Reuse SSH transports for concurrent exec sessions

internal/controller/api_vms_exec.go

@@ -5,7 +5,6 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
-	"net"
 	"net/http"
 	"strconv"
 	"time"
@@ -190,8 +189,14 @@ func (controller *Controller) newSSHExecSession(
 	sessionContext, sessionContextCancel := context.WithCancel(context.Background())
 
 	type sshExecAttempt struct {
-		portForwardConn net.Conn
-		exec            *sshexec.Exec
+		lease *execSSHTransportLease
+		exec  sshExecRunner
+	}
+
+	transportKey := execSSHTransportKey{
+		workerName:   vm.Worker,
+		vmUID:        vm.UID,
+		restartCount: vm.RestartCount,
 	}
 
 	attempt, err := retry.NewWithData[sshExecAttempt](
@@ -201,32 +206,49 @@ func (controller *Controller) newSSHExecSession(
 		retry.Attempts(0),
 		retry.LastErrorOnly(true),
 	).Do(func() (sshExecAttempt, error) {
-		portForwardConn, err := controller.portForwardConnection(
-			sessionContext,
-			waitContext,
-			vm.Worker,
-			vm.UID,
-			22,
-		)
+		lease, err := controller.execSSHPool.acquire(waitContext, transportKey, func() (execSSHTransport, error) {
+			portForwardConn, err := controller.portForwardConnection(
+				context.Background(),
+				waitContext,
+				vm.Worker,
+				vm.UID,
+				22,
+			)
+			if err != nil {
+				return nil, err
+			}
+
+			client, err := sshexec.NewClient(portForwardConn, vm.SSHUsername(), vm.SSHPassword())
+			if err != nil {
+				return nil, fmt.Errorf("failed to establish SSH connection to a VM: %w", err)
+			}
+
+			return &execSSHClientTransport{client: client}, nil
+		})
 		if err != nil {
 			return sshExecAttempt{}, err
 		}
 
-		exec, err := sshexec.New(portForwardConn, vm.SSHUsername(), vm.SSHPassword(), sshexec.Options{
+		exec, err := lease.transport().NewExec(sshexec.Options{
 			Interactive: spec.interactive,
 			TTY:         spec.tty,
 			Rows:        spec.rows,
 			Cols:        spec.cols,
 		})
 		if err != nil {
-			_ = portForwardConn.Close()
+			lease.release()
 
-			return sshExecAttempt{}, fmt.Errorf("failed to establish SSH connection to a VM: %w", err)
+			err = fmt.Errorf("failed to create SSH session for a VM: %w", err)
+			if lease.reused {
+				return sshExecAttempt{}, retry.Unrecoverable(err)
+			}
+
+			return sshExecAttempt{}, err
 		}
 
 		return sshExecAttempt{
-			portForwardConn: portForwardConn,
-			exec:            exec,
+			lease: lease,
+			exec:  exec,
 		}, nil
 	})
 	if err != nil {
@@ -242,7 +264,7 @@ func (controller *Controller) newSSHExecSession(
 		spec,
 		runCommand,
 		attempt.exec,
-		attempt.portForwardConn,
+		attempt.lease.release,
 		registry,
 		controller.execSessionExitTTL,
 		policy,

internal/controller/controller.go

@@ -66,6 +66,7 @@ type Controller struct {
 	sshNoClientAuth bool
 	sshServer       *sshserver.SSHServer
 	execSessions    *execSessionRegistry
+	execSSHPool     *execSSHTransportPool
 
 	single singleflight.Group
 
@@ -80,6 +81,7 @@ func New(opts ...Option) (*Controller, error) {
 		execSessionExitTTL:   10 * time.Minute,
 		pingInterval:         30 * time.Second,
 		execSessions:         newExecSessionRegistry(),
+		execSSHPool:          newExecSSHTransportPool(),
 		single:               singleflight.Group{},
 	}
 
@@ -313,6 +315,7 @@ func (controller *Controller) Run(ctx context.Context) error {
 		<-ctx.Done()
 
 		controller.execSessions.closeAll()
+		controller.execSSHPool.closeAll()
 
 		if err := controller.httpServer.Shutdown(ctx); err != nil {
 			controller.logger.Errorf("failed to cleanly shutdown the HTTP server: %v", err)

internal/controller/exec_sessions.go

@@ -5,7 +5,6 @@ import (
 	"errors"
 	"io"
 	"maps"
-	"net"
 	"sync"
 	"time"
 
