🐛 fix: harden connection lifecycle and add session safeguards

src/app_test.go

@@ -2,7 +2,6 @@ package main
 
 import (
 	"bytes"
-	"fmt"
 	"image"
 	"image/jpeg"
 	"os"
@@ -61,8 +60,9 @@ func TestTSFInvalid(t *testing.T) {
 	}
 
 	// Valid container but fps <= 0.
-	if err := writeTSF(path, &FramesContainer{ColorFrames: [][]byte{{1}}, FPS: 0}); err != nil {
+	rawInvalidFPS := append(tsfHeader(0, 1), 1, 0, 0, 0, 1)
-		t.Fatalf("writeTSF: %v", err)
+	if err := os.WriteFile(path, rawInvalidFPS, 0o644); err != nil {
+		t.Fatalf("WriteFile: %v", err)
 	}
 	if _, err := loadTSF(path); err == nil {
 		t.Error("expected error for fps<=0")
@@ -177,7 +177,7 @@ func TestFrameToAscii(t *testing.T) {
 		Pix:    []byte{0, 0, 0, 255, 255, 255, 255, 255},
 		Stride: 8, Rect: image.Rect(0, 0, 2, 1),
 	}
-	out := []rune(frameToAscii(img, buildRampLUT(ramp, opts)))
+	out := []rune(string(frameToAscii(img, buildRampLUT(ramp, opts))))
 	if out[0] != ramp[0] {
 		t.Errorf("dark px = %q, want %q", out[0], ramp[0])
 	}
@@ -193,7 +193,7 @@ func TestFrameToAsciiInvert(t *testing.T) {
 		Pix:    []byte{255, 255, 255, 255},
 		Stride: 4, Rect: image.Rect(0, 0, 1, 1),
 	}
-	out := []rune(frameToAscii(img, buildRampLUT(ramp, opts)))
+	out := []rune(string(frameToAscii(img, buildRampLUT(ramp, opts))))
 	if out[0] != ramp[0] {
 		t.Errorf("inverted bright = %q, want %q", out[0], ramp[0])
 	}
@@ -215,7 +215,7 @@ func TestRenderConcurrentSameKey(t *testing.T) {
 	}, newRenderCache(1<<20))
 
 	var wg sync.WaitGroup
-	results := make([]string, 32)
+	results := make([][]byte, 32)
 	for i := range results {
 		wg.Add(1)
 		go func(i int) {
@@ -230,25 +230,26 @@ func TestRenderConcurrentSameKey(t *testing.T) {
 	}
 	wg.Wait()
 	for i, got := range results {
-		if got != results[0] {
+		if !bytes.Equal(got, results[0]) {
 			t.Fatalf("result %d differs from result 0", i)
 		}
 	}
 }
 
 func TestRenderCacheEvictsByBytes(t *testing.T) {
-	budget := 3 * entryCost("k", strings.Repeat("x", 1000))
+	key := func(index int) cacheKey { return cacheKey{index: index} }
+	budget := 3 * entryCost(key(0), bytes.Repeat([]byte("x"), 1000))
 	c := newRenderCache(budget)
 	for i := range 5 {
-		c.put(fmt.Sprintf("%d", i), strings.Repeat("x", 1000))
+		c.put(key(i), bytes.Repeat([]byte("x"), 1000))
 	}
-	if c.size > budget {
+	if c.size.Load() > budget {
-		t.Errorf("size %d exceeds budget %d", c.size, budget)
+		t.Errorf("size %d exceeds budget %d", c.size.Load(), budget)
 	}
-	if _, ok := c.get("0"); ok {
+	if _, ok := c.get(key(0)); ok {
 		t.Error("oldest entry should have been evicted")
 	}
-	if _, ok := c.get("4"); !ok {
+	if _, ok := c.get(key(4)); !ok {
 		t.Error("newest entry should be cached")
 	}
 }
@@ -258,49 +259,55 @@ func TestRenderCacheDisabled(t *testing.T) {
 	if c != nil {
 		t.Fatal("zero budget should disable the cache")
 	}
-	c.put("k", "v")
+	c.put(cacheKey{}, []byte("v"))
-	if _, ok := c.get("k"); ok {
+	if _, ok := c.get(cacheKey{}); ok {
 		t.Error("nil cache should never hit")
 	}
 }
 
