sdk: add freelist tracking and ephemeral port range skipping to freeport

This should cut down on test flakiness.

Problems handled:

- If you had enough parallel test cases running, the former circular
approach to handling the port block could hand out the same port to
multiple cases before they each had a chance to bind them, leading to
one of the two tests to fail.

- The freeport library would allocate out of the ephemeral port range.
This has been corrected for Linux (which should cover CI).

- The library now waits until a formerly-in-use port is verified to be
free before putting it back into circulation.

agent/agent_test.go

@@ -1229,8 +1229,11 @@ func TestAgent_RestoreServiceWithAliasCheck(t *testing.T) {
 	testCtx, testCancel := context.WithCancel(context.Background())
 	defer testCancel()
 
-	testHTTPServer := launchHTTPCheckServer(t, testCtx)
+	testHTTPServer, returnPort := launchHTTPCheckServer(t, testCtx)
-	defer testHTTPServer.Close()
+	defer func() {
+		testHTTPServer.Close()
+		returnPort()
+	}()
 
 	registerServicesAndChecks := func(t *testing.T, a *TestAgent) {
 		// add one persistent service with a simple check
@@ -1338,8 +1341,8 @@ node_name = "` + a.Config.NodeName + `"
 	}
 }
 
-func launchHTTPCheckServer(t *testing.T, ctx context.Context) *httptest.Server {
+func launchHTTPCheckServer(t *testing.T, ctx context.Context) (srv *httptest.Server, returnPortsFn func()) {
-	ports := freeport.GetT(t, 1)
+	ports := freeport.MustTake(1)
 	port := ports[0]
 
 	addr := net.JoinHostPort("127.0.0.1", strconv.Itoa(port))
@@ -1353,12 +1356,12 @@ func launchHTTPCheckServer(t *testing.T, ctx context.Context) *httptest.Server {
 		_, _ = w.Write([]byte("OK\n"))
 	})
 
-	srv := &httptest.Server{
+	srv = &httptest.Server{
 		Listener: listener,
 		Config:   &http.Server{Handler: handler},
 	}
 	srv.Start()
-	return srv
+	return srv, func() { freeport.Return(ports) }
 }
 
 func TestAgent_AddCheck_Alias(t *testing.T) {

agent/consul/client_test.go

@@ -13,7 +13,7 @@ import (
 	"github.com/hashicorp/consul/sdk/testutil"
 	"github.com/hashicorp/consul/sdk/testutil/retry"
 	"github.com/hashicorp/consul/testrpc"
-	"github.com/hashicorp/net-rpc-msgpackrpc"
+	msgpackrpc "github.com/hashicorp/net-rpc-msgpackrpc"
 	"github.com/hashicorp/serf/serf"
 	"github.com/stretchr/testify/require"
 	"golang.org/x/time/rate"
@@ -22,15 +22,27 @@ import (
 func testClientConfig(t *testing.T) (string, *Config) {
 	dir := testutil.TempDir(t, "consul")
 	config := DefaultConfig()
+
+	ports := freeport.MustTake(2)
+
+	returnPortsFn := func() {
+		// The method of plumbing this into the client shutdown hook doesn't
+		// cover all exit points, so we insulate this against multiple
+		// invocations and then it's safe to call it a bunch of times.
+		freeport.Return(ports)
+		config.NotifyShutdown = nil // self-erasing
+	}
+	config.NotifyShutdown = returnPortsFn
+
 	config.Datacenter = "dc1"
 	config.DataDir = dir
 	config.NodeName = uniqueNodeName(t.Name())
 	config.RPCAddr = &net.TCPAddr{
 		IP:   []byte{127, 0, 0, 1},
-		Port: freeport.Get(1)[0],
+		Port: ports[0],
 	}
 	config.SerfLANConfig.MemberlistConfig.BindAddr = "127.0.0.1"
-	config.SerfLANConfig.MemberlistConfig.BindPort = freeport.Get(1)[0]
+	config.SerfLANConfig.MemberlistConfig.BindPort = ports[1]
 	config.SerfLANConfig.MemberlistConfig.ProbeTimeout = 200 * time.Millisecond
 	config.SerfLANConfig.MemberlistConfig.ProbeInterval = time.Second
 	config.SerfLANConfig.MemberlistConfig.GossipInterval = 100 * time.Millisecond
@@ -59,6 +71,7 @@ func testClientWithConfig(t *testing.T, cb func(c *Config)) (string, *Client) {
 	}
 	client, err := NewClient(config)
 	if err != nil {
+		config.NotifyShutdown()
 		t.Fatalf("err: %v", err)
 	}
 	return dir, client
@@ -416,6 +429,7 @@ func TestClient_RPC_TLS(t *testing.T) {
 	defer s1.Shutdown()
 
 	dir2, conf2 := testClientConfig(t)
+	defer conf2.NotifyShutdown()
 	conf2.VerifyOutgoing = true
 	configureTLS(conf2)
 	c1, err := NewClient(conf2)
@@ -460,6 +474,7 @@ func TestClient_RPC_RateLimit(t *testing.T) {
 	testrpc.WaitForLeader(t, s1.RPC, "dc1")
 
 	dir2, conf2 := testClientConfig(t)
+	defer conf2.NotifyShutdown()
 	conf2.RPCRate = 2
 	conf2.RPCMaxBurst = 2
 	c1, err := NewClient(conf2)
@@ -527,6 +542,7 @@ func TestClient_SnapshotRPC_RateLimit(t *testing.T) {
 	testrpc.WaitForLeader(t, s1.RPC, "dc1")
 
