fix: add write mutex for log writing to file

pkg/logger/logger.go

@@ -38,6 +38,14 @@ var (
 
 type Logger struct {
 	file *os.File
+
+	// writeMu serializes file writes.
+	// Although POSIX systems typically guarantee atomicity
+	// for single write() syscalls on regular files,
+	// Windows and some filesystems may not.
+	//
+	// This prevents log line interleaving across platforms.
+	writeMu sync.Mutex
 }
 
 type LogEntry struct {
@@ -116,13 +124,23 @@ func logMessage(level LogLevel, component string, message string, fields map[str
 		}
 	}
 
-	if logger.file != nil {
+	mu.RLock()
+	fileptr := logger.file
+
+	if fileptr != nil {
 		jsonData, err := json.Marshal(entry)
 		if err == nil {
-			logger.file.Write(append(jsonData, '\n'))
+			logger.writeMu.Lock()
+			_, err = fileptr.WriteString(string(jsonData) + "\n")
+			logger.writeMu.Unlock()
+			if err != nil {
+				log.Println("Failed to write to file:", err.Error())
+			}
 		}
 	}
 
+	mu.RUnlock()
+
 	var fieldStr string
 	if len(fields) > 0 {
 		fieldStr = " " + formatFields(fields)

pkg/logger/logger_test.go

@@ -1,6 +1,11 @@
 package logger
 
 import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+	"sync"
 	"testing"
 )
 
@@ -137,3 +142,31 @@ func TestLoggerHelperFunctions(t *testing.T) {
 	DebugC("test", "Debug with component")
 	WarnF("Warning with fields", map[string]any{"key": "value"})
 }
+
+// TestConcurrentFileLogging validates concurrent file writes are race-safe and complete.
+// Run: go test -race ./pkg/logger -run TestConcurrentFileLogging -count=1
+func TestConcurrentFileLogging(t *testing.T) {
+	dir := t.TempDir()
+	logFile := filepath.Join(dir, "test.log")
+	if err := EnableFileLogging(logFile); err != nil {
+		t.Fatal(err)
+	}
+	defer DisableFileLogging()
+	var wg sync.WaitGroup
+	for i := 0; i < 100; i++ {
+		wg.Add(1)
+		go func(n int) {
+			defer wg.Done()
+			InfoCF("test", fmt.Sprintf("msg-%d", n), map[string]any{"n": n})
+		}(i)
+	}
+	wg.Wait()
+	data, err := os.ReadFile(logFile)
+	if err != nil {
+		t.Fatal(err)
+	}
+	lines := strings.Split(strings.TrimSpace(string(data)), "\n")
+	if len(lines) != 100 {
+		t.Errorf("expected 100 log lines, got %d", len(lines))
+	}
+}