Fix surprise OutputFormat.Rel overwriting

In page.NewOutputFormat, we take an output.Format f and use it to
create a page.OutputFormat. If the format is canonical, we assign
the final OutputFormat's Rel to "canonical" rather than using
f.Rel. However, this leads to unexpected behavior for custom
output formats, where a user can define a "rel" for a format
via the config file.

For example, the standard for "humans.txt" files requires using
rel="author" in HTML "link" elements. Meanwhile, humans.txt is
usually the only format used for its content. As a result, for
Hugo configurations that define a humans.txt custom output format,
Hugo will render "link" elements to content in this format with
rel="canonical," rather than "author" as required by the standard.

This commit changes page.NewOutputFormat to check whether a given
format is user defined and, if so, skips assigning Rel to
"canonical," even if isCanonical is true.

Fixes #8030

hugolib/site_output_test.go

@@ -324,6 +324,36 @@ baseName = "customdelimbase"
 	c.Assert(outputs.Get("CUS").RelPermalink(), qt.Equals, "/blog/customdelimbase_del")
 }
 
+// Issue 8030
+func TestGetOutputFormatRel(t *testing.T) {
+	b := newTestSitesBuilder(t).
+		WithSimpleConfigFileAndSettings(map[string]interface{}{
+			"outputFormats": map[string]interface{}{
+				"humansTXT": map[string]interface{}{
+					"name":        "HUMANS",
+					"mediaType":   "text/plain",
+					"baseName":    "humans",
+					"isPlainText": true,
+					"rel":         "author",
+				},
+			},
+		}).WithTemplates("index.html", `
+{{- with ($.Site.GetPage "humans").OutputFormats.Get "humans" -}}
+<link rel="{{ .Rel }}" type="{{ .MediaType.String }}" href="{{ .Permalink }}">
+{{- end -}}
+`).WithContent("humans.md", `---
+outputs:
+- HUMANS
+---
+This is my content.
+`)
+
+	b.Build(BuildCfg{})
+	b.AssertFileContent("public/index.html", `
+<link rel="author" type="text/plain" href="/humans.txt">
+`)
+}
+
 func TestCreateSiteOutputFormats(t *testing.T) {
 	t.Run("Basic", func(t *testing.T) {
 		c := qt.New(t)

resources/page/page_outputformat.go

@@ -66,8 +66,18 @@ func (o OutputFormat) RelPermalink() string {
 }
 
 func NewOutputFormat(relPermalink, permalink string, isCanonical bool, f output.Format) OutputFormat {
+	isUserConfigured := true
+	for _, d := range output.DefaultFormats {
+		if strings.EqualFold(d.Name, f.Name) {
+			isUserConfigured = false
+		}
+	}
 	rel := f.Rel
-	if isCanonical {
+	// If the output format is the canonical format for the content, we want
+	// to specify this in the "rel" attribute of an HTML "link" element.
+	// However, for custom output formats, we don't want to surprise users by
+	// overwriting "rel"
+	if isCanonical && !isUserConfigured {
 		rel = "canonical"
 	}
 	return OutputFormat{Rel: rel, Format: f, relPermalink: relPermalink, permalink: permalink}