 	dir2, conf1 := testClientConfig(t)
+	defer conf1.NotifyShutdown()
 	conf1.RPCRate = 2
 	conf1.RPCMaxBurst = 2
 	c1, err := NewClient(conf1)
@@ -569,6 +585,7 @@ func TestClient_SnapshotRPC_TLS(t *testing.T) {
 	defer s1.Shutdown()
 
 	dir2, conf2 := testClientConfig(t)
+	defer conf2.NotifyShutdown()
 	conf2.VerifyOutgoing = true
 	configureTLS(conf2)
 	c1, err := NewClient(conf2)

agent/consul/config.go

@@ -110,6 +110,9 @@ type Config struct {
 	// configured at this point.
 	NotifyListen func()
 
+	// NotifyShutdown is called after Server is completely Shutdown.
+	NotifyShutdown func()
+
 	// RPCAddr is the RPC address used by Consul. This should be reachable
 	// by the WAN and LAN
 	RPCAddr *net.TCPAddr

agent/consul/operator_raft_endpoint_test.go

@@ -11,7 +11,7 @@ import (
 	"github.com/hashicorp/consul/agent/structs"
 	"github.com/hashicorp/consul/sdk/freeport"
 	"github.com/hashicorp/consul/testrpc"
-	"github.com/hashicorp/net-rpc-msgpackrpc"
+	msgpackrpc "github.com/hashicorp/net-rpc-msgpackrpc"
 	"github.com/hashicorp/raft"
 	"github.com/pascaldekloe/goe/verify"
 )
@@ -145,10 +145,13 @@ func TestOperator_RaftRemovePeerByAddress(t *testing.T) {
 
 	testrpc.WaitForLeader(t, s1.RPC, "dc1")
 
+	ports := freeport.MustTake(1)
+	defer freeport.Return(ports)
+
 	// Try to remove a peer that's not there.
 	arg := structs.RaftRemovePeerRequest{
 		Datacenter: "dc1",
-		Address:    raft.ServerAddress(fmt.Sprintf("127.0.0.1:%d", freeport.Get(1)[0])),
+		Address:    raft.ServerAddress(fmt.Sprintf("127.0.0.1:%d", ports[0])),
 	}
 	var reply struct{}
 	err := msgpackrpc.CallWithCodec(codec, "Operator.RaftRemovePeerByAddress", &arg, &reply)
@@ -277,7 +280,10 @@ func TestOperator_RaftRemovePeerByID(t *testing.T) {
 
 	// Add it manually to Raft.
 	{
-		future := s1.raft.AddVoter(arg.ID, raft.ServerAddress(fmt.Sprintf("127.0.0.1:%d", freeport.Get(1)[0])), 0, 0)
+		ports := freeport.MustTake(1)
+		defer freeport.Return(ports)
+
+		future := s1.raft.AddVoter(arg.ID, raft.ServerAddress(fmt.Sprintf("127.0.0.1:%d", ports[0])), 0, 0)
 		if err := future.Error(); err != nil {
 			t.Fatalf("err: %v", err)
 		}

agent/consul/server.go

@@ -843,6 +843,10 @@ func (s *Server) Shutdown() error {
 	// Close the connection pool
 	s.connPool.Shutdown()
 
+	if s.config.NotifyShutdown != nil {
+		s.config.NotifyShutdown()
+	}
+
 	return nil
 }
 

agent/consul/server_test.go

@@ -42,7 +42,17 @@ func testServerConfig(t *testing.T) (string, *Config) {
 	dir := testutil.TempDir(t, "consul")
 	config := DefaultConfig()
 
-	ports := freeport.Get(3)
+	ports := freeport.MustTake(3)
+
+	returnPortsFn := func() {
+		// The method of plumbing this into the server shutdown hook doesn't
+		// cover all exit points, so we insulate this against multiple
+		// invocations and then it's safe to call it a bunch of times.
+		freeport.Return(ports)
+		config.NotifyShutdown = nil // self-erasing
+	}
+	config.NotifyShutdown = returnPortsFn
+
 	config.NodeName = uniqueNodeName(t.Name())
 	config.Bootstrap = true
 	config.Datacenter = "dc1"
@@ -56,6 +66,7 @@ func testServerConfig(t *testing.T) (string, *Config) {
 
 	nodeID, err := uuid.GenerateUUID()
 	if err != nil {
+		returnPortsFn()
 		t.Fatal(err)
 	}
 	config.NodeID = types.NodeID(nodeID)
@@ -112,6 +123,8 @@ func testServerConfig(t *testing.T) (string, *Config) {
 		},
 	}
 
+	config.NotifyShutdown = returnPortsFn
+
 	return dir, config
 }
 
@@ -168,6 +181,7 @@ func testServerWithConfig(t *testing.T, cb func(*Config)) (string, *Server) {
 
 		srv, err = newServer(config)
 		if err != nil {
+			config.NotifyShutdown()
 			os.RemoveAll(dir)
 			r.Fatalf("err: %v", err)
 		}

agent/http_test.go

@@ -738,6 +738,7 @@ func TestParseWait(t *testing.T) {
 		t.Fatalf("Bad: %v", b)
 	}
 }
+
 func TestPProfHandlers_EnableDebug(t *testing.T) {
 	t.Parallel()
 	require := require.New(t)
@@ -751,6 +752,7 @@ func TestPProfHandlers_EnableDebug(t *testing.T) {
 
 	require.Equal(http.StatusOK, resp.Code)
 }
+
 func TestPProfHandlers_DisableDebugNoACLs(t *testing.T) {
 	t.Parallel()
 	require := require.New(t)

agent/testagent.go

@@ -52,6 +52,10 @@ type TestAgent struct {
 	// when Shutdown() is called.
 	Config *config.RuntimeConfig
 
