Fix snapshot storage bugs: prevent panic on uninitialized chunker and propagate header wait failures

- Bug 1 (7a08b411): Add nil check in StorageDiff.FileSize to prevent panic when chunker is not initialized
- Bug 2 (33d2a3b3): Change NewSnapshotSchedulingMetadata to return error, failing pause/checkpoint when header resolution fails instead of silently returning nil metadata

Author: cursoragent

packages/orchestrator/pkg/sandbox/build/storage_diff.go

@@ -128,6 +128,10 @@ func (b *StorageDiff) CachePath(context.Context) (string, error) {
 }
 
 func (b *StorageDiff) FileSize(ctx context.Context) (int64, error) {
+	if b.chunker == nil {
+		return 0, fmt.Errorf("chunker not initialized")
+	}
+
 	return b.chunker.FileSize(ctx)
 }
 

packages/orchestrator/pkg/sandbox/sandbox.go

@@ -1186,7 +1186,10 @@ func (s *Sandbox) Pause(
 	cleanup.AddNoContext(ctx, rootfsDiff.Close)
 
 	rootfsDiffHeader := NewResolvedDiffHeader(rootfsHeader)
-	schedulingMetadata := NewSnapshotSchedulingMetadata(ctx, memfileDiffHeader, rootfsDiffHeader)
+	schedulingMetadata, err := NewSnapshotSchedulingMetadata(ctx, memfileDiffHeader, rootfsDiffHeader)
+	if err != nil {
+		return nil, fmt.Errorf("failed to resolve scheduling metadata: %w", err)
+	}
 
 	metadataFileLink := template.NewLocalFileLink(cachePaths.CacheMetadata())
 	cleanup.AddNoContext(ctx, metadataFileLink.Close)

packages/orchestrator/pkg/sandbox/snapshot.go

@@ -59,12 +59,16 @@ func (s *Snapshot) Close(ctx context.Context) error {
 
 // NewSnapshotSchedulingMetadata derives the metadata from the new snapshot's
 // diff headers so the freshly created build and its contributions are included.
-func NewSnapshotSchedulingMetadata(ctx context.Context, memfileDiffHeader, rootfsDiffHeader *DiffHeader) *orchestrator.SchedulingMetadata {
+func NewSnapshotSchedulingMetadata(ctx context.Context, memfileDiffHeader, rootfsDiffHeader *DiffHeader) (*orchestrator.SchedulingMetadata, error) {
 	memfileHeader, memfileErr := memfileDiffHeader.WaitWithContext(ctx)
+	if memfileErr != nil {
+		return nil, fmt.Errorf("memfile header wait failed: %w", memfileErr)
+	}
+
 	rootfsHeader, rootfsErr := rootfsDiffHeader.WaitWithContext(ctx)
-	if memfileErr != nil || rootfsErr != nil {
-		return nil
+	if rootfsErr != nil {
+		return nil, fmt.Errorf("rootfs header wait failed: %w", rootfsErr)
 	}
 
-	return scheduling.FromHeaders(memfileHeader, rootfsHeader)
+	return scheduling.FromHeaders(memfileHeader, rootfsHeader), nil
 }