template: move the empty check into parse, which needs it when constructing

tree sets.

R=golang-dev, gri
CC=golang-dev
https://golang.org/cl/5449062

2 files changed, 59 insertions, 2 deletions

diff --git a/src/pkg/text/template/parse/parse.go b/src/pkg/text/template/parse/parse.go
index 346f613b04..4da756657d 100644
--- a/src/pkg/text/template/parse/parse.go
+++ b/src/pkg/text/template/parse/parse.go
@@ -7,6 +7,7 @@
 package parse
 
 import (
+	"bytes"
 	"fmt"
 	"runtime"
 	"strconv"
@@ -177,10 +178,37 @@ func (t *Tree) Parse(s, leftDelim, rightDelim string, treeSet map[string]*Tree,
 
 // add adds tree to the treeSet.
 func (t *Tree) add(treeSet map[string]*Tree) {
-	if _, present := treeSet[t.Name]; present {
+	tree := treeSet[t.Name]
+	if tree == nil || IsEmptyTree(tree.Root) {
+		treeSet[t.Name] = t
+		return
+	}
+	if !IsEmptyTree(t.Root) {
 		t.errorf("template: multiple definition of template %q", t.Name)
 	}
-	treeSet[t.Name] = t
+}
+
+// IsEmptyTree reports whether this tree (node) is empty of everything but space.
+func IsEmptyTree(n Node) bool {
+	switch n := n.(type) {
+	case *ActionNode:
+	case *IfNode:
+	case *ListNode:
+		for _, node := range n.Nodes {
+			if !IsEmptyTree(node) {
+				return false
+			}
+		}
+		return true
+	case *RangeNode:
+	case *TemplateNode:
+	case *TextNode:
+		return len(bytes.TrimSpace(n.Text)) == 0
+	case *WithNode:
+	default:
+		panic("unknown node: " + n.String())
+	}
+	return false
 }
 
 // parse is the top-level parser for a template, essentially the same
diff --git a/src/pkg/text/template/parse/parse_test.go b/src/pkg/text/template/parse/parse_test.go
index fc93455ecb..b70c2143d3 100644
--- a/src/pkg/text/template/parse/parse_test.go
+++ b/src/pkg/text/template/parse/parse_test.go
@@ -257,3 +257,32 @@ func TestParse(t *testing.T) {
 		}
 	}
 }
+
+type isEmptyTest struct {
+	name  string
+	input string
+	empty bool
+}
+
+var isEmptyTests = []isEmptyTest{
+	{"empty", ``, true},
+	{"nonempty", `hello`, false},
+	{"spaces only", " \t\n \t\n", true},
+	{"definition", `{{define "x"}}something{{end}}`, true},
+	{"definitions and space", "{{define `x`}}something{{end}}\n\n{{define `y`}}something{{end}}\n\n", true},
+	{"definitions and text", "{{define `x`}}something{{end}}\nx\n{{define `y`}}something{{end}}\ny\n}}", false},
+	{"definition and action", "{{define `x`}}something{{end}}{{if 3}}foo{{end}}", false},
+}
+
+func TestIsEmpty(t *testing.T) {
+	for _, test := range isEmptyTests {
+		tree, err := New("root").Parse(test.input, "", "", make(map[string]*Tree), nil)
+		if err != nil {
+			t.Errorf("%q: unexpected error: %v", test.name, err)
+			continue
+		}
+		if empty := IsEmptyTree(tree.Root); empty != test.empty {
+			t.Errorf("%q: expected %t got %t", test.name, test.empty, empty)
+		}
+	}
+}