After a power cut: remove the dead half-written backup (R-1006); a backup task from before the boot is not in flight (R-1008)

internal/backup/runner.go

@@ -58,8 +58,11 @@ type BackupRunner struct {
 	// tier cannot have that accident — its keep_last defaults to 0 (never prune), so any value
 	// there is a deliberate act.
 	allowPBSPrune bool
-	logger        *slog.Logger
-	now           func() time.Time
+	// beforeBackup runs just before the space preflight (R-1006): it removes a dead half-written vzdump of this guest on
+	// this target, so a leftover from a power cut cannot hold the space this backup needs. nil = nothing.
+	beforeBackup func(ctx context.Context, storage string, vmid int)
+	logger       *slog.Logger
+	now          func() time.Time
 	// rejected remembers the volids already announced by warnRejectedArchiveOnce, so an incomplete
 	// archive is reported ONCE rather than on every 5-minute due-check. Bounded in practice: one
 	// entry per aborted upload, and a process restart clears it. Guarded by rejectedMu because the
@@ -102,6 +105,11 @@ func NewBackupRunnerFull(api BackupAPI, target string, mode proxmox.BackupMode,
 		now: func() time.Time { return time.Now().UTC() }}
 }
 
+// SetBeforeBackup sets the step run just before the space preflight (R-1006: the dead-leftover cleanup).
+func (r *BackupRunner) SetBeforeBackup(fn func(ctx context.Context, storage string, vmid int)) {
+	r.beforeBackup = fn
+}
+
 // localPruneSpec returns the `--prune-backups` spec to apply to THIS backup, or "" to skip pruning. It
 // applies the configured retention ONLY when the target is a non-PBS storage — PBS offsite retention is a
 // separate lifecycle and must never be pruned by the per-run flag (§9). Fail-safe: if the target's type
@@ -183,6 +191,9 @@ func (r *BackupRunner) backup(ctx context.Context, vmid int, onSnapshot func())
 	// R-685 (v0.134.0): will the new archive FIT on a local target? Asked before anything runs, so a
 	// target that cannot hold it is a named SKIP with the numbers, not a nightly "No space left on
 	// device" that only the vzdump log explains (demo-hp, every night from 2026-09-23 — R-684).
+	if r.beforeBackup != nil {
+		r.beforeBackup(ctx, r.target, vmid)
+	}
 	if ok, why := r.spaceFits(ctx, vmid); !ok {
 		rec.Error = BackupSkipNoSpacePrefix + why
 		rec.DurationSeconds = time.Since(start).Seconds()