Add diff snapshots for re-pause to avoid UFFD fault-in storm

Use Firecracker's Diff snapshot type when re-pausing a previously
resumed sandbox, capturing only dirty pages instead of a full memory
dump. Chains up to 10 incremental generations before collapsing back
to a Full snapshot. Multi-generation diff files (memfile.{buildID})
are supported alongside the legacy single-file format in resume,
template creation, and snapshot existence checks.

internal/snapshot/local.go

@@ -7,6 +7,8 @@ import (
 	"os"
 	"path/filepath"
 	"syscall"
+
+	"github.com/google/uuid"
 )
 
 const (
@@ -28,11 +30,17 @@ func SnapPath(baseDir, name string) string {
 	return filepath.Join(DirPath(baseDir, name), SnapFileName)
 }
 
-// MemDiffPath returns the path to the compact memory diff file.
+// MemDiffPath returns the path to the compact memory diff file (legacy single-generation).
 func MemDiffPath(baseDir, name string) string {
 	return filepath.Join(DirPath(baseDir, name), MemDiffName)
 }
 
+// MemDiffPathForBuild returns the path to a specific generation's diff file.
+// Format: memfile.{buildID}
+func MemDiffPathForBuild(baseDir, name string, buildID uuid.UUID) string {
+	return filepath.Join(DirPath(baseDir, name), fmt.Sprintf("memfile.%s", buildID.String()))
+}
+
 // MemHeaderPath returns the path to the memory mapping header file.
 func MemHeaderPath(baseDir, name string) string {
 	return filepath.Join(DirPath(baseDir, name), MemHeaderName)
@@ -85,17 +93,38 @@ func ReadMeta(baseDir, name string) (*RootfsMeta, error) {
 
 // Exists reports whether a complete snapshot exists (all required files present).
 // Supports both legacy (rootfs.ext4) and CoW-based (rootfs.cow + rootfs.meta) snapshots.
+// Memory diff files can be either legacy "memfile" or generation-specific "memfile.{uuid}".
 func Exists(baseDir, name string) bool {
 	dir := DirPath(baseDir, name)
 
-	// Common files required by both formats.
-	for _, f := range []string{SnapFileName, MemDiffName, MemHeaderName} {
+	// snapfile and header are always required.
+	for _, f := range []string{SnapFileName, MemHeaderName} {
 		if _, err := os.Stat(filepath.Join(dir, f)); err != nil {
 			return false
 		}
 	}
 
-	// Accept either rootfs.ext4 (legacy) or rootfs.cow + rootfs.meta (dm-snapshot).
+	// Check that at least one memfile exists (legacy or generation-specific).
+	// We verify by reading the header and checking that referenced diff files exist.
+	// Fall back to checking for the legacy memfile name if header can't be read.
+	if _, err := os.Stat(filepath.Join(dir, MemDiffName)); err != nil {
+		// No legacy memfile — check if any memfile.{uuid} exists by
+		// looking for files matching the pattern.
+		matches, _ := filepath.Glob(filepath.Join(dir, "memfile.*"))
+		hasGenDiff := false
+		for _, m := range matches {
+			base := filepath.Base(m)
+			if base != MemHeaderName {
+				hasGenDiff = true
+				break
+			}
+		}
+		if !hasGenDiff {
+			return false
+		}
+	}
+
+	// Accept either rootfs.ext4 (legacy/template) or rootfs.cow + rootfs.meta (dm-snapshot).
 	if _, err := os.Stat(filepath.Join(dir, RootfsFileName)); err == nil {
 		return true
 	}
@@ -127,6 +156,37 @@ func HasCow(baseDir, name string) bool {
 	return cowErr == nil && metaErr == nil
 }
 
+// ListDiffFiles returns a map of build ID → file path for all memory diff files
+// referenced by the given header. Handles both the legacy "memfile" name
+// (single-generation) and generation-specific "memfile.{uuid}" names.
+func ListDiffFiles(baseDir, name string, header *Header) (map[string]string, error) {
+	dir := DirPath(baseDir, name)
+	result := make(map[string]string)
+
+	for _, m := range header.Mapping {
+		if m.BuildID == uuid.Nil {
+			continue // zero-fill, no file needed
+		}
+		idStr := m.BuildID.String()
+		if _, exists := result[idStr]; exists {
+			continue
+		}
+		// Try generation-specific path first, fall back to legacy.
+		genPath := filepath.Join(dir, fmt.Sprintf("memfile.%s", idStr))
+		if _, err := os.Stat(genPath); err == nil {
+			result[idStr] = genPath
+			continue
+		}
+		legacyPath := filepath.Join(dir, MemDiffName)
+		if _, err := os.Stat(legacyPath); err == nil {
+			result[idStr] = legacyPath
+			continue
+		}
+		return nil, fmt.Errorf("diff file not found for build %s", idStr)
+	}
+	return result, nil
+}
+
 // EnsureDir creates the snapshot directory if it doesn't exist.
 func EnsureDir(baseDir, name string) error {
 	dir := DirPath(baseDir, name)