fix(core): use next available port when the port for nx graph is in use (#31365)

<!-- Please make sure you have read the submission guidelines before
posting an PR -->
<!--
https://github.com/nrwl/nx/blob/master/CONTRIBUTING.md#-submitting-a-pr
-->

<!-- Please make sure that your commit message follows our format -->
<!-- Example: `fix(nx): must begin with lowercase` -->

<!-- If this is a particularly complex change or feature addition, you
can request a dedicated Nx release for this pull request branch. Mention
someone from the Nx team or the `@nrwl/nx-pipelines-reviewers` and they
will confirm if the PR warrants its own release for testing purposes,
and generate it for you if appropriate. -->

## Current Behavior
<!-- This is the behavior we have today -->
Command would fail silently with no error message
## Expected Behavior
<!-- This is the behavior we should expect with the changes in this PR
-->
Rather than erroring, Nx will find the next available port and use that.

## Related Issue(s)
<!-- Please link the issue being fixed so it gets closed when this is
merged. -->

Fixes #30915

e2e/nx/src/misc.test.ts

@@ -83,7 +83,7 @@ describe('Nx Commands', () => {
       runCLI(`generate @nx/web:app apps/${app}`);
       let url: string;
       let port: number;
-      const child_process = await runCommandUntil(
+      const childProcess = await runCommandUntil(
         `show project ${app} --web --open=false`,
         (output) => {
           console.log(output);
@@ -102,7 +102,68 @@ describe('Nx Commands', () => {
       // Check that url is alive
       const response = await fetch(url);
       expect(response.status).toEqual(200);
-      await killProcessAndPorts(child_process.pid, port);
+      await killProcessAndPorts(childProcess.pid, port);
+    }, 700000);
+
+    it('should find alternative port when default port is occupied', async () => {
+      const app = uniq('myapp');
+      runCLI(`generate @nx/web:app apps/${app}`);
+
+      const http = require('http');
+
+      // Create a server that occupies the default port 4211
+      const blockingServer = http.createServer((req, res) => {
+        res.writeHead(200, { 'Content-Type': 'text/plain' });
+        res.end('blocking server');
+      });
+
+      await new Promise<void>((resolve) => {
+        blockingServer.listen(4211, '127.0.0.1', () => {
+          console.log('Blocking server started on port 4211');
+          resolve();
+        });
+      });
+
+      let url: string;
+      let port: number;
+      let foundAlternativePort = false;
+
+      try {
+        const childProcess = await runCommandUntil(
+          `show project ${app} --web --open=false`,
+          (output) => {
+            console.log(output);
+            // Should find alternative port and show message about port being in use
+            if (output.includes('Port 4211 was already in use, using port')) {
+              foundAlternativePort = true;
+            }
+            // output should contain 'Project graph started at http://127.0.0.1:{port}'
+            if (output.includes('Project graph started at http://')) {
+              const match = /https?:\/\/[\d.]+:(?<port>\d+)/.exec(output);
+              if (match) {
+                port = parseInt(match.groups.port);
+                url = match[0];
+                return true;
+              }
+            }
+            return false;
+          }
+        );
+
+        // Verify that an alternative port was found
+        expect(foundAlternativePort).toBe(true);
+        expect(port).not.toBe(4211);
+        expect(port).toBeGreaterThan(4211);
+
+        // Check that url is alive
+        const response = await fetch(url);
+        expect(response.status).toEqual(200);
+
+        await killProcessAndPorts(childProcess.pid, port);
+      } finally {
+        // Clean up the blocking server
+        blockingServer.close();
+      }
     }, 700000);
   });
 

packages/nx/src/command-line/graph/graph.ts

@@ -21,6 +21,7 @@ import {
   parse,
   relative,
 } from 'path';
+import * as net from 'net';
 import { performance } from 'perf_hooks';
 import { readNxJson, workspaceLayout } from '../../config/configuration';
 import {
@@ -490,17 +491,29 @@ export async function generateGraph(
       !!args.file && args.file.endsWith('html') ? 'build' : 'serve'
     );
 
-    const { app, url } = await startServer(
-      html,
-      environmentJs,
-      args.host || '127.0.0.1',
-      args.port || 4211,
-      args.watch,
-      affectedProjects,
-      args.focus,
-      args.groupByFolder,
-      args.exclude
-    );
+    let app: Server;
+    let url: URL;
+    try {
+      const result = await startServer(
+        html,
+        environmentJs,
+        args.host || '127.0.0.1',
+        args.port || 4211,
+        args.watch,
+        affectedProjects,
+        args.focus,
+        args.groupByFolder,
+        args.exclude
+      );
+      app = result.app;
+      url = result.url;
+    } catch (err) {
+      output.error({
+        title: 'Failed to start graph server',
+        bodyLines: [err.message],
+      });
+      process.exit(1);
+    }
 
     url.pathname = args.view;
 
@@ -539,6 +552,33 @@ export async function generateGraph(
   }
 }
 
+function findAvailablePort(
+  startPort: number,
+  host: string = '127.0.0.1'
+): Promise<number> {
+  return new Promise((resolve, reject) => {
+    const server = net.createServer();
+
+    server.listen(startPort, host, () => {
+      const port = (server.address() as net.AddressInfo).port;
+      server.close(() => {
+        resolve(port);
+      });
+    });
+
+    server.on('error', (err: NodeJS.ErrnoException) => {
+      if (err.code === 'EADDRINUSE') {
+        // Port is in use, try the next one
+        findAvailablePort(startPort + 1, host)
+          .then(resolve)
+          .catch(reject);
+      } else {
+        reject(err);
+      }
+    });
+  });
+}
+
 async function startServer(
   html: string,
   environmentJs: string,
@@ -676,9 +716,21 @@ async function startServer(
   process.on('SIGINT', () => handleTermination(128 + 2));
   process.on('SIGTERM', () => handleTermination(128 + 15));
 
-  return new Promise<{ app: Server; url: URL }>((res) => {
-    app.listen(port, host, () => {
-      res({ app, url: new URL(`http://${host}:${port}`) });
+  // Find an available port starting from the requested port
+  const availablePort = await findAvailablePort(port, host);
+
+  return new Promise<{ app: Server; url: URL }>((res, rej) => {
+    app.on('error', (err: NodeJS.ErrnoException) => {
+      rej(err);
+    });
+
+    app.listen(availablePort, host, () => {
+      if (availablePort !== port) {
+        output.note({
+          title: `Port ${port} was already in use, using port ${availablePort} instead`,
+        });
+      }
+      res({ app, url: new URL(`http://${host}:${availablePort}`) });
     });
   });
 }