 func TestRenderCacheRejectsOversizedEntry(t *testing.T) {
 	c := newRenderCache(256)
-	c.put("big", strings.Repeat("x", 10_000))
+	c.put(cacheKey{}, bytes.Repeat([]byte("x"), 10_000))
-	if _, ok := c.get("big"); ok {
+	if _, ok := c.get(cacheKey{}); ok {
 		t.Error("entry larger than budget should not be cached")
 	}
-	if c.size != 0 {
+	if c.size.Load() != 0 {
-		t.Errorf("size = %d, want 0", c.size)
+		t.Errorf("size = %d, want 0", c.size.Load())
 	}
 }
 
 func TestConnectionTracker(t *testing.T) {
 	tr := newConnectionTracker()
-	tr.increment("1.2.3.4")
+	if _, _, ok := tr.tryAcquire("1.2.3.4", 2, 100); !ok {
-	tr.increment("1.2.3.4")
+		t.Fatal("first acquire failed")
-	if !tr.hasReachedLimits("1.2.3.4", 2, 100) {
-		t.Error("expected per-ip limit reached")
 	}
-	tr.decrement("1.2.3.4")
+	if _, _, ok := tr.tryAcquire("1.2.3.4", 2, 100); !ok {
-	tr.decrement("1.2.3.4")
+		t.Fatal("second acquire failed")
+	}
+	if _, _, ok := tr.tryAcquire("1.2.3.4", 2, 100); ok {
+		t.Error("expected per-ip limit rejection")
+	}
+	tr.release("1.2.3.4")
+	tr.release("1.2.3.4")
 	if tr.totalCount() != 0 {
 		t.Errorf("total = %d", tr.totalCount())
 	}
-	if tr.hasReachedLimits("1.2.3.4", 2, 100) {
+	if _, _, ok := tr.tryAcquire("1.2.3.4", 2, 100); !ok {
-		t.Error("should be cleared")
+		t.Error("limit should be cleared")
 	}
 }
 
-func TestClampDimension(t *testing.T) {
+func TestClampTermSize(t *testing.T) {
-	if clampDimension(0, 100) != 1 {
+	w, h := clampTermSize(1000, 500, 512, 65536, 4)
-		t.Error("floor")
+	if w < 1 || h < 1 || w > 512 || h > 512 || w*h > 65536 {
+		t.Fatalf("clamped size = %dx%d", w, h)
 	}
-	if clampDimension(500, 100) != 100 {
+	if w%4 != 0 || h%4 != 0 {
-		t.Error("ceil")
+		t.Fatalf("size is not quantized: %dx%d", w, h)
 	}
-	if clampDimension(50, 100) != 50 {
+	w, h = clampTermSize(3, 2, 100, 100, 4)
-		t.Error("passthrough")
+	if w != 3 || h != 2 {
+		t.Fatalf("small size = %dx%d", w, h)
 	}
 }
 

src/config.go

@@ -28,6 +28,8 @@ type Config struct {
 	MaxAuthAttempts     int
 	HandshakeTimeout    time.Duration
 	MaxDimension        int
+	MaxTerminalCells    int
+	SessionTimeout      time.Duration
 	RenderCacheMB       int
 	BrightnessThreshold int
 	Charset             string
@@ -126,6 +128,8 @@ func loadConfig() Config {
 		MaxAuthAttempts:     envInt("MAX_AUTH_ATTEMPTS", 6, 1, maxInt),
 		HandshakeTimeout:    envDurationMs("HANDSHAKE_TIMEOUT", 10*time.Second),
 		MaxDimension:        envInt("MAX_DIMENSION", 512, 1, 4096),
+		MaxTerminalCells:    envInt("MAX_TERMINAL_CELLS", 500*512, 1, maxInt),
+		SessionTimeout:      envDurationMs("SESSION_TIMEOUT", 10*time.Minute),
 		RenderCacheMB:       envInt("RENDER_CACHE_MB", 256, 0, maxInt),
 		BrightnessThreshold: envInt("BRIGHTNESS_THRESHOLD", 40, 0, 100),
 		Charset:             envString("CHARSET", "detailed"),

src/main.go

@@ -83,6 +83,8 @@ func generateFrames(framesDir, videoArg string, resolution int) {
 	}
 }
 
