fix(logging): make level and field-name access concurrency-safe

SetFieldName wrote a package-global map with no lock while log read it
from many goroutines, and the level, caller, and time-format setters
mutated fields without synchronisation. A runtime setter call could
crash the process with a concurrent map write.

The scalar fields now use atomics on the hot log path and the global
field-name map uses a read-write mutex. Behaviour for init-time
configuration is unchanged.

logging/logger_race_test.go

@@ -2,8 +2,12 @@ package libpack_logger
 
 import (
 	"bytes"
+	"strings"
 	"sync"
 	"testing"
+	"time"
+
+	"github.com/goccy/go-json"
 )
 
 // Test_LogConcurrentAccess verifies that the logger correctly handles concurrent access
@@ -52,3 +56,84 @@ func TestLogConcurrentAccess(t *testing.T) {
 		t.Error("Expected log output, but got none")
 	}
 }
+
+// TestConcurrentSettersDuringLog verifies that SetFieldName, SetMinLogLevel,
+// SetShowCaller, and SetTimeFormat can run concurrently with active log()
+// calls without a data race.
+//
+// Before the fix, SetFieldName wrote the package-global fieldNames map with
+// no lock while log() read it from many goroutines, and
+// SetMinLogLevel/SetShowCaller/SetTimeFormat mutated Logger fields with no
+// lock while shouldLog()/log() read them. This test fails under
+// `go test -race` on that code and passes once the setters and readers are
+// synchronized.
+func TestConcurrentSettersDuringLog(t *testing.T) {
+	// fieldNames is package-global, so snapshot and restore it: this test
+	// mutates it concurrently and must not leak state into other tests.
+	originalFieldNames := make(map[string]string)
+	for k, v := range fieldNames {
+		originalFieldNames[k] = v
+	}
+	defer func() {
+		for k, v := range originalFieldNames {
+			fieldNames[k] = v
+		}
+	}()
+
+	output := &bytes.Buffer{}
+	logger := New().SetOutput(output).SetMinLogLevel(LEVEL_DEBUG)
+
+	const (
+		numLoggers = 50
+		numSetters = 50
+		iterations = 200
+	)
+
+	var wg sync.WaitGroup
+	wg.Add(numLoggers + numSetters)
+
+	// Goroutines that continuously log, exercising log()'s reads of the
+	// global fieldNames map and the Logger's minLogLevel/showCaller/
+	// timeFormat fields.
+	for i := 0; i < numLoggers; i++ {
+		go func(id int) {
+			defer wg.Done()
+			msg := &LogMessage{Message: "concurrent setter test"}
+			for j := 0; j < iterations; j++ {
+				logger.Info(msg)
+				logger.Debug(msg)
+			}
+		}(i)
+	}
+
+	// Goroutines that continuously mutate logger-wide configuration while
+	// logging is in flight from the goroutines above.
+	for i := 0; i < numSetters; i++ {
+		go func(id int) {
+			defer wg.Done()
+			for j := 0; j < iterations; j++ {
+				logger.SetFieldName("timestamp", "ts")
+				logger.SetMinLogLevel(LEVEL_DEBUG)
+				logger.SetShowCaller(id%2 == 0)
+				logger.SetTimeFormat(time.RFC3339Nano)
+			}
+		}(i)
+	}
+
+	wg.Wait()
+
+	if output.Len() == 0 {
+		t.Fatal("Expected log output, but got none")
+	}
+
+	// Each emitted line must still be well-formed JSON: the concurrent
+	// setter calls must not corrupt log() output, only race on which
+	// setting "wins" at each point in time.
+	lines := strings.Split(strings.TrimSpace(output.String()), "\n")
+	for i, line := range lines {
+		var decoded map[string]any
+		if err := json.Unmarshal([]byte(line), &decoded); err != nil {
+			t.Fatalf("line %d is not valid JSON: %v\nline: %s", i, err, line)
+		}
+	}
+}