selfupdate: the staging folder keeps what it needs and no more (R-1368 part 2) — not released

Each good update left its staged binary in <state dir>/selfupdate for ever
(49 files / 740 MB on demo-hp). After the wrapper's commit returned success,
MaybeCommit now removes every staged binary but the running version's.

Nothing is removed while a pending marker exists or cannot be read, at a
plain start, or after a rollback. Only a regular file with the exact name
Execute writes is removed. The rollback reads pending.json and
felhom-agent.prev, no staged binary: pinned by a test on the wrapper's text
and by a test that runs the wrapper's real commit and rollback verbs in a
sandbox. Red seen three times.

No version bump, no build, no vouch. /usr/local/bin/felhom-agent.bak-* is
not reached (root-owned; R-1368 part 4).

Claude-Session: https://claude.ai/code/session_0159rPz1ZhFKsS53msqPYxtS

internal/selfupdate/commit.go

@@ -128,4 +128,11 @@ func (m *Manager) MaybeCommit(ctx context.Context) {
 	if m.onCommitted != nil {
 		m.onCommitted(ctx)
 	}
+	// R-1368 part 2: the update ended healthy — the folder keeps the running version's binary and no other
+	// (prune.go). A refusal or a failed removal is logged and changes nothing about the commit.
+	if removed, err := pruneStaged(m.stateDir, m.runningVersion, m.logger); err != nil {
+		m.logger.Warn("selfupdate: old staged binaries NOT removed after the commit", "err", err)
+	} else if len(removed) > 0 {
+		m.logger.Info("selfupdate: old staged binaries removed after the commit", "count", len(removed), "kept", "felhom-agent-"+m.runningVersion)
+	}
 }

internal/selfupdate/executor.go

@@ -103,6 +103,9 @@ func (e *Executor) Execute(ctx context.Context, op string, params json.RawMessag
 	}
 	e.logger.Debug("agent_update: op invariants passed (semver + sha format)", "version", p.Version)
 
+	// The staged file is this call's from the download to the hand-off: a prune (prune.go) waits for it.
+	stagingMu.Lock()
+	defer stagingMu.Unlock()
 	dir := stagingDir(e.stateDir)
 	if err := os.MkdirAll(dir, 0o750); err != nil {
 		return fmt.Errorf("agent_update: staging dir: %w", err)

