Make SetHeaderNormalizer safe to call while decoding

SetHeaderNormalizer reassigned the normalizer and copied a fresh sync.Map
over the struct info cache while other goroutines were using it. Keep both
in one value swapped atomically instead. Fixes #237.

csv.go

@@ -16,6 +16,7 @@ import (
 	"reflect"
 	"strings"
 	"sync"
+	"sync/atomic"
 )
 
 // FailIfUnmatchedStructTags indicates whether it is considered an error when there is an unmatched
@@ -47,17 +48,36 @@ type Normalizer func(string) string
 
 type ErrorHandler func(*csv.ParseError) bool
 
-// normalizeName function initially set to a nop Normalizer.
-var normalizeName = DefaultNameNormalizer()
+// headerNormalization is the Normalizer in use together with the struct infos
+// built with it. SetHeaderNormalizer replaces both at once, so it is safe to
+// call while other goroutines are reading or writing CSV.
+type headerNormalization struct {
+	normalize   Normalizer
+	structInfos sync.Map
+}
+
+// currentHeaderNormalization holds a *headerNormalization, initially with a nop Normalizer.
+var currentHeaderNormalization atomic.Value
+
+func init() {
+	currentHeaderNormalization.Store(&headerNormalization{normalize: DefaultNameNormalizer()})
+}
+
+func loadHeaderNormalization() *headerNormalization {
+	return currentHeaderNormalization.Load().(*headerNormalization)
+}
+
+func normalizeName(name string) string {
+	return loadHeaderNormalization().normalize(name)
+}
 
 // DefaultNameNormalizer is a nop Normalizer.
 func DefaultNameNormalizer() Normalizer { return func(s string) string { return s } }
 
 // SetHeaderNormalizer sets the normalizer used to normalize struct and header field names.
 func SetHeaderNormalizer(f Normalizer) {
-	normalizeName = f
-	// Need to clear the cache hen the header normalizer changes.
-	structInfoCache = sync.Map{}
+	// The struct infos are rebuilt from scratch, as they hold normalized names.
+	currentHeaderNormalization.Store(&headerNormalization{normalize: f})
 }
 
 // --------------------------------------------------------------------------

csv_test.go

@@ -2,6 +2,8 @@ package gocsv
 
 import (
 	"errors"
+	"strings"
+	"sync"
 	"testing"
 )
 
@@ -25,6 +27,36 @@ func TestUnmarshalToCallback_ReaderError(t *testing.T) {
 	}
 }
 
+func TestSetHeaderNormalizerWhileUnmarshaling(t *testing.T) {
+	defer SetHeaderNormalizer(DefaultNameNormalizer())
+
+	type row struct {
+		ID   string `csv:"id"`
+		Name string `csv:"name"`
+	}
+
+	var wg sync.WaitGroup
+	for i := 0; i < 50; i++ {
+		wg.Add(2)
+		go func() {
+			defer wg.Done()
+			SetHeaderNormalizer(strings.ToLower)
+		}()
+		go func() {
+			defer wg.Done()
+			var rows []row
+			if err := UnmarshalString("id,name\n1,foo\n", &rows); err != nil {
+				t.Error(err)
+				return
+			}
+			if len(rows) != 1 || rows[0].ID != "1" || rows[0].Name != "foo" {
+				t.Errorf("unexpected rows: %+v", rows)
+			}
+		}()
+	}
+	wg.Wait()
+}
+
 type errorReader struct{}
 
 func (e *errorReader) Read([]byte) (n int, err error) {

reflect.go

@@ -58,19 +58,19 @@ func removeZeroWidthChars(s string) string {
 	}, s)
 }
 
-var structInfoCache sync.Map
 var structMap = make(map[reflect.Type]*structInfo)
 var structMapMutex sync.RWMutex
 
 func getStructInfo(rType reflect.Type) *structInfo {
-	stInfo, ok := structInfoCache.Load(rType)
+	structInfos := &loadHeaderNormalization().structInfos
+	stInfo, ok := structInfos.Load(rType)
 	if ok {
 		return stInfo.(*structInfo)
 	}
 
 	fieldsList, err := getFieldInfos(rType, []int{}, []string{}, make(map[reflect.Type]bool))
 	stInfo = &structInfo{Fields: fieldsList, Err: err}
-	structInfoCache.Store(rType, stInfo)
+	structInfos.Store(rType, stInfo)
 
 	return stInfo.(*structInfo)
 }