text/template: avoid a global map to help the linker's deadcode elimination

Fixes #36021
Updates #2559
Updates #26775

Change-Id: I2e6708691311035b63866f25d5b4b3977a118290
Reviewed-on: https://go-review.googlesource.com/c/go/+/210284
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

4 files changed, 111 insertions, 28 deletions

diff --git a/src/text/template/funcs.go b/src/text/template/funcs.go
index 6a6843dfa0..fb56bc3fc6 100644
--- a/src/text/template/funcs.go
+++ b/src/text/template/funcs.go
@@ -12,6 +12,7 @@ import (
 	"net/url"
 	"reflect"
 	"strings"
+	"sync"
 	"unicode"
 	"unicode/utf8"
 )
@@ -29,31 +30,49 @@ import (
 // type can return interface{} or reflect.Value.
 type FuncMap map[string]interface{}
 
-var builtins = FuncMap{
-	"and":      and,
-	"call":     call,
-	"html":     HTMLEscaper,
-	"index":    index,
-	"slice":    slice,
-	"js":       JSEscaper,
-	"len":      length,
-	"not":      not,
-	"or":       or,
-	"print":    fmt.Sprint,
-	"printf":   fmt.Sprintf,
-	"println":  fmt.Sprintln,
-	"urlquery": URLQueryEscaper,
-
-	// Comparisons
-	"eq": eq, // ==
-	"ge": ge, // >=
-	"gt": gt, // >
-	"le": le, // <=
-	"lt": lt, // <
-	"ne": ne, // !=
-}
-
-var builtinFuncs = createValueFuncs(builtins)
+// builtins returns the FuncMap.
+// It is not a global variable so the linker can dead code eliminate
+// more when this isn't called. See golang.org/issue/36021.
+// TODO: revert this back to a global map once golang.org/issue/2559 is fixed.
+func builtins() FuncMap {
+	return FuncMap{
+		"and":      and,
+		"call":     call,
+		"html":     HTMLEscaper,
+		"index":    index,
+		"slice":    slice,
+		"js":       JSEscaper,
+		"len":      length,
+		"not":      not,
+		"or":       or,
+		"print":    fmt.Sprint,
+		"printf":   fmt.Sprintf,
+		"println":  fmt.Sprintln,
+		"urlquery": URLQueryEscaper,
+
+		// Comparisons
+		"eq": eq, // ==
+		"ge": ge, // >=
+		"gt": gt, // >
+		"le": le, // <=
+		"lt": lt, // <
+		"ne": ne, // !=
+	}
+}
+
+var builtinFuncsOnce struct {
+	sync.Once
+	v map[string]reflect.Value
+}
+
+// builtinFuncsOnce lazily computes & caches the builtinFuncs map.
+// TODO: revert this back to a global map once golang.org/issue/2559 is fixed.
+func builtinFuncs() map[string]reflect.Value {
+	builtinFuncsOnce.Do(func() {
+		builtinFuncsOnce.v = createValueFuncs(builtins())
+	})
+	return builtinFuncsOnce.v
+}
 
 // createValueFuncs turns a FuncMap into a map[string]reflect.Value
 func createValueFuncs(funcMap FuncMap) map[string]reflect.Value {
@@ -125,7 +144,7 @@ func findFunction(name string, tmpl *Template) (reflect.Value, bool) {
 			return fn, true
 		}
 	}
-	if fn := builtinFuncs[name]; fn.IsValid() {
+	if fn := builtinFuncs()[name]; fn.IsValid() {
 		return fn, true
 	}
 	return reflect.Value{}, false
diff --git a/src/text/template/link_test.go b/src/text/template/link_test.go
new file mode 100644
index 0000000000..b7415d29bb
--- /dev/null
+++ b/src/text/template/link_test.go
@@ -0,0 +1,64 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package template_test
+
+import (
+	"bytes"
+	"internal/testenv"
+	"io/ioutil"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"testing"
+)
+
+// Issue 36021: verify that text/template doesn't prevent the linker from removing
+// unused methods.
+func TestLinkerGC(t *testing.T) {
+	if testing.Short() {
+		t.Skip("skipping in short mode")
+	}
+	testenv.MustHaveGoBuild(t)
+	const prog = `package main
+
+import (
+	_ "text/template"
+)
+
+type T struct{}
+
+func (t *T) Unused() { println("THIS SHOULD BE ELIMINATED") }
+func (t *T) Used() {}
+
+var sink *T
+
+func main() {
+	var t T
+	sink = &t
+	t.Used()
+}
+`
+	td, err := ioutil.TempDir("", "text_template_TestDeadCodeElimination")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(td)
+
+	if err := ioutil.WriteFile(filepath.Join(td, "x.go"), []byte(prog), 0644); err != nil {
+		t.Fatal(err)
+	}
+	cmd := exec.Command("go", "build", "-o", "x.exe", "x.go")
+	cmd.Dir = td
+	if out, err := cmd.CombinedOutput(); err != nil {
+		t.Fatalf("go build: %v, %s", err, out)
+	}
+	slurp, err := ioutil.ReadFile(filepath.Join(td, "x.exe"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if bytes.Contains(slurp, []byte("THIS SHOULD BE ELIMINATED")) {
+		t.Error("binary contains code that should be deadcode eliminated")
+	}
+}
diff --git a/src/text/template/multi_test.go b/src/text/template/multi_test.go
index 5769470ff9..bf1f1b2701 100644
--- a/src/text/template/multi_test.go
+++ b/src/text/template/multi_test.go
@@ -242,7 +242,7 @@ func TestAddParseTree(t *testing.T) {
 		t.Fatal(err)
 	}
 	// Add a new parse tree.
-	tree, err := parse.Parse("cloneText3", cloneText3, "", "", nil, builtins)
+	tree, err := parse.Parse("cloneText3", cloneText3, "", "", nil, builtins())
 	if err != nil {
 		t.Fatal(err)
 	}
diff --git a/src/text/template/template.go b/src/text/template/template.go
index e0c096207c..ec26eb4c50 100644
--- a/src/text/template/template.go
+++ b/src/text/template/template.go
@@ -198,7 +198,7 @@ func (t *Template) Lookup(name string) *Template {
 func (t *Template) Parse(text string) (*Template, error) {
 	t.init()
 	t.muFuncs.RLock()
-	trees, err := parse.Parse(t.name, text, t.leftDelim, t.rightDelim, t.parseFuncs, builtins)
+	trees, err := parse.Parse(t.name, text, t.leftDelim, t.rightDelim, t.parseFuncs, builtins())
 	t.muFuncs.RUnlock()
 	if err != nil {
 		return nil, err