text/template, html/template: add block keyword and permit template redefinition

This change adds a new "block" keyword that permits the definition
of templates inline inside existing templates, and loosens the
restriction on template redefinition. Templates may now be redefined,
but in the html/template package they may only be redefined before
the template is executed (and therefore escaped).

The intention is that such inline templates can be redefined by
subsequent template definitions, permitting a kind of template
"inheritance" or "overlay". (See the example for details.)

Fixes #3812

Change-Id: I733cb5332c1c201c235f759cc64333462e70dc27
Reviewed-on: https://go-review.googlesource.com/14005
Reviewed-by: Rob Pike <r@golang.org>

3 files changed, 49 insertions, 3 deletions

diff --git a/src/html/template/clone_test.go b/src/html/template/clone_test.go
index c89d22a6f9..a0f1d6a048 100644
--- a/src/html/template/clone_test.go
+++ b/src/html/template/clone_test.go
@@ -78,9 +78,17 @@ func TestClone(t *testing.T) {
 	Must(t0.Parse(`{{define "lhs"}} ( {{end}}`))
 	Must(t0.Parse(`{{define "rhs"}} ) {{end}}`))
 
-	// Clone t0 as t4. Redefining the "lhs" template should fail.
+	// Clone t0 as t4. Redefining the "lhs" template should not fail.
 	t4 := Must(t0.Clone())
-	if _, err := t4.Parse(`{{define "lhs"}} FAIL {{end}}`); err == nil {
+	if _, err := t4.Parse(`{{define "lhs"}} OK {{end}}`); err != nil {
+		t.Error(`redefine "lhs": got err %v want non-nil`, err)
+	}
+	// Cloning t1 should fail as it has been executed.
+	if _, err := t1.Clone(); err == nil {
+		t.Error("cloning t1: got nil err want non-nil")
+	}
+	// Redefining the "lhs" template in t1 should fail as it has been executed.
+	if _, err := t1.Parse(`{{define "lhs"}} OK {{end}}`); err == nil {
 		t.Error(`redefine "lhs": got nil err want non-nil`)
 	}
 
diff --git a/src/html/template/example_test.go b/src/html/template/example_test.go
index a75ceec480..a7c2905098 100644
--- a/src/html/template/example_test.go
+++ b/src/html/template/example_test.go
@@ -9,6 +9,7 @@ import (
 	"html/template"
 	"log"
 	"os"
+	"strings"
 )
 
 func Example() {
@@ -120,3 +121,38 @@ func Example_escape() {
 	// %22Fran+%26+Freddie%27s+Diner%2232%3Ctasty%40example.com%3E
 
 }
+
+// The following example is duplicated in text/template; keep them in sync.
+
+func ExampleBlock() {
+	const (
+		master  = `Names:{{block "list" .}}{{"\n"}}{{range .}}{{println "-" .}}{{end}}{{end}}`
+		overlay = `{{define "list"}} {{join . ", "}}{{end}} `
+	)
+	var (
+		funcs     = template.FuncMap{"join": strings.Join}
+		guardians = []string{"Gamora", "Groot", "Nebula", "Rocket", "Star-Lord"}
+	)
+	masterTmpl, err := template.New("master").Funcs(funcs).Parse(master)
+	if err != nil {
+		log.Fatal(err)
+	}
+	overlayTmpl, err := template.Must(masterTmpl.Clone()).Parse(overlay)
+	if err != nil {
+		log.Fatal(err)
+	}
+	if err := masterTmpl.Execute(os.Stdout, guardians); err != nil {
+		log.Fatal(err)
+	}
+	if err := overlayTmpl.Execute(os.Stdout, guardians); err != nil {
+		log.Fatal(err)
+	}
+	// Output:
+	// Names:
+	// - Gamora
+	// - Groot
+	// - Nebula
+	// - Rocket
+	// - Star-Lord
+	// Names: Gamora, Groot, Nebula, Rocket, Star-Lord
+}
diff --git a/src/html/template/template.go b/src/html/template/template.go
index f9e6e43588..4c38f36e67 100644
--- a/src/html/template/template.go
+++ b/src/html/template/template.go
@@ -18,7 +18,7 @@ import (
 // Template is a specialized Template from "text/template" that produces a safe
 // HTML document fragment.
 type Template struct {
-	// Sticky error if escaping fails.
+	// Sticky error if escaping fails, or escapeOK if succeeded.
 	escapeErr error
 	// We could embed the text/template field, but it's safer not to because
 	// we need to keep our version of the name space and the underlying
@@ -170,6 +170,8 @@ func (t *Template) Parse(src string) (*Template, error) {
 		tmpl := t.set[name]
 		if tmpl == nil {
 			tmpl = t.new(name)
+		} else if tmpl.escapeErr != nil {
+			return nil, fmt.Errorf("html/template: cannot redefine %q after it has executed", name)
 		}
 		// Restore our record of this text/template to its unescaped original state.
 		tmpl.escapeErr = nil