fix: patch snapshot config paths and optimize copy with hard links

- Add patchSnapshotConfig() to update disk and tap device paths in config.json
  before restore (fixes "No such file or directory" errors when restoring)
- Change copyDir to copyDirWithLinks to use hard links for large snapshot
  files (memory-ranges is 512MB), falling back to copy if cross-device
- config.json is always copied (not linked) since we need to modify it

This enables snapshot restore to work across different instances by
updating the hardcoded device paths in the CloudHypervisor snapshot.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: alexisbouchez

runtime/drivers/vm/vm.go

@@ -2,6 +2,7 @@ package vm
 
 import (
 	"context"
+	"encoding/json"
 	"fmt"
 	"log/slog"
 	"os"
@@ -103,10 +104,15 @@ func (vm *vm) StartFromSnapshot(ctx context.Context, globalSnapshotPath, jailSna
 
 	// Copy snapshot from global storage into the jail
 	jailHostPath := getInstanceDir(vm.id) + jailSnapshotPath
-	if err := copyDir(globalSnapshotPath, jailHostPath); err != nil {
+	if err := copyDirWithLinks(globalSnapshotPath, jailHostPath); err != nil {
 		return fmt.Errorf("failed to copy snapshot to jail: %w", err)
 	}
 
+	// Patch config.json with current device paths (rootfs, tap device)
+	if err := vm.patchSnapshotConfig(jailHostPath); err != nil {
+		return fmt.Errorf("failed to patch snapshot config: %w", err)
+	}
+
 	// Chown to ravel-jailer user so CloudHypervisor can read
 	jailerUid, jailerGid, err := setupRavelJailerUser()
 	if err != nil {
@@ -116,7 +122,7 @@ func (vm *vm) StartFromSnapshot(ctx context.Context, globalSnapshotPath, jailSna
 		return fmt.Errorf("failed to chown snapshot directory: %w", err)
 	}
 
-	slog.Debug("snapshot copied to jail", "from", globalSnapshotPath, "to", jailHostPath)
+	slog.Debug("snapshot copied and patched", "from", globalSnapshotPath, "to", jailHostPath)
 
 	err = vm.cmd.Start()
 	if err != nil {
@@ -166,8 +172,66 @@ func (vm *vm) StartFromSnapshot(ctx context.Context, globalSnapshotPath, jailSna
 	return nil
 }
 
-// copyDir copies a directory from src to dst
-func copyDir(src, dst string) error {
+// patchSnapshotConfig updates the snapshot config.json with current device paths
+func (vm *vm) patchSnapshotConfig(snapshotPath string) error {
+	configPath := snapshotPath + "/config.json"
+
+	// Read the config
+	data, err := os.ReadFile(configPath)
+	if err != nil {
+		return fmt.Errorf("failed to read config.json: %w", err)
+	}
+
+	// Parse as generic JSON to preserve structure
+	var config map[string]interface{}
+	if err := json.Unmarshal(data, &config); err != nil {
+		return fmt.Errorf("failed to parse config.json: %w", err)
+	}
+
+	// Update disk path - get the current rootfs from vmConfig
+	if disks, ok := config["disks"].([]interface{}); ok && len(disks) > 0 {
+		if disk, ok := disks[0].(map[string]interface{}); ok {
+			// Get the new rootfs path from vmConfig
+			if vm.vmConfig.Disks != nil && len(*vm.vmConfig.Disks) > 0 {
+				newRootfs := (*vm.vmConfig.Disks)[0].Path
+				oldRootfs := disk["path"]
+				disk["path"] = newRootfs
+				slog.Debug("patched rootfs path", "old", oldRootfs, "new", newRootfs)
+			}
+		}
+	}
+
+	// Update tap device name
+	if nets, ok := config["net"].([]interface{}); ok && len(nets) > 0 {
+		if net, ok := nets[0].(map[string]interface{}); ok {
+			// Get the new tap device from vmConfig
+			if vm.vmConfig.Net != nil && len(*vm.vmConfig.Net) > 0 {
+				newTap := (*vm.vmConfig.Net)[0].Tap
+				if newTap != nil {
+					oldTap := net["tap"]
+					net["tap"] = *newTap
+					slog.Debug("patched tap device", "old", oldTap, "new", *newTap)
+				}
+			}
+		}
+	}
+
+	// Write back the modified config
+	newData, err := json.MarshalIndent(config, "", "  ")
+	if err != nil {
+		return fmt.Errorf("failed to marshal config.json: %w", err)
+	}
+
+	if err := os.WriteFile(configPath, newData, 0600); err != nil {
+		return fmt.Errorf("failed to write config.json: %w", err)
+	}
+
+	return nil
+}
+
+// copyDirWithLinks copies a directory from src to dst, using hard links for large files
+// config.json is always copied (not linked) since we need to modify it
+func copyDirWithLinks(src, dst string) error {
 	if err := os.MkdirAll(dst, 0755); err != nil {
 		return err
 	}
@@ -182,12 +246,24 @@ func copyDir(src, dst string) error {
 		dstPath := dst + "/" + entry.Name()
 
 		if entry.IsDir() {
-			if err := copyDir(srcPath, dstPath); err != nil {
+			if err := copyDirWithLinks(srcPath, dstPath); err != nil {
 				return err
 			}
 		} else {
-			if err := copyFile(srcPath, dstPath); err != nil {
-				return err
+			// Always copy config.json since we need to modify it
+			if entry.Name() == "config.json" {
+				if err := copyFile(srcPath, dstPath); err != nil {
+					return err
+				}
+				continue
+			}
+
+			// Try hard link first (instant, no copy), fall back to copy
+			if err := os.Link(srcPath, dstPath); err != nil {
+				// Hard link failed (maybe cross-device), fall back to copy
+				if err := copyFile(srcPath, dstPath); err != nil {
+					return err
+				}
 			}
 		}
 	}