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/14005Reviewed-by: Rob Pike <r@golang.org>

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`)
 	}
 

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
+}

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

src/text/template/doc.go

@@ -115,6 +115,14 @@ data, defined in detail below.
 		The template with the specified name is executed with dot set
 		to the value of the pipeline.
 
+	{{block "name" pipeline}} T1 {{end}}
+		A block is shorthand for defining a template
+			{{define "name"}} T1 {{end}}
+		and then executing it in place
+			{{template "name" .}}
+		The typical use is to define a set of root templates that are
+		then customized by redefining the block templates within.
+
 	{{with pipeline}} T1 {{end}}
 		If the value of the pipeline is empty, no output is generated;
 		otherwise, dot is set to the value of the pipeline and T1 is

src/text/template/example_test.go

@@ -7,6 +7,7 @@ package template_test
 import (
 	"log"
 	"os"
+	"strings"
 	"text/template"
 )
 
@@ -72,3 +73,38 @@ Josie
 	// Best wishes,
 	// Josie
 }
+
+// The following example is duplicated in html/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
+}

src/text/template/exec_test.go

@@ -1232,3 +1232,36 @@ func testBadFuncName(name string, t *testing.T) {
 	// reports an error.
 	t.Errorf("%q succeeded incorrectly as function name", name)
 }
+
+func TestBlock(t *testing.T) {
+	const (
+		input   = `a({{block "inner" .}}bar({{.}})baz{{end}})b`
+		want    = `a(bar(hello)baz)b`
+		overlay = `{{define "inner"}}foo({{.}})bar{{end}}`
+		want2   = `a(foo(goodbye)bar)b`
+	)
+	tmpl, err := New("outer").Parse(input)
+	if err != nil {
+		t.Fatal(err)
+	}
+	tmpl2, err := Must(tmpl.Clone()).Parse(overlay)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	var buf bytes.Buffer
+	if err := tmpl.Execute(&buf, "hello"); err != nil {
+		t.Fatal(err)
+	}
+	if got := buf.String(); got != want {
+		t.Errorf("got %q, want %q", got, want)
+	}
+
+	buf.Reset()
+	if err := tmpl2.Execute(&buf, "goodbye"); err != nil {
+		t.Fatal(err)
+	}
+	if got := buf.String(); got != want2 {
+		t.Errorf("got %q, want %q", got, want2)
+	}
+}

src/text/template/multi_test.go

@@ -9,7 +9,6 @@ package template
 import (
 	"bytes"
 	"fmt"
-	"strings"
 	"testing"
 	"text/template/parse"
 )
@@ -277,17 +276,11 @@ func TestRedefinition(t *testing.T) {
 	if tmpl, err = New("tmpl1").Parse(`{{define "test"}}foo{{end}}`); err != nil {
 		t.Fatalf("parse 1: %v", err)
 	}
-	if _, err = tmpl.Parse(`{{define "test"}}bar{{end}}`); err == nil {
-		t.Fatal("expected error")
+	if _, err = tmpl.Parse(`{{define "test"}}bar{{end}}`); err != nil {
+		t.Fatal("got error %v, expected nil", err)
 	}
-	if !strings.Contains(err.Error(), "redefinition") {
-		t.Fatalf("expected redefinition error; got %v", err)
-	}
-	if _, err = tmpl.New("tmpl2").Parse(`{{define "test"}}bar{{end}}`); err == nil {
-		t.Fatal("expected error")
-	}
-	if !strings.Contains(err.Error(), "redefinition") {
-		t.Fatalf("expected redefinition error; got %v", err)
+	if _, err = tmpl.New("tmpl2").Parse(`{{define "test"}}bar{{end}}`); err != nil {
+		t.Fatal("got error %v, expected nil", err)
 	}
 }
 
