Implement Fallback Icon service

src/api/icons.rs

@@ -534,7 +534,14 @@ async fn download_icon(domain: &str) -> Result<(Bytes, Option<&str>), Error> {
 
     use data_url::DataUrl;
 
-    for icon in icon_result.iconlist.iter().take(5) {
+    let mut iconlist: Vec<Icon> = icon_result.iconlist.into_iter().take(5).collect();
+    // add URL from Fallback Icon service
+    if !CONFIG.icon_service_fallback().is_empty() {
+        let fallback_url = CONFIG._icon_service_fallback_url().replace("{}", domain);
+        iconlist.push(Icon::new(0, fallback_url));
+    }
+
+    for icon in iconlist.iter() {
         if icon.href.starts_with("data:image") {
             let Ok(datauri) = DataUrl::process(&icon.href) else {
                 continue;

src/config.rs

@@ -683,6 +683,12 @@ make_config! {
         /// has been decided on, consider using permanent redirects for cacheability. The legacy codes
         /// are currently better supported by the Bitwarden clients.
         icon_redirect_code:     u32,    true,   def,    302;
+        /// Icon service to use as URL fallback for internal Icon service. Works only when icon_service is set to "internal"
+        /// Same predefined/custom values as icon_service (except "internal"). Unlike Icon service, the Fallback Icon service
+        /// does not send any redirect to the client and instead use the internal Icon service to download the icon located at the external service
+        icon_service_fallback:  String, false,  def,    String::new();
+        /// _icon_service_fallback
+        _icon_service_fallback_url: String, false,  generated,    |c| generate_icon_service_url(&c.icon_service_fallback);
         /// Positive icon cache expiry |> Number of seconds to consider that an already cached icon is fresh. After this period, the icon will be refreshed
         icon_cache_ttl:         u64,    true,   def,    2_592_000;
         /// Negative icon cache expiry |> Number of seconds before trying to download an icon that failed again.
@@ -1201,6 +1207,27 @@ fn validate_config(cfg: &ConfigItems) -> Result<(), Error> {
         }
     }
 
+    // Check if the Fallback Icon service is valid
+    let icon_service_fallback = cfg.icon_service_fallback.as_str();
+    if !icon_service_fallback.is_empty() {
+        if icon_service != "internal" {
+            err!(format!("Fallback Icon service can only be used for \"internal\" Icon service, you are currently using \"{icon_service}\""))
+        }
+        match icon_service_fallback {
+            "bitwarden" | "duckduckgo" | "google" => (),
+            _ => {
+                if !icon_service_fallback.starts_with("http") {
+                    err!(format!("Fallback Icon service URL `{icon_service_fallback}` must start with \"http\""))
+                }
+                match icon_service_fallback.matches("{}").count() {
+                    1 => (), // nominal
+                    0 => err!(format!("Fallback Icon service URL `{icon_service_fallback}` has no placeholder \"{{}}\"")),
+                    _ => err!(format!("Fallback Icon service URL `{icon_service_fallback}` has more than one placeholder \"{{}}\"")),
+                }
+            }
+        }
+    }
+
     // Check if the icon redirect code is valid
     match cfg.icon_redirect_code {
         301 | 302 | 307 | 308 => (),