(core) Add logging of errors whenever ProxyAgent is used, and a test for it.

Summary:
Also:
- Move ProxyAgent to from app/server/utils to app/server/lib, which is
  the more usual place for such classes.
- Refactor a helper (delayAbort) that node was reporting a leak in.

Test Plan: Added a test case, and tested manually.

Reviewers: JakubSerafin

Reviewed By: JakubSerafin

Subscribers: JakubSerafin, paulfitz

Differential Revision: https://phab.getgrist.com/D3897

test/server/lib/ProxyAgent.ts

@@ -1,8 +1,12 @@
-import {proxyAgent} from "app/server/utils/ProxyAgent";
+import {proxyAgent} from "app/server/lib/ProxyAgent";
+import {getAvailablePort} from "app/server/lib/serverUtils";
 import {assert} from "chai";
 import {HttpsProxyAgent} from "https-proxy-agent";
 import {HttpProxyAgent} from "http-proxy-agent";
-import {EnvironmentSnapshot} from "test/server/testUtils";
+import fetch from 'node-fetch';
+import {TestProxyServer} from 'test/server/lib/helpers/TestProxyServer';
+import {serveSomething, Serving} from 'test/server/customUtil';
+import {assertMatchArray, captureLog, EnvironmentSnapshot} from "test/server/testUtils";
 
 
 describe("ProxyAgent", function () {
@@ -35,4 +39,61 @@ describe("ProxyAgent", function () {
     const httpsProxy = proxyAgent(new URL("http://localhost:3000"));
     assert.instanceOf(httpsProxy, HttpProxyAgent);
   });
+
+  describe('proxy error handling', async function() {
+    // Handling requests
+    let serving: Serving;
+    // Proxy server emulation to test possible behaviours of real life server
+    let testProxyServer: TestProxyServer;
+
+    // Simple fetch using a proxy.
+    function testFetch(relativePath: string) {
+      const url = serving.url + relativePath;
+      return fetch(url, {
+        method: 'POST',
+        headers: {'Content-Type': 'application/json'},
+        agent: proxyAgent(new URL(url)),
+      });
+    }
+
+    beforeEach(async function () {
+      // Set up a server and a proxy.
+      const port = await getAvailablePort(22340);
+      testProxyServer = await TestProxyServer.Prepare(port);
+      serving = await serveSomething(app => {
+        app.post('/200', (_, res) => { res.sendStatus(200); res.end(); });
+        app.post('/404', (_, res) => { res.sendStatus(404); res.end(); });
+      });
+      process.env.GRIST_HTTPS_PROXY = `http://localhost:${port}`;
+    });
+
+    afterEach(async function() {
+      await serving.shutdown();
+      await testProxyServer.dispose().catch(() => {});
+    });
+
+    it("should not report error when proxy is working", async function() {
+      // Normally fetch through proxy works and produces no errors, even for failing status.
+      const logMessages1 = await captureLog('warn', async () => {
+        assert.equal((await testFetch('/200')).status, 200);
+        assert.equal((await testFetch('/404')).status, 404);
+      });
+      assert.deepEqual(logMessages1, []);
+    });
+
+    it("should report error when proxy fails", async function() {
+      // if the proxy isn't listening, fetches produces error messages.
+      await testProxyServer.dispose();
+      const logMessages2 = await captureLog('warn', async () => {
+        await assert.isRejected(testFetch('/200'), /ECONNREFUSED/);
+        await assert.isRejected(testFetch('/404'), /ECONNREFUSED/);
+      });
+
+      // We rely on "ProxyAgent error" message to detect issues with the proxy server.
+      assertMatchArray(logMessages2, [
+        /warn: ProxyAgent error.*ECONNREFUSED/,
+        /warn: ProxyAgent error.*ECONNREFUSED/,
+      ]);
+    });
+  });
 });