@@ -345,7 +338,6 @@ func TestNew(t *testing.T) {
 func TestParse(t *testing.T) {
 	// In multiple calls to Parse with the same receiver template, only one call
 	// can contain text other than space, comments, and template definitions
-	var err error
 	t1 := New("test")
 	if _, err := t1.Parse(`{{define "test"}}{{end}}`); err != nil {
 		t.Fatalf("parsing test: %s", err)
@@ -356,10 +348,4 @@ func TestParse(t *testing.T) {
 	if _, err := t1.Parse(`{{define "test"}}foo{{end}}`); err != nil {
 		t.Fatalf("parsing test: %s", err)
 	}
-	if _, err = t1.Parse(`{{define "test"}}foo{{end}}`); err == nil {
-		t.Fatal("no error from redefining a template")
-	}
-	if !strings.Contains(err.Error(), "redefinition") {
-		t.Fatalf("expected redefinition error; got %v", err)
-	}
 }

src/text/template/parse/lex.go

@@ -58,6 +58,7 @@ const (
 	itemVariable   // variable starting with '$', such as '$' or  '$1' or '$hello'
 	// Keywords appear after all the rest.
 	itemKeyword  // used only to delimit the keywords
+	itemBlock    // block keyword
 	itemDot      // the cursor, spelled '.'
 	itemDefine   // define keyword
 	itemElse     // else keyword
@@ -71,6 +72,7 @@ const (
 
 var key = map[string]itemType{
 	".":        itemDot,
+	"block":    itemBlock,
 	"define":   itemDefine,
 	"else":     itemElse,
 	"end":      itemEnd,

src/text/template/parse/lex_test.go

@@ -33,6 +33,7 @@ var itemName = map[itemType]string{
 
 	// keywords
 	itemDot:      ".",
+	itemBlock:    "block",
 	itemDefine:   "define",
 	itemElse:     "else",
 	itemIf:       "if",
@@ -58,6 +59,8 @@ type lexTest struct {
 }
 
 var (
+	tDot        = item{itemDot, 0, "."}
+	tBlock      = item{itemBlock, 0, "block"}
 	tEOF        = item{itemEOF, 0, ""}
 	tFor        = item{itemIdentifier, 0, "for"}
 	tLeft       = item{itemLeftDelim, 0, "{{"}
@@ -104,6 +107,9 @@ var lexTests = []lexTest{
 	}},
 	{"empty action", `{{}}`, []item{tLeft, tRight, tEOF}},
 	{"for", `{{for}}`, []item{tLeft, tFor, tRight, tEOF}},
+	{"block", `{{block "foo" .}}`, []item{
+		tLeft, tBlock, tSpace, {itemString, 0, `"foo"`}, tSpace, tDot, tRight, tEOF,
+	}},
 	{"quote", `{{"abc \n\t\" "}}`, []item{tLeft, tQuote, tRight, tEOF}},
 	{"raw quote", "{{" + raw + "}}", []item{tLeft, tRawQuote, tRight, tEOF}},
 	{"raw quote with newline", "{{" + rawNL + "}}", []item{tLeft, tRawQuoteNL, tRight, tEOF}},
@@ -155,7 +161,7 @@ var lexTests = []lexTest{
 	}},
 	{"dot", "{{.}}", []item{
 		tLeft,
-		{itemDot, 0, "."},
+		tDot,
 		tRight,
 		tEOF,
 	}},
@@ -169,7 +175,7 @@ var lexTests = []lexTest{
 		tLeft,
 		{itemField, 0, ".x"},
 		tSpace,
-		{itemDot, 0, "."},
+		tDot,
 		tSpace,
 		{itemNumber, 0, ".2"},
 		tSpace,
@@ -501,9 +507,9 @@ func TestShutdown(t *testing.T) {
 func (t *Tree) parseLexer(lex *lexer, text string) (tree *Tree, err error) {
 	defer t.recover(&err)
 	t.ParseName = t.Name
-	t.startParse(nil, lex)
-	t.parse(nil)
-	t.add(nil)
+	t.startParse(nil, lex, map[string]*Tree{})
+	t.parse()
+	t.add()
 	t.stopParse()
 	return t, nil
 }

src/text/template/parse/parse.go

@@ -28,6 +28,7 @@ type Tree struct {
 	token     [3]item // three-token lookahead for parser.
 	peekCount int
 	vars      []string // variables defined at the moment.
+	treeSet   map[string]*Tree
 }
 