+const frameDataWarnBytes = 2 << 30
+
 func loadAllFrames(framesDir string) []*FramesContainer {
 	entries, err := os.ReadDir(framesDir)
 	var files []string
@@ -101,6 +103,22 @@ func loadAllFrames(framesDir string) []*FramesContainer {
 			framesDir,
 		))
 	}
+	var totalBytes int64
+	for _, file := range files {
+		info, err := os.Stat(filepath.Join(framesDir, file))
+		if err != nil {
+			fail(err.Error())
+		}
+		totalBytes += info.Size()
+	}
+	if totalBytes > frameDataWarnBytes {
+		logWarn(fmt.Sprintf(
+			"Frame data is %.1f MB of mapped memory; make sure the container memory limit leaves headroom",
+			float64(totalBytes)/(1<<20),
+		))
+	} else {
+		logInfo(fmt.Sprintf("Frame data: %.1f MB", float64(totalBytes)/(1<<20)))
+	}
 
 	concurrency := min(len(files), max(1, min(runtime.NumCPU(), 4)))
 
@@ -226,14 +244,16 @@ func main() {
 		GoodbyeText:   goodbyeText,
 		VideoSets:     videoSets,
 	})
+	defer server.Close()
 
 	sigCh := make(chan os.Signal, 1)
 	signal.Notify(sigCh, syscall.SIGINT, syscall.SIGTERM)
 	go func() {
 		sig := <-sigCh
 		logInfo(fmt.Sprintf("Received %s, shutting down...", sig))
+		forceExit := time.AfterFunc(5*time.Second, func() { os.Exit(0) })
 		server.Close()
-		time.AfterFunc(5*time.Second, func() { os.Exit(0) })
+		forceExit.Stop()
 	}()
 
 	if err := server.Listen(config.Host, config.Port); err != nil {

src/performance_safeguards_test.go

@@ -0,0 +1,127 @@
+package main
+
+import (
+	"bytes"
+	"image"
+	"sync"
+	"testing"
+
+	"golang.org/x/crypto/ssh"
+)
+
+func TestConnectionTrackerConcurrentLimit(t *testing.T) {
+	tracker := newConnectionTracker()
+	start := make(chan struct{})
+	var wg sync.WaitGroup
+	var mu sync.Mutex
+	accepted := make(map[string]int)
+	for i := range 100 {
+		wg.Add(1)
+		go func(i int) {
+			defer wg.Done()
+			<-start
+			ip := string(rune('a' + i%10))
+			if _, _, ok := tracker.tryAcquire(ip, 3, 7); ok {
+				mu.Lock()
+				accepted[ip]++
+				mu.Unlock()
+			}
+		}(i)
+	}
+	close(start)
+	wg.Wait()
+
+	total := 0
+	for ip, count := range accepted {
+		total += count
+		if count > 3 {
+			t.Fatalf("IP %q acquired %d slots", ip, count)
+		}
+	}
+	if total != 7 || tracker.totalCount() != 7 {
+		t.Fatalf("accepted=%d tracked=%d, want 7", total, tracker.totalCount())
+	}
+	for ip, count := range accepted {
+		for range count {
+			tracker.release(ip)
+		}
+	}
+}
+
+func TestSessionTrackerLimits(t *testing.T) {
+	tracker := newSessionTracker()
+	first := &ssh.ServerConn{}
+	second := &ssh.ServerConn{}
+	if !tracker.tryAcquire(first, 1, 2) {
+		t.Fatal("first session rejected")
+	}
+	if tracker.tryAcquire(first, 1, 2) {
+		t.Fatal("per-connection limit was not enforced")
+	}
+	if !tracker.tryAcquire(second, 1, 2) {
+		t.Fatal("second connection session rejected")
+	}
+	if tracker.tryAcquire(&ssh.ServerConn{}, 1, 2) {
+		t.Fatal("global session limit was not enforced")
+	}
+	tracker.release(first)
+	if !tracker.tryAcquire(&ssh.ServerConn{}, 1, 2) {
+		t.Fatal("released slot was not reusable")
+	}
+}
+
+func TestTermSizeDebouncesResize(t *testing.T) {
+	size := &termSize{}
+	size.set(80, 24, 512, 500*512, true)
+	size.set(200, 100, 512, 500*512, false)
+	if w, h := size.get(); w != 80 || h != 24 {
+		t.Fatalf("debounced size = %dx%d", w, h)
+	}
+	size.set(200, 100, 512, 500*512, true)
+	if w, h := size.get(); w != 200 || h != 100 {
+		t.Fatalf("forced size = %dx%d", w, h)
+	}
+}
+
+func TestAnsi256CoalescesQuantizedColors(t *testing.T) {
+	img := &image.RGBA{
+		Pix:    []byte{96, 96, 96, 255, 100, 100, 100, 255},
+		Stride: 8,
+		Rect:   image.Rect(0, 0, 2, 1),
+	}
+	output := frameToAnsi(img, buildRampLUT([]rune(" .#"), asciiOptions{}), colorTier256)
+	if count := bytes.Count(output, []byte("\x1b[38;5;")); count != 1 {
+		t.Fatalf("color escape count = %d, want 1: %q", count, output)
+	}
+}
+
+func TestAnsiDoesNotResetEachRow(t *testing.T) {
+	img := &image.RGBA{
+		Pix:    []byte{100, 100, 100, 255, 100, 100, 100, 255},
+		Stride: 4,
+		Rect:   image.Rect(0, 0, 1, 2),
+	}
+	output := frameToAnsi(img, buildRampLUT([]rune(" .#"), asciiOptions{}), colorTierTrueColor)
+	if count := bytes.Count(output, []byte(ansiReset)); count != 1 {
+		t.Fatalf("reset count = %d, want 1: %q", count, output)
+	}
+}
+
+func TestRenderCacheAccountsRetainedCapacity(t *testing.T) {
+	cache := newRenderCache(512)
+	value := make([]byte, 1, 4096)
+	cache.put(cacheKey{}, value)
+	if _, ok := cache.get(cacheKey{}); ok {
+		t.Fatal("cache accepted an entry whose backing allocation exceeds its budget")
+	}
+	if cache.stats().Rejections != 1 {
+		t.Fatalf("rejections = %d, want 1", cache.stats().Rejections)
+	}
+}
+
+func TestSanitizeNStopsAtLimit(t *testing.T) {
+	input := "ab\x00cdefghijklmnopqrstuvwxyz"
+	if got := sanitizeN(input, 4); got != "ab�c…" {
+		t.Fatalf("sanitizeN = %q", got)
+	}
+}