+	// returnPortsFn will put the ports claimed for the test back into the
+	// general freeport pool
+	returnPortsFn func()
+
 	// LogOutput is the sink for the logs. If nil, logs are written
 	// to os.Stderr.
 	LogOutput io.Writer
@@ -150,12 +154,21 @@ func (a *TestAgent) Start() (err error) {
 		hclDataDir = `data_dir = "` + d + `"`
 	}
 
+	portsConfig, returnPortsFn := randomPortsSource(a.UseTLS)
+	a.returnPortsFn = returnPortsFn
 	a.Config = TestConfig(
-		randomPortsSource(a.UseTLS),
+		portsConfig,
 		config.Source{Name: a.Name, Format: "hcl", Data: a.HCL},
 		config.Source{Name: a.Name + ".data_dir", Format: "hcl", Data: hclDataDir},
 	)
 
+	defer func() {
+		if err != nil && a.returnPortsFn != nil {
+			a.returnPortsFn()
+			a.returnPortsFn = nil
+		}
+	}()
+
 	// write the keyring
 	if a.Key != "" {
 		writeKey := func(key, filename string) error {
@@ -286,6 +299,14 @@ func (a *TestAgent) Shutdown() error {
 		return nil
 	}
 
+	// Return ports last of all
+	defer func() {
+		if a.returnPortsFn != nil {
+			a.returnPortsFn()
+			a.returnPortsFn = nil
+		}
+	}()
+
 	// shutdown agent before endpoints
 	defer a.Agent.ShutdownEndpoints()
 	if err := a.Agent.ShutdownAgent(); err != nil {
@@ -350,27 +371,32 @@ func (a *TestAgent) consulConfig() *consul.Config {
 // chance of port conflicts for concurrently executed test binaries.
 // Instead of relying on one set of ports to be sufficient we retry
 // starting the agent with different ports on port conflict.
-func randomPortsSource(tls bool) config.Source {
+func randomPortsSource(tls bool) (src config.Source, returnPortsFn func()) {
-	ports := freeport.Get(6)
+	ports := freeport.MustTake(6)
+
+	var http, https int
 	if tls {
-		ports[1] = -1
+		http = -1
+		https = ports[2]
 	} else {
-		ports[2] = -1
+		http = ports[1]
+		https = -1
 	}
+
 	return config.Source{
 		Name:   "ports",
 		Format: "hcl",
 		Data: `
 			ports = {
 				dns = ` + strconv.Itoa(ports[0]) + `
-				http = ` + strconv.Itoa(ports[1]) + `
+				http = ` + strconv.Itoa(http) + `
-				https = ` + strconv.Itoa(ports[2]) + `
+				https = ` + strconv.Itoa(https) + `
 				serf_lan = ` + strconv.Itoa(ports[3]) + `
 				serf_wan = ` + strconv.Itoa(ports[4]) + `
 				server = ` + strconv.Itoa(ports[5]) + `
 			}
 		`,
-	}
+	}, func() { freeport.Return(ports) }
 }
 
 func NodeID() string {

connect/proxy/listener_test.go

@@ -4,13 +4,14 @@ import (
 	"bytes"
 	"context"
 	"fmt"
-	"github.com/hashicorp/consul/connect"
 	"log"
 	"net"
 	"os"
 	"testing"
 	"time"
 
+	"github.com/hashicorp/consul/connect"
+
 	metrics "github.com/armon/go-metrics"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
@@ -110,7 +111,8 @@ func TestPublicListener(t *testing.T) {
 	// Can't enable t.Parallel since we rely on the global metrics instance.
 
 	ca := agConnect.TestCA(t, nil)
-	ports := freeport.GetT(t, 1)
+	ports := freeport.MustTake(1)
+	defer freeport.Return(ports)
 
 	testApp := NewTestTCPServer(t)
 	defer testApp.Close()
@@ -162,7 +164,8 @@ func TestUpstreamListener(t *testing.T) {
 	// Can't enable t.Parallel since we rely on the global metrics instance.
 
 	ca := agConnect.TestCA(t, nil)
-	ports := freeport.GetT(t, 1)
+	ports := freeport.MustTake(1)
+	defer freeport.Return(ports)
 
 	// Run a test server that we can dial.
 	testSvr := connect.NewTestServer(t, "db", ca)

connect/proxy/proxy_test.go

@@ -22,7 +22,9 @@ func TestProxy_public(t *testing.T) {
 	t.Parallel()
 
 	require := require.New(t)
-	ports := freeport.GetT(t, 1)
+
+	ports := freeport.MustTake(1)
+	defer freeport.Return(ports)
 
 	a := agent.NewTestAgent(t, t.Name(), "")
 	defer a.Shutdown()

connect/proxy/testing.go

@@ -24,14 +24,15 @@ type TestTCPServer struct {
 	l                        net.Listener
 	stopped                  int32
 	accepted, closed, active int32
+	returnPortsFn            func()
 }
 