internal/selfupdate/prune.go

@@ -0,0 +1,96 @@
+package selfupdate
+
+import (
+	"fmt"
+	"log/slog"
+	"os"
+	"path/filepath"
+	"regexp"
+	"sort"
+	"sync"
+)
+
+// The self-update folder keeps what it needs and no more (R-1368 part 2; space list 2026-11-10, place H12).
+//
+// WHAT WAS WRONG. Execute downloads each new agent to <StateDir>/selfupdate/felhom-agent-<version> and removes that
+// file only when the download or its checksum fails. A good update left it there for ever: measured 2026-11-10, 49 /
+// 48 / 33 binaries = 740 / 724 / 509 MB on demo-hp, demo-felhom and Tester 1 — one for each version the box ever got.
+//
+// WHAT THE FOLDER IS NEEDED FOR. The root wrapper (felhom-os-apply, mode agent_update) reads the staged file ONCE,
+// copies it to its own root-owned folder and flips from that copy. After that hand-off nothing reads the staged file:
+//   - the rollback (felhom-selfupdate-guarded `rollback`) needs two files and neither is a staged binary:
+//     `pending.json` in this folder, and `/usr/local/bin/felhom-agent.prev` outside it;
+//   - the commit needs `pending.json` only.
+//
+// So a staged binary is dead weight as soon as its update ended. Pinned by TestPrune_RollbackReadsNoStagedBinary (the
+// wrapper's text) and TestPrune_RealRollbackStillWorksAfterNUpdates (the wrapper's rollback verb, run for real).
+//
+// THE RULE. After an update ended HEALTHY (the wrapper's `commit` returned success), remove every staged binary but
+// the one of the running version. Nothing is removed:
+//   - while a pending marker exists or cannot be read — an update is in flight, and that is the only time a rollback
+//     can fire;
+//   - after a rollback, or at a plain start — only the commit path calls this, so the binary of a version that failed
+//     stays for the operator to look at until the next good update;
+//   - that is not a regular file with exactly the name Execute gives a staged binary. `pending.json`, a folder, a
+//     symlink, and any name this code does not know are never touched.
+//
+// NOT REACHED BY THIS: `/usr/local/bin/felhom-agent.bak-*` (space list H13). Those files are root-owned, in a folder
+// the agent cannot write, and no code of the agent makes them (a person's copy before a hand deploy). R-1368 part 4.
+
+// stagedNameRe is the exact name Execute gives a staged binary: "felhom-agent-" + a version that versionRe accepts.
+// Kept in step with versionRe by TestPrune_NameShapeFollowsTheVersionRule.
+var stagedNameRe = regexp.MustCompile(`^felhom-agent-([0-9]+\.[0-9]+\.[0-9]+(-[0-9A-Za-z.]+)?)$`)
+
+// stagingMu makes a download and a prune take turns. Without it a prune that runs while Execute downloads the NEXT
+// version would remove that half-written file (its name is a staged binary's and it is not the running version).
+var stagingMu sync.Mutex
+
+// pruneStaged removes the staged binaries the folder no longer needs and returns the names it removed, sorted. It
+// refuses (removes nothing, returns an error) while an update is in flight. A file it cannot remove is logged and
+// skipped: a failed clean-up must never fail the commit that came before it.
+func pruneStaged(stateDir, runningVersion string, logger *slog.Logger) ([]string, error) {
+	logger = orDefaultLogger(logger)
+	stagingMu.Lock()
+	defer stagingMu.Unlock()
+
+	if !versionRe.MatchString(runningVersion) {
+		return nil, fmt.Errorf("selfupdate prune: refusing — running version %q is not a version this code knows; it could not name the file to keep", runningVersion)
+	}
+	pm, err := readPending(stateDir)
+	if err != nil {
+		return nil, fmt.Errorf("selfupdate prune: refusing — the pending marker cannot be read: %w", err)
+	}
+	if pm != nil {
+		return nil, fmt.Errorf("selfupdate prune: refusing — an update to %s is pending (a rollback may still fire)", pm.NewVersion)
+	}
+	dir := stagingDir(stateDir)
+	entries, err := os.ReadDir(dir)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return nil, nil
+		}
+		return nil, fmt.Errorf("selfupdate prune: read %s: %w", dir, err)
+	}
+	keep := "felhom-agent-" + runningVersion
+	var removed []string
+	for _, ent := range entries {
+		name := ent.Name()
+		if name == keep || !stagedNameRe.MatchString(name) {
+			continue // the running version's binary, pending.json, or a name this code does not know
+		}
+		if !ent.Type().IsRegular() {
+			logger.Warn("selfupdate prune: a staged-binary name that is not a regular file — left alone", "name", name, "type", ent.Type().String())
+			continue
+		}
+		// name came from ReadDir and matched stagedNameRe, so it holds no path separator: this is one exact file.
+		if err := os.Remove(filepath.Join(dir, name)); err != nil {
+			logger.Warn("selfupdate prune: cannot remove an old staged binary — left for the next good update", "name", name, "err", err)
+			continue
+		}
+		logger.Debug("selfupdate prune: removed an old staged binary", "name", name)
+		removed = append(removed, name)
+	}
+	sort.Strings(removed)
+	logger.Info("selfupdate prune: the self-update folder holds what it needs", "kept", keep, "removed", len(removed))
+	return removed, nil
+}

