html/template, text/template: implement break and continue for range loops

Break and continue for range loops was accepted as a proposal in June 2017.
It was implemented in CL 66410 (Oct 2017)
but then rolled back in CL 92155 (Feb 2018)
because html/template changes had not been implemented.

This CL reimplements break and continue in text/template
and then adds support for them in html/template as well.

Fixes #20531.

Change-Id: I05330482a976f1c078b4b49c2287bd9031bb7616
Reviewed-on: https://go-review.googlesource.com/c/go/+/321491
Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

5 files changed, 99 insertions, 2 deletions

diff --git a/src/text/template/parse/lex.go b/src/text/template/parse/lex.go
index 6784071b11..95e33771c0 100644
--- a/src/text/template/parse/lex.go
+++ b/src/text/template/parse/lex.go
@@ -62,6 +62,8 @@ const (
 	// Keywords appear after all the rest.
 	itemKeyword  // used only to delimit the keywords
 	itemBlock    // block keyword
+	itemBreak    // break keyword
+	itemContinue // continue keyword
 	itemDot      // the cursor, spelled '.'
 	itemDefine   // define keyword
 	itemElse     // else keyword
@@ -76,6 +78,8 @@ const (
 var key = map[string]itemType{
 	".":        itemDot,
 	"block":    itemBlock,
+	"break":    itemBreak,
+	"continue": itemContinue,
 	"define":   itemDefine,
 	"else":     itemElse,
 	"end":      itemEnd,
@@ -119,6 +123,8 @@ type lexer struct {
 	parenDepth  int       // nesting depth of ( ) exprs
 	line        int       // 1+number of newlines seen
 	startLine   int       // start line of this item
+	breakOK     bool      // break keyword allowed
+	continueOK  bool      // continue keyword allowed
 }
 
 // next returns the next rune in the input.
@@ -461,7 +467,12 @@ Loop:
 			}
 			switch {
 			case key[word] > itemKeyword:
-				l.emit(key[word])
+				item := key[word]
+				if item == itemBreak && !l.breakOK || item == itemContinue && !l.continueOK {
+					l.emit(itemIdentifier)
+				} else {
+					l.emit(item)
+				}
 			case word[0] == '.':
 				l.emit(itemField)
 			case word == "true", word == "false":
diff --git a/src/text/template/parse/lex_test.go b/src/text/template/parse/lex_test.go
index 6510eed674..df6aabffb2 100644
--- a/src/text/template/parse/lex_test.go
+++ b/src/text/template/parse/lex_test.go
@@ -35,6 +35,8 @@ var itemName = map[itemType]string{
 	// keywords
 	itemDot:      ".",
 	itemBlock:    "block",
+	itemBreak:    "break",
+	itemContinue: "continue",
 	itemDefine:   "define",
 	itemElse:     "else",
 	itemIf:       "if",
diff --git a/src/text/template/parse/node.go b/src/text/template/parse/node.go
index 177482f9b2..47268225c8 100644
--- a/src/text/template/parse/node.go
+++ b/src/text/template/parse/node.go
@@ -71,6 +71,8 @@ const (
 	NodeVariable                   // A $ variable.
 	NodeWith                       // A with action.
 	NodeComment                    // A comment.
+	NodeBreak                      // A break action.
+	NodeContinue                   // A continue action.
 )
 
 // Nodes.
@@ -907,6 +909,40 @@ func (i *IfNode) Copy() Node {
 	return i.tr.newIf(i.Pos, i.Line, i.Pipe.CopyPipe(), i.List.CopyList(), i.ElseList.CopyList())
 }
 
+// BreakNode represents a {{break}} action.
+type BreakNode struct {
+	tr *Tree
+	NodeType
+	Pos
+	Line int
+}
+
+func (t *Tree) newBreak(pos Pos, line int) *BreakNode {
+	return &BreakNode{tr: t, NodeType: NodeBreak, Pos: pos, Line: line}
+}
+
+func (b *BreakNode) Copy() Node                  { return b.tr.newBreak(b.Pos, b.Line) }
+func (b *BreakNode) String() string              { return "{{break}}" }
+func (b *BreakNode) tree() *Tree                 { return b.tr }
+func (b *BreakNode) writeTo(sb *strings.Builder) { sb.WriteString("{{break}}") }
+
+// ContinueNode represents a {{continue}} action.
+type ContinueNode struct {
+	tr *Tree
+	NodeType
+	Pos
+	Line int
+}
+
+func (t *Tree) newContinue(pos Pos, line int) *ContinueNode {
+	return &ContinueNode{tr: t, NodeType: NodeContinue, Pos: pos, Line: line}
+}
+
+func (c *ContinueNode) Copy() Node                  { return c.tr.newContinue(c.Pos, c.Line) }
+func (c *ContinueNode) String() string              { return "{{continue}}" }
+func (c *ContinueNode) tree() *Tree                 { return c.tr }
+func (c *ContinueNode) writeTo(sb *strings.Builder) { sb.WriteString("{{continue}}") }
+
 // RangeNode represents a {{range}} action and its commands.
 type RangeNode struct {
 	BranchNode
diff --git a/src/text/template/parse/parse.go b/src/text/template/parse/parse.go
index 1a63961c13..d92bed5d3d 100644
--- a/src/text/template/parse/parse.go
+++ b/src/text/template/parse/parse.go
@@ -31,6 +31,7 @@ type Tree struct {
 	vars       []string // variables defined at the moment.
 	treeSet    map[string]*Tree
 	actionLine int // line of left delim starting action
+	rangeDepth int
 	mode       Mode
 }
 