@@ -325,14 +324,14 @@ func (subscriber *execSessionSubscriber) close() {
 }
 
 type execSession struct {
-	key       execSessionKey
-	spec      execSessionSpec
-	command   string
-	exec      sshExecRunner
-	transport net.Conn
-	registry  *execSessionRegistry
-	exitTTL   time.Duration
-	policy    execSessionPolicy
+	key      execSessionKey
+	spec     execSessionSpec
+	command  string
+	exec     sshExecRunner
+	release  func()
+	registry *execSessionRegistry
+	exitTTL  time.Duration
+	policy   execSessionPolicy
 
 	ctx    context.Context
 	cancel context.CancelFunc
@@ -348,6 +347,7 @@ type execSession struct {
 	expiryTimer *time.Timer
 
 	startOnce sync.Once
+	closeOnce sync.Once
 	done      chan struct{}
 	doneOnce  sync.Once
 }
@@ -356,7 +356,7 @@ func newExecSession(
 	key execSessionKey,
 	command string,
 	exec sshExecRunner,
-	transport net.Conn,
+	release func(),
 	registry *execSessionRegistry,
 	exitTTL time.Duration,
 	policy execSessionPolicy,
@@ -370,7 +370,7 @@ func newExecSession(
 		execSessionSpec{command: command},
 		command,
 		exec,
-		transport,
+		release,
 		registry,
 		exitTTL,
 		policy,
@@ -384,7 +384,7 @@ func newExecSessionWithContextAndSpec(
 	spec execSessionSpec,
 	command string,
 	exec sshExecRunner,
-	transport net.Conn,
+	release func(),
 	registry *execSessionRegistry,
 	exitTTL time.Duration,
 	policy execSessionPolicy,
@@ -398,7 +398,7 @@ func newExecSessionWithContextAndSpec(
 		spec:        spec.clone(),
 		command:     command,
 		exec:        exec,
-		transport:   transport,
+		release:     release,
 		registry:    registry,
 		exitTTL:     exitTTL,
 		policy:      policy,
@@ -574,10 +574,7 @@ func (session *execSession) close() {
 	closeSubscribers(subscribers)
 
 	session.cancel()
-	_ = session.exec.Close()
-	if session.transport != nil {
-		_ = session.transport.Close()
-	}
+	session.closeCommandResources()
 	if session.registry != nil {
 		session.registry.remove(session.key, session)
 	}
@@ -661,6 +658,8 @@ func (session *execSession) markFinished() {
 		close(session.done)
 	})
 
+	session.closeCommandResources()
+
 	if shouldClose {
 		session.close()
 	}
@@ -693,6 +692,15 @@ func (session *execSession) dropSubscriber(subscriber *execSessionSubscriber) {
 	session.detachLocked(subscriber)
 }
 
+func (session *execSession) closeCommandResources() {
+	session.closeOnce.Do(func() {
+		_ = session.exec.Close()
+		if session.release != nil {
+			session.release()
+		}
+	})
+}
+
 func cloneExecFrame(frame *execstream.Frame) *execstream.Frame {
 	if frame == nil {
 		return nil

internal/controller/exec_sessions_test.go

@@ -386,3 +386,31 @@ func TestExecSessionFinishKeepsReconnectableSubscriberOpen(t *testing.T) {
 	require.Equal(t, execstream.FrameTypeNoMoreHistory, noMoreHistory.Type)
 	require.EqualValues(t, 2, noMoreHistory.Watermark)
 }
+
+func TestExecSessionFinishReleasesTransportWhileRetainingHistory(t *testing.T) {
+	registry := newExecSessionRegistry()
+	session := newManualExecSessionForTest(execSessionKey{vmName: "vm", sessionID: "session"}, registry)
+	var releaseCalls atomic.Int32
+	session.release = func() {
+		releaseCalls.Add(1)
+	}
+
+	session.recordFrame(&execstream.Frame{Type: execstream.FrameTypeStdout, Data: []byte("out")})
+	session.recordFrame(&execstream.Frame{
+		Type: execstream.FrameTypeExit,
+		Exit: &execstream.Exit{Code: 0},
+	})
+	session.markFinished()
+
+	require.False(t, session.closed)
+	require.EqualValues(t, 1, releaseCalls.Load())
+	require.EqualValues(t, 1, session.exec.(*fakeExec).closeCalls.Load())
+
+	subscriber, err := session.attach()
+	require.NoError(t, err)
+	session.sendHistory(subscriber, 0)
+
+	require.Equal(t, execstream.FrameTypeStdout, (<-subscriber.frames).Type)
+	require.Equal(t, execstream.FrameTypeExit, (<-subscriber.frames).Type)
+	require.Equal(t, execstream.FrameTypeNoMoreHistory, (<-subscriber.frames).Type)
+}