src/server.go

@@ -4,6 +4,8 @@ import (
 	"encoding/binary"
 	"errors"
 	"fmt"
+	"io"
+	"math"
 	"math/rand"
 	"net"
 	"strings"
@@ -14,14 +16,84 @@ import (
 )
 
 const (
-	clearScreen = "\x1b[2J\x1b[0f"
+	clearScreen         = "\x1b[2J\x1b[0f"
-	hideCursor  = "\x1b[?25l"
+	hideCursor          = "\x1b[?25l"
-	showCursor  = "\x1b[?25h"
+	showCursor          = "\x1b[?25h"
-	syncStart   = "\x1b[?2026h"
+	syncStart           = "\x1b[?2026h"
-	syncEnd     = "\x1b[?2026l"
+	syncEnd             = "\x1b[?2026l"
-	homeCursor  = "\x1b[H"
+	homeCursor          = "\x1b[H"
+	maxSessionsPerConn  = 1
+	terminalSizeQuantum = 4
+	resizeDebounce      = 200 * time.Millisecond
+	outputStallTimeout  = 15 * time.Second
 )
 
+var errOutputStalled = errors.New("SSH output stalled")
+
+func writePartsWithTimeout(
+	conn *ssh.ServerConn,
+	channel ssh.Channel,
+	timeout time.Duration,
+	parts ...string,
+) error {
+	write := func() error {
+		for _, part := range parts {
+			if _, err := io.WriteString(channel, part); err != nil {
+				return err
+			}
+		}
+		return nil
+	}
+	if timeout <= 0 {
+		return write()
+	}
+
+	fired := make(chan struct{})
+	timer := time.AfterFunc(timeout, func() {
+		_ = conn.Close()
+		close(fired)
+	})
+	err := write()
+	if timer.Stop() {
+		return err
+	}
+	<-fired
+	return errOutputStalled
+}
+
+func writeFrameWithTimeout(
+	conn *ssh.ServerConn,
+	channel ssh.Channel,
+	timeout time.Duration,
+	prefix string,
+	frame []byte,
+) error {
+	write := func() error {
+		if _, err := io.WriteString(channel, syncStart+prefix); err != nil {
+			return err
+		}
+		if _, err := channel.Write(frame); err != nil {
+			return err
+		}
+		_, err := io.WriteString(channel, syncEnd)
+		return err
+	}
+	if timeout <= 0 {
+		return write()
+	}
+	fired := make(chan struct{})
+	timer := time.AfterFunc(timeout, func() {
+		_ = conn.Close()
+		close(fired)
+	})
+	err := write()
+	if timer.Stop() {
+		return err
+	}
+	<-fired
+	return errOutputStalled
+}
+
 type ConnectionTracker struct {
 	mu     sync.Mutex
 	counts map[string]int
@@ -32,15 +104,18 @@ func newConnectionTracker() *ConnectionTracker {
 	return &ConnectionTracker{counts: make(map[string]int)}
 }
 
