hugolib: Log warning on relative front matter url with lang

Will do this for one version only, as there may be situations where this is the correct thing.

Also add some more related test cases.

Fixes #5818

hugolib/page__meta.go

@@ -21,6 +21,8 @@ import (
 	"strings"
 	"time"
 
+	"github.com/gohugoio/hugo/common/hugo"
+
 	"github.com/gohugoio/hugo/related"
 
 	"github.com/gohugoio/hugo/source"
@@ -374,11 +376,22 @@ func (pm *pageMeta) setMetadata(p *pageState, frontmatter map[string]interface{}
 			pm.urlPaths.Slug = strings.Trim(cast.ToString(v), "-")
 			pm.params[loki] = pm.Slug()
 		case "url":
-			if url := cast.ToString(v); strings.HasPrefix(url, "http://") || strings.HasPrefix(url, "https://") {
+			url := cast.ToString(v)
-				return fmt.Errorf("only relative URLs are supported, %v provided", url)
+			if strings.HasPrefix(url, "http://") || strings.HasPrefix(url, "https://") {
+				return fmt.Errorf("URLs with protocol (http*) not supported: %q. In page %q", url, p.pathOrTitle())
 			}
-			pm.urlPaths.URL = cast.ToString(v)
+			lang := p.s.GetLanguagePrefix()
-			pm.params[loki] = pm.urlPaths.URL
+			if lang != "" && !strings.HasPrefix(url, "/") && strings.HasPrefix(url, lang) {
+				if strings.HasPrefix(hugo.CurrentVersion.String(), "0.55") {
+					// We added support for page relative URLs in Hugo 0.55 and
+					// this may get its language path added twice.
+					// TODO(bep) eventually remove this.
+					p.s.Log.WARN.Printf(`Front matter in %q with the url %q with no leading / has what looks like the language prefix added. In Hugo 0.55 we added support for page relative URLs in front matter, no language prefix needed. Check the URL and consider to either add a leading / or remove the language prefix.`, p.pathOrTitle(), url)
+
+				}
+			}
+			pm.urlPaths.URL = url
+			pm.params[loki] = url
 		case "type":
 			pm.contentType = cast.ToString(v)
 			pm.params[loki] = pm.contentType

hugolib/page_permalink_test.go

@@ -108,7 +108,7 @@ Content
 func TestRelativeURLInFrontMatter(t *testing.T) {
 
 	config := `
-
+baseURL = "https://example.com"
 defaultContentLanguage = "en"
 defaultContentLanguageInSubdir = false
 
@@ -132,6 +132,8 @@ Some content.
 
 	b := newTestSitesBuilder(t).WithConfigFile("toml", config)
 	b.WithContent("content/en/blog/page1.md", fmt.Sprintf(pageTempl, "myblog/p1/"))
+	b.WithContent("content/en/blog/page2.md", fmt.Sprintf(pageTempl, "../../../../../myblog/p2/"))
+	b.WithContent("content/en/blog/page3.md", fmt.Sprintf(pageTempl, "../myblog/../myblog/p3/"))
 	b.WithContent("content/en/blog/_index.md", fmt.Sprintf(pageTempl, "this-is-my-english-blog"))
 	b.WithContent("content/nn/blog/page1.md", fmt.Sprintf(pageTempl, "myblog/p1/"))
 	b.WithContent("content/nn/blog/_index.md", fmt.Sprintf(pageTempl, "this-is-my-blog"))
@@ -139,8 +141,10 @@ Some content.
 	b.Build(BuildCfg{})
 
 	b.AssertFileContent("public/nn/myblog/p1/index.html", "Single: A page|Hello|nn|RelPermalink: /nn/myblog/p1/|")
-	b.AssertFileContent("public/nn/this-is-my-blog/index.html", "List Page 1|A page|Hello|/nn/this-is-my-blog/|")
+	b.AssertFileContent("public/nn/this-is-my-blog/index.html", "List Page 1|A page|Hello|https://example.com/nn/this-is-my-blog/|")
-	b.AssertFileContent("public/this-is-my-english-blog/index.html", "List Page 1|A page|Hello|/this-is-my-english-blog/|")
+	b.AssertFileContent("public/this-is-my-english-blog/index.html", "List Page 1|A page|Hello|https://example.com/this-is-my-english-blog/|")
-	b.AssertFileContent("public/myblog/p1/index.html", "Single: A page|Hello|en|RelPermalink: /myblog/p1/|Permalink: /myblog/p1/|")
+	b.AssertFileContent("public/myblog/p1/index.html", "Single: A page|Hello|en|RelPermalink: /myblog/p1/|Permalink: https://example.com/myblog/p1/|")
+	b.AssertFileContent("public/myblog/p2/index.html", "Single: A page|Hello|en|RelPermalink: /myblog/p2/|Permalink: https://example.com/myblog/p2/|")
+	b.AssertFileContent("public/myblog/p3/index.html", "Single: A page|Hello|en|RelPermalink: /myblog/p3/|Permalink: https://example.com/myblog/p3/|")
 
 }