feat(handlers): debounce inline queries to stop fetch storms while typing

Telegram fires an inline query on every keystroke and every prefix of a
pasted URL (status/12, status/123, ...) matches the site patterns, so
typing used to trigger a full 3-attempt fetch per keystroke. Answer only
after the query has been stable for 800ms, dedupe repeats through
Telegram's inline cache (explicit cache_time 300), and let a repeat of a
query that produced no answer retry the fetch.

crates/xmedia-bot/src/handlers.rs

@@ -764,16 +764,85 @@ pub async fn message_handler(bot: Bot, message: Message) -> Result<(), RequestEr
     respond(())
 }
 
+/// Debounce window for inline queries: Telegram fires an inline query on
+/// every keystroke, and each prefix of a pasted URL (e.g. `.../status/12`,
+/// `.../status/123`, ...) already matches the site patterns. Without a
+/// debounce every keystroke triggers a fetch (3 attempts!) of a half-typed
+/// post id. Only answer once the query has been stable for this long.
+const INLINE_DEBOUNCE: std::time::Duration = std::time::Duration::from_millis(800);
+
+/// Last seen inline query and whether it was already answered. Guards the
+/// debounce timer: a repeat of an answered query is served by Telegram's
+/// inline cache (see `cache_time`), not by another fetch.
+struct InlineDebounceState {
+    query: String,
+    answered: bool,
+}
+
+static INLINE_DEBOUNCE_STATE: LazyLock<parking_lot::Mutex<Option<InlineDebounceState>>> =
+    LazyLock::new(|| parking_lot::Mutex::new(None));
+
 pub async fn inline_query_handler(bot: Bot, query: InlineQuery) -> Result<(), RequestError> {
     if query.query.is_empty() {
         return respond(());
     }
-    // Telegram fires an inline query on every keystroke; only run a fetch
+    // Only run a fetch for something that is actually a supported post URL.
-    // (3 attempts!) for something that is actually a supported post URL, so
-    // typing does not hammer the source sites.
     if x_media::site::cache_key(&query.query).is_none() {
         return respond(());
     }
+    // Debounce: record the query and answer only after it has been stable for
+    // INLINE_DEBOUNCE (the timer below). An already-answered repeat of the
+    // same query is left to Telegram's inline cache instead of re-fetching.
+    {
+        let mut state = INLINE_DEBOUNCE_STATE.lock();
+        if let Some(prev) = state.as_ref()
+            && prev.query == query.query
+            && prev.answered
+        {
+            return respond(());
+        }
+        *state = Some(InlineDebounceState {
+            query: query.query.clone(),
+            answered: false,
+        });
+    }
+    let query_text = query.query.clone();
+    tokio::spawn(async move {
+        tokio::time::sleep(INLINE_DEBOUNCE).await;
+        // Only the last query of a typing burst survives: earlier timers see
+        // the query changed and give up without answering.
+        {
+            let mut state = INLINE_DEBOUNCE_STATE.lock();
+            let Some(state) = state.as_mut() else {
+                return;
+            };
+            if state.query != query_text || state.answered {
+                return;
+            }
+            // Claim the answer so a repeat of the same query cannot start a
+            // second fetch; reset below when no answer was produced.
+            state.answered = true;
+        }
+        match answer_inline_query(bot, query).await {
+            Ok(true) => {}
+            // No results produced (or nothing to answer): let a repeat of the
+            // same query retry the fetch.
+            Ok(false) | Err(_) => {
+                let mut state = INLINE_DEBOUNCE_STATE.lock();
+                if let Some(state) = state.as_mut()
+                    && state.query == query_text
+                {
+                    state.answered = false;
+                }
+            }
+        }
+    });
+    respond(())
+}
+
+/// Fetches the post behind an inline query and answers it. The caller has
+/// already applied the debounce. Returns `true` when an answer was sent.
+async fn answer_inline_query(bot: Bot, query: InlineQuery) -> Result<bool, RequestError> {
     log::info!("inline query: {}", query.query);
     match x_media::site::fetch(&query.query).await {
         Ok(Some(fetched)) => {
@@ -822,13 +891,18 @@ pub async fn inline_query_handler(bot: Bot, query: InlineQuery) -> Result<(), Re
                 results.push(result);
             }
             if !results.is_empty() {
-                bot.answer_inline_query(query.id, results).await?;
+                // Explicit cache window: repeats of the same query within 5
+                // minutes are served by Telegram without hitting the bot.
+                bot.answer_inline_query(query.id, results)
+                    .cache_time(300)
+                    .await?;
+                return Ok(true);
             }
         }
         Ok(None) => {}
         Err(e) => log::error!("inline fetch {}: {e}", query.query),
     }
-    respond(())
+    Ok(false)
 }
 
 pub async fn callback_query_handler(bot: Bot, query: CallbackQuery) -> Result<(), RequestError> {