Replace dynamic free-port probing with OS port-0 binding #194313

Problem

Across flutter_tools, package:dwds, dev/bots, package:web_benchmarks, and flutter/devtools, several code paths dynamically locate a "free" TCP port by probing a port (ServerSocket.bind(..., 0) or RawSocket.connect), closing the socket, and passing that port number to an HTTP server or child process (Chrome, chromedriver) to bind later. Other test harnesses hardcode fixed ports (9999, 10000, 10001), pushing downstream callers to add their own port probes.

Probing for a free port before binding has two flaws that cause recurring CI flakes:

1. TOCTOU race condition: Once the probe socket closes (or RawSocket.connect fails), the port is unreserved. Under parallel CI load, another test shard or process can claim the port before the target server or browser binds it. Worse, RawSocket.connect probes only detect sockets actively listening on localhost, missing sockets that are bound but not yet listening.
2. Address-family and dual-stack mismatch: The probe socket rarely matches the target bind's address family or socket flags. On macOS and Linux, an IPv4 socket bound to 127.0.0.1:P or 0.0.0.0:P does not conflict with an IPv6-only (v6Only: true) probe on [::1]:P.
   • For example, dwds's findUnusedPort() probes [::1] with v6Only: true, then HttpMultiServer.bind('any', port) binds [::] dual-stack (v6Only: false). If the kernel hands out a port held by an IPv4 socket, the probe succeeds on [::1] and the subsequent [::] bind fails with EADDRINUSE (errno = 48 on macOS, 98 on Linux). Under congested ephemeral port ranges, all 5 retry attempts fail in ~500ms.

The Flutter Style Guide already prohibits this pattern ("Never check if a port is available before using it... Instead, have the code that opens the port pick an available port and return it"), and dev/bots/utils.dart even names its helper findAvailablePortAndPossiblyCauseFlakyTests().

Past Flakes & Failures

Plan

Instead of probing for a free port and closing it, pass 0 directly to the bind() call (or --remote-debugging-port=0 / --port=0 flag) so the OS allocates an ephemeral port atomically for the target address family, then read back the assigned port from server.port or child stdout/stderr. Keep a bounded retry loop around multi-socket binds (HttpMultiServer.loopback).

• package:dwds: Pass port ?? 0 directly to HttpMultiServer.bind(hostname, requestedPort) inside startHttpServer instead of calling findUnusedPort().
  • Fix in pkg/dwds: https://dart-review.googlesource.com/c/sdk/+/563621
  • Publish dwds 27.2.0: https://dart-review.googlesource.com/c/sdk/+/564020
  • Roll dwds 27.2.0 into flutter_tools: Roll pub packages #194398
• package:http_multi_server: Retry HttpMultiServer._loopback when port == 0 even on in-process ListeningSocketRegistry collisions (errorCode == -1), and keep v4Server open during the retry.
  • [VM] Intermittent failure to bind to an ephemeral port dart-lang/sdk#47806
  • Fix in pkgs/http_multi_server: [http_multi_server] Retry loopback(0) when IPv6 port is already bound dart-lang/http#2025
  • Publish http_multi_server 3.2.3: [http_multi_server] Prepare to release 3.2.3 dart-lang/http#2028
  • Roll http_multi_server 3.2.3 into flutter_tools: Roll pub packages #194984
• flutter_tools Chrome launcher: Pass --remote-debugging-port=0 when debugPort == null in _spawnChromiumProcess instead of calling findFreePort(), and parse the bound host and port from Chrome's DevTools listening on ws://... stderr line.
  • Replace dynamic free-port probing with OS port-0 binding #194419
• dev/bots web runner and service worker test: Bind HttpServer.bind('localhost', 0) before constructing appUrl in dev/bots/browser.dart, support debugPort = 0 in dev/devicelab/lib/framework/browser.dart, and delete findAvailablePortAndPossiblyCauseFlakyTests().
  • findAvailablePort in test.dart / service_worker_test.dart is a race condition and likely causes flaky tests #109474
  • Replace dynamic free-port probing with OS port-0 binding #194419
• package:web_benchmarks, devicelab, and flutter/devtools: Bind HttpServer.bind('localhost', 0) first, read server.port, and launch Chrome with --remote-debugging-port=0 in package:web_benchmarks and web_benchmarks.dart instead of hardcoding 9999/10000/10001, then remove the _findAvailablePort workaround in devtools_benchmarks_test.dart.
  • [web_benchmarks] Let the OS pick ports for the benchmark server and Chrome packages#13293
• flutter_tools resolvePort cleanup: Delete the unreachable if (port == null) return os.findFreePort(); branch in devfs_config.dart (WebDevServerConfig.port is a non-null int defaulting to 0).
  • Replace dynamic free-port probing with OS port-0 binding #194419
• chromedriver test harness: Launch chromedriver --port=0 and parse started successfully on port (\d+) from stdout in web_server_test_common.dart before passing --driver-port to flutter drive, then delete the local findFreePort() helper.
  • Replace dynamic free-port probing with OS port-0 binding #194419
• flutter/packages DriveExamplesCommand: Launch chromedriver --port=0, parse started successfully on port (\d+) from stdout before passing --driver-port to flutter drive, and await chromedriver.exitCode after chromedriver.kill() in drive_examples_command.dart instead of hardcoding port 4444.
  • [tool] Let the OS pick a port for chromedriver and wait for exit packages#13355

Out of Scope

• External binaries that do not report an OS-allocated port: iproxy (ios/devices.dart) and user-defined custom-device forwarding commands (custom_devices.dart) require a concrete host port on the CLI and do not emit a dynamically bound port. They must retain findFreePort() with retry re-probing.
• Explicit user-supplied port flags (--web-port, --dds-port, --host-vmservice-port): When omitted, flutter_tools already defaults all three to port 0. When a user or test explicitly passes a port number (e.g., observatory_port_test.dart testing that --dds-port=N is honored), the tool must bind N.