-func (t *ConnectionTracker) increment(ip string) int {
+func (t *ConnectionTracker) tryAcquire(ip string, maxPerIP, maxTotal int) (int, int, bool) {
 	t.mu.Lock()
 	defer t.mu.Unlock()
+	if t.total >= maxTotal || t.counts[ip] >= maxPerIP {
+		return t.counts[ip], t.total, false
+	}
 	t.counts[ip]++
 	t.total++
-	return t.counts[ip]
+	return t.counts[ip], t.total, true
 }
 
-func (t *ConnectionTracker) decrement(ip string) {
+func (t *ConnectionTracker) release(ip string) {
 	t.mu.Lock()
 	defer t.mu.Unlock()
 	if _, ok := t.counts[ip]; !ok {
@@ -61,10 +136,40 @@ func (t *ConnectionTracker) totalCount() int {
 	return t.total
 }
 
-func (t *ConnectionTracker) hasReachedLimits(ip string, maxPerIP, maxTotal int) bool {
+type SessionTracker struct {
+	mu      sync.Mutex
+	perConn map[*ssh.ServerConn]int
+	total   int
+}
+
+func newSessionTracker() *SessionTracker {
+	return &SessionTracker{perConn: make(map[*ssh.ServerConn]int)}
+}
+
+func (t *SessionTracker) tryAcquire(conn *ssh.ServerConn, maxPerConn, maxTotal int) bool {
 	t.mu.Lock()
 	defer t.mu.Unlock()
-	return t.total >= maxTotal || t.counts[ip] >= maxPerIP
+	if t.total >= maxTotal || t.perConn[conn] >= maxPerConn {
+		return false
+	}
+	t.perConn[conn]++
+	t.total++
+	return true
+}
+
+func (t *SessionTracker) release(conn *ssh.ServerConn) {
+	t.mu.Lock()
+	defer t.mu.Unlock()
+	count := t.perConn[conn]
+	if count <= 0 {
+		return
+	}
+	if count == 1 {
+		delete(t.perConn, conn)
+	} else {
+		t.perConn[conn] = count - 1
+	}
+	t.total--
 }
 
 type frameSet struct {
@@ -76,10 +181,16 @@ type Server struct {
 	config    Config
 	sshConfig *ssh.ServerConfig
 	sets      []frameSet
+	cache     *renderCache
 	tracker   *ConnectionTracker
+	sessions  *SessionTracker
 	fakeLogin *string
 	goodbye   *string
+	mu        sync.Mutex
 	listener  net.Listener
+	conns     map[net.Conn]struct{}
+	connWG    sync.WaitGroup
+	closing   bool
 	closeOnce sync.Once
 }
 
@@ -92,14 +203,25 @@ type ServerDeps struct {
 	VideoSets     []*FramesContainer
 }
 
-func clampDimension(value, max int) int {
+func clampTermSize(cols, rows, maxDimension, maxCells, quantum int) (int, int) {
-	if value < 1 {
+	cols = max(cols, 1)
-		return 1
+	rows = max(rows, 1)
+	scale := min(1.0, float64(maxDimension)/float64(cols), float64(maxDimension)/float64(rows))
+	area := float64(cols) * float64(rows)
+	if area*scale*scale > float64(maxCells) {
+		scale = min(scale, math.Sqrt(float64(maxCells)/area))
 	}
-	if value > max {
+	cols = max(1, int(math.Floor(float64(cols)*scale)))
-		return max
+	rows = max(1, int(math.Floor(float64(rows)*scale)))
+	if quantum > 1 {
+		if cols >= quantum {
+			cols -= cols % quantum
+		}
+		if rows >= quantum {
+			rows -= rows % quantum
+		}
 	}
-	return value
+	return cols, rows
 }
 