 // NewTestTCPServer opens as a listening socket on the given address and returns
 // a TestTCPServer serving requests to it. The server is already started and can
 // be stopped by calling Close().
 func NewTestTCPServer(t testing.T) *TestTCPServer {
-	port := freeport.GetT(t, 1)
+	ports := freeport.MustTake(1)
-	addr := TestLocalAddr(port[0])
+	addr := TestLocalAddr(ports[0])
 
 	l, err := net.Listen("tcp", addr)
 	require.NoError(t, err)
@@ -39,6 +40,7 @@ func NewTestTCPServer(t testing.T) *TestTCPServer {
 	log.Printf("test tcp server listening on %s", addr)
 	s := &TestTCPServer{
 		l:             l,
+		returnPortsFn: func() { freeport.Return(ports) },
 	}
 	go s.accept()
 
@@ -51,6 +53,10 @@ func (s *TestTCPServer) Close() {
 	if s.l != nil {
 		s.l.Close()
 	}
+	if s.returnPortsFn != nil {
+		s.returnPortsFn()
+		s.returnPortsFn = nil
+	}
 }
 
 // Addr returns the address that this server is listening on.

connect/testing.go

@@ -95,6 +95,7 @@ type TestServer struct {
 	Listening chan struct{}
 
 	l             net.Listener
+	returnPortsFn func()
 	stopFlag      int32
 	stopChan      chan struct{}
 }
@@ -102,7 +103,7 @@ type TestServer struct {
 // NewTestServer returns a TestServer. It should be closed when test is
 // complete.
 func NewTestServer(t testing.T, service string, ca *structs.CARoot) *TestServer {
-	ports := freeport.GetT(t, 1)
+	ports := freeport.MustTake(1)
 	return &TestServer{
 		Service:       service,
 		CA:            ca,
@@ -110,6 +111,7 @@ func NewTestServer(t testing.T, service string, ca *structs.CARoot) *TestServer
 		TLSCfg:        TestTLSConfig(t, service, ca),
 		Addr:          fmt.Sprintf("127.0.0.1:%d", ports[0]),
 		Listening:     make(chan struct{}),
+		returnPortsFn: func() { freeport.Return(ports) },
 	}
 }
 
@@ -186,6 +188,10 @@ func (s *TestServer) Close() error {
 		if s.l != nil {
 			s.l.Close()
 		}
+		if s.returnPortsFn != nil {
+			s.returnPortsFn()
+			s.returnPortsFn = nil
+		}
 		close(s.stopChan)
 	}
 	return nil

sdk/freeport/ephemeral_fallback.go

@@ -0,0 +1,7 @@
+//+build !linux
+
+package freeport
+
+func getEphemeralPortRange() (int, int, error) {
+	return 0, 0, nil
+}

sdk/freeport/ephemeral_linux.go

@@ -0,0 +1,36 @@
+//+build linux
+
+package freeport
+
+import (
+	"fmt"
+	"os/exec"
+	"regexp"
+	"strconv"
+)
+
+const ephemeralPortRangeSysctlKey = "net.ipv4.ip_local_port_range"
+
+var ephemeralPortRangePatt = regexp.MustCompile(`^\s*(\d+)\s+(\d+)\s*$`)
+
+func getEphemeralPortRange() (int, int, error) {
+	cmd := exec.Command("sysctl", "-n", ephemeralPortRangeSysctlKey)
+	out, err := cmd.Output()
+	if err != nil {
+		return 0, 0, err
+	}
+
+	val := string(out)
+
+	m := ephemeralPortRangePatt.FindStringSubmatch(val)
+	if m != nil {
+		min, err1 := strconv.Atoi(m[1])
+		max, err2 := strconv.Atoi(m[2])
+
+		if err1 == nil && err2 == nil {
+			return min, max, nil
+		}
+	}
+
+	return 0, 0, fmt.Errorf("unexpected sysctl value %q for key %q", val, ephemeralPortRangeSysctlKey)
+}

sdk/freeport/ephemeral_linux_test.go

@@ -0,0 +1,18 @@
+//+build linux
+
+package freeport
+
+import (
+	"testing"
+)
+
+func TestGetEphemeralPortRange(t *testing.T) {
+	min, max, err := getEphemeralPortRange()
+	if err != nil {
+		t.Fatalf("err: %v", err)
+	}
+	if min <= 0 || max <= 0 || min > max {
+		t.Fatalf("unexpected values: min=%d, max=%d", min, max)
+	}
+	t.Logf("min=%d, max=%d", min, max)
+}

sdk/freeport/freeport.go

@@ -3,9 +3,12 @@
 package freeport
 
 import (
+	"container/list"
 	"fmt"
 	"math/rand"
 	"net"
+	"os"
+	"runtime"
 	"sync"
 	"time"
 
@@ -14,12 +17,10 @@ import (
 
 const (
 	// blockSize is the size of the allocated port block. ports are given out
-	// consecutively from that block with roll-over for the lifetime of the
+	// consecutively from that block and after that point in a LRU fashion.
-	// application/test run.
 	blockSize = 1500
 
-	// maxBlocks is the number of available port blocks.
+	// maxBlocks is the number of available port blocks before exclusions.
-	// lowPort + maxBlocks * blockSize must be less than 65535.
 	maxBlocks = 30
 
 	// lowPort is the lowest port number that should be used.
@@ -31,31 +32,158 @@ const (
 )
 
 var (
+	// effectiveMaxBlocks is the number of available port blocks.
+	// lowPort + effectiveMaxBlocks * blockSize must be less than 65535.
+	effectiveMaxBlocks int
+
 	// firstPort is the first port of the allocated block.
 	firstPort int
 
