📦 build: add memory limit and session safeguard settings

.env.example

@@ -1,12 +1,7 @@
 HOST=0.0.0.0
 PORT=22
 
-# Generation settings
+# Playback settings
-# Stored frame resolution in pixels. Higher = sharper but bigger .tsf files.
-# regenerate your frames after changing it.
-FRAME_RESOLUTION=512
-
-# Playback settings.
 # Playthroughs before the session is closed (0, unlimited).
 MAX_LOOP=5
 # Whether to keep looping the same frame set or pick a random one after each
@@ -30,7 +25,17 @@ INVERT=false
 FORCE_GRAYSCALE=false
 
 # Max rendered width/height in characters.
-MAX_DIMENSION=1080
+MAX_DIMENSION=512
+# Max rendered area (columns x rows); larger terminals are scaled down.
+MAX_TERMINAL_CELLS=256000
+
+# Memory budget in MB for the rendered-frame cache (0 disables caching).
+RENDER_CACHE_MB=256
+
+# Go soft memory limit; set below your container limit to avoid OOM kills.
+GOMEMLIMIT=1GiB
+# Docker Compose container memory limit. Leave headroom for mapped frame files.
+MEMORY_LIMIT=2g
 
 # Connection limits. New connections over a limit are dropped immediately.
 # Max simultaneous connections from a single client IP.
@@ -42,6 +47,8 @@ MAX_TOTAL_CONNECTIONS=1000
 MAX_AUTH_ATTEMPTS=3
 # SSH handshake deadline in ms (0 to disable).
 HANDSHAKE_TIMEOUT=30000
+# Maximum session lifetime in ms (0 to disable).
+SESSION_TIMEOUT=600000
 
 # Log attempted usernames/passwords.
 LOG_CREDENTIALS=true

README.md

@@ -25,7 +25,7 @@ Generate a frame set from a video through container image
 
 ```sh
 docker run --rm -v ./video.mp4:/home/app/video.mp4 -v ./frames:/home/app/frames \
-    ghcr.io/yuzuzensai/trollssh:latest trollssh --generate --video video.mp4
+    ghcr.io/yuzuzensai/trollssh:v1.0.1 trollssh --generate --video video.mp4 --resolution 512
 ```
 
 This writes `frames/<name>.tsf`, a simple container of color JPEG frames plus
@@ -51,13 +51,20 @@ ssh anyone@localhost
 
 ## Configuration
 
-Configuration is via environment variables, loaded from a `.env` file if one
+Server configuration is via environment variables, loaded from a `.env` file
-exists (see [`.env.example`](.env.example) for the full annotated list).
+if one exists (see [`.env.example`](.env.example) for the full annotated
-Durations are in milliseconds.
+list). Durations are in milliseconds.
 
 Host keys (`data/id_rsa`, `data/id_ed25519`) are generated on first run and
 reused afterwards.
 
+Frame generation is configured with flags:
+
+| Flag                 | Default | Description                                        |
+| -------------------- | ------- | -------------------------------------------------- |
+| `--generate`, `-g`   |         | Generate a `.tsf` frame set instead of serving     |
+| `--video`, `-v`      |         | Source video path                                  |
+| `--resolution`, `-r` | `512`   | Stored frame max dimension in pixels. Higher = sharper but bigger `.tsf` files and slower rendering |
 
 ## Customization
 
@@ -75,7 +82,7 @@ Requirements: Go 1.25+ and `ffmpeg` / `ffprobe` on `PATH` (only for
 `--generate`).
 
 ```sh
-go run ./src --generate --video video.mp4
+go run ./src --generate --video video.mp4 --resolution 512
 go run ./src
 ```
 

docker-compose.yaml

@@ -1,8 +1,9 @@
 services:
     trollssh:
-        image: ghcr.io/yuzuzensai/trollssh:latest
+        image: ghcr.io/yuzuzensai/trollssh:v1.0.1
         container_name: trollssh
         restart: unless-stopped
+        mem_limit: ${MEMORY_LIMIT:-2g}
         ports:
             - "22:22"
         env_file:

src/app_test.go

@@ -1,9 +1,13 @@
 package main
 
 import (
+	"bytes"
+	"image"
+	"image/jpeg"
 	"os"
 	"path/filepath"
 	"strings"
+	"sync"
 	"testing"
 	"time"
 )
@@ -56,8 +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")
@@ -168,7 +173,11 @@ func TestFrameToAscii(t *testing.T) {
 	// Below threshold -> first ramp char; full brightness -> last.
 	opts := asciiOptions{brightnessThreshold: 40, charset: "standard"}
 	ramp := []rune(resolveCharset("standard"))
-	out := []rune(frameToAscii([]byte{0, 255}, opts))
+	img := &image.RGBA{
+		Pix:    []byte{0, 0, 0, 255, 255, 255, 255, 255},
+		Stride: 8, Rect: image.Rect(0, 0, 2, 1),
+	}
+	out := []rune(string(frameToAscii(img, buildRampLUT(ramp, opts))))
 	if out[0] != ramp[0] {
 		t.Errorf("dark px = %q, want %q", out[0], ramp[0])
 	}
@@ -180,38 +189,125 @@ func TestFrameToAscii(t *testing.T) {
 func TestFrameToAsciiInvert(t *testing.T) {
 	opts := asciiOptions{brightnessThreshold: 40, charset: "standard", invert: true}
 	ramp := []rune(resolveCharset("standard"))
-	out := []rune(frameToAscii([]byte{255}, opts))
+	img := &image.RGBA{
+		Pix:    []byte{255, 255, 255, 255},
+		Stride: 4, Rect: image.Rect(0, 0, 1, 1),
+	}
+	out := []rune(string(frameToAscii(img, buildRampLUT(ramp, opts))))
 	if out[0] != ramp[0] {
 		t.Errorf("inverted bright = %q, want %q", out[0], ramp[0])
 	}
 }
 
-func TestConnectionTracker(t *testing.T) {
+func TestRenderConcurrentSameKey(t *testing.T) {
-	tr := newConnectionTracker()
+	var jpegBuf bytes.Buffer
-	tr.increment("1.2.3.4")
+	src := image.NewRGBA(image.Rect(0, 0, 16, 16))
-	tr.increment("1.2.3.4")
+	for i := range src.Pix {
-	if !tr.hasReachedLimits("1.2.3.4", 2, 100) {
+		src.Pix[i] = byte(i * 7)
-		t.Error("expected per-ip limit reached")
 	}
-	tr.decrement("1.2.3.4")
+	if err := jpeg.Encode(&jpegBuf, src, nil); err != nil {
-	tr.decrement("1.2.3.4")
+		t.Fatal(err)
-	if tr.totalCount() != 0 {
-		t.Errorf("total = %d", tr.totalCount())
 	}
-	if tr.hasReachedLimits("1.2.3.4", 2, 100) {
+
-		t.Error("should be cleared")
+	r := newFrameRenderer(0, [][]byte{jpegBuf.Bytes()}, asciiOptions{
+		brightnessThreshold: 40,
+		charset:             "standard",
+	}, newRenderCache(1<<20))
+
+	var wg sync.WaitGroup
+	results := make([][]byte, 32)
+	for i := range results {
+		wg.Add(1)
+		go func(i int) {
+			defer wg.Done()
+			ascii, err := r.render(0, 20, 10, false, colorTierTrueColor)
+			if err != nil {
+				t.Error(err)
+				return
+			}
+			results[i] = ascii
+		}(i)
+	}
+	wg.Wait()
+	for i, got := range results {
+		if !bytes.Equal(got, results[0]) {
+			t.Fatalf("result %d differs from result 0", i)
+		}
 	}
 }
 
