fix(auth): allow anonymous read-only API access

Match the Python client for read-only commands by falling back to
anonymous API requests when no valid login cache is available.

Keep mutating commands login-only while letting ls, labs, metadata,
and file-metadata resolve laboratories and folders without a cached
token.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/cache/mod.rs

@@ -185,6 +185,43 @@ fn load_cache_from_dir(remote: &str, config_dir: &Path) -> Result<Cache, anyhow:
     Ok(cache)
 }
 
+fn load_cache_if_present_from_dir(
+    remote: &str,
+    config_dir: &Path,
+) -> Result<Option<Cache>, anyhow::Error> {
+    let cache_path = cache_file_path_in(config_dir, remote);
+    let snapshot = match read_cache_snapshot(&cache_path) {
+        Ok(snapshot) => snapshot,
+        Err(e) if e.kind() == std::io::ErrorKind::NotFound => {
+            invalidate_cached_entry(config_dir, remote);
+            return Ok(None);
+        }
+        Err(e) => return Err(e.into()),
+    };
+
+    if let Some(cache) = cached_entry(config_dir, remote, &snapshot) {
+        return Ok(Some(cache));
+    }
+
+    let data = match fs::read_to_string(&cache_path) {
+        Ok(data) => data,
+        Err(e) if e.kind() == std::io::ErrorKind::NotFound => {
+            invalidate_cached_entry(config_dir, remote);
+            return Ok(None);
+        }
+        Err(e) => return Err(e.into()),
+    };
+    let cache = match parse_cache(remote, &data) {
+        Ok(cache) => cache,
+        Err(_) => {
+            remove_cache_in_dir(remote, config_dir)?;
+            return Ok(None);
+        }
+    };
+    update_cached_entry(config_dir, remote, snapshot, cache.clone());
+    Ok(Some(cache))
+}
+
 fn persist_cache_in_dir(
     remote: &str,
     config_dir: &Path,
@@ -247,6 +284,7 @@ pub async fn load_cache_with_token_refresh(remote: &str) -> Result<Cache, anyhow
     let lock = get_remote_lock(remote);
     let _guard = lock.lock().await;
 
+    ensure_cache_dir(&cache_dir_path(&crate::settings::SETTINGS.config_dirname))?;
     let lock_path = cache_file_path(remote).with_extension("lock");
     use fs2::FileExt;
     let lock_file = fs::OpenOptions::new()
@@ -280,9 +318,71 @@ pub async fn load_cache_with_token_refresh(remote: &str) -> Result<Cache, anyhow
     result
 }
 
+async fn load_cache_with_token_refresh_optional_from_dir(
+    remote: &str,
+    config_dir: &Path,
+) -> Result<Option<Cache>, anyhow::Error> {
+    let lock = get_remote_lock(remote);
+    let _guard = lock.lock().await;
+
+    ensure_cache_dir(&cache_dir_path(config_dir))?;
+    let lock_path = cache_file_path_in(config_dir, remote).with_extension("lock");
+    use fs2::FileExt;
+    let lock_file = fs::OpenOptions::new()
+        .write(true)
+        .create(true)
+        .open(&lock_path)?;
+    lock_file.lock_exclusive()?;
+
+    let result: Result<Option<Cache>, anyhow::Error> = async {
+        let Some(mut cache) = load_cache_if_present_from_dir(remote, config_dir)? else {
+            return Ok(None);
+        };
+
+        if crate::token::is_expired(&cache.token.refresh) {
+            remove_cache_in_dir(remote, config_dir)?;
+            return Ok(None);
+        }
+
+        if crate::token::is_refresh_required(&cache.token.access, &cache.token.refresh) {
+            cache = refresh_and_persist_in_dir(remote, config_dir, &cache).await?;
+        }
+
+        Ok(Some(cache))
+    }
+    .await;
+
+    lock_file.unlock()?;
+    result
+}
+
+/// Load cache when present and refresh its token if needed.
+///
+/// Unlike `load_cache_with_token_refresh`, this returns `Ok(None)` when the user
+/// is effectively anonymous: no cache file exists, the cache is invalid, or the
+/// refresh token has already expired. This mirrors the Python client behavior
+/// used by read-only commands.
+pub async fn load_cache_with_token_refresh_optional(
+    remote: &str,
+) -> Result<Option<Cache>, anyhow::Error> {
+    load_cache_with_token_refresh_optional_from_dir(
+        remote,
+        &crate::settings::SETTINGS.config_dirname,
+    )
+    .await
+}
+
 /// Call the token-refresh endpoint and write the new access token back to the
 /// cache file.  The caller must already hold the per-remote async mutex.
 async fn refresh_and_persist(remote: &str, cache: &Cache) -> Result<Cache, anyhow::Error> {
+    refresh_and_persist_in_dir(remote, &crate::settings::SETTINGS.config_dirname, cache).await
+}
+
+async fn refresh_and_persist_in_dir(
+    remote: &str,
+    config_dir: &Path,
+    cache: &Cache,
+) -> Result<Cache, anyhow::Error> {
     let url = crate::commands::config::get_remote_url(remote)?
         .ok_or_else(|| anyhow!("Remote `{}` is not configured.", remote))?;
     let conn = MDRSConnection::new(&url);
@@ -298,7 +398,7 @@ async fn refresh_and_persist(remote: &str, cache: &Cache) -> Result<Cache, anyho
         &updated_cache.laboratories,
     );
 
