refactor(uffd): extract Prefault to its own file (#2441)

Move Prefault and the directDataSource helper from userfaultfd.go
into a dedicated prefault.go. Pure file move, no behavior change.

Author: ValentaTomas

packages/orchestrator/pkg/sandbox/uffd/userfaultfd/prefault.go

@@ -0,0 +1,41 @@
+package userfaultfd
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/e2b-dev/infra/packages/orchestrator/pkg/sandbox/block"
+)
+
+// Prefault proactively copies a page to guest memory at the given offset.
+// This is used to speed up sandbox starts by prefetching pages that are known to be needed.
+// Returns nil on success, or if the page is already mapped (EEXIST is handled gracefully).
+func (u *Userfaultfd) Prefault(ctx context.Context, offset int64, data []byte) error {
+	ctx, span := tracer.Start(ctx, "prefault page")
+	defer span.End()
+
+	addr, err := u.ma.GetHostVirtAddr(offset)
+	if err != nil {
+		return fmt.Errorf("failed to get host virtual address: %w", err)
+	}
+
+	if len(data) != int(u.pageSize) {
+		return fmt.Errorf("data length (%d) does not match pagesize (%d)", len(data), u.pageSize)
+	}
+
+	return u.faultPage(ctx, addr, offset, directDataSource{data, int64(u.pageSize)}, nil, block.Prefetch)
+}
+
+// directDataSource wraps a byte slice to implement block.Slicer for prefaulting.
+type directDataSource struct {
+	data     []byte
+	pagesize int64
+}
+
+func (d directDataSource) Slice(_ context.Context, _, _ int64) ([]byte, error) {
+	return d.data, nil
+}
+
+func (d directDataSource) BlockSize() int64 {
+	return d.pagesize
+}

packages/orchestrator/pkg/sandbox/uffd/userfaultfd/userfaultfd.go

@@ -288,39 +288,6 @@ func (u *Userfaultfd) PrefetchData() block.PrefetchData {
 	return u.prefetchTracker.PrefetchData()
 }
 
-// Prefault proactively copies a page to guest memory at the given offset.
-// This is used to speed up sandbox starts by prefetching pages that are known to be needed.
-// Returns nil on success, or if the page is already mapped (EEXIST is handled gracefully).
-func (u *Userfaultfd) Prefault(ctx context.Context, offset int64, data []byte) error {
-	ctx, span := tracer.Start(ctx, "prefault page")
-	defer span.End()
-
-	addr, err := u.ma.GetHostVirtAddr(offset)
-	if err != nil {
-		return fmt.Errorf("failed to get host virtual address: %w", err)
-	}
-
-	if len(data) != int(u.pageSize) {
-		return fmt.Errorf("data length (%d) does not match pagesize (%d)", len(data), u.pageSize)
-	}
-
-	return u.faultPage(ctx, addr, offset, directDataSource{data, int64(u.pageSize)}, nil, block.Prefetch)
-}
-
-// directDataSource wraps a byte slice to implement block.Slicer for prefaulting.
-type directDataSource struct {
-	data     []byte
-	pagesize int64
-}
-
-func (d directDataSource) Slice(_ context.Context, _, _ int64) ([]byte, error) {
-	return d.data, nil
-}
-
-func (d directDataSource) BlockSize() int64 {
-	return d.pagesize
-}
-
 func (u *Userfaultfd) faultPage(
 	ctx context.Context,
 	addr uintptr,