OPML nested past 100 levels is refused before it is parsed (#322)

The opml crate's Outline reader (hard-xml's derive) recurses once per
<outline> level with no limit. A 2 MB upload holds tens of thousands of
levels, enough to overflow a tokio worker's 2 MiB stack, and a stack
overflow aborts the process, not the task. A fetched OPML (up to 50 MB)
reached the same parse on every scan, so a publisher could keep the
daemon in a crash loop.

feed::read_opml counts nesting with quick-xml's streaming reader, which
does not recurse, and refuses past 100 before handing the text to the
opml crate. Every place that parsed OPML goes through it: the upload,
`ipx opml`, a fetched OPML's feeds and its title. The limit also bounds
collect_outlines' recursion and the drop of the nested Vec.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/feed/src/lib.rs

@@ -357,10 +357,37 @@ pub fn is_opml(bytes: &[u8]) -> bool {
     text.contains("<opml")
 }
 
+/// How deep an OPML document's elements may nest. Real ones are a folder or two deep.
+const MAX_OPML_DEPTH: usize = 100;
+
+/// Parses an OPML document, refusing one nested deeper than `MAX_OPML_DEPTH`. The opml crate's
+/// reader recurses once per level with no limit, and a few MB of `<outline>` overflowed the
+/// stack, which aborts the whole daemon, and again on each restart for a subscribed feed (#322).
+pub fn read_opml(text: &str) -> Result<opml::OPML> {
+    use quick_xml::events::Event;
+    let mut reader = quick_xml::Reader::from_str(text);
+    let mut depth = 0usize;
+    loop {
+        match reader.read_event() {
+            Ok(Event::Start(_)) => {
+                depth += 1;
+                if depth > MAX_OPML_DEPTH {
+                    anyhow::bail!("that OPML nests more than {MAX_OPML_DEPTH} levels deep");
+                }
+            }
+            Ok(Event::End(_)) => depth = depth.saturating_sub(1),
+            // Malformed XML is the opml crate's to describe.
+            Ok(Event::Eof) | Err(_) => break,
+            Ok(_) => {}
+        }
+    }
+    opml::OPML::from_str(text).map_err(|e| anyhow!("{e}"))
+}
+
 /// The feeds listed in an OPML document, as (title, xml_url), walking nested folders.
 pub fn parse_opml(bytes: &[u8]) -> Result<Vec<(String, String)>> {
     let text = String::from_utf8_lossy(bytes);
-    let doc = opml::OPML::from_str(&text).map_err(|e| anyhow!("that does not parse as OPML: {e}"))?;
+    let doc = read_opml(&text).map_err(|e| anyhow!("that does not parse as OPML: {e}"))?;
     let mut out = vec![];
     collect_outlines(&doc.body.outlines, &mut out);
     Ok(out)
@@ -380,7 +407,7 @@ pub fn collect_outlines(outlines: &[opml::Outline], out: &mut Vec<(String, Strin
 /// The <head><title> of an OPML document.
 pub fn opml_title(bytes: &[u8]) -> Option<String> {
     let text = String::from_utf8_lossy(bytes);
-    let doc = opml::OPML::from_str(&text).ok()?;
+    let doc = read_opml(&text).ok()?;
     doc.head.and_then(|h| h.title).map(|t| t.trim().to_owned()).filter(|t| !t.is_empty())
 }
 
@@ -1809,6 +1836,22 @@ Some **bold** and [a link](https://x.example/).</description></item>
         assert_eq!((p1.season, p1.episode), (Some(8), Some(1)));
     }
 
+    #[test]
+    fn deeply_nested_opml_is_refused_before_it_is_parsed() {
+        let nest = |n: usize| {
+            format!(
+                r#"<opml version="2.0"><head><title>T</title></head><body>{}<outline text="f" xmlUrl="https://e.com/f"/>{}</body></opml>"#,
+                r#"<outline text="a">"#.repeat(n),
+                "</outline>".repeat(n)
+            )
+        };
+        assert_eq!(parse_opml(nest(5).as_bytes()).unwrap().len(), 1, "a few folders deep is fine");
+        // Deep enough to overflow a 2 MiB stack in the opml crate's reader without the check.
+        let deep = nest(100_000);
+        assert!(parse_opml(deep.as_bytes()).unwrap_err().to_string().contains("levels deep"));
+        assert_eq!(opml_title(deep.as_bytes()), None);
+    }
+
     #[test]
     fn opml_is_recognised_and_its_feeds_listed() {
         let xml = br#"<opml version="2.0"><head><title>My Subscriptions</title></head><body>