-func TestClampDimension(t *testing.T) {
+func TestRenderCacheEvictsByBytes(t *testing.T) {
-	if clampDimension(0, 100) != 1 {
+	key := func(index int) cacheKey { return cacheKey{index: index} }
-		t.Error("floor")
+	budget := 3 * entryCost(key(0), bytes.Repeat([]byte("x"), 1000))
+	c := newRenderCache(budget)
+	for i := range 5 {
+		c.put(key(i), bytes.Repeat([]byte("x"), 1000))
 	}
-	if clampDimension(500, 100) != 100 {
+	if c.size.Load() > budget {
-		t.Error("ceil")
+		t.Errorf("size %d exceeds budget %d", c.size.Load(), budget)
 	}
-	if clampDimension(50, 100) != 50 {
+	if _, ok := c.get(key(0)); ok {
-		t.Error("passthrough")
+		t.Error("oldest entry should have been evicted")
+	}
+	if _, ok := c.get(key(4)); !ok {
+		t.Error("newest entry should be cached")
+	}
+}
+
+func TestRenderCacheDisabled(t *testing.T) {
+	c := newRenderCache(0)
+	if c != nil {
+		t.Fatal("zero budget should disable the cache")
+	}
+	c.put(cacheKey{}, []byte("v"))
+	if _, ok := c.get(cacheKey{}); ok {
+		t.Error("nil cache should never hit")
+	}
+}
+
+func TestRenderCacheRejectsOversizedEntry(t *testing.T) {
+	c := newRenderCache(256)
+	c.put(cacheKey{}, bytes.Repeat([]byte("x"), 10_000))
+	if _, ok := c.get(cacheKey{}); ok {
+		t.Error("entry larger than budget should not be cached")
+	}
+	if c.size.Load() != 0 {
+		t.Errorf("size = %d, want 0", c.size.Load())
+	}
+}
+
+func TestConnectionTracker(t *testing.T) {
+	tr := newConnectionTracker()
+	if _, _, ok := tr.tryAcquire("1.2.3.4", 2, 100); !ok {
+		t.Fatal("first acquire failed")
+	}
+	if _, _, ok := tr.tryAcquire("1.2.3.4", 2, 100); !ok {
+		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 _, _, ok := tr.tryAcquire("1.2.3.4", 2, 100); !ok {
+		t.Error("limit should be cleared")
+	}
+}
+
+func TestClampTermSize(t *testing.T) {
+	w, h := clampTermSize(1000, 500, 512, 65536, 4)
+	if w < 1 || h < 1 || w > 512 || h > 512 || w*h > 65536 {
+		t.Fatalf("clamped size = %dx%d", w, h)
+	}
+	if w%4 != 0 || h%4 != 0 {
+		t.Fatalf("size is not quantized: %dx%d", w, h)
+	}
+	w, h = clampTermSize(3, 2, 100, 100, 4)
+	if w != 3 || h != 2 {
+		t.Fatalf("small size = %dx%d", w, h)
 	}
 }
 

src/config.go

@@ -28,7 +28,9 @@ type Config struct {
 	MaxAuthAttempts     int
 	HandshakeTimeout    time.Duration
 	MaxDimension        int
-	FrameResolution     int
+	MaxTerminalCells    int
+	SessionTimeout      time.Duration
+	RenderCacheMB       int
 	BrightnessThreshold int
 	Charset             string
 	Invert              bool
@@ -126,7 +128,9 @@ 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),
-		FrameResolution:     envInt("FRAME_RESOLUTION", 360, 16, 1080),
+		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"),
 		Invert:              envBool("INVERT", false),

src/frameformat.go

@@ -6,16 +6,68 @@ import (
 	"fmt"
 	"math"
 	"os"
+	"sync"
 )
 
 // .tsf container, little-endian: "TSFR" | version uint16 | fps float64 |
 // count uint32 | count × (colorLen uint32, color JPEG).
 const (
-	tsfMagic   = "TSFR"
+	tsfMagic         = "TSFR"
-	tsfVersion = 1
+	tsfVersion       = 1
+	maxTSFFPS        = 240
+	maxTSFFrameCount = 10_000_000
 )
 
+type frameFile struct {
+	data    []byte
+	cleanup func() error
+	once    sync.Once
+	err     error
+}
+
+func (f *frameFile) Close() error {
+	if f == nil {
+		return nil
+	}
+	f.once.Do(func() {
+		if f.cleanup != nil {
+			f.err = f.cleanup()
+		}
+		f.data = nil
+	})
+	return f.err
+}
+
+var frameFileOwners sync.Map // map[*FramesContainer]*frameFile
+
+func (data *FramesContainer) Close() error {
+	if data == nil {
+		return nil
+	}
+	owner, ok := frameFileOwners.LoadAndDelete(data)
+	if !ok {
+		return nil
+	}
+	data.ColorFrames = nil
+	return owner.(*frameFile).Close()
+}
+
 func writeTSF(output string, data *FramesContainer) error {
+	if data == nil {
+		return fmt.Errorf("cannot write nil frames container")
+	}
+	if math.IsNaN(data.FPS) || math.IsInf(data.FPS, 0) || data.FPS <= 0 || data.FPS > maxTSFFPS {
+		return fmt.Errorf("cannot write .tsf: fps must be finite, positive, and at most %d", maxTSFFPS)
+	}
+	if len(data.ColorFrames) > maxTSFFrameCount || uint64(len(data.ColorFrames)) > math.MaxUint32 {
+		return fmt.Errorf("cannot write .tsf: frame count exceeds limit")
+	}
+	for i, frame := range data.ColorFrames {
+		if uint64(len(frame)) > math.MaxUint32 {
+			return fmt.Errorf("cannot write .tsf: frame %d length exceeds uint32", i)
+		}
+	}
+
 	f, err := os.Create(output)
 	if err != nil {
 		return err
@@ -48,10 +100,17 @@ func writeTSF(output string, data *FramesContainer) error {
 }
 