internal/controller/exec_ssh_pool.go

@@ -0,0 +1,182 @@
+package controller
+
+import (
+	"context"
+	"sync"
+
+	"github.com/cirruslabs/orchard/internal/controller/sshexec"
+)
+
+type execSSHTransport interface {
+	NewExec(options sshexec.Options) (sshExecRunner, error)
+	Close() error
+}
+
+type execSSHClientTransport struct {
+	client *sshexec.Client
+}
+
+func (transport *execSSHClientTransport) NewExec(options sshexec.Options) (sshExecRunner, error) {
+	return transport.client.NewExec(options)
+}
+
+func (transport *execSSHClientTransport) Close() error {
+	return transport.client.Close()
+}
+
+type execSSHTransportKey struct {
+	workerName   string
+	vmUID        string
+	restartCount uint64
+}
+
+type execSSHTransportEntry struct {
+	key       execSSHTransportKey
+	transport execSSHTransport
+	refs      int
+	closed    bool
+}
+
+type execSSHTransportCreation struct {
+	done chan struct{}
+	err  error
+}
+
+type execSSHTransportPool struct {
+	mu       sync.Mutex
+	entries  map[execSSHTransportKey]*execSSHTransportEntry
+	creating map[execSSHTransportKey]*execSSHTransportCreation
+}
+
+func newExecSSHTransportPool() *execSSHTransportPool {
+	return &execSSHTransportPool{
+		entries:  map[execSSHTransportKey]*execSSHTransportEntry{},
+		creating: map[execSSHTransportKey]*execSSHTransportCreation{},
+	}
+}
+
+func (pool *execSSHTransportPool) acquire(
+	ctx context.Context,
+	key execSSHTransportKey,
+	create func() (execSSHTransport, error),
+) (*execSSHTransportLease, error) {
+	for {
+		pool.mu.Lock()
+
+		if entry, ok := pool.entries[key]; ok {
+			entry.refs++
+			pool.mu.Unlock()
+
+			return &execSSHTransportLease{
+				pool:   pool,
+				entry:  entry,
+				reused: true,
+			}, nil
+		}
+
+		if creation, ok := pool.creating[key]; ok {
+			pool.mu.Unlock()
+
+			select {
+			case <-ctx.Done():
+				return nil, ctx.Err()
+			case <-creation.done:
+				if creation.err != nil {
+					return nil, creation.err
+				}
+			}
+
+			continue
+		}
+
+		creation := &execSSHTransportCreation{done: make(chan struct{})}
+		pool.creating[key] = creation
+		pool.mu.Unlock()
+
+		transport, err := create()
+
+		pool.mu.Lock()
+		delete(pool.creating, key)
+		creation.err = err
+
+		var entry *execSSHTransportEntry
+		if err == nil {
+			entry = &execSSHTransportEntry{
+				key:       key,
+				transport: transport,
+				refs:      1,
+			}
+			pool.entries[key] = entry
+		}
+
+		close(creation.done)
+		pool.mu.Unlock()
+
+		if err != nil {
+			return nil, err
+		}
+
+		return &execSSHTransportLease{
+			pool:  pool,
+			entry: entry,
+		}, nil
+	}
+}
+
+func (pool *execSSHTransportPool) release(entry *execSSHTransportEntry) {
+	var transport execSSHTransport
+
+	pool.mu.Lock()
+	if entry.closed {
+		pool.mu.Unlock()
+
+		return
+	}
+
+	entry.refs--
+	if entry.refs == 0 {
+		entry.closed = true
+		if pool.entries[entry.key] == entry {
+			delete(pool.entries, entry.key)
+		}
+		transport = entry.transport
+	}
+	pool.mu.Unlock()
+
+	if transport != nil {
+		_ = transport.Close()
+	}
+}
+
+func (pool *execSSHTransportPool) closeAll() {
+	pool.mu.Lock()
+	entries := make([]*execSSHTransportEntry, 0, len(pool.entries))
+	for key, entry := range pool.entries {
+		entry.closed = true
+		delete(pool.entries, key)
+		entries = append(entries, entry)
+	}
+	pool.mu.Unlock()
+
+	for _, entry := range entries {
+		_ = entry.transport.Close()
+	}
+}
+
+type execSSHTransportLease struct {
+	pool   *execSSHTransportPool
+	entry  *execSSHTransportEntry
+	reused bool
+
+	releaseOnce sync.Once
+}
+
+func (lease *execSSHTransportLease) transport() execSSHTransport {
+	return lease.entry.transport
+}
+
+func (lease *execSSHTransportLease) release() {
+	lease.releaseOnce.Do(func() {
+		lease.pool.release(lease.entry)
+	})
+}

