make MErrror thread safe

go.mod

@@ -1 +1,5 @@
 module github.com/wiggin77/merror
+
+go 1.15
+
+require github.com/stretchr/testify v1.6.1

go.sum

@@ -0,0 +1,10 @@
+github.com/davecgh/go-spew v1.1.0 h1:ZDRjVQ15GmhC3fiQ8ni8+OwkZQO4DARzQgrnXU1Liz8=
+github.com/davecgh/go-spew v1.1.0/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
+github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
+github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
+github.com/stretchr/objx v0.1.0/go.mod h1:HFkY916IF+rwdDfMAkV7OtwuqBVzrE8GR6GFx+wExME=
+github.com/stretchr/testify v1.6.1 h1:hDPOHmpOpP40lSULcqw7IrRb/u7w6RpDC9399XyoNd0=
+github.com/stretchr/testify v1.6.1/go.mod h1:6Fq8oRcR53rry900zMqJjRRixrwX3KX962/h/Wwjteg=
+gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
+gopkg.in/yaml.v3 v3.0.0-20200313102051-9f266ea9e77c h1:dUUwHk2QECo/6vqA44rthZ8ie2QXMNeKRTHCNY2nXvo=
+gopkg.in/yaml.v3 v3.0.0-20200313102051-9f266ea9e77c/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=

merror.go

@@ -1,9 +1,13 @@
 package merror
 
+import "sync"
+
 // MError represents zero or more errors that can be
 // accumulated via the `Append` method.
 type MError struct {
-	cap       int
+	cap int
+
+	mux       sync.RWMutex
 	errors    []error
 	overflow  int
 	formatter FormatterFunc
@@ -34,33 +38,52 @@ func (me *MError) Append(err error) {
 	if err == nil {
 		return
 	}
+
+	me.mux.Lock()
+	defer me.mux.Unlock()
+
 	if me.cap > 0 && len(me.errors) >= me.cap {
 		me.overflow++
 	} else {
 		me.errors = append(me.errors, err)
 	}
 }
 
-// Errors returns an array of the `error` instances that have been
+// Errors returns a slice of the `error` instances that have been
 // appended to this `MError`.
 func (me *MError) Errors() []error {
-	return me.errors
+	me.mux.RLock()
+	defer me.mux.RUnlock()
+
+	errs := make([]error, len(me.errors))
+	copy(errs, me.errors)
+
+	return errs
 }
 
 // Len returns the number of errors that have been appended.
 func (me *MError) Len() int {
+	me.mux.RLock()
+	defer me.mux.RUnlock()
+
 	return len(me.errors)
 }
 
 // Overflow returns the number of errors that have been truncated
 // because maximum capacity was exceeded.
 func (me *MError) Overflow() int {
+	me.mux.RLock()
+	defer me.mux.RUnlock()
+
 	return me.overflow
 }
 
 // SetFormatter sets the `FormatterFunc` to be used when `Error` is
 // called. The previous `FormatterFunc` is returned.
 func (me *MError) SetFormatter(f FormatterFunc) (old FormatterFunc) {
+	me.mux.Lock()
+	defer me.mux.Unlock()
+
 	old = me.formatter
 	me.formatter = f
 	return
@@ -69,7 +92,14 @@ func (me *MError) SetFormatter(f FormatterFunc) (old FormatterFunc) {
 // ErrorOrNil returns nil if this `MError` contains no errors,
 // otherwise this `MError` is returned.
 func (me *MError) ErrorOrNil() error {
-	if me == nil || len(me.errors) == 0 {
+	if me == nil {
+		return nil
+	}
+
+	me.mux.RLock()
+	defer me.mux.RUnlock()
+
+	if len(me.errors) == 0 {
 		return nil
 	}
 	return me
@@ -79,6 +109,9 @@ func (me *MError) ErrorOrNil() error {
 // The output format depends on the `Formatter` set for this
 // merror instance, or the global formatter if none set.
 func (me *MError) Error() string {
+	me.mux.RLock()
+	defer me.mux.RUnlock()
+
 	f := me.formatter
 	if f == nil {
 		f = GlobalFormatter

merror_test.go

@@ -0,0 +1,33 @@
+package merror
+
+import (
+	"fmt"
+	"sync"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+const (
+	NumThreads = 25
+	NumErrors  = 5
+)
+
+func TestRace(t *testing.T) {
+	merr := New()
+	wg := &sync.WaitGroup{}
+	wg.Add(NumThreads)
+
+	for i := 0; i < NumThreads; i++ {
+		go func(id int) {
+			defer wg.Done()
+			for j := 0; j < NumErrors; j++ {
+				merr.Append(fmt.Errorf("Goroutine: %d   ErrorNum: %d", id, j))
+			}
+		}(i)
+	}
+
+	wg.Wait()
+
+	assert.Equal(t, NumThreads*NumErrors, len(merr.Errors()))
+}