platform/app: fix nvbandwidth split fallback, blackbox discovery churn, and add blocking sync brackets around load steps

- gpuBandwidthSocketGroups: a single GPU whose NUMA node fails to resolve
  no longer collapses the whole per-socket nvbandwidth split into one
  fallback pass — it now folds into the last resolved group instead,
  preserving isolation for the sockets that did resolve.
- blackbox discoverMarkedTargets: skip mounting/unmounting devices that
  already have a running worker on every 2s discovery tick. This was
  observed hammering the same USB target continuously (mount+unmount
  every ~2s for the whole session) and contending with the worker's own
  sync cycle, plausibly explaining multi-minute sync cycles seen on a
  real crash bundle.
- syncFilesystem now calls syscall.Sync() directly instead of spawning
  /bin/sync per copied file; blackbox mounts removable targets with
  -o sync so writes are durable without relying on the app-level sync as
  the primary mechanism.
- New platform.SetSyncBracketHook / satJob.syncBracket: blocks (with a
  bounded timeout) on blackbox actually reaching removable media right
  before and right after a diagnostic's real load step (nvbandwidth,
  memtester, stress-ng, dcgmi diag, nccl, smartctl/nvme self-test...),
  instead of only firing a fire-and-forget kick after the job's own log
  file is written. A crash mid-load now has durable evidence the load
  started, not just whatever streamed to the RAM-backed export dir before
  blackbox's next scheduled cycle.

Found investigating a real support bundle where blackbox's last
successful sync (19:55:25) predated both the previous job finishing and
the crashing nvbandwidth job starting (19:56:57) — none of the crash
window ever reached durable media.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

audit/internal/app/blackbox.go

@@ -15,6 +15,7 @@ import (
 	"sort"
 	"strings"
 	"sync"
+	"syscall"
 	"time"
 
 	"bee/audit/internal/platform"
@@ -33,6 +34,13 @@ const (
 	// output reaches removable media promptly instead of waiting out
 	// whatever the current adaptive flush period happens to be.
 	blackboxKickFileName = ".blackbox-kick"
+
+	// blackboxSyncBracketTimeout bounds how long a syncBracket-marked SAT job
+	// (see platform.SetSyncBracketHook) blocks waiting for blackbox to catch
+	// up before/after the actual load step. Generous relative to the normal
+	// sub-second kick-to-sync latency, but bounded so a genuinely stuck or
+	// unplugged target can't hang the diagnostic indefinitely.
+	blackboxSyncBracketTimeout = 20 * time.Second
 )
 
 // blackboxKickPollInterval is how often an idle worker checks the kick file
@@ -53,6 +61,64 @@ func requestBlackboxSync(exportDir string) {
 	_ = os.WriteFile(path, []byte(now.Format(time.RFC3339Nano)+"\n"), 0644)
 }
 
+// blackboxSyncWaitPollInterval is how often requestBlackboxSyncAndWait
+// re-reads the state file while waiting for a sync to catch up. A package
+// var so tests can shrink it.
+var blackboxSyncWaitPollInterval = 200 * time.Millisecond
+
+// requestBlackboxSyncAndWait kicks every enrolled blackbox target and blocks
+// until each one has completed a sync cycle that started at or after this
+// call, or until timeout elapses. Use this (instead of the fire-and-forget
+// requestBlackboxSync) around an event where losing the data to a crash
+// before it reaches removable media would matter — e.g. bracket a risky load
+// step so its own SAT job directory (and the fact that it started at all) is
+// durable on the flash drive before the load runs, and durable again once it
+// finishes.
+//
+// Returns nil once caught up. Returns an error naming what didn't catch up
+// (timeout, or a target stuck "degraded") — callers should treat that as
+// best-effort informational (log it) rather than fail the job over it: a
+// blackbox problem should not block the diagnostic the operator actually
+// asked for.
+func requestBlackboxSyncAndWait(exportDir, statePath string, timeout time.Duration) error {
+	requestedAt := blackboxNow()
+	requestBlackboxSync(exportDir)
+
+	deadline := time.Now().Add(timeout)
+	for {
+		state, err := ReadBlackboxState(statePath)
+		if err == nil {
+			if ok, pending := blackboxStateCaughtUp(state, requestedAt); ok {
+				return nil
+			} else if time.Now().After(deadline) {
+				return fmt.Errorf("blackbox sync did not catch up within %s (pending: %s)", timeout, pending)
+			}
+		} else if time.Now().After(deadline) {
+			return fmt.Errorf("blackbox sync wait: could not read state after %s: %w", timeout, err)
+		}
+		time.Sleep(blackboxSyncWaitPollInterval)
+	}
+}
+
+// blackboxStateCaughtUp reports whether every non-degraded enrolled target
+// has synced at or after requestedAt. No targets enrolled counts as caught
+// up (nothing to wait for — e.g. no removable media plugged in). A target
+// stuck "degraded" (mount/copy failing) is skipped rather than waited on
+// forever; its enrollment ID is named in the returned pending string so
+// callers can log which target is the reason a real wait timed out.
+func blackboxStateCaughtUp(state BlackboxState, requestedAt time.Time) (bool, string) {
+	for _, t := range state.Targets {
+		if t.Status == "degraded" {
+			continue
+		}
+		syncedAt, err := time.Parse(time.RFC3339Nano, t.LastSyncAtUTC)
+		if err != nil || syncedAt.Before(requestedAt) {
+			return false, t.EnrollmentID
+		}
+	}
+	return true, ""
+}
+
 // blackboxKickModTime returns the kick file's mtime, or the zero Time if it
 // doesn't exist yet (nothing has requested a sync since boot).
 func blackboxKickModTime(exportDir string) time.Time {
@@ -270,7 +336,7 @@ func DisableBlackboxTarget(device, enrollmentID string) error {
 }
 
 func (rt *blackboxRuntime) reconcile() {
-	discovered, _ := rt.discoverMarkedTargets()
+	discovered, _ := rt.discoverMarkedTargets(rt.trackedDevices())
 
 	rt.mu.Lock()
 	defer rt.mu.Unlock()
@@ -311,7 +377,35 @@ func (rt *blackboxRuntime) stopAll() {
 	}
 }
 
