fallback to smaller media when file too large

crates/x-media/src/media.rs

@@ -14,6 +14,24 @@ impl Media {
             Media::Animated { thumbnail_url, .. } => Some(thumbnail_url),
         }
     }
+
+    /// A smaller variant of this media's file (used as the fallback when the
+    /// primary URL or upload exceeds Telegram's size limits). None when no
+    /// smaller variant exists (videos, animated gifs).
+    pub fn smaller_url(&self) -> Option<&str> {
+        match self {
+            Media::Illustration {
+                url,
+                fallback_url,
+                thumbnail_url,
+                ..
+            } => fallback_url
+                .as_deref()
+                .or(thumbnail_url.as_deref())
+                .filter(|smaller| *smaller != url),
+            Media::Video { .. } | Media::Animated { .. } => None,
+        }
+    }
 }
 
 #[derive(Debug)]

crates/x-media/src/site/mod.rs

@@ -195,6 +195,19 @@ async fn fetch_once(url: &str) -> Result<Option<Fetched>, FetchError> {
 /// fetch of a media URL is blocked (hotlink protection), the bot downloads
 /// the file itself and uploads it via multipart. Site-appropriate headers:
 /// pixiv image hosts need the `Referer` header.
+/// Returns the Content-Length of a media URL, or `None` when the server does
+/// not report one. Used to check whether a file fits Telegram's size limits
+/// before downloading/uploading it.
+pub async fn media_size(url: &str) -> Result<Option<u64>, FetchError> {
+    let mut request = CLIENT.get(url);
+    let lower = url.to_ascii_lowercase();
+    if lower.contains("pximg.net") {
+        request = request.header("Referer", "https://www.pixiv.net/");
+    }
+    let response = request.send().await?;
+    Ok(response.content_length())
+}
+
 pub async fn download_media(url: &str) -> Result<bytes::Bytes, FetchError> {
     let mut request = CLIENT.get(url);
     let lower = url.to_ascii_lowercase();

crates/x-media/src/site/twitter/interface.rs

@@ -129,7 +129,9 @@ impl Tweet {
                     title: None,
                     url: original_twimg_url(&item.media_url_https),
                     thumbnail_url: None,
-                    fallback_url: None,
+                    // The param-less base URL is a reduced-size variant;
+                    // used as the fallback when the original is too large.
+                    fallback_url: Some(item.media_url_https.clone()),
                 }),
                 "video" => media.push(Media::Video {
                     title: None,