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>

12 files changed, 232 insertions, 4 deletions

diff --git a/src/html/template/context.go b/src/html/template/context.go
index f7d4849928..aaa7d08359 100644
--- a/src/html/template/context.go
+++ b/src/html/template/context.go
@@ -6,6 +6,7 @@ package template
 
 import (
 	"fmt"
+	"text/template/parse"
 )
 
 // context describes the state an HTML parser must be in when it reaches the
@@ -22,6 +23,7 @@ type context struct {
 	jsCtx   jsCtx
 	attr    attr
 	element element
+	n       parse.Node // for range break/continue
 	err     *Error
 }
 
@@ -141,6 +143,8 @@ const (
 	// stateError is an infectious error state outside any valid
 	// HTML/CSS/JS construct.
 	stateError
+	// stateDead marks unreachable code after a {{break}} or {{continue}}.
+	stateDead
 )
 
 // isComment is true for any state that contains content meant for template
diff --git a/src/html/template/escape.go b/src/html/template/escape.go
index 8739735cb7..6dea79c7b5 100644
--- a/src/html/template/escape.go
+++ b/src/html/template/escape.go
@@ -97,6 +97,15 @@ type escaper struct {
 	actionNodeEdits   map[*parse.ActionNode][]string
 	templateNodeEdits map[*parse.TemplateNode]string
 	textNodeEdits     map[*parse.TextNode][]byte
+	// rangeContext holds context about the current range loop.
+	rangeContext *rangeContext
+}
+
+// rangeContext holds information about the current range loop.
+type rangeContext struct {
+	outer     *rangeContext // outer loop
+	breaks    []context     // context at each break action
+	continues []context     // context at each continue action
 }
 
 // makeEscaper creates a blank escaper for the given set.
@@ -109,6 +118,7 @@ func makeEscaper(n *nameSpace) escaper {
 		map[*parse.ActionNode][]string{},
 		map[*parse.TemplateNode]string{},
 		map[*parse.TextNode][]byte{},
+		nil,
 	}
 }
 
@@ -124,8 +134,16 @@ func (e *escaper) escape(c context, n parse.Node) context {
 	switch n := n.(type) {
 	case *parse.ActionNode:
 		return e.escapeAction(c, n)
+	case *parse.BreakNode:
+		c.n = n
+		e.rangeContext.breaks = append(e.rangeContext.breaks, c)
+		return context{state: stateDead}
 	case *parse.CommentNode:
 		return c
+	case *parse.ContinueNode:
+		c.n = n
+		e.rangeContext.continues = append(e.rangeContext.breaks, c)
+		return context{state: stateDead}
 	case *parse.IfNode:
 		return e.escapeBranch(c, &n.BranchNode, "if")
 	case *parse.ListNode:
@@ -427,6 +445,12 @@ func join(a, b context, node parse.Node, nodeName string) context {
 	if b.state == stateError {
 		return b
 	}
+	if a.state == stateDead {
+		return b
+	}
+	if b.state == stateDead {
+		return a
+	}
 	if a.eq(b) {
 		return a
 	}
@@ -466,14 +490,27 @@ func join(a, b context, node parse.Node, nodeName string) context {
 
 // escapeBranch escapes a branch template node: "if", "range" and "with".
 func (e *escaper) escapeBranch(c context, n *parse.BranchNode, nodeName string) context {
+	if nodeName == "range" {
+		e.rangeContext = &rangeContext{outer: e.rangeContext}
+	}
 	c0 := e.escapeList(c, n.List)
-	if nodeName == "range" && c0.state != stateError {
+	if nodeName == "range" {
+		if c0.state != stateError {
+			c0 = joinRange(c0, e.rangeContext)
+		}
+		e.rangeContext = e.rangeContext.outer
+		if c0.state == stateError {
+			return c0
+		}
+
 		// The "true" branch of a "range" node can execute multiple times.
 		// We check that executing n.List once results in the same context
 		// as executing n.List twice.
+		e.rangeContext = &rangeContext{outer: e.rangeContext}
 		c1, _ := e.escapeListConditionally(c0, n.List, nil)
 		c0 = join(c0, c1, n, nodeName)
 		if c0.state == stateError {
+			e.rangeContext = e.rangeContext.outer
 			// Make clear that this is a problem on loop re-entry
 			// since developers tend to overlook that branch when
 			// debugging templates.
@@ -481,11 +518,39 @@ func (e *escaper) escapeBranch(c context, n *parse.BranchNode, nodeName string)
 			c0.err.Description = "on range loop re-entry: " + c0.err.Description
 			return c0
 		}
+		c0 = joinRange(c0, e.rangeContext)
+		e.rangeContext = e.rangeContext.outer
+		if c0.state == stateError {
+			return c0
+		}
 	}
 	c1 := e.escapeList(c, n.ElseList)
 	return join(c0, c1, n, nodeName)
 }
 