 func loadTSF(filename string) (*FramesContainer, error) {
-	raw, err := os.ReadFile(filename)
+	file, err := readFrameFile(filename)
 	if err != nil {
 		return nil, err
 	}
+	owned := false
+	defer func() {
+		if !owned {
+			_ = file.Close()
+		}
+	}()
+	raw := file.data
 	invalid := func() error {
 		return fmt.Errorf("invalid frames file %q: corrupt .tsf container", filename)
 	}
@@ -65,27 +124,40 @@ func loadTSF(filename string) (*FramesContainer, error) {
 	}
 	fps := math.Float64frombits(binary.LittleEndian.Uint64(raw[6:]))
 	count := binary.LittleEndian.Uint32(raw[14:])
-
+	if math.IsNaN(fps) || math.IsInf(fps, 0) || fps <= 0 || fps > maxTSFFPS {
-	colorFrames := make([][]byte, 0, count)
-	off := 18
-	for range count {
-		if off+4 > len(raw) {
-			return nil, invalid()
-		}
-		n := int(binary.LittleEndian.Uint32(raw[off:]))
-		off += 4
-		if off+n > len(raw) {
-			return nil, invalid()
-		}
-		colorFrames = append(colorFrames, raw[off:off+n])
-		off += n
-	}
-
-	if len(colorFrames) == 0 || fps <= 0 {
 		return nil, fmt.Errorf(
-			"invalid frames file %q: expected non-empty frames and a positive fps",
+			"invalid frames file %q: fps must be finite, greater than 0, and at most %d",
-			filename,
+			filename, maxTSFFPS,
 		)
 	}
-	return &FramesContainer{ColorFrames: colorFrames, FPS: fps}, nil
+	if count == 0 {
+		return nil, fmt.Errorf("invalid frames file %q: expected non-empty frames", filename)
+	}
+	if count > maxTSFFrameCount || uint64(count) > uint64((len(raw)-18)/4) {
+		return nil, invalid()
+	}
+
+	colorFrames := make([][]byte, 0, int(count))
+	off := 18
+	for range count {
+		if len(raw)-off < 4 {
+			return nil, invalid()
+		}
+		n := uint64(binary.LittleEndian.Uint32(raw[off:]))
+		off += 4
+		if n > uint64(len(raw)-off) {
+			return nil, invalid()
+		}
+		nativeLen := int(n)
+		colorFrames = append(colorFrames, raw[off:off+nativeLen])
+		off += nativeLen
+	}
+
+	if off != len(raw) {
+		return nil, invalid()
+	}
+	data := &FramesContainer{ColorFrames: colorFrames, FPS: fps}
+	frameFileOwners.Store(data, file)
+	owned = true
+	return data, nil
 }

