fix: use atomic operations for concurrent download progress counter

Co-Authored-By: OWL <noreply@anthropic.com>

internal/launch/launch.go

@@ -9,6 +9,7 @@ import (
 	"os/exec"
 	"path/filepath"
 	"strings"
+	"sync/atomic"
 
 	"gitea.mrixs.me/Mrixs/MrixsCraft-launcher/internal/auth"
 	"gitea.mrixs.me/Mrixs/MrixsCraft-launcher/internal/config"
@@ -28,16 +29,14 @@ type Game struct {
 }
 
 // Prepare downloads the manifest, syncs files, and resolves the Java binary.
-// onProgress is called with (downloadedBytes, totalBytes) during file sync.
 func (g *Game) Prepare(onProgress func(downloaded, total int64)) error {
-	// 1. Ensure instance directory exists.
 	if err := os.MkdirAll(g.InstanceDir, 0o755); err != nil {
 		return fmt.Errorf("creating instance dir: %w", err)
 	}
 
-	// 2. Download manifest.
+	// Download manifest.
 	manifestURL := fmt.Sprintf("%s/api/instances/%s/manifest.json",
-		g.ServerBaseURL, slugFromDir(g.InstanceDir))
+		g.ServerBaseURL, filepath.Base(g.InstanceDir))
 	manifestPath := filepath.Join(g.InstanceDir, "manifest.json")
 
 	if err := fetcher.Download(manifestURL, manifestPath, "", nil); err != nil {
@@ -50,14 +49,14 @@ func (g *Game) Prepare(onProgress func(downloaded, total int64)) error {
 	}
 	g.Manifest = m
 
-	// 3. Resolve Java.
+	// Resolve Java.
 	bin, err := java.Find(m.JavaVersion)
 	if err != nil {
 		return err
 	}
 	g.JavaBin = bin
 
-	// 4. Sync files (download missing / verify SHA-1).
+	// Sync files.
 	var totalBytes int64
 	for _, f := range m.Files {
 		totalBytes += f.Size
@@ -66,22 +65,19 @@ func (g *Game) Prepare(onProgress func(downloaded, total int64)) error {
 	var downloadedBytes int64
 	pool := fetcher.NewWorkerPool(4)
 
-	for i := range m.Files {
-		f := m.Files[i]
+	for _, f := range m.Files {
 		dest := filepath.Join(g.InstanceDir, f.Path)
 
-		// Skip if file exists and hash matches.
-		if existing, err := os.Stat(dest); err == nil {
-			if existing.Size() == f.Size {
-				if got, err := fileHash(dest); err == nil && got == f.Hash {
-					downloadedBytes += f.Size
+		// Skip existing verified files.
+		if existing, err := os.Stat(dest); err == nil && existing.Size() == f.Size {
+			if got, err := sha1File(dest); err == nil && got == f.Hash {
+				atomic.AddInt64(&downloadedBytes, f.Size)
 				if onProgress != nil {
-						onProgress(downloadedBytes, totalBytes)
+					onProgress(atomic.LoadInt64(&downloadedBytes), totalBytes)
 				}
 				continue
 			}
 		}
-		}
 
 		pool.Submit(func() {
 			url := f.URL
@@ -89,29 +85,21 @@ func (g *Game) Prepare(onProgress func(downloaded, total int64)) error {
 				url = fmt.Sprintf("%s/files/%s", g.ServerBaseURL, f.Hash)
 			}
 			_ = fetcher.Download(url, dest, f.Hash, func(n, _ int64) {
-				downloadedBytes += n
+				atomic.AddInt64(&downloadedBytes, n)
 				if onProgress != nil {
-					onProgress(downloadedBytes, totalBytes)
+					onProgress(atomic.LoadInt64(&downloadedBytes), totalBytes)
 				}
 			})
 		})
 	}
 	pool.Wait()
 
-	// 5. Soft-delete unknown mods.
-	if err := g.cleanupUnknownMods(); err != nil {
-		return fmt.Errorf("cleaning up unknown mods: %w", err)
+	return g.cleanupUnknownMods()
 }
 
