ADR 0061. Recovery was the most systemd-specific part of the host, and it is
the part that must work on a machine where nothing else does -- which made
unit-file syntax a poor place for it, because syntax cannot be tested and the
one time it runs is the one time nobody can afford it wrong.
So StartLimitBurst and OnFailure move into a launcher script that init starts
instead of the host. The unit drops to start-at-boot and restart-on-exit, which
OpenRC, runit, s6 and an Android init.rc can all express. Everything 0059
decided is kept: two watchdogs, roll back once, recovery is local, the rollback
shares no code with the host.
The counter is the whole mechanism, so it is what the tests are mostly about.
Three real problems came out of writing them:
A counter file holding "1 2" became "12" -- `tr -d [:space:]` concatenates
rather than rejecting -- which is past the limit, so a HEALTHY node rolled
itself back. Now it reads the first field and insists on a plain integer.
The corrupt-counter test used "not-a-number", which shell arithmetic happens to
evaluate to 0, so it passed with the guard removed and proved nothing. Replaced
with values that discriminate: "5x" errors under set -e and kills the launcher,
and "0x10" is read as HEX 16 -- past the limit, so again a healthy node rolls
back.
And the test harness itself was wrong. With `set -e` and a bare launcher call,
removing a guard killed the script at the first corrupt case and silently
skipped everything after -- reporting a full pass over tests that never ran.
Every launcher call now records its failure instead of aborting. Same class as
the placebo assertion found last time, and the reason to keep injecting faults
rather than trusting green.
Both scripts run in `make check`. 27 launcher tests, 9 rollback tests, all
confirmed to bite.
ADR 0059's recovery path: the pieces that run when the host will not start.
internal/upgrade -- two facts, neither of them the host judging its health.
Whether the executable this process started from has been replaced on disk, and
which version last completed a reconcile.
The first design was wrong and the tests caught it, not review. It asked
/proc/self/exe whether it was marked deleted. That is Linux procfs behaviour
rather than a fact about files, and it catches only unlink -- a binary swapped
by rename onto the same path reads as untouched, which is exactly what a
package manager does. Now the identity is captured at start and compared later:
no procfs, and neither case missed.
known-good is one bare line. The reader is a shell script on a machine where
the host is failing to start, so it must not need a parser to be present and
working. Written only after a clean apply, which is the whole claim -- not
health, because a disconnected node is ordinary and a failing resource is the
machine's problem rather than the binary's.
packaging/ -- the unit, the rollback unit, and the rollback script. The script
shares no code with the host and calls none of it: a binary that cannot start
cannot be its own recovery. POSIX sh, nothing that has to be installed. The
unit carries Restart=always with a comment saying why on-failure would break
every upgrade.
Both are tested and both sets of tests were confirmed to bite. Injecting five
faults broke exactly the intended tests -- except one, and chasing why it did
not found a placebo assertion I had written: `check "exits zero" ... "0" "0"`
compares a literal to itself and can never fail. Replaced with the real exit
code, after which the injection bites.
Also caught: an injection that produced a build failure rather than a test
failure, which my grep read as "no failure". Re-run so it compiled, and the
test did bite.
The script test runs in `make check`, so it is a gate rather than something
that was run once.
Verified against the real binary: known-good is written beside the store after
a clean apply and is NOT written after a failed one.
jschoubben