agent: the restore test takes only archives of a guest that still exists (R-689, second half)

Measured on demo-hp right after v0.135.0: with the golden skipped the pick fell to a leftover
archive of guest 9100, deleted in August. "does not exist" skips it; any other lookup error
makes the tier unknown. Red-proofed.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0159rPz1ZhFKsS53msqPYxtS

internal/backup/backup_test.go

@@ -2,6 +2,7 @@ package backup
 
 import (
 	"context"
+	"fmt"
 	"encoding/json"
 	"errors"
 	"io"
@@ -21,6 +22,7 @@ type fakeBackupAPI struct {
 	vzdumpErr  error
 	waitErr    error
 	cfg        proxmox.GuestConfig
+	goneGuests map[int]bool // R-689: vmids whose config lookup answers "does not exist"
 	cfgErr     error
 	content    []proxmox.StorageContent
 	contentErr error
@@ -43,7 +45,10 @@ func (f *fakeBackupAPI) WaitTask(_ context.Context, _ string, _ proxmox.WaitOpti
 	}
 	return proxmox.TaskStatus{Status: "stopped", ExitStatus: "OK"}, f.waitErr
 }
-func (f *fakeBackupAPI) GuestConfig(_ context.Context, _ int) (proxmox.GuestConfig, error) {
+func (f *fakeBackupAPI) GuestConfig(_ context.Context, vmid int) (proxmox.GuestConfig, error) {
+	if f.goneGuests[vmid] { // R-689: PVE's answer for a deleted guest
+		return proxmox.GuestConfig{}, fmt.Errorf("proxmox: GET /nodes/n/lxc/%d/config -> HTTP 500: Configuration file 'nodes/n/lxc/%d.conf' does not exist", vmid, vmid)
+	}
 	return f.cfg, f.cfgErr
 }
 func (f *fakeBackupAPI) StorageContent(_ context.Context, _ string) ([]proxmox.StorageContent, error) {

internal/backup/r689_golden_test.go

@@ -2,6 +2,7 @@ package backup
 
 import (
 	"context"
+	"fmt"
 	"testing"
 	"time"
 
@@ -54,3 +55,39 @@ func TestR689_GuestBackupArchiveShapes(t *testing.T) {
 		}
 	}
 }
+
+// R-689 (v0.136.0) — the measured demo-hp shape right after v0.135.0: the golden (skipped), a leftover archive
+// of guest 9100 deleted in August (settled), and today's archive of 9201 (not settled yet). The pick must be
+// NOTHING — never the deleted guest's archive. With 9201's archive settled, that one.
+//
+// COMPANION RED-PROOF (REPORT.md): drop the known-guest check — the pick is the 9100 leftover.
+func TestR689_AnArchiveOfADeletedGuestIsNeverPicked(t *testing.T) {
+	const day = int64(86400)
+	now := int64(1790476000)
+	api := &fakeBackupAPI{goneGuests: map[int]bool{9100: true}, content: []proxmox.StorageContent{
+		{VolID: "local:backup/felhom-golden-0.236.0.tar.zst", Content: "backup", Size: 654115664, CTime: now - 14*day},
+		{VolID: "local:backup/vzdump-lxc-9100-2026_08_21-17_59_15.tar.zst", Content: "backup", VMID: 9100, Size: 656970239, CTime: now - 37*day},
+		{VolID: "local:backup/vzdump-lxc-9201-2026_09_27-04_35_47.tar.zst", Content: "backup", VMID: 9201, Size: 8 << 30, CTime: now - 7*3600},
+	}}
+	r := NewBackupRunner(api, "local", proxmox.ModeSnapshot, "", "keep-last=1", quiet())
+	got, _, err := r.PickSettledRestoreCandidateOn(context.Background(), "local", time.Unix(now-day, 0).UTC())
+	if err != nil || got != "" {
+		t.Fatalf("picked %q err=%v — a deleted guest's archive proves nothing about this box", got, err)
+	}
+	got, _, _ = r.PickSettledRestoreCandidateOn(context.Background(), "local", time.Unix(now, 0).UTC())
+	if got != "local:backup/vzdump-lxc-9201-2026_09_27-04_35_47.tar.zst" {
+		t.Fatalf("with 9201's archive settled the pick is %q", got)
+	}
+}
+
+// Any OTHER lookup failure is not "the guest is gone": the tier must read UNKNOWN (an error), never
+// "nothing to prove".
+func TestR689_AGuestLookupFailureIsUnknownNotEmpty(t *testing.T) {
+	api := &fakeBackupAPI{cfgErr: fmt.Errorf("proxmox: connection refused"), content: []proxmox.StorageContent{
+		{VolID: "local:backup/vzdump-lxc-9201-x.tar.zst", Content: "backup", VMID: 9201, Size: 8 << 30, CTime: 10},
+	}}
+	r := NewBackupRunner(api, "local", proxmox.ModeSnapshot, "", "keep-last=1", quiet())
+	if _, _, err := r.PickSettledRestoreCandidateOn(context.Background(), "local", time.Time{}); err == nil {
+		t.Fatal("a failed guest lookup read as a clean answer")
+	}
+}

internal/backup/runner.go

@@ -359,6 +359,7 @@ func (r *BackupRunner) PickSettledRestoreCandidateOn(ctx context.Context, target
 	}
 	var best string
 	var bestCTime int64 = -1
+	known := map[int]bool{} // vmid → the guest exists on this node (asked once per vmid per pick)
 	for _, e := range contents {
 		if e.Content != "backup" {
 			continue
@@ -371,6 +372,25 @@ func (r *BackupRunner) PickSettledRestoreCandidateOn(ctx context.Context, target
 			r.noteNotAGuestBackupOnce(e, why)
 			continue
 		}
+		// R-689 (v0.136.0): … OF A GUEST THAT STILL EXISTS here. Measured on demo-hp 2026-09-27 right after
+		// v0.135.0: with the golden skipped, the pick fell to `vzdump-lxc-9100-2026_08_21…`, a leftover of a
+		// guest deleted in August — proving nothing about any guest this box runs. "Does not exist" skips the
+		// archive; any OTHER lookup failure is returned, so the tier reads UNKNOWN, never "nothing to prove".
+		if _, seen := known[e.VMID]; !seen {
+			_, err := r.api.GuestConfig(ctx, e.VMID)
+			switch {
+			case err == nil:
+				known[e.VMID] = true
+			case strings.Contains(err.Error(), "does not exist"):
+				known[e.VMID] = false
+			default:
+				return "", time.Time{}, fmt.Errorf("checking whether guest %d still exists: %w", e.VMID, err)
+			}
+		}
+		if !known[e.VMID] {
+			r.noteNotAGuestBackupOnce(e, fmt.Sprintf("guest %d no longer exists on this node", e.VMID))
+			continue
+		}
 		if !notAfter.IsZero() && e.CTime > notAfter.Unix() {
 			continue // not settled yet — a newer archive is not a reason to re-prove an older one
 		}