internal/selfupdate/prune_test.go

@@ -0,0 +1,373 @@
+package selfupdate
+
+import (
+	"bytes"
+	"context"
+	"fmt"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"regexp"
+	"sort"
+	"strings"
+	"testing"
+)
+
+// R-1368 part 2 — the self-update folder keeps what it needs and no more (prune.go).
+//
+// RED-PROOFS (each observed 2026-11-10, then restored):
+//   - drop the pruneStaged call from MaybeCommit → TestPrune_AfterNUpdatesTheFolderHoldsOneBinary fails with
+//     "after update 1 (0.170.0) the folder holds [… felhom-agent-0.169.0 felhom-agent-0.170.0 …]", and
+//     TestPrune_RealRollbackStillWorksAfterNUpdates fails too;
+//   - drop the pending-marker refusal from pruneStaged → TestPrune_RefusesWhileAnUpdateIsPending fails with
+//     "removed [...] while an update was pending";
+//   - drop the IsRegular check → TestPrune_AfterNUpdatesTheFolderHoldsOneBinary fails with "a symlink with a
+//     staged-binary name was removed".
+
+// committingWrapper is the guarded wrapper's `commit` as far as this package can see it: it clears the pending marker.
+type committingWrapper struct {
+	stateDir string
+	calls    int
+}
+
+func (w *committingWrapper) Run(_ context.Context, _ string, args ...string) ([]byte, []byte, error) {
+	if len(args) == 1 && args[0] == "commit" {
+		w.calls++
+		if err := os.Remove(pendingPath(w.stateDir)); err != nil {
+			return nil, []byte(err.Error()), err
+		}
+	}
+	return []byte("ok"), nil, nil
+}
+
+func stage(t *testing.T, stateDir, version string) {
+	t.Helper()
+	if err := os.MkdirAll(stagingDir(stateDir), 0o750); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(filepath.Join(stagingDir(stateDir), "felhom-agent-"+version), []byte("binary "+version), 0o755); err != nil {
+		t.Fatal(err)
+	}
+}
+
+func folder(t *testing.T, stateDir string) []string {
+	t.Helper()
+	ents, err := os.ReadDir(stagingDir(stateDir))
+	if err != nil {
+		t.Fatal(err)
+	}
+	var out []string
+	for _, e := range ents {
+		out = append(out, e.Name())
+	}
+	sort.Strings(out)
+	return out
+}
+
+// The consequence: after N good updates the folder holds ONE staged binary — the running version's — and every file
+// that is not a staged binary is still there.
+func TestPrune_AfterNUpdatesTheFolderHoldsOneBinary(t *testing.T) {
+	stateDir := t.TempDir()
+	dir := stagingDir(stateDir)
+	if err := os.MkdirAll(dir, 0o750); err != nil {
+		t.Fatal(err)
+	}
+	// Bystanders: names this code does not know, and two things that carry a staged binary's NAME but are not files.
+	outside := filepath.Join(t.TempDir(), "precious")
+	if err := os.WriteFile(outside, []byte("not ours"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	for _, n := range []string{"notes.txt", "felhom-agent-0.1.0.partial", "felhom-agent.bak-0.139.0"} {
+		if err := os.WriteFile(filepath.Join(dir, n), []byte("x"), 0o644); err != nil {
+			t.Fatal(err)
+		}
+	}
+	if err := os.Mkdir(filepath.Join(dir, "felhom-agent-9.9.9"), 0o750); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.Symlink(outside, filepath.Join(dir, "felhom-agent-8.8.8")); err != nil {
+		t.Fatal(err)
+	}
+	bystanders := []string{"felhom-agent-0.1.0.partial", "felhom-agent-8.8.8", "felhom-agent-9.9.9", "felhom-agent.bak-0.139.0", "notes.txt"}
+
+	versions := []string{"0.170.0", "0.171.0", "0.172.0", "0.172.1", "0.173.0-rc.1", "0.173.0"}
+	prev := "0.169.0"
+	stage(t, stateDir, prev) // what an old agent left behind
+	for i, v := range versions {
+		stage(t, stateDir, v)                                                                                   // Execute downloaded it
+		writePending(t, stateDir, PendingMarker{OldVersion: prev, NewVersion: v, SHA256: "ab", AppliedAt: "t"}) // the wrapper flipped
+		wrap := &committingWrapper{stateDir: stateDir}
+		m := newMgr(t, stateDir, v, wrap, nil)
+		m.MaybeCommit(context.Background())
+		if wrap.calls != 1 {
+			t.Fatalf("update %d (%s): commit calls = %d, want 1", i+1, v, wrap.calls)
+		}