src/frameformat_hardening_test.go

@@ -0,0 +1,82 @@
+package main
+
+import (
+	"encoding/binary"
+	"math"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+func tsfHeader(fps float64, count uint32) []byte {
+	raw := make([]byte, 18)
+	copy(raw, tsfMagic)
+	binary.LittleEndian.PutUint16(raw[4:], tsfVersion)
+	binary.LittleEndian.PutUint64(raw[6:], math.Float64bits(fps))
+	binary.LittleEndian.PutUint32(raw[14:], count)
+	return raw
+}
+
+func writeRawTSF(t *testing.T, raw []byte) string {
+	t.Helper()
+	path := filepath.Join(t.TempDir(), "frames.tsf")
+	if err := os.WriteFile(path, raw, 0o644); err != nil {
+		t.Fatal(err)
+	}
+	return path
+}
+
+func TestTSFRejectsInvalidFPS(t *testing.T) {
+	for _, fps := range []float64{math.NaN(), math.Inf(1), math.Inf(-1), -1, 0, 240.01} {
+		raw := append(tsfHeader(fps, 1), 0, 0, 0, 0)
+		if _, err := loadTSF(writeRawTSF(t, raw)); err == nil {
+			t.Errorf("loadTSF accepted fps %v", fps)
+		}
+	}
+}
+
+func TestTSFRejectsImpossibleCountsAndLengths(t *testing.T) {
+	if _, err := loadTSF(writeRawTSF(t, tsfHeader(30, math.MaxUint32))); err == nil {
+		t.Fatal("loadTSF accepted impossible frame count")
+	}
+
+	raw := append(tsfHeader(30, 1), 0xff, 0xff, 0xff, 0xff)
+	if _, err := loadTSF(writeRawTSF(t, raw)); err == nil {
+		t.Fatal("loadTSF accepted overflowing frame length")
+	}
+}
+
+func TestTSFCloseReleasesOwnedFrames(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "frames.tsf")
+	if err := writeTSF(path, &FramesContainer{FPS: 30, ColorFrames: [][]byte{{1, 2, 3}}}); err != nil {
+		t.Fatal(err)
+	}
+	frames, err := loadTSF(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got := frames.ColorFrames[0]; len(got) != 3 || got[0] != 1 {
+		t.Fatalf("unexpected zero-copy frame data: %v", got)
+	}
+	if err := frames.Close(); err != nil {
+		t.Fatalf("Close: %v", err)
+	}
+	if frames.ColorFrames != nil {
+		t.Fatal("Close retained references to released frame data")
+	}
+	if err := frames.Close(); err != nil {
+		t.Fatalf("second Close: %v", err)
+	}
+}
+
+func TestTSFWriteRejectsInvalidHeaderValuesBeforeCreate(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "frames.tsf")
+	err := writeTSF(path, &FramesContainer{FPS: math.NaN(), ColorFrames: [][]byte{{1}}})
+	if err == nil || !strings.Contains(err.Error(), "fps") {
+		t.Fatalf("writeTSF error = %v", err)
+	}
+	if _, err := os.Stat(path); !os.IsNotExist(err) {
+		t.Fatalf("invalid write created output: %v", err)
+	}
+}

src/frames.go

@@ -3,12 +3,15 @@ package main
 import (
 	"bytes"
 	"container/list"
-	"fmt"
 	"image"
 	"image/color"
 	"image/jpeg"
+	"strconv"
 	"strings"
 	"sync"
+	"sync/atomic"
+	"time"
+	"unicode/utf8"
 
 	"golang.org/x/image/draw"
 )
@@ -62,22 +65,52 @@ func resolveCharset(charset string) string {
 	return charset
 }
 
-func resizeFrame(frame []byte, width, height int, keepAspectRatio bool, tier colorTier) (draw.Image, error) {
+const maxPooledBuffer = 4 << 20
+
+var pixPool sync.Pool
+
+func getPixBuf(n int) []byte {
+	if v := pixPool.Get(); v != nil {
+		if b := *v.(*[]byte); cap(b) >= n {
+			return b[:n]
+		}
+	}
+	return make([]byte, n)
+}
+
+func putPixBuf(b []byte) {
+	if cap(b) <= maxPooledBuffer {
+		pixPool.Put(&b)
+	}
+}
+
+var outPool sync.Pool
+
+func getOutBuf(capacity int) []byte {
+	if v := outPool.Get(); v != nil {
+		if b := *v.(*[]byte); cap(b) >= capacity {
+			return b[:0]
+		}
+	}
+	return make([]byte, 0, capacity)
+}
+
+func putOutBuf(b []byte) {
+	if cap(b) <= maxPooledBuffer {
+		outPool.Put(&b)
+	}
+}
+
+func resizeFrame(frame, pix []byte, width, height int, keepAspectRatio bool) (*image.RGBA, error) {
 	src, err := jpeg.Decode(bytes.NewReader(frame))
 	if err != nil {
 		return nil, err
 	}
-	var dst draw.Image
+	rect := image.Rect(0, 0, width, height)
-	var bg color.Color
+
-	if tier == colorTierNone {
+	dst := &image.RGBA{Pix: pix[:4*width*height], Stride: 4 * width, Rect: rect}
-		dst = image.NewGray(image.Rect(0, 0, width, height))
-		bg = color.Gray{0}
-	} else {
-		dst = image.NewNRGBA(image.Rect(0, 0, width, height))
-		bg = color.Black
-	}
 	if keepAspectRatio {
-		draw.Draw(dst, dst.Bounds(), image.NewUniform(bg), image.Point{}, draw.Src)
+		draw.Draw(dst, dst.Bounds(), image.NewUniform(color.Black), image.Point{}, draw.Src)
 		sb := src.Bounds()
 		sw, sh := sb.Dx(), sb.Dy()
 		scale := min(float64(width)/float64(sw), float64(height)/float64(sh))
@@ -98,6 +131,15 @@ type asciiOptions struct {
 	invert              bool
 }
 