@@ -224,6 +225,8 @@ func (t *Tree) startParse(funcs []map[string]interface{}, lex *lexer, treeSet ma
 	t.vars = []string{"$"}
 	t.funcs = funcs
 	t.treeSet = treeSet
+	lex.breakOK = !t.hasFunction("break")
+	lex.continueOK = !t.hasFunction("continue")
 }
 
 // stopParse terminates parsing.
@@ -386,6 +389,10 @@ func (t *Tree) action() (n Node) {
 	switch token := t.nextNonSpace(); token.typ {
 	case itemBlock:
 		return t.blockControl()
+	case itemBreak:
+		return t.breakControl(token.pos, token.line)
+	case itemContinue:
+		return t.continueControl(token.pos, token.line)
 	case itemElse:
 		return t.elseControl()
 	case itemEnd:
@@ -405,6 +412,32 @@ func (t *Tree) action() (n Node) {
 	return t.newAction(token.pos, token.line, t.pipeline("command", itemRightDelim))
 }
 
+// Break:
+//	{{break}}
+// Break keyword is past.
+func (t *Tree) breakControl(pos Pos, line int) Node {
+	if token := t.next(); token.typ != itemRightDelim {
+		t.unexpected(token, "in {{break}}")
+	}
+	if t.rangeDepth == 0 {
+		t.errorf("{{break}} outside {{range}}")
+	}
+	return t.newBreak(pos, line)
+}
+
+// Continue:
+//	{{continue}}
+// Continue keyword is past.
+func (t *Tree) continueControl(pos Pos, line int) Node {
+	if token := t.next(); token.typ != itemRightDelim {
+		t.unexpected(token, "in {{continue}}")
+	}
+	if t.rangeDepth == 0 {
+		t.errorf("{{continue}} outside {{range}}")
+	}
+	return t.newContinue(pos, line)
+}
+
 // Pipeline:
 //	declarations? command ('|' command)*
 func (t *Tree) pipeline(context string, end itemType) (pipe *PipeNode) {
@@ -480,8 +513,14 @@ func (t *Tree) checkPipeline(pipe *PipeNode, context string) {
 func (t *Tree) parseControl(allowElseIf bool, context string) (pos Pos, line int, pipe *PipeNode, list, elseList *ListNode) {
 	defer t.popVars(len(t.vars))
 	pipe = t.pipeline(context, itemRightDelim)
+	if context == "range" {
+		t.rangeDepth++
+	}
 	var next Node
 	list, next = t.itemList()
+	if context == "range" {
+		t.rangeDepth--
+	}
 	switch next.Type() {
 	case nodeEnd: //done
 	case nodeElse:
@@ -523,7 +562,8 @@ func (t *Tree) ifControl() Node {
 //	{{range pipeline}} itemList {{else}} itemList {{end}}
 // Range keyword is past.
 func (t *Tree) rangeControl() Node {
-	return t.newRange(t.parseControl(false, "range"))
+	r := t.newRange(t.parseControl(false, "range"))
+	return r
 }
 
 // With:
diff --git a/src/text/template/parse/parse_test.go b/src/text/template/parse/parse_test.go
index 9b1be272e5..c3679a08de 100644
--- a/src/text/template/parse/parse_test.go
+++ b/src/text/template/parse/parse_test.go
@@ -230,6 +230,10 @@ var parseTests = []parseTest{
 		`{{range $x := .SI}}{{.}}{{end}}`},
 	{"range 2 vars", "{{range $x, $y := .SI}}{{.}}{{end}}", noError,
 		`{{range $x, $y := .SI}}{{.}}{{end}}`},
+	{"range with break", "{{range .SI}}{{.}}{{break}}{{end}}", noError,
+		`{{range .SI}}{{.}}{{break}}{{end}}`},
+	{"range with continue", "{{range .SI}}{{.}}{{continue}}{{end}}", noError,
+		`{{range .SI}}{{.}}{{continue}}{{end}}`},
 	{"constants", "{{range .SI 1 -3.2i true false 'a' nil}}{{end}}", noError,
 		`{{range .SI 1 -3.2i true false 'a' nil}}{{end}}`},
 	{"template", "{{template `x`}}", noError,
@@ -279,6 +283,10 @@ var parseTests = []parseTest{
 	{"adjacent args", "{{printf 3`x`}}", hasError, ""},
 	{"adjacent args with .", "{{printf `x`.}}", hasError, ""},
 	{"extra end after if", "{{if .X}}a{{else if .Y}}b{{end}}{{end}}", hasError, ""},
+	{"break outside range", "{{range .}}{{end}} {{break}}", hasError, ""},
+	{"continue outside range", "{{range .}}{{end}} {{continue}}", hasError, ""},
+	{"break in range else", "{{range .}}{{else}}{{break}}{{end}}", hasError, ""},
+	{"continue in range else", "{{range .}}{{else}}{{continue}}{{end}}", hasError, ""},
 	// Other kinds of assignments and operators aren't available yet.
 	{"bug0a", "{{$x := 0}}{{$x}}", noError, "{{$x := 0}}{{$x}}"},
 	{"bug0b", "{{$x += 1}}{{$x}}", hasError, ""},