 	// lockLn is the system-wide mutex for the port block.
 	lockLn net.Listener
 
-	// mu guards nextPort
+	// mu guards:
+	// - pendingPorts
+	// - freePorts
+	// - total
 	mu sync.Mutex
 
 	// once is used to do the initialization on the first call to retrieve free
 	// ports
 	once sync.Once
 
-	// port is the last allocated port.
+	// condNotEmpty is a condition variable to wait for freePorts to be not
-	port int
+	// empty. Linked to 'mu'
+	condNotEmpty *sync.Cond
+
+	// freePorts is a FIFO of all currently free ports. Take from the front,
+	// and return to the back.
+	freePorts *list.List
+
+	// pendingPorts is a FIFO of recently freed ports that have not yet passed
+	// the not-in-use check.
+	pendingPorts *list.List
+
+	// total is the total number of available ports in the block for use.
+	total int
 )
 
 // initialize is used to initialize freeport.
 func initialize() {
-	if lowPort+maxBlocks*blockSize > 65535 {
+	var err error
+	effectiveMaxBlocks, err = adjustMaxBlocks()
+	if err != nil {
+		panic("freeport: ephemeral port range detection failed: " + err.Error())
+	}
+	if effectiveMaxBlocks < 0 {
+		panic("freeport: no blocks of ports available outside of ephemeral range")
+	}
+	if lowPort+effectiveMaxBlocks*blockSize > 65535 {
 		panic("freeport: block size too big or too many blocks requested")
 	}
 
 	rand.Seed(time.Now().UnixNano())
 	firstPort, lockLn = alloc()
+
+	condNotEmpty = sync.NewCond(&mu)
+	freePorts = list.New()
+	pendingPorts = list.New()
+
+	// fill with all available free ports
+	for port := firstPort + 1; port < firstPort+blockSize; port++ {
+		if used := isPortInUse(port); !used {
+			freePorts.PushBack(port)
+		}
+	}
+	total = freePorts.Len()
+
+	go checkFreedPorts()
+}
+
+// reset will reverse the setup from initialize() and then redo it (for tests)
+func reset() {
+	mu.Lock()
+	defer mu.Unlock()
+
+	logf("INFO", "resetting the freeport package state")
+
+	effectiveMaxBlocks = 0
+	firstPort = 0
+	if lockLn != nil {
+		lockLn.Close()
+		lockLn = nil
+	}
+
+	once = sync.Once{}
+
+	freePorts = nil
+	pendingPorts = nil
+	total = 0
+}
+
+func checkFreedPorts() {
+	ticker := time.NewTicker(250 * time.Millisecond)
+	for {
+		<-ticker.C
+		checkFreedPortsOnce()
+	}
+}
+
+func checkFreedPortsOnce() {
+	mu.Lock()
+	defer mu.Unlock()
+
+	pending := pendingPorts.Len()
+	remove := make([]*list.Element, 0, pending)
+	for elem := pendingPorts.Front(); elem != nil; elem = elem.Next() {
+		port := elem.Value.(int)
+		if used := isPortInUse(port); !used {
+			freePorts.PushBack(port)
+			remove = append(remove, elem)
+		}
+	}
+
+	retained := pending - len(remove)
+
+	if retained > 0 {
+		logf("WARN", "%d out of %d pending ports are still in use; something probably didn't wait around for the port to be closed!", retained, pending)
+	}
+
+	if len(remove) == 0 {
+		return
+	}
+
+	for _, elem := range remove {
+		pendingPorts.Remove(elem)
+	}
+
+	condNotEmpty.Broadcast()
+}
+
+// adjustMaxBlocks avoids having the allocation ranges overlap the ephemeral
+// port range.
+func adjustMaxBlocks() (int, error) {
+	ephemeralPortMin, ephemeralPortMax, err := getEphemeralPortRange()
+	if err != nil {
+		return 0, err
+	}
+
+	if ephemeralPortMin <= 0 || ephemeralPortMax <= 0 {
+		logf("INFO", "ephemeral port range detection not configured for GOOS=%q", runtime.GOOS)
+		return maxBlocks, nil
+	}
+
+	logf("INFO", "detected ephemeral port range of [%d, %d]", ephemeralPortMin, ephemeralPortMax)
+	for block := 0; block < maxBlocks; block++ {
+		min := lowPort + block*blockSize
+		max := min + blockSize
+		overlap := intervalOverlap(min, max-1, ephemeralPortMin, ephemeralPortMax)
+		if overlap {
+			logf("INFO", "reducing max blocks from %d to %d to avoid the ephemeral port range", maxBlocks, block)
+			return block, nil
+		}
+	}
+	return maxBlocks, nil
 }
 
 // alloc reserves a port block for exclusive use for the lifetime of the
@@ -64,76 +192,154 @@ func initialize() {
 // be automatically released when the application terminates.
 func alloc() (int, net.Listener) {
 	for i := 0; i < attempts; i++ {
-		block := int(rand.Int31n(int32(maxBlocks)))
+		block := int(rand.Int31n(int32(effectiveMaxBlocks)))
 		firstPort := lowPort + block*blockSize
 		ln, err := net.ListenTCP("tcp", tcpAddr("127.0.0.1", firstPort))
 		if err != nil {
 			continue
 		}
-		// log.Printf("[DEBUG] freeport: allocated port block %d (%d-%d)", block, firstPort, firstPort+blockSize-1)
+		// logf("DEBUG", "allocated port block %d (%d-%d)", block, firstPort, firstPort+blockSize-1)
 		return firstPort, ln
 	}
 	panic("freeport: cannot allocate port block")
 }
 
