Get rid of allowlist check allocs

src/api/client_server/media.rs

@@ -242,37 +242,29 @@ async fn get_url_preview(url: &Url) -> Result<UrlPreviewData> {
     }
 }
 
+/// Verify that the given URL's host is in the allow list.
 fn url_preview_allowed(url: &Url) -> bool {
-    const DEFAULT_ALLOWLIST: &[&str] = &[
-        "matrix.org",
-        "mastodon.social",
-        "youtube.com",
-        "wikipedia.org",
-    ];
+    // host's existence is already verified in get_media_preview_route, unwrap is safe
+    let host = url.host_str().unwrap().to_lowercase();
+    let host_parts_iter = host
+        .char_indices()
+        .filter_map(|(i, c)| {
+            if i == 0 {
+                Some(host.as_str())
+            }
+            else if c == '.' {
+                Some(&host[i+1..])
+            }
+            else {
+                None
+            }
+        })
+        .rev().skip(1); // don't match TLDs
 
-    let mut host = match url.host_str() {
-        None => return false,
-        Some(h) => h.to_lowercase(),
-    };
-
-    let allowlist = services().globals.url_preview_allowlist();
-    if allowlist.contains(&"*".to_owned()) {
-        return true;
-    }
-    while !host.is_empty() {
-        if allowlist.contains(&host) {
-            return true;
-        }
-        if allowlist.contains(&"default".to_owned()) && DEFAULT_ALLOWLIST.contains(&host.as_str()) {
-            return true;
-        }
-        /* also check higher level domains, so that e.g. `en.m.wikipedia.org` is matched by `wikipedia.org` on allowlist. */
-        host = match host.split_once('.') {
-            None => return false,
-            Some((_, domain)) => domain.to_owned(),
-        }
-    }
-    false
+    let ret = ["*"].into_iter().chain(host_parts_iter).any(|nld| {
+        services().globals.url_preview_allowlist().any(|a| a == nld)
+    });
+    ret // temp variable to avoid returning from the closure
 }
 
 /// # `GET /_matrix/media/r0/preview_url`

src/service/globals/mod.rs

@@ -324,8 +324,8 @@ impl Service {
         self.config.allow_federation
     }
 
-    pub fn url_preview_allowlist(&self) -> &Vec<String> {
-        &self.config.url_preview_allowlist
+    pub fn url_preview_allowlist(&self) -> impl Iterator<Item=&str> {
+        self.config.url_preview_allowlist.iter().map(|x| x.as_str())
     }
 
     pub fn allow_room_creation(&self) -> bool {