 // Copy returns a copy of the Tree. Any parsing state is discarded.
@@ -205,11 +206,12 @@ func (t *Tree) recover(errp *error) {
 }
 
 // startParse initializes the parser, using the lexer.
-func (t *Tree) startParse(funcs []map[string]interface{}, lex *lexer) {
+func (t *Tree) startParse(funcs []map[string]interface{}, lex *lexer, treeSet map[string]*Tree) {
 	t.Root = nil
 	t.lex = lex
 	t.vars = []string{"$"}
 	t.funcs = funcs
+	t.treeSet = treeSet
 }
 
 // stopParse terminates parsing.
@@ -217,6 +219,7 @@ func (t *Tree) stopParse() {
 	t.lex = nil
 	t.vars = nil
 	t.funcs = nil
+	t.treeSet = nil
 }
 
 // Parse parses the template definition string to construct a representation of
@@ -226,19 +229,19 @@ func (t *Tree) stopParse() {
 func (t *Tree) Parse(text, leftDelim, rightDelim string, treeSet map[string]*Tree, funcs ...map[string]interface{}) (tree *Tree, err error) {
 	defer t.recover(&err)
 	t.ParseName = t.Name
-	t.startParse(funcs, lex(t.Name, text, leftDelim, rightDelim))
+	t.startParse(funcs, lex(t.Name, text, leftDelim, rightDelim), treeSet)
 	t.text = text
-	t.parse(treeSet)
-	t.add(treeSet)
+	t.parse()
+	t.add()
 	t.stopParse()
 	return t, nil
 }
 
-// add adds tree to the treeSet.
-func (t *Tree) add(treeSet map[string]*Tree) {
-	tree := treeSet[t.Name]
+// add adds tree to t.treeSet.
+func (t *Tree) add() {
+	tree := t.treeSet[t.Name]
 	if tree == nil || IsEmptyTree(tree.Root) {
-		treeSet[t.Name] = t
+		t.treeSet[t.Name] = t
 		return
 	}
 	if !IsEmptyTree(t.Root) {
@@ -274,7 +277,7 @@ func IsEmptyTree(n Node) bool {
 // parse is the top-level parser for a template, essentially the same
 // as itemList except it also parses {{define}} actions.
 // It runs to EOF.
-func (t *Tree) parse(treeSet map[string]*Tree) (next Node) {
+func (t *Tree) parse() (next Node) {
 	t.Root = t.newList(t.peek().pos)
 	for t.peek().typ != itemEOF {
 		if t.peek().typ == itemLeftDelim {
@@ -283,8 +286,8 @@ func (t *Tree) parse(treeSet map[string]*Tree) (next Node) {
 				newT := New("definition") // name will be updated once we know it.
 				newT.text = t.text
 				newT.ParseName = t.ParseName
-				newT.startParse(t.funcs, t.lex)
-				newT.parseDefinition(treeSet)
+				newT.startParse(t.funcs, t.lex, t.treeSet)
+				newT.parseDefinition()
 				continue
 			}
 			t.backup2(delim)
@@ -300,9 +303,9 @@ func (t *Tree) parse(treeSet map[string]*Tree) (next Node) {
 }
 
 // parseDefinition parses a {{define}} ...  {{end}} template definition and
-// installs the definition in the treeSet map.  The "define" keyword has already
+// installs the definition in t.treeSet. The "define" keyword has already
 // been scanned.
-func (t *Tree) parseDefinition(treeSet map[string]*Tree) {
+func (t *Tree) parseDefinition() {
 	const context = "define clause"
 	name := t.expectOneOf(itemString, itemRawString, context)
 	var err error
@@ -316,7 +319,7 @@ func (t *Tree) parseDefinition(treeSet map[string]*Tree) {
 	if end.Type() != nodeEnd {
 		t.errorf("unexpected %s in %s", end, context)
 	}
-	t.add(treeSet)
+	t.add()
 	t.stopParse()
 }
 
