/app0.raw
LogExtraFields=PORTABLE_EXTENSION=app0.raw
LogExtraFields=PORTABLE_EXTENSION_NAME_AND_VERSION=app_0

ExtensionImages=/home/bluca/git/systemd/app1.raw
LogExtraFields=PORTABLE_EXTENSION=app1.raw
LogExtraFields=PORTABLE_EXTENSION_NAME_AND_VERSION=app_1

1 files changed, 20 insertions, 2 deletions

diff --git a/docs/PORTABLE_SERVICES.md b/docs/PORTABLE_SERVICES.md
index 7936eebccb..8d65c9002d 100644
--- a/docs/PORTABLE_SERVICES.md
+++ b/docs/PORTABLE_SERVICES.md
@@ -345,15 +345,33 @@ was loaded from. In case extensions are used, additionally there will be a
 (i.e.: `RootImage=` or `RootDirectory=`), and one `PORTABLE_EXTENSION=` field per
 each extension image used.
 
+The `os-release` file from the portable image will be parsed and added as structured
+metadata to the journal log entries. The parsed fields will be the first ID field which
+is set from the set of `IMAGE_ID` and `ID` in this order of preference, and the first
+version field which is set from a set of `IMAGE_VERSION`, `VERSION_ID`, and `BUILD_ID`
+in this order of preference. The ID and version, if any, are concatenated with an
+underscore (`_`) as separator. If only either one is found, it will be used by itself.
+The field will be named `PORTABLE_NAME_AND_VERSION=`.
+
+In case extensions are used, the same fields in the same order are, but prefixed by
+`SYSEXT_`, are parsed from each `extension-release` file, and are appended to the
+journal as log entries, using `PORTABLE_EXTENSION_NAME_AND_VERSION=` as the field
+name. The base layer's field will be named `PORTABLE_ROOT_NAME_AND_VERSION=` instead
+of `PORTABLE_NAME_AND_VERSION=` in this case.
+
 For example, a portable service `app0` using two extensions `app0.raw` and
-`app1.raw`, and a base layer `base.raw`, will create log entries with the
-following fields:
+`app1.raw` (with `SYSEXT_ID=app`, and `SYSEXT_VERSION_ID=` `0` and `1` in their
+respective extension-releases), and a base layer `base.raw` (with `VERSION_ID=10` and
+`ID=debian` in `os-release`), will create log entries with the following fields:
 
 ```
 PORTABLE=app0.raw
 PORTABLE_ROOT=base.raw
+PORTABLE_ROOT_NAME_AND_VERSION=debian_10
 PORTABLE_EXTENSION=app0.raw
+PORTABLE_EXTENSION_NAME_AND_VERSION=app_0
 PORTABLE_EXTENSION=app1.raw
+PORTABLE_EXTENSION_NAME_AND_VERSION=app_1
 ```
 
 ## Links