Implement server rotation for classworks cloud provider

Co-authored-by: Sunwuyuan <88357633+Sunwuyuan@users.noreply.github.com>

src/utils/serverRotation.js

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+/**
+ * Server rotation utility for Classworks Cloud provider
+ * Provides fallback mechanism across multiple server endpoints
+ */
+
+import { getSetting } from "./settings";
+
+// Server list for classworkscloud provider (in priority order)
+const CLASSWORKS_CLOUD_SERVERS = [
+  "https://kv-service.houlang.cloud",
+  "https://kv-service.wuyuan.dev",
+];
+
+/**
+ * Get the list of servers to try for the given provider
+ * @param {string} provider - The provider type
+ * @returns {string[]} Array of server URLs to try
+ */
+export function getServerList(provider) {
+  if (provider === "classworkscloud") {
+    return [...CLASSWORKS_CLOUD_SERVERS];
+  }
+  
+  // For other providers, use the configured domain
+  const domain = getSetting("server.domain");
+  return domain ? [domain] : [];
+}
+
+/**
+ * Try an operation with server rotation fallback
+ * @param {Function} operation - Async function that takes a serverUrl and returns a promise
+ * @param {Object} options - Options
+ * @param {string} options.provider - Provider type (optional, defaults to current setting)
+ * @param {Function} options.onServerTried - Callback called when a server is tried (optional)
+ * @returns {Promise} Result from the first successful server, or throws the last error
+ */
+export async function tryWithRotation(operation, options = {}) {
+  const provider = options.provider || getSetting("server.provider");
+  const onServerTried = options.onServerTried || (() => {});
+  
+  const servers = getServerList(provider);
+  const triedServers = [];
+  let lastError = null;
+
+  for (const serverUrl of servers) {
+    try {
+      triedServers.push({ url: serverUrl, status: "trying" });
+      onServerTried({ url: serverUrl, status: "trying", tried: [...triedServers] });
+      
+      const result = await operation(serverUrl);
+      
+      triedServers[triedServers.length - 1].status = "success";
+      onServerTried({ url: serverUrl, status: "success", tried: [...triedServers] });
+      
+      return result;
+    } catch (error) {
+      lastError = error;
+      triedServers[triedServers.length - 1].status = "failed";
+      triedServers[triedServers.length - 1].error = error.message || String(error);
+      onServerTried({ url: serverUrl, status: "failed", error, tried: [...triedServers] });
+      
+      // Continue to next server
+      console.warn(`Server ${serverUrl} failed:`, error.message);
+    }
+  }
+
+  // All servers failed
+  console.error("All servers failed. Tried:", triedServers);
+  const error = lastError || new Error("All servers failed");
+  error.triedServers = triedServers;
+  throw error;
+}
+
+/**
+ * Get the effective server URL for the current provider
+ * For classworkscloud, returns the first server in the list
+ * For other providers, returns the configured domain
+ * @returns {string} Server URL
+ */
+export function getEffectiveServerUrl() {
+  const provider = getSetting("server.provider");
+  
+  if (provider === "classworkscloud") {
+    return CLASSWORKS_CLOUD_SERVERS[0];
+  }
+  
+  return getSetting("server.domain") || "";
+}
+
+/**
+ * Check if rotation is enabled for the current provider
+ * @returns {boolean}
+ */
+export function isRotationEnabled() {
+  const provider = getSetting("server.provider");
+  return provider === "classworkscloud";
+}