text/template/parse, html/template: copy Tree.text during html template clone

The root cause of the panic reported in https://code.google.com/p/go/issues/detail?id=5980
is that parse's Tree.Text wasn't being copied during the clone.

Fix this by adding and using a Copy method for parse.Tree.

Fixes #5980.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/12420044

2 files changed, 29 insertions, 0 deletions

diff --git a/src/pkg/text/template/parse/parse.go b/src/pkg/text/template/parse/parse.go
index be83e77cf5..34112fb7b3 100644
--- a/src/pkg/text/template/parse/parse.go
+++ b/src/pkg/text/template/parse/parse.go
@@ -30,6 +30,19 @@ type Tree struct {
 	vars      []string // variables defined at the moment.
 }
 
+// Copy returns a copy of the Tree. Any parsing state is discarded.
+func (t *Tree) Copy() *Tree {
+	if t == nil {
+		return nil
+	}
+	return &Tree{
+		Name:      t.Name,
+		ParseName: t.ParseName,
+		Root:      t.Root.CopyList(),
+		text:      t.text,
+	}
+}
+
 // Parse returns a map from template name to parse.Tree, created by parsing the
 // templates described in the argument string. The top-level template will be
 // given the specified name. If an error is encountered, parsing stops and an
diff --git a/src/pkg/text/template/parse/parse_test.go b/src/pkg/text/template/parse/parse_test.go
index 049e65c7d3..ba1a18ec54 100644
--- a/src/pkg/text/template/parse/parse_test.go
+++ b/src/pkg/text/template/parse/parse_test.go
@@ -332,6 +332,22 @@ func TestIsEmpty(t *testing.T) {
 	}
 }
 
+func TestErrorContextWithTreeCopy(t *testing.T) {
+	tree, err := New("root").Parse("{{if true}}{{end}}", "", "", make(map[string]*Tree), nil)
+	if err != nil {
+		t.Fatalf("unexpected tree parse failure: %v", err)
+	}
+	treeCopy := tree.Copy()
+	wantLocation, wantContext := tree.ErrorContext(tree.Root.Nodes[0])
+	gotLocation, gotContext := treeCopy.ErrorContext(treeCopy.Root.Nodes[0])
+	if wantLocation != gotLocation {
+		t.Errorf("wrong error location want %q got %q", wantLocation, gotLocation)
+	}
+	if wantContext != gotContext {
+		t.Errorf("wrong error location want %q got %q", wantContext, gotContext)
+	}
+}
+
 // All failures, and the result is a string that must appear in the error message.
 var errorTests = []parseTest{
 	// Check line numbers are accurate.