+func joinRange(c0 context, rc *rangeContext) context {
+	// Merge contexts at break and continue statements into overall body context.
+	// In theory we could treat breaks differently from continues, but for now it is
+	// enough to treat them both as going back to the start of the loop (which may then stop).
+	for _, c := range rc.breaks {
+		c0 = join(c0, c, c.n, "range")
+		if c0.state == stateError {
+			c0.err.Line = c.n.(*parse.BreakNode).Line
+			c0.err.Description = "at range loop break: " + c0.err.Description
+			return c0
+		}
+	}
+	for _, c := range rc.continues {
+		c0 = join(c0, c, c.n, "range")
+		if c0.state == stateError {
+			c0.err.Line = c.n.(*parse.ContinueNode).Line
+			c0.err.Description = "at range loop continue: " + c0.err.Description
+			return c0
+		}
+	}
+	return c0
+}
+
 // escapeList escapes a list template node.
 func (e *escaper) escapeList(c context, n *parse.ListNode) context {
 	if n == nil {
@@ -493,6 +558,9 @@ func (e *escaper) escapeList(c context, n *parse.ListNode) context {
 	}
 	for _, m := range n.Nodes {
 		c = e.escape(c, m)
+		if c.state == stateDead {
+			break
+		}
 	}
 	return c
 }
@@ -503,6 +571,7 @@ func (e *escaper) escapeList(c context, n *parse.ListNode) context {
 // which is the same as whether e was updated.
 func (e *escaper) escapeListConditionally(c context, n *parse.ListNode, filter func(*escaper, context) bool) (context, bool) {
 	e1 := makeEscaper(e.ns)
+	e1.rangeContext = e.rangeContext
 	// Make type inferences available to f.
 	for k, v := range e.output {
 		e1.output[k] = v
diff --git a/src/html/template/escape_test.go b/src/html/template/escape_test.go
index fbc84a7592..3b0aa8c846 100644
--- a/src/html/template/escape_test.go
+++ b/src/html/template/escape_test.go
@@ -920,6 +920,22 @@ func TestErrors(t *testing.T) {
 			"<a href='/foo?{{range .Items}}&{{.K}}={{.V}}{{end}}'>",
 			"",
 		},
+		{
+			"{{range .Items}}<a{{if .X}}{{end}}>{{end}}",
+			"",
+		},
+		{
+			"{{range .Items}}<a{{if .X}}{{end}}>{{continue}}{{end}}",
+			"",
+		},
+		{
+			"{{range .Items}}<a{{if .X}}{{end}}>{{break}}{{end}}",
+			"",
+		},
+		{
+			"{{range .Items}}<a{{if .X}}{{end}}>{{if .X}}{{break}}{{end}}{{end}}",
+			"",
+		},
 		// Error cases.
 		{
 			"{{if .Cond}}<a{{end}}",
@@ -956,6 +972,14 @@ func TestErrors(t *testing.T) {
 			"z:2:8: on range loop re-entry: {{range}} branches",
 		},
 		{
+			"{{range .Items}}<a{{if .X}}{{break}}{{end}}>{{end}}",
+			"z:1:29: at range loop break: {{range}} branches end in different contexts",
+		},
+		{
+			"{{range .Items}}<a{{if .X}}{{continue}}{{end}}>{{end}}",
+			"z:1:29: at range loop continue: {{range}} branches end in different contexts",
+		},
+		{
 			"<a b=1 c={{.H}}",
 			"z: ends in a non-text context: {stateAttr delimSpaceOrTagEnd",
 		},
diff --git a/src/html/template/exec_test.go b/src/html/template/exec_test.go
index 888587335d..523340bac9 100644
--- a/src/html/template/exec_test.go
+++ b/src/html/template/exec_test.go
@@ -567,6 +567,8 @@ var execTests = []execTest{
 	{"range empty no else", "{{range .SIEmpty}}-{{.}}-{{end}}", "", tVal, true},
 	{"range []int else", "{{range .SI}}-{{.}}-{{else}}EMPTY{{end}}", "-3--4--5-", tVal, true},
 	{"range empty else", "{{range .SIEmpty}}-{{.}}-{{else}}EMPTY{{end}}", "EMPTY", tVal, true},
+	{"range []int break else", "{{range .SI}}-{{.}}-{{break}}NOTREACHED{{else}}EMPTY{{end}}", "-3-", tVal, true},
+	{"range []int continue else", "{{range .SI}}-{{.}}-{{continue}}NOTREACHED{{else}}EMPTY{{end}}", "-3--4--5-", tVal, true},
 	{"range []bool", "{{range .SB}}-{{.}}-{{end}}", "-true--false-", tVal, true},
 	{"range []int method", "{{range .SI | .MAdd .I}}-{{.}}-{{end}}", "-20--21--22-", tVal, true},
 	{"range map", "{{range .MSI}}-{{.}}-{{end}}", "-1--3--2-", tVal, true},
diff --git a/src/text/template/doc.go b/src/text/template/doc.go
index 0ea132e8e6..10093881fb 100644
--- a/src/text/template/doc.go
+++ b/src/text/template/doc.go
@@ -112,6 +112,14 @@ data, defined in detail in the corresponding sections that follow.
 		T0 is executed; otherwise, dot is set to the successive elements
 		of the array, slice, or map and T1 is executed.
 
