Fix scheduling metadata being nil on sandbox create

When GetTemplate is called for a new sandbox, headers were pre-filled
with nil using resolvedHeader(nil), which immediately marked the SetOnce
as 'done'. When Fetch loaded actual headers from storage, they couldn't
update the SetOnce because it was already set. This caused
SchedulingMetadata() to return nil on create.

Changed GetTemplate to pass unresolved SetOnce instances and modified
Fetch to load headers from storage when they are unset.

Author: cursoragent

packages/orchestrator/pkg/sandbox/template/cache.go

@@ -193,8 +193,8 @@ func (c *Cache) GetTemplate(
 	storageTemplate, err := newTemplateFromStorage(
 		c.config.BuilderConfig,
 		buildID,
-		resolvedHeader(nil),
-		resolvedHeader(nil),
+		utils.NewSetOnce[*header.Header](),
+		utils.NewSetOnce[*header.Header](),
 		persistence,
 		c.blockMetrics,
 		nil,

packages/orchestrator/pkg/sandbox/template/storage_template.go

@@ -176,14 +176,28 @@ func (t *storageTemplate) Fetch(ctx context.Context, buildStore *build.DiffStore
 	})
 
 	wg.Go(func() error {
-		memHdr, hdrErr := t.memfileHeader.WaitWithContext(ctx)
+		memHdr, hdrErr := t.memfileHeader.Result()
 		if hdrErr != nil {
-			errMsg := fmt.Errorf("failed to resolve memfile header: %w", hdrErr)
-			if err := t.memfile.SetError(errMsg); err != nil {
-				return fmt.Errorf("failed to set memfile error: %w", errors.Join(errMsg, err))
+			if _, notSet := hdrErr.(utils.NotSetError); notSet {
+				// Header not set yet, load it from storage
+				loadedHdr, loadErr := header.LoadHeader(ctx, t.persistence, t.paths.MemfileHeader())
+				if loadErr != nil && !errors.Is(loadErr, storage.ErrObjectNotExist) {
+					errMsg := fmt.Errorf("failed to load memfile header: %w", loadErr)
+					if err := t.memfile.SetError(errMsg); err != nil {
+						return fmt.Errorf("failed to set memfile error: %w", errors.Join(errMsg, err))
+					}
+					return nil
+				}
+				// Set the header (may be nil if not found)
+				_ = t.memfileHeader.SetValue(loadedHdr)
+				memHdr = loadedHdr
+			} else {
+				errMsg := fmt.Errorf("failed to resolve memfile header: %w", hdrErr)
+				if err := t.memfile.SetError(errMsg); err != nil {
+					return fmt.Errorf("failed to set memfile error: %w", errors.Join(errMsg, err))
+				}
+				return nil
 			}
-
-			return nil
 		}
 
 		memfileStorage, memfileErr := NewStorage(
@@ -214,14 +228,28 @@ func (t *storageTemplate) Fetch(ctx context.Context, buildStore *build.DiffStore
 	})
 
 	wg.Go(func() error {
-		rootHdr, hdrErr := t.rootfsHeader.WaitWithContext(ctx)
+		rootHdr, hdrErr := t.rootfsHeader.Result()
 		if hdrErr != nil {
-			errMsg := fmt.Errorf("failed to resolve rootfs header: %w", hdrErr)
-			if err := t.rootfs.SetError(errMsg); err != nil {
-				return fmt.Errorf("failed to set rootfs error: %w", errors.Join(errMsg, err))
+			if _, notSet := hdrErr.(utils.NotSetError); notSet {
+				// Header not set yet, load it from storage
+				loadedHdr, loadErr := header.LoadHeader(ctx, t.persistence, t.paths.RootfsHeader())
+				if loadErr != nil && !errors.Is(loadErr, storage.ErrObjectNotExist) {
+					errMsg := fmt.Errorf("failed to load rootfs header: %w", loadErr)
+					if err := t.rootfs.SetError(errMsg); err != nil {
+						return fmt.Errorf("failed to set rootfs error: %w", errors.Join(errMsg, err))
+					}
+					return nil
+				}
+				// Set the header (may be nil if not found)
+				_ = t.rootfsHeader.SetValue(loadedHdr)
+				rootHdr = loadedHdr
+			} else {
+				errMsg := fmt.Errorf("failed to resolve rootfs header: %w", hdrErr)
+				if err := t.rootfs.SetError(errMsg); err != nil {
+					return fmt.Errorf("failed to set rootfs error: %w", errors.Join(errMsg, err))
+				}
+				return nil
 			}
-
-			return nil
 		}
 
 		rootfsStorage, rootfsErr := NewStorage(