+		want := append([]string{"felhom-agent-" + v}, bystanders...)
+		sort.Strings(want)
+		got := folder(t, stateDir)
+		if strings.Join(got, " ") != strings.Join(want, " ") {
+			if _, err := os.Lstat(filepath.Join(dir, "felhom-agent-8.8.8")); err != nil {
+				t.Fatalf("a symlink with a staged-binary name was removed: %v", err)
+			}
+			t.Fatalf("after update %d (%s) the folder holds %v, want %v", i+1, v, got, want)
+		}
+		prev = v
+	}
+	if b, err := os.ReadFile(outside); err != nil || string(b) != "not ours" {
+		t.Fatalf("the file behind the symlink changed: %q %v", b, err)
+	}
+}
+
+// While an update is pending — the only time a rollback can fire — nothing is removed.
+func TestPrune_RefusesWhileAnUpdateIsPending(t *testing.T) {
+	stateDir := t.TempDir()
+	for _, v := range []string{"0.170.0", "0.171.0", "0.172.0"} {
+		stage(t, stateDir, v)
+	}
+	writePending(t, stateDir, PendingMarker{OldVersion: "0.171.0", NewVersion: "0.172.0", SHA256: "ab", AppliedAt: "t"})
+	before := folder(t, stateDir)
+
+	removed, err := pruneStaged(stateDir, "0.172.0", nil)
+	if err == nil || len(removed) != 0 {
+		t.Fatalf("removed %v while an update was pending (err=%v) — want a refusal", removed, err)
+	}
+	if got := folder(t, stateDir); strings.Join(got, " ") != strings.Join(before, " ") {
+		t.Fatalf("removed %v while an update was pending; folder now %v", before, got)
+	}
+
+	// The same through the commit path: a `commit` that reports success but leaves the marker (fakeWrapper does) must
+	// not lead to a prune.
+	m := newMgr(t, stateDir, "0.172.0", &fakeWrapper{}, nil)
+	m.MaybeCommit(context.Background())
+	if got := folder(t, stateDir); strings.Join(got, " ") != strings.Join(before, " ") {
+		t.Fatalf("the commit path pruned with the marker still there; folder now %v", got)
+	}
+
+	// A marker nobody can read is a refusal too.
+	if err := os.WriteFile(pendingPath(stateDir), []byte("{not json"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	if removed, err := pruneStaged(stateDir, "0.172.0", nil); err == nil || len(removed) != 0 {
+		t.Fatalf("an unreadable marker must refuse; removed %v err=%v", removed, err)
+	}
+}
+
+// Only a commit prunes: a plain start, a version mismatch and a failed commit remove nothing — so the binary of a
+// version that was rolled back stays for the operator.
+func TestPrune_OnlyAHealthyCommitPrunes(t *testing.T) {
+	cases := []struct {
+		name    string
+		pending *PendingMarker
+		running string
+		wrap    WrapperRunner
+	}{
+		{"plain start, no marker (also the state after a rollback)", nil, "0.171.0", &fakeWrapper{}},
+		{"marker names another version", &PendingMarker{OldVersion: "0.171.0", NewVersion: "0.172.0"}, "0.171.0", &fakeWrapper{}},
+		{"commit fails", &PendingMarker{OldVersion: "0.171.0", NewVersion: "0.172.0"}, "0.172.0", &fakeWrapper{err: fmt.Errorf("sudo: no")}},
+	}
+	for _, c := range cases {
+		t.Run(c.name, func(t *testing.T) {
+			stateDir := t.TempDir()
+			for _, v := range []string{"0.170.0", "0.171.0", "0.172.0"} {
+				stage(t, stateDir, v)
+			}
+			if c.pending != nil {
+				writePending(t, stateDir, *c.pending)
+			}
+			before := folder(t, stateDir)
+			newMgr(t, stateDir, c.running, c.wrap, nil).MaybeCommit(context.Background())
+			if got := folder(t, stateDir); strings.Join(got, " ") != strings.Join(before, " ") {
+				t.Fatalf("folder changed without a healthy commit: %v → %v", before, got)
+			}
+		})
+	}
+	// A running version this code cannot name a file for: refuse, do not guess.
+	stateDir := t.TempDir()
+	stage(t, stateDir, "0.170.0")
+	if removed, err := pruneStaged(stateDir, "dev", nil); err == nil || len(removed) != 0 {
+		t.Fatalf("running version \"dev\" must refuse; removed %v err=%v", removed, err)
+	}
+}
+
+// The name rule and the version rule say the same thing: every name Execute can write is a name the prune knows, and
+// nothing else is.
+func TestPrune_NameShapeFollowsTheVersionRule(t *testing.T) {