@@ -358,6 +361,8 @@ func (t *Tree) textOrAction() Node {
 // First word could be a keyword such as range.
 func (t *Tree) action() (n Node) {
 	switch token := t.nextNonSpace(); token.typ {
+	case itemBlock:
+		return t.blockControl()
 	case itemElse:
 		return t.elseControl()
 	case itemEnd:
@@ -522,13 +527,51 @@ func (t *Tree) elseControl() Node {
 	return t.newElse(t.expect(itemRightDelim, "else").pos, t.lex.lineNumber())
 }
 
+// Block:
+//	{{block stringValue pipeline}}
+// Block keyword is past.
+// The name must be something that can evaluate to a string.
+// The pipeline is mandatory.
+func (t *Tree) blockControl() Node {
+	const context = "block clause"
+
+	token := t.nextNonSpace()
+	name := t.parseTemplateName(token, context)
+	pipe := t.pipeline(context)
+
+	block := New(name) // name will be updated once we know it.
+	block.text = t.text
+	block.ParseName = t.ParseName
+	block.startParse(t.funcs, t.lex, t.treeSet)
+	var end Node
+	block.Root, end = block.itemList()
+	if end.Type() != nodeEnd {
+		t.errorf("unexpected %s in %s", end, context)
+	}
+	block.add()
+	block.stopParse()
+
+	return t.newTemplate(token.pos, t.lex.lineNumber(), name, pipe)
+}
+
 // Template:
 //	{{template stringValue pipeline}}
 // Template keyword is past.  The name must be something that can evaluate
 // to a string.
 func (t *Tree) templateControl() Node {
-	var name string
+	const context = "template clause"
 	token := t.nextNonSpace()
+	name := t.parseTemplateName(token, context)
+	var pipe *PipeNode
+	if t.nextNonSpace().typ != itemRightDelim {
+		t.backup()
+		// Do not pop variables; they persist until "end".
+		pipe = t.pipeline(context)
+	}
+	return t.newTemplate(token.pos, t.lex.lineNumber(), name, pipe)
+}
+
+func (t *Tree) parseTemplateName(token item, context string) (name string) {
 	switch token.typ {
 	case itemString, itemRawString:
 		s, err := strconv.Unquote(token.val)
@@ -537,15 +580,9 @@ func (t *Tree) templateControl() Node {
 		}
 		name = s
 	default:
-		t.unexpected(token, "template invocation")
-	}
-	var pipe *PipeNode
-	if t.nextNonSpace().typ != itemRightDelim {
-		t.backup()
-		// Do not pop variables; they persist until "end".
-		pipe = t.pipeline("template")
+		t.unexpected(token, context)
 	}
-	return t.newTemplate(token.pos, t.lex.lineNumber(), name, pipe)
+	return
 }
 
 // command:

src/text/template/parse/parse_test.go

@@ -235,6 +235,8 @@ var parseTests = []parseTest{
 	{"comment trim left", "x \r\n\t{{- /* hi */}}", noError, `"x"`},
 	{"comment trim right", "{{/* hi */ -}}\n\n\ty", noError, `"y"`},
 	{"comment trim left and right", "x \r\n\t{{- /* */ -}}\n\n\ty", noError, `"x""y"`},
+	{"block definition", `{{block "foo" .}}hello{{end}}`, noError,
+		`{{template "foo" .}}`},
 	// Errors.
 	{"unclosed action", "hello{{range", hasError, ""},
 	{"unmatched end", "{{end}}", hasError, ""},
@@ -284,6 +286,8 @@ var parseTests = []parseTest{
 	{"wrong pipeline boolean", "{{.|true}}", hasError, ""},
 	{"wrong pipeline nil", "{{'c'|nil}}", hasError, ""},
 	{"empty pipeline", `{{printf "%d" ( ) }}`, hasError, ""},
+	// Missing pipeline in block
+	{"block definition", `{{block "foo"}}hello{{end}}`, hasError, ""},
 }
 
