metropolis/pkg/freeport/freeport.go

// Copyright 2020 The Monogon Project Authors.
//
// SPDX-License-Identifier: Apache-2.0
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package freeport

import (
	"io"
	"net"
)

// AllocateTCPPort allocates a TCP port on the looopback address, and starts a temporary listener on it. That listener
// is returned to the caller alongside with the allocated port number. The listener must be closed right before
// the port is used by the caller. This naturally still leaves a race condition window where that port number
// might be snatched up by some other process, but there doesn't seem to be a better way to do this.
func AllocateTCPPort() (uint16, io.Closer, error) {
	addr, err := net.ResolveTCPAddr("tcp", "127.0.0.1:0")
	if err != nil {
		return 0, nil, err
	}

	l, err := net.ListenTCP("tcp", addr)
	if err != nil {
		return 0, nil, err
	}
	return uint16(l.Addr().(*net.TCPAddr).Port), l, nil
}

// MustConsume takes the result of AllocateTCPPort, closes the listener and returns the allocated port.
// If anything goes wrong (port could not be allocated or closed) it will panic.
func MustConsume(port uint16, lis io.Closer, err error) int {
	if err != nil {
		panic(err)
	}
	if err := lis.Close(); err != nil {
		panic(err)
	}
	return int(port)
}