+// MustTake is the same as Take except it panics on error.
+func MustTake(n int) (ports []int) {
+	ports, err := Take(n)
+	if err != nil {
+		panic(err)
+	}
+	return ports
+}
+
+// Take returns a list of free ports from the allocated port block. It is safe
+// to call this method concurrently. Ports have been tested to be available on
+// 127.0.0.1 TCP but there is no guarantee that they will remain free in the
+// future.
+func Take(n int) (ports []int, err error) {
+	if n <= 0 {
+		return nil, fmt.Errorf("freeport: cannot take %d ports", n)
+	}
+
+	mu.Lock()
+	defer mu.Unlock()
+
+	// Reserve a port block
+	once.Do(initialize)
+
+	if n > total {
+		return nil, fmt.Errorf("freeport: block size too small")
+	}
+
+	for len(ports) < n {
+		for freePorts.Len() == 0 {
+			if total == 0 {
+				return nil, fmt.Errorf("freeport: impossible to satisfy request; there are no actual free ports in the block anymore")
+			}
+			condNotEmpty.Wait()
+		}
+
+		elem := freePorts.Front()
+		freePorts.Remove(elem)
+		port := elem.Value.(int)
+
+		if used := isPortInUse(port); used {
+			// Something outside of the test suite has stolen this port, possibly
+			// due to assignment to an ephemeral port, remove it completely.
+			logf("WARN", "leaked port %d due to theft; removing from circulation", port)
+			total--
+			continue
+		}
+
+		ports = append(ports, port)
+	}
+
+	// logf("DEBUG", "free ports: %v", ports)
+	return ports, nil
+}
+
+// peekFree returns the next port that will be returned by Take to aid in testing.
+func peekFree() int {
+	mu.Lock()
+	defer mu.Unlock()
+	return freePorts.Front().Value.(int)
+}
+
+// peekAllFree returns all free ports that could be returned by Take to aid in testing.
+func peekAllFree() []int {
+	mu.Lock()
+	defer mu.Unlock()
+
+	var out []int
+	for elem := freePorts.Front(); elem != nil; elem = elem.Next() {
+		port := elem.Value.(int)
+		out = append(out, port)
+	}
+
+	return out
+}
+
+// stats returns diagnostic data to aid in testing
+func stats() (numTotal, numPending, numFree int) {
+	mu.Lock()
+	defer mu.Unlock()
+	return total, pendingPorts.Len(), freePorts.Len()
+}
+
+// Return returns a block of ports back to the general pool. These ports should
+// have been returned from a call to Take().
+func Return(ports []int) {
+	if len(ports) == 0 {
+		return // convenience short circuit for test ergonomics
+	}
+
+	mu.Lock()
+	defer mu.Unlock()
+
+	for _, port := range ports {
+		if port > firstPort && port < firstPort+blockSize {
+			pendingPorts.PushBack(port)
+		}
+	}
+}
+
+func isPortInUse(port int) bool {
+	ln, err := net.ListenTCP("tcp", tcpAddr("127.0.0.1", port))
+	if err != nil {
+		return true
+	}
+	ln.Close()
+	return false
+}
+
 func tcpAddr(ip string, port int) *net.TCPAddr {
 	return &net.TCPAddr{IP: net.ParseIP(ip), Port: port}
 }
 
-// Get wraps the Free function and panics on any failure retrieving ports.
+// intervalOverlap returns true if the doubly-inclusive integer intervals
-func Get(n int) (ports []int) {
+// represented by [min1, max1] and [min2, max2] overlap.
-	ports, err := Free(n)
+func intervalOverlap(min1, max1, min2, max2 int) bool {
-	if err != nil {
+	if min1 > max1 {
-		panic(err)
+		logf("WARN", "interval1 is not ordered [%d, %d]", min1, max1)
+		return false
 	}
-
+	if min2 > max2 {
-	return ports
+		logf("WARN", "interval2 is not ordered [%d, %d]", min2, max2)
+		return false
+	}
+	return min1 <= max2 && min2 <= max1
 }
 
-// GetT is suitable for use when retrieving unused ports in tests. If there is
+func logf(severity string, format string, a ...interface{}) {
-// an error retrieving free ports, the test will be failed.
+	fmt.Fprintf(os.Stderr, "["+severity+"] freeport: "+format+"\n", a...)
-func GetT(t testing.T, n int) (ports []int) {
-	ports, err := Free(n)
-	if err != nil {
-		t.Fatalf("Failed retrieving free port: %v", err)
-	}
-
-	return ports
 }
 
-// Free returns a list of free ports from the allocated port block. It is safe
+// Deprecated: Please use Take/Return calls instead.
-// to call this method concurrently. Ports have been tested to be available on
+func Get(n int) (ports []int) { return MustTake(n) }
-// 127.0.0.1 TCP but there is no guarantee that they will remain free in the
-// future.
-func Free(n int) (ports []int, err error) {
-	mu.Lock()
-	defer mu.Unlock()
 
-	if n > blockSize-1 {
+// Deprecated: Please use Take/Return calls instead.
-		return nil, fmt.Errorf("freeport: block size too small")
+func GetT(t testing.T, n int) (ports []int) { return MustTake(n) }
-	}
 