internal/controller/exec_ssh_pool_test.go

@@ -0,0 +1,186 @@
+package controller
+
+import (
+	"context"
+	"errors"
+	"sync"
+	"sync/atomic"
+	"testing"
+
+	"github.com/cirruslabs/orchard/internal/controller/sshexec"
+	"github.com/stretchr/testify/require"
+)
+
+type fakeExecSSHTransport struct {
+	newExec func(sshexec.Options) (sshExecRunner, error)
+
+	closeCalls atomic.Int32
+}
+
+func (transport *fakeExecSSHTransport) NewExec(options sshexec.Options) (sshExecRunner, error) {
+	if transport.newExec != nil {
+		return transport.newExec(options)
+	}
+
+	return &fakeExec{}, nil
+}
+
+func (transport *fakeExecSSHTransport) Close() error {
+	transport.closeCalls.Add(1)
+
+	return nil
+}
+
+func TestExecSSHTransportPoolConcurrentAcquireReusesOneTransport(t *testing.T) {
+	pool := newExecSSHTransportPool()
+	key := execSSHTransportKey{workerName: "worker", vmUID: "vm", restartCount: 1}
+
+	createStarted := make(chan struct{})
+	releaseCreate := make(chan struct{})
+	var createCalls atomic.Int32
+	transport := &fakeExecSSHTransport{}
+
+	create := func() (execSSHTransport, error) {
+		if createCalls.Add(1) == 1 {
+			close(createStarted)
+		}
+		<-releaseCreate
+
+		return transport, nil
+	}
+
+	const leasesCount = 16
+	leases := make([]*execSSHTransportLease, leasesCount)
+	errCh := make(chan error, leasesCount)
+
+	var wg sync.WaitGroup
+	wg.Add(leasesCount)
+	for i := range leasesCount {
+		go func() {
+			defer wg.Done()
+
+			lease, err := pool.acquire(context.Background(), key, create)
+			if err != nil {
+				errCh <- err
+
+				return
+			}
+
+			leases[i] = lease
+		}()
+	}
+
+	<-createStarted
+	close(releaseCreate)
+	wg.Wait()
+	close(errCh)
+
+	for err := range errCh {
+		require.NoError(t, err)
+	}
+	require.EqualValues(t, 1, createCalls.Load())
+
+	for _, lease := range leases {
+		require.NotNil(t, lease)
+		require.Same(t, transport, lease.transport())
+		lease.release()
+	}
+}
+
+func TestExecSSHTransportPoolClosesOnLastRelease(t *testing.T) {
+	pool := newExecSSHTransportPool()
+	key := execSSHTransportKey{workerName: "worker", vmUID: "vm", restartCount: 1}
+	transport := &fakeExecSSHTransport{}
+
+	create := func() (execSSHTransport, error) {
+		return transport, nil
+	}
+
+	firstLease, err := pool.acquire(context.Background(), key, create)
+	require.NoError(t, err)
+	secondLease, err := pool.acquire(context.Background(), key, create)
+	require.NoError(t, err)
+
+	firstLease.release()
+	require.EqualValues(t, 0, transport.closeCalls.Load())
+
+	secondLease.release()
+	require.EqualValues(t, 1, transport.closeCalls.Load())
+	require.Empty(t, pool.entries)
+}
+
+func TestExecSSHTransportPoolSeparatesVMIncarnations(t *testing.T) {
+	pool := newExecSSHTransportPool()
+	var createCalls atomic.Int32
+
+	create := func() (execSSHTransport, error) {
+		createCalls.Add(1)
+
+		return &fakeExecSSHTransport{}, nil
+	}
+
+	firstLease, err := pool.acquire(context.Background(),
+		execSSHTransportKey{workerName: "worker", vmUID: "vm", restartCount: 1}, create)
+	require.NoError(t, err)
+	secondLease, err := pool.acquire(context.Background(),
+		execSSHTransportKey{workerName: "worker", vmUID: "vm", restartCount: 2}, create)
+	require.NoError(t, err)
+	defer firstLease.release()
+	defer secondLease.release()
+
+	require.EqualValues(t, 2, createCalls.Load())
+	require.NotSame(t, firstLease.transport(), secondLease.transport())
+}
+
+func TestExecSSHTransportPoolKeepsSharedTransportAfterSessionCreationFailure(t *testing.T) {
+	pool := newExecSSHTransportPool()
+	key := execSSHTransportKey{workerName: "worker", vmUID: "vm", restartCount: 1}
+	var createCalls atomic.Int32
+	transport := &fakeExecSSHTransport{
+		newExec: func(sshexec.Options) (sshExecRunner, error) {
+			return nil, errors.New("failed to open channel")
+		},
+	}
+
+	create := func() (execSSHTransport, error) {
+		createCalls.Add(1)
+
+		return transport, nil
+	}
+
+	activeLease, err := pool.acquire(context.Background(), key, create)
+	require.NoError(t, err)
+	failedLease, err := pool.acquire(context.Background(), key, create)
+	require.NoError(t, err)
+	require.True(t, failedLease.reused)
+
+	_, err = failedLease.transport().NewExec(sshexec.Options{})
+	require.ErrorContains(t, err, "failed to open channel")
+	failedLease.release()
+
+	require.EqualValues(t, 1, createCalls.Load())
+	require.EqualValues(t, 0, transport.closeCalls.Load())
+	require.Len(t, pool.entries, 1)
+
+	activeLease.release()
+	require.EqualValues(t, 1, transport.closeCalls.Load())
+}
+
+func TestExecSSHTransportPoolCloseAllClosesActiveTransports(t *testing.T) {
+	pool := newExecSSHTransportPool()
+	transport := &fakeExecSSHTransport{}
+
+	lease, err := pool.acquire(context.Background(),
+		execSSHTransportKey{workerName: "worker", vmUID: "vm", restartCount: 1},
+		func() (execSSHTransport, error) {
+			return transport, nil
+		})
+	require.NoError(t, err)
+
+	pool.closeAll()
+	require.EqualValues(t, 1, transport.closeCalls.Load())
+	require.Empty(t, pool.entries)
+
+	lease.release()
+	require.EqualValues(t, 1, transport.closeCalls.Load())
+}

