Fail with error when double-rendering text in markdownify/RenderString

This commit prevents the most commons case of infinite recursion in link render hooks when the `linkify` option is enabled (see below). This is always a user error, but getting a `stack overflow` (the current stack limit in Go is 1 GB on 64-bit, 250 MB on 32-bit) error isn't very helpful. This fix will not prevent all such errors, though, but we may do better once #9570 is in place.

So, these will fail:

```
<a href="{{ .Destination | safeURL }}" >{{ .Text | markdownify }}</a>
<a href="{{ .Destination | safeURL }}" >{{ .Text | .Page.RenderString }}</a>
```

`.Text` is already rendered to `HTML`. The above needs to be rewritten to:

```
<a href="{{ .Destination | safeURL }}" >{{ .Text | safeHTML }}</a>
<a href="{{ .Destination | safeURL }}" >{{ .Text | safeHTML }}</a>
```

Fixes #8959

1 files changed, 10 insertions, 2 deletions

diff --git a/hugolib/page__per_output.go b/hugolib/page__per_output.go
index d79b152f3..a7ad2a245 100644
--- a/hugolib/page__per_output.go
+++ b/hugolib/page__per_output.go
@@ -24,6 +24,7 @@ import (
 	"unicode/utf8"
 
 	"github.com/gohugoio/hugo/common/text"
+	"github.com/gohugoio/hugo/common/types/hstring"
 	"github.com/gohugoio/hugo/identity"
 	"github.com/mitchellh/mapstructure"
 	"github.com/pkg/errors"
@@ -351,8 +352,16 @@ func (p *pageContentOutput) RenderString(args ...interface{}) (template.HTML, er
 		}
 	}
 
+	contentToRender := args[sidx]
+
+	if _, ok := contentToRender.(hstring.RenderedString); ok {
+		// This content is already rendered, this is potentially
+		// a infinite recursion.
+		return "", errors.New("text is already rendered, repeating it may cause infinite recursion")
+	}
+
 	var err error
-	s, err = cast.ToStringE(args[sidx])
+	s, err = cast.ToStringE(contentToRender)
 	if err != nil {
 		return "", err
 	}
@@ -515,7 +524,6 @@ func (p *pageContentOutput) initRenderHooks() error {
 					}
 				}
 			}
-
 			if !found1 {
 				if tp == hooks.CodeBlockRendererType {
 					// No user provided tempplate for code blocks, so we use the native Go code version -- which is also faster.