Revise the fix for shortcode vs output format nilpointer

We do lazy initialization and (potentially) reuse of an output format's rendered content. We do this evaluation when we
start a new rendering a new output format. There are, however, situation where these borders gets crossed (e.g.
accessing content from another output format). We have a check for this in place for most cases, but not the content
rendering of inner markdown blocks inside shortcodes. This patch applies that same logic to the newly introduced
RenderContent method (which is not available from the templates).

Fixes #10391

hugolib/page.go

@@ -909,6 +909,7 @@ func (p *pageState) shiftToOutputFormat(isRenderingSite bool, idx int) error {
 				}
 				return cp, nil
 			})
+			p.pageOutput.contentRenderer = lcp
 			p.pageOutput.ContentProvider = lcp
 			p.pageOutput.TableOfContentsProvider = lcp
 			p.pageOutput.PageRenderProvider = lcp

hugolib/page__output.go

@@ -81,6 +81,7 @@ type pageOutput struct {
 
 	// These interface provides the functionality that is specific for this
 	// output format.
+	contentRenderer page.ContentRenderer
 	pagePerOutputProviders
 	page.ContentProvider
 	page.TableOfContentsProvider
@@ -94,10 +95,12 @@ func (p *pageOutput) initContentProvider(cp *pageContentOutput) {
 	if cp == nil {
 		return
 	}
+	p.contentRenderer = cp
 	p.ContentProvider = cp
 	p.TableOfContentsProvider = cp
 	p.PageRenderProvider = cp
 	p.cp = cp
+
 }
 
 func (p *pageOutput) enablePlaceholders() {

hugolib/page__per_output.go

@@ -123,7 +123,7 @@ func newPageContentOutput(p *pageState, po *pageOutput) (*pageContentOutput, err
 		isHTML := cp.p.m.markup == "html"
 
 		if !isHTML {
-			r, err := cp.renderContent(cp.workContent, true)
+			r, err := po.contentRenderer.RenderContent(cp.workContent, true)
 			if err != nil {
 				return err
 			}
@@ -183,7 +183,7 @@ func newPageContentOutput(p *pageState, po *pageOutput) (*pageContentOutput, err
 				}
 			}
 		} else if cp.p.m.summary != "" {
-			b, err := cp.renderContent([]byte(cp.p.m.summary), false)
+			b, err := po.contentRenderer.RenderContent([]byte(cp.p.m.summary), false)
 			if err != nil {
 				return err
 			}
@@ -629,7 +629,7 @@ func (p *pageContentOutput) setAutoSummary() error {
 	return nil
 }
 
-func (cp *pageContentOutput) renderContent(content []byte, renderTOC bool) (converter.Result, error) {
+func (cp *pageContentOutput) RenderContent(content []byte, renderTOC bool) (converter.Result, error) {
 	if err := cp.initRenderHooks(); err != nil {
 		return nil, err
 	}

hugolib/shortcode.go

@@ -381,19 +381,8 @@ func renderShortcode(
 		// Pre Hugo 0.55 this was the behaviour even for the outer-most
 		// shortcode.
 		if sc.doMarkup && (level > 0 || sc.configVersion() == 1) {
-
-			cp := p.pageOutput.cp
-			if cp == nil {
-				var err error
-				cp, err = newPageContentOutput(p, p.pageOutput)
-				if err != nil {
-					return "", false, err
-				}
-				p.pageOutput.initContentProvider(cp)
-			}
-
 			var err error
-			b, err := p.pageOutput.cp.renderContent([]byte(inner), false)
+			b, err := p.pageOutput.contentRenderer.RenderContent([]byte(inner), false)
 			if err != nil {
 				return "", false, err
 			}

resources/page/page.go

@@ -19,6 +19,7 @@ import (
 	"html/template"
 
 	"github.com/gohugoio/hugo/identity"
+	"github.com/gohugoio/hugo/markup/converter"
 
 	"github.com/bep/gitmap"
 	"github.com/gohugoio/hugo/config"
@@ -105,6 +106,11 @@ type ContentProvider interface {
 	Len() int
 }
 
+// ContentRenderer provides the content rendering methods for some content.
+type ContentRenderer interface {
+	RenderContent(content []byte, renderTOC bool) (converter.Result, error)
+}
+
 // FileProvider provides the source file.
 type FileProvider interface {
 	File() source.File

resources/page/page_lazy_contentprovider.go

@@ -17,6 +17,7 @@ import (
 	"html/template"
 
 	"github.com/gohugoio/hugo/lazy"
+	"github.com/gohugoio/hugo/markup/converter"
 )
 
 // OutputFormatContentProvider represents the method set that is "outputFormat aware" and that we
@@ -24,6 +25,14 @@ import (
 // Note that this set is currently not complete, but should cover the most common use cases.
 // For the others, the implementation will be from the page.NoopPage.
 type OutputFormatContentProvider interface {
+	OutputFormatPageContentProvider
+
+	// for internal use.
+	ContentRenderer
+}
+
+// OutputFormatPageContentProvider holds the exported methods from Page that are "outputFormat aware".
+type OutputFormatPageContentProvider interface {
 	ContentProvider
 	TableOfContentsProvider
 	PageRenderProvider
@@ -46,7 +55,7 @@ type LazyContentProvider struct {
 func NewLazyContentProvider(f func() (OutputFormatContentProvider, error)) *LazyContentProvider {
 	lcp := LazyContentProvider{
 		init: lazy.New(),
-		cp:   NopPage,
+		cp:   NopCPageContentRenderer,
 	}
 	lcp.init.Add(func() (any, error) {
 		cp, err := f()
@@ -122,3 +131,8 @@ func (lcp *LazyContentProvider) TableOfContents() template.HTML {
 	lcp.init.Do()
 	return lcp.cp.TableOfContents()
 }
+
+func (lcp *LazyContentProvider) RenderContent(content []byte, renderTOC bool) (converter.Result, error) {
+	lcp.init.Do()
+	return lcp.cp.RenderContent(content, renderTOC)
+}

resources/page/page_nop.go

@@ -16,10 +16,12 @@
 package page
 
 import (
+	"bytes"
 	"html/template"
 	"time"
 
 	"github.com/gohugoio/hugo/identity"
+	"github.com/gohugoio/hugo/markup/converter"
 
 	"github.com/gohugoio/hugo/hugofs/files"
 	"github.com/gohugoio/hugo/tpl"
@@ -41,7 +43,15 @@ import (
 )
 
 var (
-	NopPage Page = new(nopPage)
+	NopPage                 Page            = new(nopPage)
+	NopContentRenderer      ContentRenderer = new(nopContentRenderer)
+	NopCPageContentRenderer                 = struct {
+		OutputFormatPageContentProvider
+		ContentRenderer
+	}{
+		NopPage,
+		NopContentRenderer,
+	}
 	NilPage *nopPage
 )
 
@@ -513,3 +523,10 @@ func (p *nopPage) WordCount() int {
 func (p *nopPage) GetIdentity() identity.Identity {
 	return identity.NewPathIdentity("content", "foo/bar.md")
 }
+
+type nopContentRenderer int
+
+func (r *nopContentRenderer) RenderContent(content []byte, renderTOC bool) (converter.Result, error) {
+	b := &bytes.Buffer{}
+	return b, nil
+}