+	{{break}}
+		The innermost {{range pipeline}} loop is ended early, stopping the
+		current iteration and bypassing all remaining iterations.
+
+	{{continue}}
+		The current iteration of the innermost {{range pipeline}} loop is
+		stopped, and the loop starts the next iteration.
+
 	{{template "name"}}
 		The template with the specified name is executed with nil data.
 
diff --git a/src/text/template/exec.go b/src/text/template/exec.go
index 6e005b57d7..e03920964e 100644
--- a/src/text/template/exec.go
+++ b/src/text/template/exec.go
@@ -5,6 +5,7 @@
 package template
 
 import (
+	"errors"
 	"fmt"
 	"internal/fmtsort"
 	"io"
@@ -243,6 +244,12 @@ func (t *Template) DefinedTemplates() string {
 	return b.String()
 }
 
+// Sentinel errors for use with panic to signal early exits from range loops.
+var (
+	walkBreak    = errors.New("break")
+	walkContinue = errors.New("continue")
+)
+
 // Walk functions step through the major pieces of the template structure,
 // generating output as they go.
 func (s *state) walk(dot reflect.Value, node parse.Node) {
@@ -255,7 +262,11 @@ func (s *state) walk(dot reflect.Value, node parse.Node) {
 		if len(node.Pipe.Decl) == 0 {
 			s.printValue(node, val)
 		}
+	case *parse.BreakNode:
+		panic(walkBreak)
 	case *parse.CommentNode:
+	case *parse.ContinueNode:
+		panic(walkContinue)
 	case *parse.IfNode:
 		s.walkIfOrWith(parse.NodeIf, dot, node.Pipe, node.List, node.ElseList)
 	case *parse.ListNode:
@@ -334,6 +345,11 @@ func isTrue(val reflect.Value) (truth, ok bool) {
 
 func (s *state) walkRange(dot reflect.Value, r *parse.RangeNode) {
 	s.at(r)
+	defer func() {
+		if r := recover(); r != nil && r != walkBreak {
+			panic(r)
+		}
+	}()
 	defer s.pop(s.mark())
 	val, _ := indirect(s.evalPipeline(dot, r.Pipe))
 	// mark top of stack before any variables in the body are pushed.
@@ -347,8 +363,14 @@ func (s *state) walkRange(dot reflect.Value, r *parse.RangeNode) {
 		if len(r.Pipe.Decl) > 1 {
 			s.setTopVar(2, index)
 		}
+		defer s.pop(mark)
+		defer func() {
+			// Consume panic(walkContinue)
+			if r := recover(); r != nil && r != walkContinue {
+				panic(r)
+			}
+		}()
 		s.walk(elem, r.List)
-		s.pop(mark)
 	}
 	switch val.Kind() {
 	case reflect.Array, reflect.Slice:
diff --git a/src/text/template/exec_test.go b/src/text/template/exec_test.go
index ae67b9334f..93fd54e84d 100644
--- a/src/text/template/exec_test.go
+++ b/src/text/template/exec_test.go
@@ -568,6 +568,8 @@ var execTests = []execTest{
 	{"range empty no else", "{{range .SIEmpty}}-{{.}}-{{end}}", "", tVal, true},
 	{"range []int else", "{{range .SI}}-{{.}}-{{else}}EMPTY{{end}}", "-3--4--5-", tVal, true},
 	{"range empty else", "{{range .SIEmpty}}-{{.}}-{{else}}EMPTY{{end}}", "EMPTY", tVal, true},
+	{"range []int break else", "{{range .SI}}-{{.}}-{{break}}NOTREACHED{{else}}EMPTY{{end}}", "-3-", tVal, true},
+	{"range []int continue else", "{{range .SI}}-{{.}}-{{continue}}NOTREACHED{{else}}EMPTY{{end}}", "-3--4--5-", tVal, true},
 	{"range []bool", "{{range .SB}}-{{.}}-{{end}}", "-true--false-", tVal, true},
 	{"range []int method", "{{range .SI | .MAdd .I}}-{{.}}-{{end}}", "-20--21--22-", tVal, true},
 	{"range map", "{{range .MSI}}-{{.}}-{{end}}", "-1--3--2-", tVal, true},
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, ""},