(core) tweak handler for aborted connections to work on modern node

Summary:
It became hard to detect aborted connections in node 16.
In node 14, req.on('close', ...) did the job. Thid diff adds a
work-around, until a better way is discovered or added.
Aborting a req will typically lead to 'close' being called
on the response, without writableFinished being set.

 - https://github.com/nodejs/node/issues/38924
 - https://github.com/nodejs/node/issues/40775

Test Plan:
existing DocApiForwarder test passes; manually
checking on various node versions.

Reviewers: JakubSerafin

Reviewed By: JakubSerafin

Differential Revision: https://phab.getgrist.com/D3923

app/gen-server/lib/DocApiForwarder.ts

@@ -10,6 +10,7 @@ import { IDocWorkerMap } from "app/server/lib/DocWorkerMap";
 import { expressWrap } from "app/server/lib/expressWrap";
 import { GristServer } from "app/server/lib/GristServer";
 import { getAssignmentId } from "app/server/lib/idUtils";
+import { addAbortHandler } from "app/server/lib/requestUtils";
 
 /**
  * Forwards all /api/docs/:docId/tables requests to the doc worker handling the :docId document. Makes
@@ -101,7 +102,7 @@ export class DocApiForwarder {
 
     // If the original request is aborted, abort the forwarded request too. (Currently this only
     // affects some export/download requests which can abort long-running work.)
-    req.on('close', () => controller.abort());
+    addAbortHandler(req, res, () => controller.abort());
 
     const options: RequestInit = {
       method: req.method,

app/server/lib/ExportXLSX.ts

@@ -14,6 +14,7 @@
 import {ActiveDoc} from 'app/server/lib/ActiveDoc';
 import {ActiveDocSource, ActiveDocSourceDirect, DownloadOptions, ExportParameters} from 'app/server/lib/Export';
 import log from 'app/server/lib/log';
+import {addAbortHandler} from 'app/server/lib/requestUtils';
 import * as express from 'express';
 import contentDisposition from 'content-disposition';
 import {Rpc} from 'grain-rpc';
@@ -78,7 +79,7 @@ export async function streamXLSX(activeDoc: ActiveDoc, req: express.Request,
       req.off('close', cancelWorker);
     });
 
-    req.on('close', cancelWorker);
+    addAbortHandler(req, outputStream, cancelWorker);
 
     const run = (method: string, ...args: any[]) => exportPool.run({port: port2, testDates, args}, {
       name: method,

app/server/lib/requestUtils.ts

@@ -9,6 +9,7 @@ import log from 'app/server/lib/log';
 import {Permit} from 'app/server/lib/Permit';
 import {Request, Response} from 'express';
 import _ from 'lodash';
+import {Writable} from 'stream';
 
 // log api details outside of dev environment (when GRIST_HOSTED_VERSION is set)
 const shouldLogApiDetails = Boolean(process.env.GRIST_HOSTED_VERSION);
@@ -335,3 +336,19 @@ export function clearSessionCacheIfNeeded(req: Request, options?: {
 }) {
   (req as RequestWithGrist).gristServer?.getSessions().clearCacheIfNeeded(options);
 }
+
+export function addAbortHandler(req: Request, res: Writable, op: () => void) {
+  // It became hard to detect aborted connections in node 16.
+  // In node 14, req.on('close', ...) did the job.
+  // The following is a work-around, until a better way is discovered
+  // or added. Aborting a req will typically lead to 'close' being called
+  // on the response, without writableFinished being set.
+  //   https://github.com/nodejs/node/issues/38924
+  //   https://github.com/nodejs/node/issues/40775
+  res.on('close', () => {
+    const aborted = !res.writableFinished;
+    if (aborted) {
+      op();
+    }
+  });
+}