chore: Avoid getport race conditions when starting anvil (#11077)

Our test suite uses `startAnvil` all over the place to start an anvil
instance in the background. This was using `get-port`, which was subject
to race conditions when checking for port availability if we ran
multiple tests in parallel.

This PR removes `get-port` and instead uses port zero to ask the OS to
allocate a port for us, which should be free of race conditions (see
[this SO answer](https://unix.stackexchange.com/a/55918)).

Author: spalladino

yarn-project/ethereum/package.json

@@ -33,7 +33,6 @@
     "@aztec/l1-artifacts": "workspace:^",
     "@viem/anvil": "^0.0.10",
     "dotenv": "^16.0.3",
-    "get-port": "^7.1.0",
     "tslib": "^2.4.0",
     "viem": "^2.7.15",
     "zod": "^3.23.8"

yarn-project/ethereum/src/test/start_anvil.test.ts

@@ -5,6 +5,15 @@ import { startAnvil } from './start_anvil.js';
 describe('start_anvil', () => {
   it('starts anvil on a free port', async () => {
     const { anvil, rpcUrl } = await startAnvil();
+
+    const port = parseInt(new URL(rpcUrl).port);
+    expect(port).toBeLessThan(65536);
+    expect(port).toBeGreaterThan(1024);
+    expect(anvil.port).toEqual(port);
+
+    const host = new URL(rpcUrl).hostname;
+    expect(anvil.host).toEqual(host);
+
     const publicClient = createPublicClient({ transport: http(rpcUrl) });
     const chainId = await publicClient.getChainId();
     expect(chainId).toEqual(31337);

yarn-project/ethereum/src/test/start_anvil.ts

@@ -2,38 +2,47 @@ import { makeBackoff, retry } from '@aztec/foundation/retry';
 import { fileURLToPath } from '@aztec/foundation/url';
 
 import { type Anvil, createAnvil } from '@viem/anvil';
-import getPort from 'get-port';
 import { dirname, resolve } from 'path';
 
 /**
  * Ensures there's a running Anvil instance and returns the RPC URL.
  */
 export async function startAnvil(l1BlockTime?: number): Promise<{ anvil: Anvil; rpcUrl: string }> {
-  let ethereumHostPort: number | undefined;
-
   const anvilBinary = resolve(dirname(fileURLToPath(import.meta.url)), '../../', 'scripts/anvil_kill_wrapper.sh');
 
+  let port: number | undefined;
+
   // Start anvil.
   // We go via a wrapper script to ensure if the parent dies, anvil dies.
   const anvil = await retry(
     async () => {
-      ethereumHostPort = await getPort();
       const anvil = createAnvil({
         anvilBinary,
-        port: ethereumHostPort,
+        port: 0,
         blockTime: l1BlockTime,
+        stopTimeout: 1000,
+      });
+
+      // Listen to the anvil output to get the port.
+      const removeHandler = anvil.on('message', (message: string) => {
+        if (port === undefined && message.includes('Listening on')) {
+          port = parseInt(message.match(/Listening on ([^:]+):(\d+)/)![2]);
+        }
       });
       await anvil.start();
+      removeHandler();
+
       return anvil;
     },
     'Start anvil',
     makeBackoff([5, 5, 5]),
   );
 
-  if (!ethereumHostPort) {
+  if (!port) {
     throw new Error('Failed to start anvil');
   }
 
-  const rpcUrl = `http://127.0.0.1:${ethereumHostPort}`;
-  return { anvil, rpcUrl };
+  // Monkeypatch the anvil instance to include the actually assigned port
+  Object.defineProperty(anvil, 'port', { value: port, writable: false });
+  return { anvil, rpcUrl: `http://127.0.0.1:${port}` };
 }