Fix TestMendFunctionDirect race condition on macOS CI

Add explicit file system synchronization to fix flaky tests failing on macOS CI but passing locally.
The issue was a race condition between file operations (truncate/rename) and subsequent verification reads.

Changes:
- Add defer reader.Close() in analyzeWARCFile to properly clean up gzip decompressor state
- Add file.Sync() in truncateFile to ensure truncation is persisted before subsequent reads
- Add syncDirectory() helper and call it after rename to ensure metadata is persisted
- Add 'path' package import for directory path manipulation

This ensures file operations complete before verification reads files, eliminating platform-specific timing differences.

Author: CorentinB

cmd/warc/mend/mend.go

@@ -6,6 +6,7 @@ import (
 	"io"
 	"log/slog"
 	"os"
+	"path"
 	"strings"
 	"time"
 
@@ -200,6 +201,13 @@ func mend(cmd *cobra.Command, files []string) {
 					}
 					slog.Info("removed .open suffix", "from", filepath, "to", result.newName)
 					stats.renamedFiles++
+
+					// Sync directory to ensure rename is persisted to disk before verification reads the file
+					dirPath := path.Dir(result.newName)
+					if err := syncDirectory(dirPath); err != nil {
+						slog.Warn("failed to sync directory after rename", "file", filepath, "dir", dirPath, "error", err)
+						// Continue anyway - sync is a performance optimization, not critical for correctness
+					}
 				}
 			}
 		} else {
@@ -294,6 +302,7 @@ func analyzeWARCFile(filepath string, verbose bool, force bool) mendResult {
 		return result
 	}
 	defer f.Close()
+	defer reader.Close()
 
 	var lastValidEndPos int64 = 0
 
@@ -377,6 +386,20 @@ func analyzeWARCFile(filepath string, verbose bool, force bool) mendResult {
 	return result
 }
 
+// syncDirectory syncs the directory to ensure file operations (rename, truncate) are persisted to disk
+func syncDirectory(dirPath string) error {
+	dir, err := os.Open(dirPath)
+	if err != nil {
+		return fmt.Errorf("failed to open directory for sync: %w", err)
+	}
+	defer dir.Close()
+
+	if err := dir.Sync(); err != nil {
+		return fmt.Errorf("failed to sync directory: %w", err)
+	}
+	return nil
+}
+
 func truncateFile(filepath string, position int64) error {
 	file, err := os.OpenFile(filepath, os.O_RDWR, 0)
 	if err != nil {
@@ -388,6 +411,11 @@ func truncateFile(filepath string, position int64) error {
 		return fmt.Errorf("failed to truncate file: %w", err)
 	}
 
+	// Sync to disk to ensure changes are persisted before subsequent operations
+	if err := file.Sync(); err != nil {
+		return fmt.Errorf("failed to sync file after truncation: %w", err)
+	}
+
 	return nil
 }