rendering: Avoid prepending URL prefix to links that start with a scheme (#817)

Links that start with a scheme (e.g., `tel:18008675309`) inadvertently
had a URL prefix prepended. Previously, only `mailto:` was handled, but
given the sheer number of [registered URI schemes][uri-schemes], a loose
pattern matcher is used to detect schemes instead.

External links, as identified by the renderer, are now limited to `http`
and `https` schemes.

Fixes #747 and fixes #816.

[uri-schemes]: https://www.iana.org/assignments/uri-schemes/uri-schemes.xhtml

components/rendering/src/markdown.rs

@@ -1,4 +1,5 @@
 use pulldown_cmark as cmark;
+use regex::Regex;
 use slug::slugify;
 use syntect::easy::HighlightLines;
 use syntect::html::{
@@ -60,11 +61,31 @@ fn find_anchor(anchors: &[String], name: String, level: u8) -> String {
     find_anchor(anchors, name, level + 1)
 }
 
+// Returns whether the given string starts with a schema.
+//
+// Although there exists [a list of registered URI schemes][uri-schemes], a link may use arbitrary,
+// private schemes. This function checks if the given string starts with something that just looks
+// like a scheme, i.e., a case-insensitive identifier followed by a colon.
+//
+// [uri-schemes]: https://www.iana.org/assignments/uri-schemes/uri-schemes.xhtml
+fn starts_with_schema(s: &str) -> bool {
+    lazy_static! {
+        static ref PATTERN: Regex = Regex::new(r"^[0-9A-Za-z\-]+:").unwrap();
+    }
+
+    PATTERN.is_match(s)
+}
+
 // Colocated asset links refers to the files in the same directory,
 // there it should be a filename only
 fn is_colocated_asset_link(link: &str) -> bool {
     !link.contains('/')  // http://, ftp://, ../ etc
-        && !link.starts_with("mailto:")
+        && !starts_with_schema(link)
+}
+
+// Returns whether a link starts with an HTTP(s) scheme.
+fn is_external_link(link: &str) -> bool {
+    link.starts_with("http:") || link.starts_with("https:")
 }
 
 fn fix_link(
@@ -103,7 +124,7 @@ fn fix_link(
     } else if is_colocated_asset_link(&link) {
         format!("{}{}", context.current_page_permalink, link)
     } else {
-        if !link.starts_with('#') && !link.starts_with("mailto:") {
+        if is_external_link(link) {
             external_links.push(link.to_owned());
         }
         link.to_string()
@@ -328,3 +349,41 @@ pub fn markdown_to_html(content: &str, context: &RenderContext) -> Result<Render
         })
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_starts_with_schema() {
+        // registered
+        assert!(starts_with_schema("https://example.com/"));
+        assert!(starts_with_schema("ftp://example.com/"));
+        assert!(starts_with_schema("mailto:user@example.com"));
+        assert!(starts_with_schema("xmpp:node@example.com"));
+        assert!(starts_with_schema("tel:18008675309"));
+        assert!(starts_with_schema("sms:18008675309"));
+        assert!(starts_with_schema("h323:user@example.com"));
+
+        // arbitrary
+        assert!(starts_with_schema("zola:post?content=hi"));
+
+        // case-insensitive
+        assert!(starts_with_schema("MailTo:user@example.com"));
+        assert!(starts_with_schema("MAILTO:user@example.com"));
+    }
+
+    #[test]
+    fn test_is_external_link() {
+        assert!(is_external_link("http://example.com/"));
+        assert!(is_external_link("https://example.com/"));
+        assert!(is_external_link("https://example.com/index.html#introduction"));
+
+        assert!(!is_external_link("mailto:user@example.com"));
+        assert!(!is_external_link("tel:18008675309"));
+
+        assert!(!is_external_link("#introduction"));
+
+        assert!(!is_external_link("http.jpg"))
+    }
+}

components/rendering/tests/markdown.rs

@@ -821,12 +821,34 @@ fn doesnt_try_to_highlight_content_from_shortcode() {
 //}
 
 // https://github.com/getzola/zola/issues/747
+// https://github.com/getzola/zola/issues/816
 #[test]
 fn leaves_custom_url_scheme_untouched() {
+    let content = r#"[foo@bar.tld](xmpp:foo@bar.tld)
+
+[(123) 456-7890](tel:+11234567890)
+
+[blank page](about:blank)
+"#;
+
     let tera_ctx = Tera::default();
-    let permalinks_ctx = HashMap::new();
     let config = Config::default();
-    let context = RenderContext::new(&tera_ctx, &config, "", &permalinks_ctx, InsertAnchor::None);
-    let res = render_content("[foo@bar.tld](xmpp:foo@bar.tld)", &context).unwrap();
-    assert_eq!(res.body, "<p><a href=\"xmpp:foo@bar.tld\">foo@bar.tld</a></p>\n");
+    let permalinks_ctx = HashMap::new();
+
+    let context = RenderContext::new(
+        &tera_ctx,
+        &config,
+        "https://vincent.is/",
+        &permalinks_ctx,
+        InsertAnchor::None,
+    );
+
+    let res = render_content(content, &context).unwrap();
+
+    let expected = r#"<p><a href="xmpp:foo@bar.tld">foo@bar.tld</a></p>
+<p><a href="tel:+11234567890">(123) 456-7890</a></p>
+<p><a href="about:blank">blank page</a></p>
+"#;
+
+    assert_eq!(res.body, expected);
 }