 func createServer(deps ServerDeps) *Server {
@@ -155,9 +277,12 @@ func createServer(deps ServerDeps) *Server {
 		config:    config,
 		sshConfig: sshConfig,
 		sets:      sets,
+		cache:     cache,
 		tracker:   newConnectionTracker(),
+		sessions:  newSessionTracker(),
 		fakeLogin: deps.FakeLoginText,
 		goodbye:   deps.GoodbyeText,
+		conns:     make(map[net.Conn]struct{}),
 	}
 }
 
@@ -174,7 +299,14 @@ func (s *Server) Listen(host string, port int) error {
 	if err != nil {
 		return err
 	}
+	s.mu.Lock()
+	if s.closing {
+		s.mu.Unlock()
+		_ = listener.Close()
+		return nil
+	}
 	s.listener = listener
+	s.mu.Unlock()
 	logInfo(fmt.Sprintf("TrollSSH listening on %s:%d", host, port))
 	for {
 		conn, err := listener.Accept()
@@ -184,29 +316,68 @@ func (s *Server) Listen(host string, port int) error {
 			}
 			return err
 		}
-		go s.handleConn(conn)
+		ip := hostOnly(conn.RemoteAddr().String())
+		activeForIP, total, ok := s.tracker.tryAcquire(ip, s.config.MaxConnections, s.config.MaxTotalConnections)
+		if !ok {
+			_ = conn.Close()
+			logWarn("Connection rejected (limit reached) from", ip)
+			continue
+		}
+		s.mu.Lock()
+		if s.closing {
+			s.mu.Unlock()
+			s.tracker.release(ip)
+			_ = conn.Close()
+			continue
+		}
+		s.conns[conn] = struct{}{}
+		s.connWG.Add(1)
+		s.mu.Unlock()
+		go s.handleConn(conn, ip, activeForIP, total)
 	}
 }
 
 func (s *Server) Close() {
 	s.closeOnce.Do(func() {
-		if s.listener != nil {
+		s.mu.Lock()
-			_ = s.listener.Close()
+		s.closing = true
+		listener := s.listener
+		conns := make([]net.Conn, 0, len(s.conns))
+		for conn := range s.conns {
+			conns = append(conns, conn)
+		}
+		s.mu.Unlock()
+		if listener != nil {
+			_ = listener.Close()
+		}
+		for _, conn := range conns {
+			_ = conn.Close()
+		}
+		s.connWG.Wait()
+		stats := s.cache.stats()
+		if stats.Hits+stats.Misses > 0 {
+			logInfo(fmt.Sprintf(
+				"Render cache: size=%.1fMB hits=%d misses=%d evictions=%d rejected=%d renders=%d render_time=%s",
+				float64(stats.SizeBytes)/(1<<20), stats.Hits, stats.Misses, stats.Evictions,
+				stats.Rejections, stats.Renders, stats.RenderTime,
+			))
+		}
+		for _, set := range s.sets {
+			if err := set.data.Close(); err != nil {
+				logWarn("Failed to release frame set", set.data.Name, sanitize(err.Error()))
+			}
 		}
 	})
 }
 
-func (s *Server) handleConn(conn net.Conn) {
+func (s *Server) handleConn(conn net.Conn, ip string, activeForIP, total int) {
-	ip := hostOnly(conn.RemoteAddr().String())
+	defer func() {
-
+		s.tracker.release(ip)
-	if s.tracker.hasReachedLimits(ip, s.config.MaxConnections, s.config.MaxTotalConnections) {
+		s.mu.Lock()
-		_ = conn.Close()
+		delete(s.conns, conn)
-		logWarn("Connection rejected (limit reached) from", ip)
+		s.mu.Unlock()
-		return
+		s.connWG.Done()
-	}
+	}()
-
-	activeForIP := s.tracker.increment(ip)
-	defer s.tracker.decrement(ip)
 
 	if s.config.HandshakeTimeout > 0 {
 		_ = conn.SetDeadline(time.Now().Add(s.config.HandshakeTimeout))
@@ -229,35 +400,58 @@ func (s *Server) handleConn(conn net.Conn) {
 	setIndex := rand.Intn(len(s.sets))
 	logInfo(fmt.Sprintf(
 		"New connection from %s (ip=%d, total=%d) -> playing %q",
-		ip, activeForIP, s.tracker.totalCount(), s.sets[setIndex].data.Name,
+		ip, activeForIP, total, s.sets[setIndex].data.Name,
 	))
 
 	go ssh.DiscardRequests(reqs)
 
