Guard concurrent groupnames computation (#67)

Author: anuraaga

concurrency_test.go

@@ -31,3 +31,20 @@ func TestConcurrency(t *testing.T) {
 
 	wg.Wait()
 }
+
+func TestConcurrenctSubgroupNames(t *testing.T) {
+	r := MustCompile("(?P<foo>foo)(?P<bar>bar)(?P<baz>baz)")
+
+	wg := &sync.WaitGroup{}
+	for i := 0; i < 20; i++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			if len(r.SubexpNames()) != 4 {
+				t.Error("wrong number of subgroup names")
+			}
+		}()
+	}
+
+	wg.Wait()
+}

internal/re2.go

@@ -5,6 +5,7 @@ import (
 	"runtime"
 	"strconv"
 	"strings"
+	"sync"
 	"sync/atomic"
 	"unicode"
 	"unicode/utf8"
@@ -17,8 +18,9 @@ type Regexp struct {
 
 	expr string
 
-	numMatches int
-	groupNames []string
+	numMatches     int
+	groupNames     []string
+	groupNamesOnce sync.Once
 
 	abi *libre2ABI
 
@@ -698,9 +700,9 @@ func (re *Regexp) Split(s string, n int) []string {
 // Since the Regexp as a whole cannot be named, names[0] is always
 // the empty string. The slice should not be modified.
 func (re *Regexp) SubexpNames() []string {
-	if re.groupNames == nil {
+	re.groupNamesOnce.Do(func() {
 		re.groupNames = subexpNames(re.abi, re.ptr, re.numMatches)
-	}
+	})
 	return re.groupNames
 }