internal/controller/sshexec/sshexec.go

@@ -10,6 +10,7 @@ import (
 	"slices"
 	"sort"
 	"strings"
+	"sync"
 
 	"github.com/cirruslabs/orchard/internal/execstream"
 	"golang.org/x/crypto/ssh"
@@ -28,16 +29,24 @@ type Options struct {
 }
 
 type Exec struct {
-	sshClient   *ssh.Client
 	sshSession  *ssh.Session
 	stdout      io.Reader
 	stderr      io.Reader
 	stdin       io.WriteCloser
 	stdinReader *io.PipeReader
 	tty         bool
+	closeOwner  func() error
 }
 
-func New(netConn net.Conn, user string, password string, options Options) (*Exec, error) {
+type Client struct {
+	netConn   net.Conn
+	sshClient *ssh.Client
+
+	closeOnce sync.Once
+	closeErr  error
+}
+
+func NewClient(netConn net.Conn, user string, password string) (*Client, error) {
 	// Establish an SSH connection
 	sshConn, sshChans, sshReqs, err := ssh.NewClientConn(netConn, "", &ssh.ClientConfig{
 		HostKeyCallback: func(hostname string, remote net.Addr, key ssh.PublicKey) error {
@@ -49,21 +58,29 @@ func New(netConn net.Conn, user string, password string, options Options) (*Exec
 		},
 	})
 	if err != nil {
+		_ = netConn.Close()
+
 		return nil, fmt.Errorf("failed to create an SSH connection: %w", err)
 	}
 
