include mxcs from dangling thumbnails in service::media::iter_all

When requesting remote thumbnails over federation, we can end up with a
thumbnail in the media db without an associated original file. Because
of this, skipping thumbnails is insufficient to get a list of all MXCs.

src/service/media.rs

@@ -169,11 +169,27 @@ impl Service {
 
     /// List all media stored in the database.
     ///
-    /// Each MXC is list once. Thumbnails are not included separately from the
+    /// Each MXC is listed once. Thumbnails are not included separately from the
     /// original media.
     #[tracing::instrument(skip(self))]
     pub(crate) fn iter_all(&self) -> impl Iterator<Item = Result<OwnedMxcUri>> {
-        self.db.all_file_metadata().map(|media| media.map(|(mxc, ..)| mxc))
+        let mut prev_mxc = None;
+        self.db
+            .all_file_metadata()
+            .map(|media| media.map(|(mxc, ..)| mxc))
+            .filter(move |mxc| {
+                if let Ok(mxc) = mxc {
+                    // Skip mxcs that we have already seen. All files associated
+                    // with a given mxc should appear consecutively in the db
+                    // iterator, so we only need to check against the previous
+                    // value.
+                    if prev_mxc.as_ref() == Some(mxc) {
+                        return false;
+                    }
+                    prev_mxc = Some(mxc.clone());
+                }
+                true
+            })
     }
 
     /// Returns width, height of the thumbnail and whether it should be cropped.