+	var sessionWG sync.WaitGroup
 	for newChannel := range chans {
 		if newChannel.ChannelType() != "session" {
 			_ = newChannel.Reject(ssh.UnknownChannelType, "unknown channel type")
 			continue
 		}
-		channel, requests, err := newChannel.Accept()
+		if !s.sessions.tryAcquire(sshConn, maxSessionsPerConn, s.config.MaxTotalConnections) {
-		if err != nil {
+			_ = newChannel.Reject(ssh.ResourceShortage, "session limit reached")
 			continue
 		}
-		go s.handleSession(sshConn, channel, requests, ip, setIndex)
+		channel, requests, err := newChannel.Accept()
+		if err != nil {
+			s.sessions.release(sshConn)
+			continue
+		}
+		sessionWG.Add(1)
+		go func() {
+			defer sessionWG.Done()
+			defer s.sessions.release(sshConn)
+			var timer *time.Timer
+			if s.config.SessionTimeout > 0 {
+				timer = time.AfterFunc(s.config.SessionTimeout, func() { _ = sshConn.Close() })
+				defer timer.Stop()
+			}
+			s.handleSession(sshConn, channel, requests, ip, setIndex)
+		}()
 	}
+	_ = sshConn.Close()
+	sessionWG.Wait()
 	logInfo("Client closed connection from", ip)
 }
 
 type termSize struct {
-	mu     sync.Mutex
+	mu      sync.Mutex
-	width  int
+	width   int
-	height int
+	height  int
+	updated time.Time
 }
 
-func (t *termSize) set(w, h, maxDim int) {
+func (t *termSize) set(w, h, maxDimension, maxCells int, force bool) {
 	t.mu.Lock()
-	t.width = clampDimension(w, maxDim)
+	if !force && time.Since(t.updated) < resizeDebounce {
-	t.height = clampDimension(h, maxDim)
+		t.mu.Unlock()
+		return
+	}
+	t.width, t.height = clampTermSize(w, h, maxDimension, maxCells, terminalSizeQuantum)
+	t.updated = time.Now()
 	t.mu.Unlock()
 }
 
@@ -304,20 +498,22 @@ func (s *Server) handleSession(
 	ip string,
 	initialSetIndex int,
 ) {
+	defer func() { _ = channel.Close() }()
 	size := &termSize{}
-	size.set(80, 24, s.config.MaxDimension)
+	size.set(80, 24, s.config.MaxDimension, s.config.MaxTerminalCells, true)
 	tier := colorTierTrueColor
 	if s.config.ForceGrayscale {
 		tier = colorTierNone
 	}
 
 	started := false
+	var playDone chan struct{}
 	for req := range requests {
 		switch req.Type {
 		case "pty-req":
 			logDebug("Opening pty for session", ip)
 			if cols, rows, ok := parseDims(req.Payload); ok {
-				size.set(cols, rows, s.config.MaxDimension)
+				size.set(cols, rows, s.config.MaxDimension, s.config.MaxTerminalCells, true)
 			}
 			if term, ok := parsePtyTerm(req.Payload); ok {
 				tier = detectColorTier(term)
@@ -331,7 +527,7 @@ func (s *Server) handleSession(
 			if len(req.Payload) >= 8 {
 				cols := int(binary.BigEndian.Uint32(req.Payload))
 				rows := int(binary.BigEndian.Uint32(req.Payload[4:]))
-				size.set(cols, rows, s.config.MaxDimension)
+				size.set(cols, rows, s.config.MaxDimension, s.config.MaxTerminalCells, false)
 			}
 			if req.WantReply {
 				_ = req.Reply(true, nil)
@@ -348,14 +544,24 @@ func (s *Server) handleSession(
 			_ = req.Reply(true, nil)
 			if !started {
 				started = true
-				go s.playVideo(sshConn, channel, size, ip, initialSetIndex, false, tier)
+				playDone = make(chan struct{})
+				playTier := tier
+				go func(tier colorTier) {
+					defer close(playDone)
+					s.playVideo(sshConn, channel, size, ip, initialSetIndex, false, tier)
+				}(playTier)
 			}
 		case "shell":
 			logDebug("Opening shell for session", ip)
 			_ = req.Reply(true, nil)
 			if !started {
 				started = true
-				go s.playVideo(sshConn, channel, size, ip, initialSetIndex, false, tier)
+				playDone = make(chan struct{})
+				playTier := tier
+				go func(tier colorTier) {
+					defer close(playDone)
+					s.playVideo(sshConn, channel, size, ip, initialSetIndex, false, tier)
+				}(playTier)
 			}
 		default:
 			if req.WantReply {
@@ -363,6 +569,10 @@ func (s *Server) handleSession(
 			}
 		}
 	}
+	_ = channel.Close()
+	if playDone != nil {
+		<-playDone
+	}
 }
 