+	for _, v := range []string{"0.70.1", "0.172.1", "10.0.0", "0.70.2-crash", "0.173.0-rc.1"} {
+		if !versionRe.MatchString(v) {
+			t.Fatalf("test input %q is not a version versionRe accepts", v)
+		}
+		if !stagedNameRe.MatchString("felhom-agent-" + v) {
+			t.Errorf("Execute can stage felhom-agent-%s but the prune does not know that name", v)
+		}
+	}
+	for _, n := range []string{"pending.json", "felhom-agent", "felhom-agent-", "felhom-agent-1.2", "felhom-agent-0.1.0.partial",
+		"felhom-agent.bak-0.139.0", "felhom-agent.prev", "felhom-agent-0.1.0/x", "../felhom-agent-0.1.0", "felhom-agent-0.1.0 ", "xfelhom-agent-0.1.0"} {
+		if stagedNameRe.MatchString(n) {
+			t.Errorf("the prune would treat %q as a staged binary", n)
+		}
+	}
+	m := stagedNameRe.FindStringSubmatch("felhom-agent-0.173.0-rc.1")
+	if m == nil || !versionRe.MatchString(m[1]) {
+		t.Fatalf("the version part of a staged name must satisfy versionRe; got %v", m)
+	}
+}
+
+func wrapperText(t *testing.T) string {
+	t.Helper()
+	b, err := os.ReadFile(filepath.Join("..", "..", "configs", "felhom-selfupdate-guarded"))
+	if err != nil {
+		t.Fatalf("the guarded wrapper is the other half of this proof and must be readable: %v", err)
+	}
+	return string(b)
+}
+
+// The stated dependency, pinned: the wrapper's `rollback` and `commit` read the pending marker and `.prev`, and no
+// staged binary. If either verb starts to read the staging folder, this fails and prune.go's rule must be looked at.
+func TestPrune_RollbackReadsNoStagedBinary(t *testing.T) {
+	s := wrapperText(t)
+	for _, want := range []string{"\nBIN=/usr/local/bin/felhom-agent\n", "\nPREV=$BIN.prev\n", "\nSTAGING=/var/lib/felhom-agent/selfupdate\n", "\nPENDING=$STAGING/pending.json\n"} {
+		if !strings.Contains(s, want) {
+			t.Fatalf("the wrapper no longer holds the line %q — the folder's rule (prune.go) was written against it", strings.TrimSpace(want))
+		}
+	}
+	for _, verb := range []string{"commit", "rollback"} {
+		i := strings.Index(s, "\n"+verb+")\n")
+		if i < 0 {
+			t.Fatalf("no %s) block in the wrapper", verb)
+		}
+		block := s[i:]
+		j := strings.Index(block, "\n    ;;\n")
+		if j < 0 {
+			t.Fatalf("no end of the %s) block", verb)
+		}
+		block = block[:j]
+		if !strings.Contains(block, "$PENDING") {
+			t.Fatalf("%s) does not read $PENDING any more", verb)
+		}
+		code := regexp.MustCompile(`(?m)^\s*#.*$`).ReplaceAllString(block, "")
+		for _, bad := range []string{"STAGING", "staged", "felhom-agent-"} {
+			if strings.Contains(code, bad) {
+				t.Fatalf("%s) now mentions %q — it may read a staged binary, and the prune removes those", verb, bad)
+			}
+		}
+	}
+	if stagedNameRe.MatchString("pending.json") || stagedNameRe.MatchString("felhom-agent.prev") {
+		t.Fatal("the prune would remove a file the rollback needs")
+	}
+}
+
+// The consequence, with the wrapper's own code: after N good updates (each committed by the REAL `commit` verb and
+// pruned), update N+1 goes bad and the REAL `rollback` verb puts the previous binary back. The wrapper runs from a copy
+// whose four fixed paths point into a temp folder; systemctl, logger and chown are stubs.
+func TestPrune_RealRollbackStillWorksAfterNUpdates(t *testing.T) {
+	sh, err := exec.LookPath("sh")
+	if err != nil {
+		t.Skip("no sh on this machine")
+	}
+	root := t.TempDir()
+	stateDir := filepath.Join(root, "state")
+	binDir := filepath.Join(root, "bin")
+	stubs := filepath.Join(root, "stubs")
+	for _, d := range []string{stagingDir(stateDir), binDir, stubs, filepath.Join(root, "rootstaging")} {
+		if err := os.MkdirAll(d, 0o750); err != nil {
+			t.Fatal(err)
+		}
+	}
+	live := filepath.Join(binDir, "felhom-agent")
+	s := wrapperText(t)
+	for old, repl := range map[string]string{
+		"\nBIN=/usr/local/bin/felhom-agent\n":                    "\nBIN=" + live + "\n",
+		"\nSTAGING=/var/lib/felhom-agent/selfupdate\n":           "\nSTAGING=" + stagingDir(stateDir) + "\n",
