Icon theme fallback for missing themes

Fixes: pop-os#48

This PR implements a last resort fallback to the default GTK icon theme
if COSMIC's theme is missing.

The user is warned if COSMIC's themes aren't available since a correct
installation should have them much like GTK should always have Adwaita.

src/lib.rs

@@ -71,6 +71,9 @@ pub fn main() -> Result<(), Box<dyn std::error::Error>> {
     let mut settings = Settings::default();
     settings = settings.theme(config.app_theme.theme());
     settings = settings.size_limits(Limits::NONE.min_width(360.0).min_height(180.0));
+    if let Some(fallback_theme) = crate::mime_icon::fallback_theme() {
+        settings = settings.default_icon_theme(fallback_theme);
+    }
 
     let flags = Flags {
         config_handler,

src/mime_icon.rs

@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: GPL-3.0-only
 
-use cosmic::widget::icon;
+use cosmic::{config::CosmicTk, cosmic_config::ConfigGet, widget::icon};
 use mime_guess::Mime;
 use once_cell::sync::Lazy;
 use std::{collections::HashMap, path::Path, sync::Mutex};
@@ -69,3 +69,103 @@ pub fn mime_icon(mime: Mime, size: u16) -> icon::Handle {
         None => icon::from_name(FALLBACK_MIME_ICON).size(size).handle(),
     }
 }
+
+pub fn fallback_theme() -> Option<String> {
+    // Icon precedence
+    // 1. User's preferred theme (automatic via libcosmic)
+    // 2. COSMIC default (automatic)
+    // 3. GTK default
+    if CosmicTk::config()
+        .and_then(|config| config.get::<String>("icon_theme"))
+        .is_err()
+        && icon::from_name("folder").path().is_none()
+    {
+        log::warn!("COSMIC's default theme is missing, but it should be available as part of a correct installation");
+
+        // Try GTK as a last resort.
+        let icon_theme = gtk_icon_theme();
+        if icon_theme.is_none() {
+            log::warn!("Could not find a valid icon theme. Install COSMIC's icon theme or set a default with cosmic-settings.");
+        }
+        icon_theme
+    } else {
+        None
+    }
+}
+
+fn gtk_icon_theme() -> Option<String> {
+    let gsettings = std::process::Command::new("gsettings")
+        .args(["get", "org.gnome.desktop.interface", "icon-theme"])
+        .output()
+        .ok()?;
+
+    if gsettings.status.success() {
+        let name = String::from_utf8(gsettings.stdout)
+            .ok()?
+            .trim()
+            .trim_matches('\'')
+            .to_owned();
+        Some(name)
+    } else {
+        None
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use cosmic::widget;
+
+    use super::fallback_theme;
+
+    #[test]
+    fn basic_icon_lookup_never_fails() {
+        if let Some(gtk_theme) = fallback_theme() {
+            cosmic::icon_theme::set_default(gtk_theme);
+        }
+
+        // Fallback
+        widget::icon::from_name("text-x-generic")
+            .path()
+            .expect("`text-x-generic` should always be found");
+
+        // Trash
+        widget::icon::from_name("user-trash-full-symbolic")
+            .path()
+            .expect("`user-trash-full-symbolic` should always be found");
+        widget::icon::from_name("user-trash-symbolic")
+            .path()
+            .expect("`user-trash-symbolic` should always be found");
+
+        // Folders
+        widget::icon::from_name("folder")
+            .path()
+            .expect("`folder` should always be found");
+        widget::icon::from_name("folder-documents-symbolic")
+            .path()
+            .expect("`folder-documents-symbolic` should always be found");
+        widget::icon::from_name("folder-download-symbolic")
+            .path()
+            .expect("`folder-documents-symbolic` should always be found");
+        widget::icon::from_name("folder-music-symbolic")
+            .path()
+            .expect("`folder-music-symbolic` should always be found");
+        widget::icon::from_name("folder-pictures-symbolic")
+            .path()
+            .expect("`folder-pictures-symbolic` should always be found");
+        widget::icon::from_name("folder-publicshare-symbolic")
+            .path()
+            .expect("`folder-publicshare-symbolic` should always be found");
+        widget::icon::from_name("folder-templates-symbolic")
+            .path()
+            .expect("`folder-templates-symbolic` should always be found");
+        widget::icon::from_name("folder-videos-symbolic")
+            .path()
+            .expect("`folder-videos-symbolic` should always be found");
+        widget::icon::from_name("user-desktop-symbolic")
+            .path()
+            .expect("`user-desktop-symbolic` should always be found");
+        widget::icon::from_name("user-home-symbolic")
+            .path()
+            .expect("`user-home-symblic` should always be found");
+    }
+}