-    persist_cache(remote, &updated_cache)?;
+    persist_cache_in_dir(remote, config_dir, &updated_cache)?;
 
     Ok(updated_cache)
 }
@@ -312,11 +412,26 @@ pub fn create_authenticated_conn(
     remote: &str,
     cache: &Cache,
 ) -> Result<MDRSConnection, anyhow::Error> {
+    Ok(create_remote_conn(remote)?.with_token(cache.token.access.clone()))
+}
+
+/// Create an unauthenticated `MDRSConnection` for the given remote label.
+pub fn create_remote_conn(remote: &str) -> Result<MDRSConnection, anyhow::Error> {
     let url = crate::commands::config::get_remote_url(remote)?
         .ok_or_else(|| anyhow!("Remote `{}` is not configured.", remote))?;
-    Ok(MDRSConnection::new(&url)
-        .with_remote(remote)
-        .with_token(cache.token.access.clone()))
+    Ok(MDRSConnection::new(&url).with_remote(remote))
+}
+
+/// Create a connection for read-only commands, attaching a bearer token only
+/// when a valid login cache is available.
+pub async fn create_readonly_conn(
+    remote: &str,
+) -> Result<(MDRSConnection, Option<Cache>), anyhow::Error> {
+    let conn = create_remote_conn(remote)?;
+    match load_cache_with_token_refresh_optional(remote).await? {
+        Some(cache) => Ok((conn.with_token(cache.token.access.clone()), Some(cache))),
+        None => Ok((conn, None)),
+    }
 }
 
 #[cfg(test)]
@@ -438,4 +553,54 @@ mod tests {
                 .contains(&format!("Not logged in to `{remote}`"))
         );
     }
+
+    fn make_jwt_with_exp(exp: i64) -> String {
+        use base64::{Engine, engine::general_purpose::URL_SAFE_NO_PAD};
+
+        let header = URL_SAFE_NO_PAD.encode(r#"{"alg":"none","typ":"JWT"}"#);
+        let payload = URL_SAFE_NO_PAD.encode(format!(r#"{{"exp":{exp}}}"#));
+        format!("{header}.{payload}.")
+    }
+
+    #[test]
+    fn load_cache_if_present_returns_none_when_cache_missing() {
+        let dir = tempdir().unwrap();
+        let remote = remote_name("missing", dir.path());
+
+        let loaded = load_cache_if_present_from_dir(&remote, dir.path()).unwrap();
+
+        assert!(loaded.is_none());
+    }
+
+    #[test]
+    fn load_cache_if_present_clears_invalid_cache() {
+        let dir = tempdir().unwrap();
+        let remote = remote_name("invalid", dir.path());
+        let cache_dir = cache_dir_path(dir.path());
+        ensure_cache_dir(&cache_dir).unwrap();
+        let cache_path = cache_file_path_in(dir.path(), &remote);
+        fs::write(&cache_path, b"{invalid json").unwrap();
+
+        let loaded = load_cache_if_present_from_dir(&remote, dir.path()).unwrap();
+
+        assert!(loaded.is_none());
+        assert!(!cache_path.exists());
+    }
+
+    #[tokio::test]
+    async fn optional_cache_load_treats_expired_session_as_anonymous() {
+        let dir = tempdir().unwrap();
+        let remote = remote_name("expired", dir.path());
+        let mut cache = sample_cache("alice");
+        cache.token.access = make_jwt_with_exp(0);
+        cache.token.refresh = make_jwt_with_exp(0);
+        persist_cache_in_dir(&remote, dir.path(), &cache).unwrap();
+
+        let loaded = load_cache_with_token_refresh_optional_from_dir(&remote, dir.path())
+            .await
+            .unwrap();
+
+        assert!(loaded.is_none());
+        assert!(!cache_file_path_in(dir.path(), &remote).exists());
+    }
 }