helpers: Improve schema detection when creating relative URLs

Fixes #11080

helpers/url.go

@@ -152,7 +152,13 @@ func (p *PathSpec) getBaseURLRoot(path string) string {
 func (p *PathSpec) RelURL(in string, addLanguage bool) string {
 	baseURL := p.getBaseURLRoot(in)
 	canonifyURLs := p.Cfg.CanonifyURLs()
-	if (!strings.HasPrefix(in, baseURL) && strings.HasPrefix(in, "http")) || strings.HasPrefix(in, "//") {
+
+	url, err := url.Parse(in)
+	if err != nil {
+		return in
+	}
+
+	if (!strings.HasPrefix(in, baseURL) && url.IsAbs()) || strings.HasPrefix(in, "//") {
 		return in
 	}
 

helpers/url_test.go

@@ -177,6 +177,10 @@ func doTestRelURL(t testing.TB, defaultInSubDir, addLanguage, multilingual bool,
 		{"/foo/bar", "https://example.org/foo/", false, "MULTI/foo/bar"},
 		{"foo/bar", "https://example.org/foo/", false, "/fooMULTI/foo/bar"},
 
+		// Issue 11080
+		{"mailto:[email protected]", "http://base/", false, "mailto:[email protected]"},
+		{"ftp://b.com/a.txt", "http://base/", false, "ftp://b.com/a.txt"},
+
 		{"/test/foo", "http://base/", false, "MULTI/test/foo"},
 		{"/" + lang + "/test/foo", "http://base/", false, "/" + lang + "/test/foo"},
 		{lang + "/test/foo", "http://base/", false, "/" + lang + "/test/foo"},