-	sshClient := ssh.NewClient(sshConn, sshChans, sshReqs)
+	return &Client{
+		netConn:   netConn,
+		sshClient: ssh.NewClient(sshConn, sshChans, sshReqs),
+	}, nil
+}
+
+func (client *Client) NewExec(options Options) (*Exec, error) {
+	if client == nil || client.sshClient == nil {
+		return nil, errors.New("SSH client is not initialized")
+	}
 
 	// Create a new SSH session
-	sshSession, err := sshClient.NewSession()
+	sshSession, err := client.sshClient.NewSession()
 	if err != nil {
-		_ = sshClient.Close()
-
 		return nil, fmt.Errorf("failed to create an SSH session: %w", err)
 	}
 
 	exec := &Exec{
-		sshClient:  sshClient,
 		sshSession: sshSession,
 		tty:        options.TTY,
 	}
@@ -83,7 +100,6 @@ func New(netConn net.Conn, user string, password string, options Options) (*Exec
 			ssh.TerminalModes{},
 		); err != nil {
 			_ = sshSession.Close()
-			_ = sshClient.Close()
 
 			return nil, fmt.Errorf("failed to request PTY for the SSH session: %w", err)
 		}
@@ -92,7 +108,6 @@ func New(netConn net.Conn, user string, password string, options Options) (*Exec
 	exec.stdout, err = sshSession.StdoutPipe()
 	if err != nil {
 		_ = sshSession.Close()
-		_ = sshClient.Close()
 
 		return nil, fmt.Errorf("failed to create standard output pipe "+
 			"for the SSH session: %w", err)
@@ -101,7 +116,6 @@ func New(netConn net.Conn, user string, password string, options Options) (*Exec
 	exec.stderr, err = sshSession.StderrPipe()
 	if err != nil {
 		_ = sshSession.Close()
-		_ = sshClient.Close()
 
 		return nil, fmt.Errorf("failed to create standard error pipe "+
 			"for the SSH session: %w", err)
@@ -110,6 +124,45 @@ func New(netConn net.Conn, user string, password string, options Options) (*Exec
 	return exec, nil
 }
 
+func (client *Client) Close() error {
+	if client == nil {
+		return nil
+	}
+
+	client.closeOnce.Do(func() {
+		if client.sshClient != nil {
+			client.closeErr = client.sshClient.Close()
+			if client.closeErr == nil {
+				return
+			}
+		}
+
+		if client.netConn != nil {
+			client.closeErr = errors.Join(client.closeErr, client.netConn.Close())
+		}
+	})
+
+	return client.closeErr
+}
+
+func New(netConn net.Conn, user string, password string, options Options) (*Exec, error) {
+	client, err := NewClient(netConn, user, password)
+	if err != nil {
+		return nil, err
+	}
+
+	exec, err := client.NewExec(options)
+	if err != nil {
+		_ = client.Close()
+
+		return nil, err
+	}
+
+	exec.closeOwner = client.Close
+
+	return exec, nil
+}
+
 func (exec *Exec) Stdin() io.WriteCloser {
 	return exec.stdin
 }
@@ -285,11 +338,15 @@ func (exec *Exec) Close() error {
 		_ = exec.stdinReader.Close()
 	}
 
-	if err := exec.sshSession.Close(); err != nil {
-		_ = exec.sshClient.Close()
+	sessionErr := exec.sshSession.Close()
+	if exec.closeOwner != nil {
+		ownerErr := exec.closeOwner()
+		if sessionErr != nil {
+			return sessionErr
+		}
 
-		return err
+		return ownerErr
 	}
 
-	return exec.sshClient.Close()
+	return sessionErr
 }

internal/tests/exec_ssh_server_test.go

@@ -17,11 +17,23 @@ type execSSHServer struct {
 	config   *ssh.ServerConfig
 
 	rejectFirstConnections atomic.Int32
+	successfulConnections  atomic.Int32
+	acceptedSessions       atomic.Int32
+	releaseSessions        <-chan struct{}
+	done                   chan struct{}
 
 	wg sync.WaitGroup
 }
 
 func startExecSSHServer(t *testing.T, rejectFirstConnections int32) *execSSHServer {
+	return startExecSSHServerWithSessionGate(t, rejectFirstConnections, nil)
+}
+
+func startExecSSHServerWithSessionGate(
+	t *testing.T,
+	rejectFirstConnections int32,
+	releaseSessions <-chan struct{},
+) *execSSHServer {
 	t.Helper()
 
