hugolib: Fix deadlock when content building times out

Fixes #5375

hugolib/page.go

@@ -290,10 +290,11 @@ func (p *Page) initContent() {
 		defer cancel()
 		c := make(chan error, 1)
 
+		p.contentInitMu.Lock()
+		defer p.contentInitMu.Unlock()
+
 		go func() {
 			var err error
-			p.contentInitMu.Lock()
-			defer p.contentInitMu.Unlock()
 
 			err = p.prepareForRender()
 			if err != nil {
@@ -301,6 +302,12 @@ func (p *Page) initContent() {
 				return
 			}
 
+			select {
+			case <-ctx.Done():
+				return
+			default:
+			}
+
 			if len(p.summary) == 0 {
 				if err = p.setAutoSummary(); err != nil {
 					err = p.errorf(err, "failed to set auto summary")