Add only what is a feed or links one; refuse the rest (#102)

Adding an address looked for the feed a web page links and, finding none, added the address
as it was: every check then failed, and the sidebar called it a feed that had moved. cnn.com
is one; its page links no feed. find_feed replaces feed_behind_page: the address is added if
it is a feed or an OPML list, the feed its page links if it is a web page that links one (and
that is a feed), and otherwise the add is refused with the reason, from the web page (400, the
dialog stays open) and from `ipx add`.

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

src/feed.rs

@@ -394,20 +394,38 @@ fn alternate_feed_link(bytes: &[u8]) -> Option<String> {
     None
 }
 
-/// The feed a web page links as its own, for someone who pasted a site's address instead of
-/// its feed's. Anything else, or a page that cannot be read, comes back as given, and adding
-/// it goes on as before.
-pub async fn feed_behind_page(client: &reqwest::Client, url: &str) -> String {
-    let found = async {
-        let resp = client.get(url).timeout(std::time::Duration::from_secs(20)).send().await.ok()?;
+/// What someone asked to add, as a feed: the address itself when it is a feed or an OPML list,
+/// otherwise the feed its web page links as its own, otherwise an error and nothing is added.
+/// A page that linked no feed used to be added as it was and failed on every check, called a
+/// feed that moved (#102); cnn.com is one.
+pub async fn find_feed(client: &reqwest::Client, url: &str) -> Result<String> {
+    let read = |u: String| async move {
+        let resp = client
+            .get(&u)
+            .timeout(std::time::Duration::from_secs(30))
+            .send()
+            .await
+            .context("connecting")?;
+        anyhow::ensure!(resp.status().is_success(), "HTTP {}", resp.status());
         let base = resp.url().clone();
-        let page = resp.bytes().await.ok()?;
-        if !looks_like_html(&page) {
-            return None;
-        }
-        base.join(&alternate_feed_link(&page)?).ok()
+        anyhow::Ok((base, resp.bytes().await.context("reading")?))
     };
-    found.await.map(String::from).unwrap_or_else(|| url.to_owned())
+    let is_feed = |b: &[u8]| is_opml(b) || parse(b).is_ok();
+    let (base, body) = read(url.to_owned()).await.with_context(|| format!("could not read {url}"))?;
+    if is_feed(&body) {
+        return Ok(url.to_owned());
+    }
+    if !looks_like_html(&body) {
+        let why = parse(&body).err().map(|e| format!("{e:#}")).unwrap_or_default();
+        anyhow::bail!("{url} is not a feed: {why}");
+    }
+    let Some(href) = alternate_feed_link(&body) else {
+        anyhow::bail!("{url} is a web page that links no feed, so there is nothing to subscribe to");
+    };
+    let linked = base.join(&href).with_context(|| format!("{url} links {href} as its feed, which is not an address"))?;
+    let (_, feed) = read(linked.to_string()).await.with_context(|| format!("{url} links {linked} as its feed, but"))?;
+    anyhow::ensure!(is_feed(&feed), "{url} links {linked} as its feed, but that is not a feed either");
+    Ok(linked.into())
 }
 
 /// Artwork for a feed that has none: the icon its site's page names, or else the site's