-	// Reserve a port block
+// Deprecated: Please use Take/Return calls instead.
-	once.Do(initialize)
+func Free(n int) (ports []int, err error) { return MustTake(n), nil }
-
-	for len(ports) < n {
-		port++
-
-		// roll-over the port
-		if port < firstPort+1 || port >= firstPort+blockSize {
-			port = firstPort + 1
-		}
-
-		// if the port is in use then skip it
-		ln, err := net.ListenTCP("tcp", tcpAddr("127.0.0.1", port))
-		if err != nil {
-			// log.Println("[DEBUG] freeport: port already in use: ", port)
-			continue
-		}
-		ln.Close()
-
-		ports = append(ports, port)
-	}
-	// log.Println("[DEBUG] freeport: free ports:", ports)
-	return ports, nil
-}

sdk/freeport/freeport_test.go

@@ -0,0 +1,231 @@
+package freeport
+
+import (
+	"fmt"
+	"io"
+	"net"
+	"testing"
+
+	"github.com/hashicorp/consul/sdk/testutil/retry"
+)
+
+func TestTakeReturn(t *testing.T) {
+	// NOTE: for global var reasons this cannot execute in parallel
+	// t.Parallel()
+
+	// Since this test is destructive (i.e. it leaks all ports) it means that
+	// any other test cases in this package will not function after it runs. To
+	// help out we reset the global state after we run this test.
+	defer reset()
+
+	// OK: do a simple take/return cycle to trigger the package initialization
+	func() {
+		ports, err := Take(1)
+		if err != nil {
+			t.Fatalf("err: %v", err)
+		}
+		defer Return(ports)
+
+		if len(ports) != 1 {
+			t.Fatalf("expected %d but got %d ports", 1, len(ports))
+		}
+	}()
+
+	waitForStatsReset := func() (numTotal int) {
+		t.Helper()
+		numTotal, numPending, numFree := stats()
+		if numTotal != numFree+numPending {
+			t.Fatalf("expected total (%d) and free+pending (%d) ports to match", numTotal, numFree+numPending)
+		}
+		retry.Run(t, func(r *retry.R) {
+			numTotal, numPending, numFree = stats()
+			if numPending != 0 {
+				r.Fatalf("pending is still non zero: %d", numPending)
+			}
+			if numTotal != numFree {
+				r.Fatalf("total (%d) does not equal free (%d)", numTotal, numFree)
+			}
+		})
+		return numTotal
+	}
+
+	// Reset
+	numTotal := waitForStatsReset()
+
+	// --------------------
+	// OK: take the max
+	func() {
+		ports, err := Take(numTotal)
+		if err != nil {
+			t.Fatalf("err: %v", err)
+		}
+		defer Return(ports)
+
+		if len(ports) != numTotal {
+			t.Fatalf("expected %d but got %d ports", numTotal, len(ports))
+		}
+	}()
+
+	// Reset
+	numTotal = waitForStatsReset()
+
+	expectError := func(expected string, got error) {
+		t.Helper()
+		if got == nil {
+			t.Fatalf("expected error but was nil")
+		}
+		if got.Error() != expected {
+			t.Fatalf("expected error %q but got %q", expected, got.Error())
+		}
+	}
+
+	// --------------------
+	// ERROR: take too many ports
+	func() {
+		ports, err := Take(numTotal + 1)
+		defer Return(ports)
+		expectError("freeport: block size too small", err)
+	}()
+
+	// --------------------
+	// ERROR: invalid ports request (negative)
+	func() {
+		_, err := Take(-1)
+		expectError("freeport: cannot take -1 ports", err)
+	}()
+
+	// --------------------
+	// ERROR: invalid ports request (zero)
+	func() {
+		_, err := Take(0)
+		expectError("freeport: cannot take 0 ports", err)
+	}()
+
+	// --------------------
+	// OK: Steal a port under the covers and let freeport detect the theft and compensate
+	leakedPort := peekFree()
+	func() {
+		leakyListener, err := net.ListenTCP("tcp", tcpAddr("127.0.0.1", leakedPort))
+		if err != nil {
+			t.Fatalf("err: %v", err)
+		}
+		defer leakyListener.Close()
+
+		func() {
+			ports, err := Take(3)
+			if err != nil {
+				t.Fatalf("err: %v", err)
+			}
+			defer Return(ports)
+
+			if len(ports) != 3 {
+				t.Fatalf("expected %d but got %d ports", 3, len(ports))
+			}
+
+			for _, port := range ports {
+				if port == leakedPort {
+					t.Fatalf("did not expect for Take to return the leaked port")
+				}
+			}
+		}()
+
+		newNumTotal := waitForStatsReset()
+		if newNumTotal != numTotal-1 {
+			t.Fatalf("expected total to drop to %d but got %d", numTotal-1, newNumTotal)
+		}
+		numTotal = newNumTotal // update outer variable for later tests
+	}()
+
+	// --------------------
+	// OK: sequence it so that one Take must wait on another Take to Return.
+	func() {
+		mostPorts, err := Take(numTotal - 5)
+		if err != nil {
+			t.Fatalf("err: %v", err)
+		}
+
+		type reply struct {
+			ports []int
+			err   error
+		}
+		ch := make(chan reply, 1)
+		go func() {
+			ports, err := Take(10)
+			ch <- reply{ports: ports, err: err}
+		}()
+
+		Return(mostPorts)
+
+		r := <-ch
+		if r.err != nil {
+			t.Fatalf("err: %v", r.err)
+		}
+		defer Return(r.ports)
+
+		if len(r.ports) != 10 {
+			t.Fatalf("expected %d ports but got %d", 10, len(r.ports))
+		}
+	}()
+
+	// Reset
+	numTotal = waitForStatsReset()
+
+	// --------------------
+	// ERROR: Now we end on the crazy "Ocean's 11" level port theft where we
+	// orchestrate a situation where all ports are stolen and we don't find out
+	// until Take.
+	func() {
+		// 1. Grab all of the ports.
+		allPorts := peekAllFree()
+
+		// 2. Leak all of the ports
+		leaked := make([]io.Closer, 0, len(allPorts))
+		defer func() {
+			for _, c := range leaked {
+				c.Close()
+			}
+		}()
+		for _, port := range allPorts {
+			ln, err := net.ListenTCP("tcp", tcpAddr("127.0.0.1", port))
+			if err != nil {
+				t.Fatalf("err: %v", err)
+			}
+			leaked = append(leaked, ln)
+		}
+
+		// 3. Request 1 port which will detect the leaked ports and fail.
+		_, err := Take(1)
+		expectError("freeport: impossible to satisfy request; there are no actual free ports in the block anymore", err)
+
+		// 4. Wait for the block to zero out.
+		newNumTotal := waitForStatsReset()
+		if newNumTotal != 0 {
+			t.Fatalf("expected total to drop to %d but got %d", 0, newNumTotal)
+		}
+	}()
+}
+
+func TestIntervalOverlap(t *testing.T) {
+	cases := []struct {
+		min1, max1, min2, max2 int
+		overlap                bool
+	}{
+		{0, 0, 0, 0, true},
+		{1, 1, 1, 1, true},
+		{1, 3, 1, 3, true},  // same
+		{1, 3, 4, 6, false}, // serial
+		{1, 4, 3, 6, true},  // inner overlap
+		{1, 6, 3, 4, true},  // nest
+	}
+
+	for _, tc := range cases {
+		t.Run(fmt.Sprintf("%d:%d vs %d:%d", tc.min1, tc.max1, tc.min2, tc.max2), func(t *testing.T) {
+			if tc.overlap != intervalOverlap(tc.min1, tc.max1, tc.min2, tc.max2) { // 1 vs 2
+				t.Fatalf("expected %v but got %v", tc.overlap, !tc.overlap)
+			}
+			if tc.overlap != intervalOverlap(tc.min2, tc.max2, tc.min1, tc.max1) { // 2 vs 1
+				t.Fatalf("expected %v but got %v", tc.overlap, !tc.overlap)
+			}
+		})
+	}
+}

