agent v0.25.0: slice 10 P2 — bind enrolled user-data drives into the guest

POST /disks/guest-attach binds an enrolled drive's felhom-data namespace into
the guest (Model A: felhom-data is the bind source mounted at /mnt/<name>, so
only Felhom's namespace crosses in). GuestBinder does mkdir+chown(100000)+pct set
(RW bind) via the fenced runner. Idempotent, free-slot selection, path-validated.
Spike-proven on 9201. Pairs with controller P2C + golden /mnt:rslave (P2B).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/localapi/disks_test.go

@@ -372,6 +372,96 @@ func TestEject_RoleGated(t *testing.T) {
 	d4.mu.Unlock()
 }
 
+// ---- guest data-drive passthrough (slice 10 P2) -----------------------------------------
+
+type fakeGuestAttacher struct {
+	mu    sync.Mutex
+	calls []struct {
+		vmid        int
+		slot, where string
+	}
+}
+
+func (f *fakeGuestAttacher) AttachBind(_ context.Context, vmid int, mountKey, where string) error {
+	f.mu.Lock()
+	defer f.mu.Unlock()
+	f.calls = append(f.calls, struct {
+		vmid        int
+		slot, where string
+	}{vmid, mountKey, where})
+	return nil
+}
+func (f *fakeGuestAttacher) count() int { f.mu.Lock(); defer f.mu.Unlock(); return len(f.calls) }
+
+func newAttachServer(t *testing.T, ga GuestAttacher, mounts map[int]map[string]string) http.Handler {
+	t.Helper()
+	srv, err := NewServer(Options{
+		ListenAddr: "127.0.0.1:0", Guests: &fakeGuestsCfg{mounts: mounts}, Backups: &fakeBackups{},
+		Store: &fakeStore{}, Storage: fakeStorage{}, Tokens: staticTokens{"A": 8200, "B": 9300},
+		GuestAttach: ga, Logger: slog.New(slog.NewTextHandler(io.Discard, nil)),
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	srv.baseCtx = context.Background()
+	return srv.Handler()
+}
+
+// A first attach picks the lowest free slot (mp0; mp9 bootstrap is taken) and calls the binder.
+func TestGuestAttach_PicksFreeSlotAndBinds(t *testing.T) {
+	ga := &fakeGuestAttacher{}
+	h := newAttachServer(t, ga, map[int]map[string]string{
+		8200: {"mp9": "/var/lib/.../bootstrap,mp=/etc/felhom-bootstrap,ro=1"},
+	})
+	w := do(t, h, "POST", "/disks/guest-attach", "A", `{"where":"/mnt/felhom-usb"}`)
+	if w.Code != http.StatusOK {
+		t.Fatalf("attach: got %d want 200 (%s)", w.Code, w.Body.String())
+	}
+	if ga.count() != 1 || ga.calls[0].slot != "mp0" || ga.calls[0].where != "/mnt/felhom-usb" || ga.calls[0].vmid != 8200 {
+		t.Fatalf("AttachBind not called with mp0/where/vmid: %+v", ga.calls)
+	}
+}
+
+// An already-bound drive is idempotent: returns the existing slot, binder NOT called again.
+func TestGuestAttach_Idempotent(t *testing.T) {
+	ga := &fakeGuestAttacher{}
+	h := newAttachServer(t, ga, map[int]map[string]string{
+		8200: {"mp0": "/mnt/felhom-usb/felhom-data,mp=/mnt/felhom-usb"},
+	})
+	w := do(t, h, "POST", "/disks/guest-attach", "A", `{"where":"/mnt/felhom-usb"}`)
+	if w.Code != http.StatusOK {
+		t.Fatalf("idempotent attach: got %d want 200 (%s)", w.Code, w.Body.String())
+	}
+	if !strings.Contains(w.Body.String(), `"already":true`) {
+		t.Fatalf("expected already:true: %s", w.Body.String())
+	}
+	if ga.count() != 0 {
+		t.Fatalf("AttachBind must NOT be called for an already-bound drive: %+v", ga.calls)
+	}
+}
+
+// A hostile/invalid where is refused with no binder call.
+func TestGuestAttach_RejectsBadPath(t *testing.T) {
+	ga := &fakeGuestAttacher{}
+	h := newAttachServer(t, ga, map[int]map[string]string{8200: {}})
+	for _, bad := range []string{`{"where":"/mnt/../etc"}`, `{"where":"/etc/passwd"}`, `{"where":"/mnt/a/b"}`, `{"where":""}`} {
+		if w := do(t, h, "POST", "/disks/guest-attach", "A", bad); w.Code != http.StatusBadRequest {
+			t.Fatalf("bad where %s: got %d want 400", bad, w.Code)
+		}
+	}
+	if ga.count() != 0 {
+		t.Fatal("binder called for an invalid path")
+	}
+}
+
+// Not configured (no GuestAttach dep) → 503.
+func TestGuestAttach_NotConfigured(t *testing.T) {
+	h := newAttachServer(t, nil, map[int]map[string]string{8200: {}})
+	if w := do(t, h, "POST", "/disks/guest-attach", "A", `{"where":"/mnt/felhom-usb"}`); w.Code != http.StatusServiceUnavailable {
+		t.Fatalf("unconfigured: got %d want 503", w.Code)
+	}
+}
+
 // ---- auth / config ----------------------------------------------------------------------
 
 func TestDisks_CrossGuest403(t *testing.T) {