(core) change handling of server access prior to full configuration

Summary:
Recently, the server became more strict about not responding to
requests before being fully configured. This is a problem when
a doc worker is trying to check whether it has become available
from a load balancer.

This change gives access to health checks prior to configuration
being complete. Otherwise, app endpoints accessed before full
configuration return a 503.

A flag is added to /status to allow checking explicitly for
the server being ready and configured.

Test Plan: manual

Reviewers: dsagal

Reviewed By: dsagal

Subscribers: dsagal

Differential Revision: https://phab.getgrist.com/D4103

app/server/lib/FlexServer.ts

@@ -173,8 +173,6 @@ export class FlexServer implements GristServer {
   private _getLogoutRedirectUrl: (req: express.Request, nextUrl: URL) => Promise<string>;
   private _sendAppPage: (req: express.Request, resp: express.Response, options: ISendAppPageOptions) => Promise<void>;
   private _getLoginSystem?: () => Promise<GristLoginSystem>;
-  // Called by ready() to allow requests to be served.
-  private _ready: () => void;
   // Set once ready() is called
   private _isReady: boolean = false;
 
@@ -183,20 +181,6 @@ export class FlexServer implements GristServer {
     this.app = express();
     this.app.set('port', port);
 
-    // Before doing anything, we pause any request handling to wait
-    // for the server being entirely ready. The specific reason to do
-    // so is because, if we are serving plugins, and using an
-    // OS-assigned port to do so, we won't know the URL to use for
-    // plugins until quite late. But it seems a nice thing to
-    // guarantee in general.
-    const readyPromise = new Promise(resolve => {
-      this._ready = () => resolve(undefined);
-    });
-    this.app.use(async (_req, _res, next) => {
-      await readyPromise;
-      next();
-    });
-
     this.appRoot = getAppRoot();
     this.host = process.env.GRIST_HOST || "localhost";
     log.info(`== Grist version is ${version.version} (commit ${version.gitcommit})`);
@@ -452,6 +436,7 @@ export class FlexServer implements GristServer {
     // /status/hooks allows the tests to wait for them to be ready.
     // If db=1 query parameter is included, status will include the status of DB connection.
     // If redis=1 query parameter is included, status will include the status of the Redis connection.
+    // If ready=1 query parameter is included, status will include whether the server is fully ready.
     this.app.get('/status(/hooks)?', async (req, res) => {
       const checks = new Map<string, Promise<boolean>|boolean>();
       const timeout = optIntegerParam(req.query.timeout, 'timeout') || 10_000;
@@ -473,6 +458,9 @@ export class FlexServer implements GristServer {
       if (isParameterOn(req.query.redis)) {
         checks.set('redis', asyncCheck(this._docWorkerMap.getRedisClient()?.pingAsync()));
       }
+      if (isParameterOn(req.query.ready)) {
+        checks.set('ready', this._isReady);
+      }
       let extra = '';
       let ok = true;
       // If we had any extra check, collect their status to report them.
@@ -493,6 +481,18 @@ export class FlexServer implements GristServer {
     });
   }
 
+  public denyRequestsIfNotReady() {
+    this.app.use((_req, res, next) => {
+      if (!this._isReady) {
+        // If ready() hasn't been called yet, don't continue, and
+        // give a clear error. This is to avoid exposing the service
+        // in a partially configured form.
+        return res.status(503).json({error: 'Service unavailable during start up'});
+      }
+      next();
+    });
+  }
+
   public testAddRouter() {
     if (this._check('router')) { return; }
     this.app.get('/test/router', (req, res) => {
@@ -1568,9 +1568,7 @@ export class FlexServer implements GristServer {
   }
 
   public ready() {
-    if (this._isReady) { return; }
     this._isReady = true;
-    this._ready();
   }
 
   public checkOptionCombinations() {

app/server/mergedServerMain.ts

@@ -104,6 +104,7 @@ export async function main(port: number, serverTypes: ServerType[],
   }
 
   server.addHealthCheck();
+  server.denyRequestsIfNotReady();
 
   if (includeHome || includeStatic || includeApp) {
     server.setDirectory();