 var builtins = map[string]interface{}{
@@ -457,3 +461,26 @@ func TestErrors(t *testing.T) {
 		}
 	}
 }
+
+func TestBlock(t *testing.T) {
+	const (
+		input = `a{{block "inner" .}}bar{{.}}baz{{end}}b`
+		outer = `a{{template "inner" .}}b`
+		inner = `bar{{.}}baz`
+	)
+	treeSet := make(map[string]*Tree)
+	tmpl, err := New("outer").Parse(input, "", "", treeSet, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if g, w := tmpl.Root.String(), outer; g != w {
+		t.Errorf("outer template = %q, want %q", g, w)
+	}
+	inTmpl := treeSet["inner"]
+	if inTmpl == nil {
+		t.Fatal("block did not define template")
+	}
+	if g, w := inTmpl.Root.String(), inner; g != w {
+		t.Errorf("inner template = %q, want %q", g, w)
+	}
+}

src/text/template/template.go

@@ -5,7 +5,6 @@
 package template
 
 import (
-	"fmt"
 	"reflect"
 	"sync"
 	"text/template/parse"
@@ -117,11 +116,10 @@ func (t *Template) copy(c *common) *Template {
 
 // AddParseTree adds parse tree for template with given name and associates it with t.
 // If the template does not already exist, it will create a new one.
-// It is an error to reuse a name except to overwrite an empty template.
+// If the template does exist, it will be replaced.
 func (t *Template) AddParseTree(name string, tree *parse.Tree) (*Template, error) {
 	t.init()
 	// If the name is the name of this template, overwrite this template.
-	// The associate method checks it's not a redefinition.
 	nt := t
 	if name != t.name {
 		nt = t.New(name)
@@ -185,11 +183,7 @@ func (t *Template) Lookup(name string) *Template {
 
 // Parse defines the template by parsing the text. Nested template definitions will be
 // associated with the top-level template t. Parse may be called multiple times
-// to parse definitions of templates to associate with t. It is an error if a
-// resulting template is non-empty (contains content other than template
-// definitions) and would replace a non-empty template with the same name.
-// (In multiple calls to Parse with the same receiver template, only one call
-// can contain text other than space, comments, and template definitions.)
+// to parse definitions of templates to associate with t.
 func (t *Template) Parse(text string) (*Template, error) {
 	t.init()
 	t.muFuncs.RLock()
@@ -208,25 +202,17 @@ func (t *Template) Parse(text string) (*Template, error) {
 }
 
 // associate installs the new template into the group of templates associated
-// with t. It is an error to reuse a name except to overwrite an empty
-// template. The two are already known to share the common structure.
-// The boolean return value reports wither to store this tree as t.Tree.
+// with t. The two are already known to share the common structure.
+// The boolean return value reports whether to store this tree as t.Tree.
 func (t *Template) associate(new *Template, tree *parse.Tree) (bool, error) {
 	if new.common != t.common {
 		panic("internal error: associate not common")
 	}
-	name := new.name
-	if old := t.tmpl[name]; old != nil {
-		oldIsEmpty := parse.IsEmptyTree(old.Root)
-		newIsEmpty := parse.IsEmptyTree(tree.Root)
-		if newIsEmpty {
-			// Whether old is empty or not, new is empty; no reason to replace old.
+	if t.tmpl[new.name] != nil && parse.IsEmptyTree(tree.Root) {
+		// If a template by that name exists,
+		// don't replace it with an empty template.
 		return false, nil
 	}
-		if !oldIsEmpty {
-			return false, fmt.Errorf("template: redefinition of template %q", name)
-		}
-	}
-	t.tmpl[name] = new
+	t.tmpl[new.name] = new
 	return true, nil
 }