Gateway: fallback routes per host, for the laptop's VPN address

gateway/src/t3/host.ts

@@ -26,8 +26,11 @@ export type HostStatus =
 export interface HostConfig {
   id: string;
   label: string;
-  /** The server's HTTP origin, such as `https://t3mac.gbrown.org`. */
-  url: string;
+  /**
+   * The server's HTTP origins, preferred first, such as a LAN route then a VPN route. Every one
+   * must reach the same T3 server, since they share the environment ID and the token.
+   */
+  urls: string[];
   token: string;
   environmentId: string;
 }
@@ -49,6 +52,8 @@ export interface HostOptions {
 const BACKOFF_MIN_MS = 1_000;
 const BACKOFF_MAX_MS = 5 * 60_000;
 const AUTH_RETRY_MS = 10 * 60_000;
+/** How long one route gets to answer before the next is tried. */
+const ROUTE_TIMEOUT_MS = 5_000;
 
 export class HostError extends Error {
   constructor(
@@ -73,11 +78,17 @@ export class T3Host {
   private stopped = false;
   private attempt = 0;
   private retryTimer: ReturnType<typeof setTimeout> | null = null;
+  /** The route this connection uses. Chosen afresh, preferred first, on every connect. */
+  private activeUrl: string;
 
   constructor(
     readonly config: HostConfig,
     private readonly options: HostOptions = {},
-  ) {}
+  ) {
+    const [first] = config.urls;
+    if (!first) throw new Error(`${config.label} has no URL`);
+    this.activeUrl = first;
+  }
 
   get id() {
     return this.config.id;
@@ -87,6 +98,11 @@ export class T3Host {
     return this.config.label;
   }
 
+  /** The route in use, or the one that will be tried first. */
+  get url() {
+    return this.activeUrl;
+  }
+
   start() {
     this.stopped = false;
     void this.connect();
@@ -130,7 +146,7 @@ export class T3Host {
   }
 
   private async http<T>(path: string, init: RequestInit = {}): Promise<T> {
-    const response = await fetch(new URL(path, this.config.url), {
+    const response = await fetch(new URL(path, this.activeUrl), {
       ...init,
       headers: {
         authorization: `Bearer ${this.config.token}`,
@@ -155,7 +171,7 @@ export class T3Host {
     if (this.stopped) return;
     this.setStatus("connecting", this.lastError);
     try {
-      const descriptor = await this.http<EnvironmentDescriptor>("/.well-known/t3/environment");
+      const descriptor = await this.pickRoute();
       const protocol = descriptor.orchestrationProtocolVersion ?? 1;
       if (protocol !== ORCHESTRATION_PROTOCOL_VERSION) {
         throw new HostError(`speaks orchestration protocol ${protocol}, the gateway speaks ${ORCHESTRATION_PROTOCOL_VERSION}`, 426);
@@ -164,7 +180,7 @@ export class T3Host {
         throw new HostError(`is environment ${descriptor.environmentId}, expected ${this.config.environmentId}`, 409);
       }
       const { ticket } = await this.http<{ ticket: string }>("/api/auth/websocket-ticket/rpc", { method: "POST" });
-      const wsUrl = new URL("/ws", this.config.url);
+      const wsUrl = new URL("/ws", this.activeUrl);
       wsUrl.protocol = wsUrl.protocol === "https:" ? "wss:" : "ws:";
       wsUrl.searchParams.set("wsTicket", ticket);
       wsUrl.searchParams.set("orchestrationProtocol", String(ORCHESTRATION_PROTOCOL_VERSION));
@@ -187,6 +203,26 @@ export class T3Host {
     }
   }
 
+  /** Uses the first route whose well-known descriptor answers, and returns that descriptor. */
+  private async pickRoute(): Promise<EnvironmentDescriptor> {
+    const failures: string[] = [];
+    for (const url of this.config.urls) {
+      try {
+        const response = await fetch(new URL("/.well-known/t3/environment", url), {
+          signal: AbortSignal.timeout(ROUTE_TIMEOUT_MS),
+        });
+        if (!response.ok) throw new Error(`returned ${response.status}`);
+        const descriptor = (await response.json()) as EnvironmentDescriptor;
+        if (url !== this.activeUrl) this.options.log?.(`${this.label}: using ${url}`);
+        this.activeUrl = url;
+        return descriptor;
+      } catch (error) {
+        failures.push(`${new URL(url).host} ${error instanceof Error ? error.message : String(error)}`);
+      }
+    }
+    throw new HostError(`no route answered (${failures.join(", ")})`, 503);
+  }
+
   private subscribe(socket: RpcSocket) {
     const input = this.hasSnapshot && this.sequence !== null ? { afterSequence: this.sequence } : {};
     const { done } = socket.stream("orchestration.subscribeShell", input, (value) =>