fix(desktop): write preferences owner-only on Unix (#200) (#235)

state.json holds online owner tokens. The temporary file is now created
0600 (and narrowed if a crash left one behind) before anything is written,
and the rename carries that mode to state.json, so an older
world-readable file is replaced on the next save.

Co-authored-by: Justin John <babyjjbabyjj549@gmail.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

apps/figcraft/src/services.rs

@@ -141,17 +141,31 @@ pub fn save_state(app: &FigApp) {
 }
 
 /// Write `prefs` to `dir` atomically (a temporary file renamed over the old one: a crash leaves
-/// either version whole).
-fn write_prefs(dir: &Path, prefs: &Prefs) {
+/// either version whole). They hold online owner tokens, so on Unix only the owner can read them.
+pub(crate) fn write_prefs(dir: &Path, prefs: &Prefs) {
     let tmp = dir.join("state.json.tmp");
     let written = std::fs::create_dir_all(dir)
-        .and_then(|()| std::fs::write(&tmp, json!({"prefs": prefs}).to_string()))
+        .and_then(|()| write_private(&tmp, json!({"prefs": prefs}).to_string().as_bytes()))
         .and_then(|()| std::fs::rename(&tmp, dir.join(STATE)));
     if let Err(e) = written {
         log::warn!("preferences: couldn't save them in {}: {e}", dir.display());
     }
 }
 
+/// Write `bytes` to `path`, owner read/write only on Unix from before the first byte (a leftover
+/// file from a crash is narrowed too: the mode given at open only applies to new files).
+fn write_private(path: &Path, bytes: &[u8]) -> std::io::Result<()> {
+    use std::io::Write;
+    let mut o = std::fs::OpenOptions::new();
+    o.write(true).create(true).truncate(true);
+    #[cfg(unix)]
+    std::os::unix::fs::OpenOptionsExt::mode(&mut o, 0o600);
+    let mut f = o.open(path)?;
+    #[cfg(unix)]
+    f.set_permissions(std::os::unix::fs::PermissionsExt::from_mode(0o600))?;
+    f.write_all(bytes)
+}
+
 impl Services for Desktop {
     fn pick_open(&mut self, title: &str, exts: &[&str]) -> Option<PathBuf> {
         rfd::FileDialog::new().set_title(title).add_filter("Supported files", exts).pick_file()

apps/figcraft/src/services_tests.rs

@@ -66,3 +66,19 @@ fn plugins_are_kept_when_the_app_is_killed() {
     assert!(!dir.join("state.json.tmp").exists(), "no temporary file is left behind");
     let _ = std::fs::remove_dir_all(&dir);
 }
+
+#[cfg(unix)]
+#[test]
+fn preferences_are_readable_by_their_owner_only() {
+    use std::os::unix::fs::PermissionsExt;
+    let dir = root("private");
+    let mode = |p: &std::path::Path| std::fs::metadata(p).unwrap().permissions().mode() & 0o777;
+    // An older world-readable file and a leftover temporary one from a crash.
+    for name in ["state.json", "state.json.tmp"] {
+        std::fs::write(dir.join(name), "{}").unwrap();
+        std::fs::set_permissions(dir.join(name), std::fs::Permissions::from_mode(0o644)).unwrap();
+    }
+    crate::services::write_prefs(&dir, &figcraft_engine::Prefs::default());
+    assert_eq!(mode(&dir.join("state.json")), 0o600);
+    let _ = std::fs::remove_dir_all(&dir);
+}