-func (rt *blackboxRuntime) discoverMarkedTargets() ([]discoveredBlackboxTarget, error) {
+// trackedDevices returns the already-enrolled targets, keyed by device path,
+// for every worker currently running. Used by discoverMarkedTargets to skip
+// re-mounting devices a worker already owns.
+func (rt *blackboxRuntime) trackedDevices() map[string]discoveredBlackboxTarget {
+	rt.mu.Lock()
+	defer rt.mu.Unlock()
+	known := make(map[string]discoveredBlackboxTarget, len(rt.workers))
+	for _, worker := range rt.workers {
+		worker.mu.Lock()
+		if worker.target.Device != "" {
+			known[worker.target.Device] = discoveredBlackboxTarget{
+				marker: worker.marker,
+				target: worker.target,
+			}
+		}
+		worker.mu.Unlock()
+	}
+	return known
+}
+
+// discoverMarkedTargets probes removable media for the bee-blackbox marker.
+// known holds devices a worker is already running against (see
+// trackedDevices) — those are reported back as-is, without mounting, since
+// mounting/unmounting the same device on every discovery tick (every
+// blackboxDiscoverInterval) fights the worker's own mount for the device and
+// was observed to slow its actual sync cycle by an order of magnitude on
+// FUSE-backed filesystems (NTFS via ntfs-3g). Only devices with no running
+// worker get probed.
+func (rt *blackboxRuntime) discoverMarkedTargets(known map[string]discoveredBlackboxTarget) ([]discoveredBlackboxTarget, error) {
 	targets, err := rt.system.ListRemovableTargets()
 	if err != nil {
 		return nil, err
@@ -322,6 +416,11 @@ func (rt *blackboxRuntime) discoverMarkedTargets() ([]discoveredBlackboxTarget,
 		if target.Device == "" {
 			continue
 		}
+		if cached, ok := known[target.Device]; ok {
+			cached.target = target
+			out = append(out, cached)
+			continue
+		}
 		mountpoint, mountedByBee, err := ensureMountedTarget(target, "probe")
 		if err != nil {
 			continue
@@ -578,7 +677,11 @@ func (rt *blackboxRuntime) persistStateLocked() {
 			Mountpoint:   worker.mountpoint,
 		}
 		if !worker.lastSyncAt.IsZero() {
-			targetState.LastSyncAtUTC = worker.lastSyncAt.Format(time.RFC3339)
+			// Nanosecond precision, not RFC3339's default seconds — a sync
+			// that lands in the same wall-clock second as a kick request
+			// must still compare as "after" it (see blackboxStateCaughtUp),
+			// which second-only precision can get wrong.
+			targetState.LastSyncAtUTC = worker.lastSyncAt.Format(time.RFC3339Nano)
 		}
 		if worker.lastDuration > 0 {
 			targetState.LastCycleDuration = worker.lastDuration.String()
@@ -647,7 +750,15 @@ func ensureMountedTarget(target platform.RemovableTarget, suffix string) (mountp
 	if err := os.MkdirAll(mountpoint, 0755); err != nil {
 		return "", false, err
 	}
-	if raw, err := blackboxExecCommand("mount", target.Device, mountpoint).CombinedOutput(); err != nil {
+	// -o sync makes every write to the target synchronous at the VFS layer
+	// (no page-cache write-back to lose on a hard reset) instead of relying
+	// solely on the explicit syscall.Sync() calls in writeFileAtomic/
+	// unmountTarget to flush it after the fact. Those explicit syncs stay in
+	// place as a fallback (e.g. for target.Mountpoint above, an
+	// already-mounted filesystem we don't control the options of) — with -o
+	// sync already doing the work, they become a fast no-op most of the time
+	// instead of the primary durability mechanism.
+	if raw, err := blackboxExecCommand("mount", "-o", "sync", target.Device, mountpoint).CombinedOutput(); err != nil {
 		return "", false, formatBlackboxMountTargetError(target, string(raw), err)
 	}
 	if err := ensureWritableBlackboxMountpoint(mountpoint); err != nil {
@@ -658,7 +769,7 @@ func ensureMountedTarget(target platform.RemovableTarget, suffix string) (mountp
 }
 
 func unmountTarget(mountpoint string) error {
-	_ = blackboxExecCommand("sync").Run()
+	syscall.Sync()
 	raw, err := blackboxExecCommand("umount", mountpoint).CombinedOutput()
 	if err != nil {
 		msg := strings.TrimSpace(string(raw))
@@ -812,8 +923,15 @@ func writeFileAtomic(path string, data []byte, perm os.FileMode) error {
 	return syncFilesystem(filepath.Dir(path))
 }
 
+// syncFilesystem flushes pending writes to durable storage. Uses the sync(2)
+// syscall directly rather than spawning /bin/sync — writeFileAtomic calls
+// this once per copied file (syncDirectoryTree can touch hundreds of files
+// per cycle), and syscall.Sync() does the same flush without a fork+exec per
+// call. path is unused (sync(2) always flushes system-wide; kept as a
+// parameter for call-site clarity about which tree the caller cares about).
 func syncFilesystem(path string) error {
-	return blackboxExecCommand("sync").Run()
+	syscall.Sync()
+	return nil
 }
 
 func ensureWritableBlackboxMountpoint(mountpoint string) error {