text/template: add lock for Template.tmpl to fix data race

This adds a new lock protecting "tmpl".

This is a copy of https://golang.org/cl/257817 by Andreas Fleig,
updated for current tip, and updated to start running the
html/template TestEscapeRace test.

Thanks to @bep for providing the test case.

Fixes #39807

Change-Id: Ic8874484290283a49116812eeaffb8608346dc70
Reviewed-on: https://go-review.googlesource.com/c/go/+/316669
Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

src/html/template/exec_test.go

@@ -1720,8 +1720,6 @@ var v = "v";
 `
 
 func TestEscapeRace(t *testing.T) {
-	t.Skip("this test currently fails with -race; see issue #39807")
-
 	tmpl := New("")
 	_, err := tmpl.New("templ.html").Parse(raceText)
 	if err != nil {

src/text/template/exec.go

@@ -179,10 +179,7 @@ func errRecover(errp *error) {
 // A template may be executed safely in parallel, although if parallel
 // executions share a Writer the output may be interleaved.
 func (t *Template) ExecuteTemplate(wr io.Writer, name string, data interface{}) error {
-	var tmpl *Template
-	if t.common != nil {
-		tmpl = t.tmpl[name]
-	}
+	tmpl := t.Lookup(name)
 	if tmpl == nil {
 		return fmt.Errorf("template: no template %q associated with template %q", name, t.name)
 	}
@@ -230,6 +227,8 @@ func (t *Template) DefinedTemplates() string {
 		return ""
 	}
 	var b strings.Builder
+	t.muTmpl.RLock()
+	defer t.muTmpl.RUnlock()
 	for name, tmpl := range t.tmpl {
 		if tmpl.Tree == nil || tmpl.Root == nil {
 			continue
@@ -401,7 +400,7 @@ func (s *state) walkRange(dot reflect.Value, r *parse.RangeNode) {
 
 func (s *state) walkTemplate(dot reflect.Value, t *parse.TemplateNode) {
 	s.at(t)
-	tmpl := s.tmpl.tmpl[t.Name]
+	tmpl := s.tmpl.Lookup(t.Name)
 	if tmpl == nil {
 		s.errorf("template %q not defined", t.Name)
 	}

src/text/template/exec_test.go

@@ -12,6 +12,7 @@ import (
 	"io"
 	"reflect"
 	"strings"
+	"sync"
 	"testing"
 )
 
@@ -1732,3 +1733,40 @@ func TestIssue43065(t *testing.T) {
 		t.Errorf("%s", err)
 	}
 }
+
+// Issue 39807: data race in html/template & text/template
+func TestIssue39807(t *testing.T) {
+	var wg sync.WaitGroup
+
+	tplFoo, err := New("foo").Parse(`{{ template "bar" . }}`)
+	if err != nil {
+		t.Error(err)
+	}
+
+	tplBar, err := New("bar").Parse("bar")
+	if err != nil {
+		t.Error(err)
+	}
+
+	gofuncs := 10
+	numTemplates := 10
+
+	for i := 1; i <= gofuncs; i++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			for j := 0; j < numTemplates; j++ {
+				_, err := tplFoo.AddParseTree(tplBar.Name(), tplBar.Tree)
+				if err != nil {
+					t.Error(err)
+				}
+				err = tplFoo.Execute(io.Discard, nil)
+				if err != nil {
+					t.Error(err)
+				}
+			}
+		}()
+	}
+
+	wg.Wait()
+}

src/text/template/template.go

@@ -13,6 +13,7 @@ import (
 // common holds the information shared by related templates.
 type common struct {
 	tmpl   map[string]*Template // Map from name to defined templates.
+	muTmpl sync.RWMutex         // protects tmpl
 	option option
 	// We use two maps, one for parsing and one for execution.
 	// This separation makes the API cleaner since it doesn't
@@ -88,6 +89,8 @@ func (t *Template) Clone() (*Template, error) {
 	if t.common == nil {
 		return nt, nil
 	}
+	t.muTmpl.RLock()
+	defer t.muTmpl.RUnlock()
 	for k, v := range t.tmpl {
 		if k == t.name {
 			nt.tmpl[t.name] = nt
@@ -124,6 +127,8 @@ func (t *Template) copy(c *common) *Template {
 // its definition. If it has been defined and already has that name, the existing
 // definition is replaced; otherwise a new template is created, defined, and returned.
 func (t *Template) AddParseTree(name string, tree *parse.Tree) (*Template, error) {
+	t.muTmpl.Lock()
+	defer t.muTmpl.Unlock()
 	t.init()
 	nt := t
 	if name != t.name {
@@ -142,6 +147,8 @@ func (t *Template) Templates() []*Template {
 		return nil
 	}
 	// Return a slice so we don't expose the map.
+	t.muTmpl.RLock()
+	defer t.muTmpl.RUnlock()
 	m := make([]*Template, 0, len(t.tmpl))
 	for _, v := range t.tmpl {
 		m = append(m, v)
@@ -182,6 +189,8 @@ func (t *Template) Lookup(name string) *Template {
 	if t.common == nil {
 		return nil
 	}
+	t.muTmpl.RLock()
+	defer t.muTmpl.RUnlock()
 	return t.tmpl[name]
 }