text/template: special depth limit for WASM

Deeply nested parenthesized expressions in templates can cause stack
overflows in WebAssembly environments, which typically have more
restricted stack space than native platforms.

This change introduces a separate, lower maximum depth constant for
WASM (1000 levels vs 10000 for other platforms), and modifies the
tests to use the appropriate limit based on the current architecture.

Similar to the approach used in encoding/xml package.

Author: thevilledev

src/text/template/parse/parse.go

@@ -46,6 +46,7 @@ const (
 // maxExpressionParenDepth is the maximum depth of nested parenthesized expressions.
 // It is used to prevent stack overflows from deep finite recursion in the parser.
 const maxExpressionParenDepth = 10000
+const maxExpressionParenDepthWasm = 1000 // Lower limit for WASM environments
 
 // Copy returns a copy of the [Tree]. Any parsing state is discarded.
 func (t *Tree) Copy() *Tree {
@@ -793,7 +794,8 @@ func (t *Tree) term() Node {
 		}
 		return number
 	case itemLeftParen:
-		if t.parenDepth >= maxExpressionParenDepth {
+		if t.parenDepth >= maxExpressionParenDepth ||
+			runtime.GOARCH == "wasm" && t.parenDepth >= maxExpressionParenDepthWasm {
 			t.errorf("max expression depth exceeded")
 		}
 		t.parenDepth++

src/text/template/parse/parse_test.go

@@ -7,6 +7,7 @@ package parse
 import (
 	"flag"
 	"fmt"
+	"runtime"
 	"strings"
 	"testing"
 )
@@ -330,12 +331,12 @@ var parseTests = []parseTest{
 
 	// Parenthesis nesting depth tests
 	{"paren nesting normal", "{{( ( ( ( (1) ) ) ) )}}", noError, "{{(((((1)))))}}"},
-	{"paren nesting at limit", "{{" + buildNestedParenExpression(10000, "1") + "}}", noError, "{{" + buildNestedParenExpression(10000, "1") + "}}"},
-	{"paren nesting exceeds limit", "{{" + buildNestedParenExpression(10001, "1") + "}}", hasError, "template: test:1: max expression depth exceeded"},
+	{"paren nesting at limit", "{{" + buildNestedParenExpression(getMaxParenDepth(), "1") + "}}", noError, "{{" + buildNestedParenExpression(getMaxParenDepth(), "1") + "}}"},
+	{"paren nesting exceeds limit", "{{" + buildNestedParenExpression(getMaxParenDepth()+1, "1") + "}}", hasError, "template: test:1: max expression depth exceeded"},
 	{"paren nesting in pipeline", "{{ ( ( ( ( (1) ) ) ) ) | printf }}", noError, "{{(((((1))))) | printf}}"},
-	{"paren nesting in pipeline exceeds limit", "{{ " + buildNestedParenExpression(10001, "1") + " | printf }}", hasError, "template: test:1: max expression depth exceeded"},
+	{"paren nesting in pipeline exceeds limit", "{{ " + buildNestedParenExpression(getMaxParenDepth()+1, "1") + " | printf }}", hasError, "template: test:1: max expression depth exceeded"},
 	{"paren nesting with other constructs", "{{if " + buildNestedParenExpression(5, "true") + "}}YES{{end}}", noError, "{{if " + buildNestedParenExpression(5, "true") + "}}\"YES\"{{end}}"},
-	{"paren nesting with other constructs exceeds limit", "{{if " + buildNestedParenExpression(10001, "true") + "}}YES{{end}}", hasError, "template: test:1: max expression depth exceeded"},
+	{"paren nesting with other constructs exceeds limit", "{{if " + buildNestedParenExpression(getMaxParenDepth()+1, "true") + "}}YES{{end}}", hasError, "template: test:1: max expression depth exceeded"},
 }
 
 var builtins = map[string]any{
@@ -730,3 +731,12 @@ func BenchmarkListString(b *testing.B) {
 func buildNestedParenExpression(depth int, content string) string {
 	return strings.Repeat("(", depth) + content + strings.Repeat(")", depth)
 }
+
+// getMaxParenDepth returns the appropriate parenthesis nesting depth limit
+// based on the current architecture.
+func getMaxParenDepth() int {
+	if runtime.GOARCH == "wasm" {
+		return maxExpressionParenDepthWasm
+	}
+	return maxExpressionParenDepth
+}