sdk/testutil/server.go

@@ -104,6 +104,7 @@ type TestServerConfig struct {
 	ReadyTimeout        time.Duration          `json:"-"`
 	Stdout, Stderr      io.Writer              `json:"-"`
 	Args                []string               `json:"-"`
+	ReturnPorts         func()                 `json:"-"`
 }
 
 type TestACLs struct {
@@ -138,7 +139,8 @@ func defaultServerConfig() *TestServerConfig {
 		panic(err)
 	}
 
-	ports := freeport.Get(6)
+	ports := freeport.MustTake(6)
+
 	return &TestServerConfig{
 		NodeName:          "node-" + nodeID,
 		NodeID:            nodeID,
@@ -167,6 +169,9 @@ func defaultServerConfig() *TestServerConfig {
 				"cluster_id": "11111111-2222-3333-4444-555555555555",
 			},
 		},
+		ReturnPorts: func() {
+			freeport.Return(ports)
+		},
 	}
 }
 
@@ -244,6 +249,7 @@ func newTestServerConfigT(t *testing.T, cb ServerConfigCallback) (*TestServer, e
 	}
 
 	cfg := defaultServerConfig()
+
 	cfg.DataDir = filepath.Join(tmpdir, "data")
 	if cb != nil {
 		cb(cfg)
@@ -251,6 +257,7 @@ func newTestServerConfigT(t *testing.T, cb ServerConfigCallback) (*TestServer, e
 
 	b, err := json.Marshal(cfg)
 	if err != nil {
+		cfg.ReturnPorts()
 		os.RemoveAll(tmpdir)
 		return nil, errors.Wrap(err, "failed marshaling json")
 	}
@@ -258,6 +265,7 @@ func newTestServerConfigT(t *testing.T, cb ServerConfigCallback) (*TestServer, e
 	log.Printf("CONFIG JSON: %s", string(b))
 	configFile := filepath.Join(tmpdir, "config.json")
 	if err := ioutil.WriteFile(configFile, b, 0644); err != nil {
+		cfg.ReturnPorts()
 		os.RemoveAll(tmpdir)
 		return nil, errors.Wrap(err, "failed writing config content")
 	}
@@ -278,6 +286,7 @@ func newTestServerConfigT(t *testing.T, cb ServerConfigCallback) (*TestServer, e
 	cmd.Stdout = stdout
 	cmd.Stderr = stderr
 	if err := cmd.Start(); err != nil {
+		cfg.ReturnPorts()
 		os.RemoveAll(tmpdir)
 		return nil, errors.Wrap(err, "failed starting command")
 	}
@@ -319,6 +328,7 @@ func newTestServerConfigT(t *testing.T, cb ServerConfigCallback) (*TestServer, e
 // Stop stops the test Consul server, and removes the Consul data
 // directory once we are done.
 func (s *TestServer) Stop() error {
+	defer s.Config.ReturnPorts()
 	defer os.RemoveAll(s.tmpdir)
 
 	// There was no process