docs/AUTOMATIC_BOOT_ASSESSMENT: describe how to integrate a DE viability check

I reordered the component list to match chronological order: we first install
an entry, then boot it, then the checks happen, etc. Before it was
ordered by "importance", but that is harder to follow.

docs/AUTOMATIC_BOOT_ASSESSMENT.md

@@ -20,12 +20,32 @@ other boot loaders or take actions outside of the boot loader.
 
 Here's a brief overview of the complete set of components:
 
+* The
+  [`kernel-install(8)`](https://www.freedesktop.org/software/systemd/man/kernel-install.html)
+  script can optionally create boot loader entries that carry an initial boot
+  counter (the initial counter is configurable in `/etc/kernel/tries`).
+
 * The
   [`systemd-boot(7)`](https://www.freedesktop.org/software/systemd/man/systemd-boot.html)
-  boot loader optionally maintains a per-boot-loader-entry counter that is
+  boot loader optionally maintains a per-boot-loader-entry counter described by
-  decreased by one on each attempt to boot the entry, prioritizing entries that
+  the [Boot Loader Specification](BOOT_LOADER_SPECIFICATION.md#boot-counting)
-  have non-zero counters over those which already reached a counter of zero
+  that is decreased by one on each attempt to boot the entry, prioritizing
-  when choosing the entry to boot.
+  entries that have non-zero counters over those which already reached a
+  counter of zero when choosing the entry to boot.
+
+* The `boot-complete.target` target unit (see
+  [`systemd.special(7)`](https://www.freedesktop.org/software/systemd/man/systemd.special.html))
+  serves as a generic extension point both for units that are necessary to
+  consider a boot successful (e.g. `systemd-boot-check-no-failures.service`
+  described below), and units that want to act only if the boot is
+  successful (e.g. `systemd-bless-boot.service` described below).
+
+* The
+  [`systemd-boot-check-no-failures.service(8)`](https://www.freedesktop.org/software/systemd/man/systemd-boot-check-no-failures.service.html)
+  service is a simple service health check tool. When enabled it becomes an
+  indirect dependency of `systemd-bless-boot.service` (by means of
+  `boot-complete.target`, see below), ensuring that the boot will not be
+  considered successful if there are any failed services.
 
 * The
   [`systemd-bless-boot.service(8)`](https://www.freedesktop.org/software/systemd/man/systemd-bless-boot.service.html)
@@ -38,26 +58,6 @@ Here's a brief overview of the complete set of components:
   generator automatically pulls in `systemd-bless-boot.service` when use of
   `systemd-boot` with boot counting enabled is detected.
 
-* The
-  [`systemd-boot-check-no-failures.service(8)`](https://www.freedesktop.org/software/systemd/man/systemd-boot-check-no-failures.service.html)
-  service is a simple health check tool that determines whether the boot
-  completed successfully. When enabled it becomes an indirect dependency of
-  `systemd-bless-boot.service` (by means of `boot-complete.target`, see
-  below), ensuring that the boot will not be considered successful if there are
-  any failed services.
-
-* The `boot-complete.target` target unit (see
-  [`systemd.special(7)`](https://www.freedesktop.org/software/systemd/man/systemd.special.html))
-  serves as a generic extension point both for units that are necessary to
-  consider a boot successful (example: `systemd-boot-check-no-failures.service`
-  as described above), and units that want to act only if the boot is
-  successful (example: `systemd-bless-boot.service` as described above).
-
-* The
-  [`kernel-install(8)`](https://www.freedesktop.org/software/systemd/man/kernel-install.html)
-  script can optionally create boot loader entries that carry an initial boot
-  counter (the initial counter is configurable in `/etc/kernel/tries`).
-
 ## Details
 
 As described in [Boot Loader Specification](BOOT_LOADER_SPECIFICATION.md#boot-counting),
@@ -132,21 +132,31 @@ scenario the first 4 steps are the same as above:
    that are required to succeed for the boot process to be considered
    successful. One such unit is `systemd-boot-check-no-failures.service`.
 
-9. `systemd-boot-check-no-failures.service` is run after all its own
+9. The graphical desktop environment installed on the machine starts a
+   service called `graphical-session-good.service`, which is also ordered before
+   `boot-complete.target`, that registers a D-Bus endpoint.
+
+10. `systemd-boot-check-no-failures.service` is run after all its own
    dependencies completed, and assesses that the boot completed
    successfully. It hence exits cleanly.
 
-10. This allows `boot-complete.target` to be reached. This signifies to the
+11. `graphical-session-good.service` waits for a user to log in. In the user
+   desktop environment, one minute after the user has logged in and started the
+   first program, a user service is invoked which makes a D-Bus call to
+   `graphical-session-good.service`. Upon receiving that call,
+   `graphical-session-good.service` exits cleanly.
+
+12. This allows `boot-complete.target` to be reached. This signifies to the
     system that this boot attempt shall be considered successful.
 
-11. Which in turn permits `systemd-bless-boot.service` to run. It now
+13. Which in turn permits `systemd-bless-boot.service` to run. It now
     determines which boot loader entry file was used to boot the system, and
     renames it dropping the counter tag. Thus
     `4.14.11-300.fc27.x86_64+1-2.conf` is renamed to
     `4.14.11-300.fc27.x86_64.conf`. From this moment boot counting is turned
     off for this entry.
 
-12. On the following boot (and all subsequent boots after that) the entry is
+14. On the following boot (and all subsequent boots after that) the entry is
     now seen with boot counting turned off, no further renaming takes place.
 
 ## How to adapt this scheme to other setups
@@ -176,6 +186,13 @@ are a couple of recommendations.
    good. Note that the target unit shall pull in these boot checking units, not
    the other way around.
 
+   Depending on the setup, it may be most convenient to pull in such units
+   through normal enablement symlinks, or during early boot using a
+   [`generator`](https://www.freedesktop.org/software/systemd/man/systemd.generator.html),
+   or even during later boot. In the last case, care must be taken to ensure
+   that the start job is created before `boot-complete.target` has been
+   reached.
+
 3. To support additional components that shall only run on boot success, simply
    wrap them in a unit and order them after `boot-complete.target`, pulling it
    in.