 func (s *Server) pickNextSetIndex(exclude int) int {
@@ -391,11 +601,16 @@ func (s *Server) playVideo(
 	w, h := size.get()
 	logDebug(fmt.Sprintf("Terminal size %dx%d for %s", w, h, ip))
 
-	defer func() { _, _ = channel.Write([]byte(showCursor)) }()
+	defer func() {
+		_ = writePartsWithTimeout(sshConn, channel, outputStallTimeout, showCursor)
+	}()
 
 	if s.fakeLogin != nil {
-		_, _ = channel.Write([]byte(clearScreen))
+		if err := writePartsWithTimeout(
-		_, _ = channel.Write([]byte(*s.fakeLogin))
+			sshConn, channel, outputStallTimeout, clearScreen, *s.fakeLogin,
+		); err != nil {
+			return
+		}
 	}
 
 	done := make(chan struct{})
@@ -439,13 +654,19 @@ func (s *Server) playVideo(
 		}
 	}()
 
+	loginTimer := time.NewTimer(config.LoginDelay)
 	select {
-	case <-time.After(config.LoginDelay):
+	case <-loginTimer.C:
 	case <-done:
+		if !loginTimer.Stop() {
+			<-loginTimer.C
+		}
 		return
 	}
 
-	_, _ = channel.Write([]byte(hideCursor))
+	if err := writePartsWithTimeout(sshConn, channel, outputStallTimeout, hideCursor); err != nil {
+		return
+	}
 
 	frameInterval := func() time.Duration {
 		return time.Duration(float64(time.Second) / current.data.FPS)
@@ -457,8 +678,6 @@ func (s *Server) playVideo(
 	currentFrame := 0
 	loopCount := 0
 	lastW, lastH := 0, 0
-	var writeBuf []byte
-
 	for {
 		select {
 		case <-done:
@@ -489,11 +708,9 @@ func (s *Server) playVideo(
 				prefix = clearScreen
 				lastW, lastH = w, h
 			}
-			writeBuf = append(writeBuf[:0], syncStart...)
+			if err := writeFrameWithTimeout(
-			writeBuf = append(writeBuf, prefix...)
+				sshConn, channel, outputStallTimeout, prefix, ascii,
-			writeBuf = append(writeBuf, ascii...)
+			); err != nil {
-			writeBuf = append(writeBuf, syncEnd...)
-			if _, err := channel.Write(writeBuf); err != nil {
 				closeSession()
 				return
 			}
@@ -506,11 +723,27 @@ func (s *Server) playVideo(
 			currentFrame = 0
 			loopCount++
 			if config.MaxLoop > 0 && loopCount >= config.MaxLoop {
-				_, _ = channel.Write([]byte(showCursor + clearScreen))
+				if err := writePartsWithTimeout(
+					sshConn, channel, outputStallTimeout, showCursor, clearScreen,
+				); err != nil {
+					return
+				}
 				if s.goodbye != nil {
-					_, _ = channel.Write([]byte(*s.goodbye))
+					if err := writePartsWithTimeout(
+						sshConn, channel, outputStallTimeout, *s.goodbye,
+					); err != nil {
+						return
+					}
+				}
+				closeTimer := time.NewTimer(time.Second)
+				select {
+				case <-closeTimer.C:
+				case <-done:
+					if !closeTimer.Stop() {
+						<-closeTimer.C
+					}
+					return
 				}
-				time.Sleep(1 * time.Second)
 				logInfo("Playback finished, closing session", ip)
 				_ = channel.Close()
 				_ = sshConn.Close()