 	listener, err := net.Listen("tcp", "127.0.0.1:0")
@@ -34,7 +46,9 @@ func startExecSSHServer(t *testing.T, rejectFirstConnections int32) *execSSHServ
 	require.NoError(t, err)
 
 	server := &execSSHServer{
-		listener: listener,
+		listener:        listener,
+		releaseSessions: releaseSessions,
+		done:            make(chan struct{}),
 		config: &ssh.ServerConfig{
 			PasswordCallback: func(conn ssh.ConnMetadata, password []byte) (*ssh.Permissions, error) {
 				if conn.User() != "admin" || string(password) != "admin" {
@@ -52,6 +66,7 @@ func startExecSSHServer(t *testing.T, rejectFirstConnections int32) *execSSHServ
 	go server.run()
 
 	t.Cleanup(func() {
+		close(server.done)
 		require.NoError(t, server.listener.Close())
 		server.wg.Wait()
 	})
@@ -94,6 +109,7 @@ func (server *execSSHServer) serve(conn net.Conn) {
 	if err != nil {
 		return
 	}
+	server.successfulConnections.Add(1)
 	defer serverConn.Close()
 
 	go ssh.DiscardRequests(requests)
@@ -109,17 +125,18 @@ func (server *execSSHServer) serve(conn net.Conn) {
 		if err != nil {
 			continue
 		}
+		server.acceptedSessions.Add(1)
 
 		server.wg.Add(1)
 		go func() {
 			defer server.wg.Done()
 
-			serveExecSSHSession(channel, requests)
+			server.serveExecSSHSession(channel, requests)
 		}()
 	}
 }
 
-func serveExecSSHSession(channel ssh.Channel, requests <-chan *ssh.Request) {
+func (server *execSSHServer) serveExecSSHSession(channel ssh.Channel, requests <-chan *ssh.Request) {
 	defer channel.Close()
 
 	for request := range requests {
@@ -127,6 +144,13 @@ func serveExecSSHSession(channel ssh.Channel, requests <-chan *ssh.Request) {
 		case "exec":
 			_ = request.Reply(true, nil)
 			_, _ = io.WriteString(channel, "ok")
+			if server.releaseSessions != nil {
+				select {
+				case <-server.releaseSessions:
+				case <-server.done:
+					return
+				}
+			}
 			_, _ = channel.SendRequest("exit-status", false, ssh.Marshal(struct {
 				Status uint32
 			}{Status: 0}))

internal/tests/exec_test.go

@@ -7,6 +7,7 @@ import (
 	"fmt"
 	"net"
 	"sync"
+	"sync/atomic"
 	"testing"
 	"time"
 
@@ -228,10 +229,14 @@ func TestVMExecScript(t *testing.T) {
 }
 
 func TestVMExecManyConcurrentSessions(t *testing.T) {
-	sshServer := startExecSSHServer(t, 24)
+	releaseSessions := make(chan struct{})
+	sshServer := startExecSSHServerWithSessionGate(t, 0, releaseSessions)
+	var vmDialCalls atomic.Int32
 
 	devClient, vmName := prepareForSyntheticExec(t, dialer.DialFunc(
 		func(ctx context.Context, network string, addr string) (net.Conn, error) {
+			vmDialCalls.Add(1)
+
 			var netDialer net.Dialer
 
 			return netDialer.DialContext(ctx, network, sshServer.Addr())
@@ -301,12 +306,21 @@ func TestVMExecManyConcurrentSessions(t *testing.T) {
 	}
 
 	close(start)
+	require.Eventually(t, func() bool {
+		return sshServer.acceptedSessions.Load() == concurrentExecs
+	}, 30*time.Second, 10*time.Millisecond)
+	require.EqualValues(t, 1, vmDialCalls.Load())
+	require.EqualValues(t, 1, sshServer.successfulConnections.Load())
+	close(releaseSessions)
+
 	wg.Wait()
 	close(errCh)
 
 	for err := range errCh {
 		require.NoError(t, err)
 	}
+
+	require.EqualValues(t, concurrentExecs, sshServer.acceptedSessions.Load())
 }
 
 func TestVMExecSessionReconnectHistory(t *testing.T) {