markup/goldmark: Fix mangling of headers/links in render hooks

```bash

name                    old time/op    new time/op    delta
SiteWithRenderHooks-10    11.9ms ± 1%    11.9ms ± 1%    ~     (p=0.486 n=4+4)

name                    old alloc/op   new alloc/op   delta
SiteWithRenderHooks-10    11.2MB ± 0%    11.3MB ± 0%  +0.16%  (p=0.029 n=4+4)

name                    old allocs/op  new allocs/op  delta
SiteWithRenderHooks-10      145k ± 0%      145k ± 0%  +0.14%  (p=0.029 n=4+4)
```

Fixes #9504

markup/goldmark/convert.go

@@ -47,8 +47,7 @@ import (
 // Provider is the package entry point.
 var Provider converter.ProviderProvider = provide{}
 
-type provide struct {
+type provide struct{}
-}
 
 func (p provide) New(cfg converter.ProviderConfig) (converter.Provider, error) {
 	md := newMarkdown(cfg)
@@ -199,10 +198,21 @@ func (b *bufWriter) Flush() error {
 
 type renderContext struct {
 	*bufWriter
-	pos int
+	positions []int
 	renderContextData
 }
 
+func (ctx *renderContext) pushPos(n int) {
+	ctx.positions = append(ctx.positions, n)
+}
+
+func (ctx *renderContext) popPos() int {
+	i := len(ctx.positions) - 1
+	p := ctx.positions[i]
+	ctx.positions = ctx.positions[:i]
+	return p
+}
+
 type renderContextData interface {
 	RenderContext() converter.RenderContext
 	DocumentContext() converter.DocumentContext

markup/goldmark/integration_test.go

@@ -61,6 +61,42 @@ foo
 	`)
 }
 
+// Issue 9504
+func TestLinkInTitle(t *testing.T) {
+	t.Parallel()
+
+	files := `
+-- config.toml --
+-- content/p1.md --
+---
+title: "p1"
+---
+## Hello [Test](https://example.com)
+-- layouts/_default/single.html --
+{{ .Content }}
+-- layouts/_default/_markup/render-heading.html --
+<h{{ .Level }} id="{{ .Anchor | safeURL }}">
+  {{ .Text | safeHTML }}
+  <a class="anchor" href="#{{ .Anchor | safeURL }}">#</a>
+</h{{ .Level }}>
+-- layouts/_default/_markup/render-link.html --
+<a href="{{ .Destination | safeURL }}"{{ with .Title}} title="{{ . }}"{{ end }}>{{ .Text | safeHTML }}</a>
+
+`
+
+	b := hugolib.NewIntegrationTestBuilder(
+		hugolib.IntegrationTestConfig{
+			T:           t,
+			TxtarString: files,
+			NeedsOsFS:   false,
+		},
+	).Build()
+
+	b.AssertFileContent("public/p1/index.html",
+		"<h2 id=\"hello-testhttpsexamplecom\">\n  Hello <a href=\"https://example.com\">Test</a>\n\n  <a class=\"anchor\" href=\"#hello-testhttpsexamplecom\">#</a>\n</h2>",
+	)
+}
+
 func BenchmarkSiteWithRenderHooks(b *testing.B) {
 	files := `
 -- config.toml --

markup/goldmark/render_hooks.go

@@ -144,16 +144,13 @@ func (r *hookedRenderer) renderAttributesForNode(w util.BufWriter, node ast.Node
 	renderAttributes(w, false, node.Attributes()...)
 }
 
-var (
-
 // Attributes with special meaning that does not make sense to render in HTML.
-	attributeExcludes = map[string]bool{
+var attributeExcludes = map[string]bool{
 	"hl_lines":    true,
 	"hl_style":    true,
 	"linenos":     true,
 	"linenostart": true,
 }
-)
 
 func renderAttributes(w util.BufWriter, skipClass bool, attributes ...ast.Attribute) {
 	for _, attr := range attributes {
@@ -197,12 +194,13 @@ func (r *hookedRenderer) renderImage(w util.BufWriter, source []byte, node ast.N
 
 	if entering {
 		// Store the current pos so we can capture the rendered text.
-		ctx.pos = ctx.Buffer.Len()
+		ctx.pushPos(ctx.Buffer.Len())
 		return ast.WalkContinue, nil
 	}
 
-	text := ctx.Buffer.Bytes()[ctx.pos:]
+	pos := ctx.popPos()
-	ctx.Buffer.Truncate(ctx.pos)
+	text := ctx.Buffer.Bytes()[pos:]
+	ctx.Buffer.Truncate(pos)
 
 	err := h.ImageRenderer.RenderLink(
 		w,
@@ -263,12 +261,13 @@ func (r *hookedRenderer) renderLink(w util.BufWriter, source []byte, node ast.No
 
 	if entering {
 		// Store the current pos so we can capture the rendered text.
-		ctx.pos = ctx.Buffer.Len()
+		ctx.pushPos(ctx.Buffer.Len())
 		return ast.WalkContinue, nil
 	}
 
-	text := ctx.Buffer.Bytes()[ctx.pos:]
+	pos := ctx.popPos()
-	ctx.Buffer.Truncate(ctx.pos)
+	text := ctx.Buffer.Bytes()[pos:]
+	ctx.Buffer.Truncate(pos)
 
 	err := h.LinkRenderer.RenderLink(
 		w,
@@ -395,12 +394,13 @@ func (r *hookedRenderer) renderHeading(w util.BufWriter, source []byte, node ast
 
 	if entering {
 		// Store the current pos so we can capture the rendered text.
-		ctx.pos = ctx.Buffer.Len()
+		ctx.pushPos(ctx.Buffer.Len())
 		return ast.WalkContinue, nil
 	}
 
-	text := ctx.Buffer.Bytes()[ctx.pos:]
+	pos := ctx.popPos()
-	ctx.Buffer.Truncate(ctx.pos)
+	text := ctx.Buffer.Bytes()[pos:]
+	ctx.Buffer.Truncate(pos)
 	// All ast.Heading nodes are guaranteed to have an attribute called "id"
 	// that is an array of bytes that encode a valid string.
 	anchori, _ := n.AttributeString("id")
@@ -440,8 +440,7 @@ func (r *hookedRenderer) renderHeadingDefault(w util.BufWriter, source []byte, n
 	return ast.WalkContinue, nil
 }
 
-type links struct {
+type links struct{}
-}
 
 // Extend implements goldmark.Extender.
 func (e *links) Extend(m goldmark.Markdown) {