+		"\nROOT_STAGING=/var/lib/felhom-os-apply/agent-update\n": "\nROOT_STAGING=" + filepath.Join(root, "rootstaging") + "\n",
+	} {
+		if strings.Count(s, old) != 1 {
+			t.Fatalf("the wrapper's path line %q is not there exactly once — this test cannot sandbox it", strings.TrimSpace(old))
+		}
+		s = strings.Replace(s, old, repl, 1)
+	}
+	// Safety: no assignment in the copy may still point at a real system path (PREV and PENDING are built from BIN and
+	// STAGING, so they follow).
+	for _, line := range strings.Split(s, "\n") {
+		if regexp.MustCompile(`^[A-Z_]+=/`).MatchString(line) && !strings.Contains(line, root) {
+			t.Fatalf("the sandboxed wrapper still holds a real path: %q", line)
+		}
+	}
+	script := filepath.Join(root, "wrapper.sh")
+	if err := os.WriteFile(script, []byte(s), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	stubLog := filepath.Join(root, "stub.log")
+	for _, n := range []string{"systemctl", "logger", "chown", "systemd-run"} {
+		body := "#!/bin/sh\necho \"" + n + " $*\" >> '" + stubLog + "'\nexit 0\n"
+		if err := os.WriteFile(filepath.Join(stubs, n), []byte(body), 0o755); err != nil {
+			t.Fatal(err)
+		}
+	}
+	run := func(verb string) (string, error) {
+		cmd := exec.Command(sh, script, verb)
+		cmd.Env = []string{"PATH=" + stubs + ":/usr/bin:/bin"}
+		out, err := cmd.CombinedOutput()
+		return string(out), err
+	}
+	// applied is what the wrapper's `apply` leaves behind (it needs root, so the test writes the same four things).
+	applied := func(oldV, newV string) {
+		cur, err := os.ReadFile(live)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if err := os.WriteFile(live+".prev", cur, 0o755); err != nil {
+			t.Fatal(err)
+		}
+		if err := os.WriteFile(live, []byte("binary "+newV), 0o755); err != nil {
+			t.Fatal(err)
+		}
+		writePending(t, stateDir, PendingMarker{OldVersion: oldV, NewVersion: newV, SHA256: "ab", AppliedAt: "t"})
+	}
+	if err := os.WriteFile(live, []byte("binary 0.169.0"), 0o755); err != nil {
+		t.Fatal(err)
+	}
+
+	prev := "0.169.0"
+	for _, v := range []string{"0.170.0", "0.171.0", "0.172.0", "0.172.1"} {
+		stage(t, stateDir, v)
+		applied(prev, v)
+		m := newMgr(t, stateDir, v, runnerFunc(func(args ...string) ([]byte, []byte, error) {
+			out, err := run(args[0])
+			return []byte(out), nil, err
+		}), nil)
+		m.MaybeCommit(context.Background()) // the REAL commit verb, then the prune
+		if got := folder(t, stateDir); len(got) != 1 || got[0] != "felhom-agent-"+v {
+			t.Fatalf("after the good update to %s the folder holds %v", v, got)
+		}
+		prev = v
+	}
+
+	// Update N+1 is applied and goes bad: systemd's OnFailure runs the wrapper's rollback.
+	stage(t, stateDir, "0.173.0")
+	applied("0.172.1", "0.173.0")
+	out, err := run("rollback")
+	if err != nil {
+		t.Fatalf("the real rollback verb failed after the prunes: %v\n%s", err, out)
+	}
+	if b, _ := os.ReadFile(live); string(b) != "binary 0.172.1" {
+		t.Fatalf("after the rollback the live binary is %q, want the previous version's", b)
+	}
+	if p, _ := readPending(stateDir); p != nil {
+		t.Fatal("the rollback left the pending marker")
+	}
+	if b, _ := os.ReadFile(stubLog); !bytes.Contains(b, []byte("systemctl restart felhom-agent.service")) {
+		t.Fatalf("the rollback did not restart the agent; stub log:\n%s", b)
+	}
+	// The rolled-back agent starts: no marker, so no commit and no prune — the bad version's binary stays to be read.
+	newMgr(t, stateDir, "0.172.1", &fakeWrapper{}, nil).MaybeCommit(context.Background())
+	if got := folder(t, stateDir); strings.Join(got, " ") != "felhom-agent-0.172.1 felhom-agent-0.173.0" {
+		t.Fatalf("after the rollback the folder holds %v, want the running version and the one that failed", got)
+	}
+}
+
+// runnerFunc adapts a function to WrapperRunner; it gets the verb arguments only.
+type runnerFunc func(args ...string) ([]byte, []byte, error)
+
+func (f runnerFunc) Run(_ context.Context, _ string, args ...string) ([]byte, []byte, error) {
+	return f(args...)
+}