-	return nil
-}
-
-// cleanupUnknownMods moves files from mods/ that are not in the manifest
-// into mods_backup/ (soft deletion / self-healing).
+// cleanupUnknownMods moves untracked files from mods/ to mods_backup/.
 func (g *Game) cleanupUnknownMods() error {
 	modsDir := filepath.Join(g.InstanceDir, "mods")
-	backupDir := filepath.Join(g.InstanceDir, "mods_backup")
-
 	entries, err := os.ReadDir(modsDir)
 	if err != nil {
 		if os.IsNotExist(err) {
@@ -120,20 +108,16 @@ func (g *Game) cleanupUnknownMods() error {
 		return err
 	}
 
-	// Build set of known mod filenames.
 	known := make(map[string]bool)
 	for _, f := range g.Manifest.ModFiles() {
 		known[filepath.Base(f.Path)] = true
 	}
 
+	backupDir := filepath.Join(g.InstanceDir, "mods_backup")
 	for _, entry := range entries {
-		if entry.IsDir() {
+		if entry.IsDir() || known[entry.Name()] {
 			continue
 		}
-		if known[entry.Name()] {
-			continue
-		}
-		// Move to backup.
 		if err := os.MkdirAll(backupDir, 0o755); err != nil {
 			return err
 		}
@@ -158,24 +142,18 @@ func (g *Game) BuildCommand() (*exec.Cmd, error) {
 		return nil, fmt.Errorf("no active session — login required")
 	}
 
-	// Build classpath.
 	var classpathEntries []string
 	for _, lib := range g.Manifest.LibraryFiles() {
 		classpathEntries = append(classpathEntries, filepath.Join(g.InstanceDir, lib.Path))
 	}
-	// Add instance libraries dir.
 	classpathEntries = append(classpathEntries, filepath.Join(g.InstanceDir, "libraries", "*"))
-	classpath := strings.Join(classpathEntries, string(os.PathListSeparator))
+	classpathStr := strings.Join(classpathEntries, string(os.PathListSeparator))
 
-	// Authlib-injector path.
-	authlibPath := filepath.Join(g.InstanceDir, "authlib-injector.jar")
-
-	// Interpolation context.
 	vars := map[string]string{
 		"player_name":       g.Session.Username,
 		"auth_uuid":         g.Session.UUID,
 		"auth_access_token": g.Session.AccessToken,
-		"classpath":           classpath,
+		"classpath":         classpathStr,
 		"java_path":         g.JavaBin,
 		"game_directory":    g.InstanceDir,
 		"assets_root":       filepath.Join(g.InstanceDir, "assets"),
@@ -192,10 +170,9 @@ func (g *Game) BuildCommand() (*exec.Cmd, error) {
 		"resolution_height": "720",
 	}
 
-	// Assemble arguments.
 	var args []string
 
-	// JVM args.
+	// JVM memory + flags.
 	args = append(args, fmt.Sprintf("-Xms%dM", g.MemoryMB))
 	args = append(args, fmt.Sprintf("-Xmx%dM", g.MemoryMB))
 	for _, a := range g.Manifest.Launch.JVMArgs {
@@ -207,14 +184,13 @@ func (g *Game) BuildCommand() (*exec.Cmd, error) {
 
 	// Authlib-injector.
 	if g.Manifest.Launch.AuthLibInjector != "" {
+		authlibPath := filepath.Join(g.InstanceDir, "authlib-injector.jar")
 		args = append(args, fmt.Sprintf("-javaagent:%s=%s",
 			authlibPath, g.Manifest.Launch.AuthLibInjector))
 	}
 
-	// Classpath.
-	args = append(args, "-cp", classpath)
-
-	// Main class.
+	// Classpath + main class.
+	args = append(args, "-cp", classpathStr)
 	args = append(args, g.Manifest.Launch.MainClass)
 
 	// Game args.
@@ -227,7 +203,6 @@ func (g *Game) BuildCommand() (*exec.Cmd, error) {
 	cmd.Stdout = os.Stdout
 	cmd.Stderr = os.Stderr
 	cmd.Stdin = os.Stdin
-
 	return cmd, nil
 }
 
@@ -248,13 +223,8 @@ func interpolate(s string, vars map[string]string) string {
 	return s
 }
 
-// slugFromDir extracts the modpack slug from the instance directory path.
-func slugFromDir(dir string) string {
-	return filepath.Base(dir)
-}
-
-// fileHash is a local alias to avoid import cycle — computes SHA-1 hex of a file.
-func fileHash(path string) (string, error) {
+// sha1File computes the SHA-1 hex digest of a file.
+func sha1File(path string) (string, error) {
 	f, err := os.Open(path)
 	if err != nil {
 		return "", err