+func buildRampLUT(ramp []rune, options asciiOptions) *[101][]byte {
+	var lut [101][]byte
+	for b := range lut {
+		index := rampIndex(b, options.brightnessThreshold, len(ramp), options.invert)
+		lut[b] = utf8.AppendRune(nil, ramp[index])
+	}
+	return &lut
+}
+
 func rampIndex(brightness, threshold, total int, invert bool) int {
 	var index int
 	if brightness < threshold {
@@ -114,27 +156,38 @@ func rampIndex(brightness, threshold, total int, invert bool) int {
 	return index
 }
 
-func frameToAscii(pixels []byte, options asciiOptions) string {
+func frameToAscii(img *image.RGBA, rampLUT *[101][]byte) []byte {
-	ramp := []rune(resolveCharset(options.charset))
+	pix := img.Pix
-	total := len(ramp)
+	maxCharBytes := 1
-	var b strings.Builder
+	for _, char := range rampLUT {
-	for _, p := range pixels {
+		maxCharBytes = max(maxCharBytes, len(char))
-		brightness := int(p) * 100 / 255
-		index := rampIndex(brightness, options.brightnessThreshold, total, options.invert)
-		b.WriteRune(ramp[index])
 	}
-	return b.String()
+	buf := getOutBuf(len(pix) / 4 * maxCharBytes)
+	for o := 0; o < len(pix); o += 4 {
+		brightness := (int(pix[o])*299 + int(pix[o+1])*587 + int(pix[o+2])*114) / 255 / 10
+		buf = append(buf, rampLUT[brightness]...)
+	}
+	output := bytes.Clone(buf)
+	putOutBuf(buf)
+	return output
 }
 
 const ansiReset = "\x1b[0m"
 
 var ansi256Levels = [6]int{0, 95, 135, 175, 215, 255}
 
-func quantize256(r, g, b uint8) int {
+var decimal = func() (t [256]string) {
-	toLevel := func(v uint8) int {
+	for i := range t {
+		t[i] = strconv.Itoa(i)
+	}
+	return
+}()
+
+var ansi256Cube = func() (t [256]uint8) {
+	for v := range t {
 		best, bestDist := 0, 1<<30
 		for i, l := range ansi256Levels {
-			d := int(v) - l
+			d := v - l
 			if d < 0 {
 				d = -d
 			}
@@ -142,105 +195,288 @@ func quantize256(r, g, b uint8) int {
 				bestDist, best = d, i
 			}
 		}
-		return best
+		t[v] = uint8(best)
 	}
-	return 16 + 36*toLevel(r) + 6*toLevel(g) + toLevel(b)
+	return
+}()
+
+func quantize256(r, g, b uint8) int {
+	return 16 + 36*int(ansi256Cube[r]) + 6*int(ansi256Cube[g]) + int(ansi256Cube[b])
 }
 
-func frameToAnsi(img *image.NRGBA, options asciiOptions, tier colorTier) string {
+func appendColor(buf []byte, r, g, b uint8, tier colorTier) []byte {
-	ramp := []rune(resolveCharset(options.charset))
+	if tier == colorTierTrueColor {
-	total := len(ramp)
+		buf = append(buf, "\x1b[38;2;"...)
+		buf = append(buf, decimal[r]...)
+		buf = append(buf, ';')
+		buf = append(buf, decimal[g]...)
+		buf = append(buf, ';')
+		buf = append(buf, decimal[b]...)
+	} else {
+		buf = append(buf, "\x1b[38;5;"...)
+		buf = append(buf, decimal[quantize256(r, g, b)]...)
+	}
+	return append(buf, 'm')
+}
+
+func frameToAnsi(img *image.RGBA, rampLUT *[101][]byte, tier colorTier) []byte {
 	bounds := img.Bounds()
-	var b strings.Builder
+	bytesPerCell := 11
+	if tier == colorTierTrueColor {
+		bytesPerCell = 16
+	}
+	buf := getOutBuf(bounds.Dx() * bounds.Dy() * bytesPerCell)
 	var lastR, lastG, lastB uint8
+	last256 := -1
 	first := true
 	for y := bounds.Min.Y; y < bounds.Max.Y; y++ {
+		o := img.PixOffset(bounds.Min.X, y)
 		for x := bounds.Min.X; x < bounds.Max.X; x++ {
-			o := img.PixOffset(x, y)
 			r, g, bl := img.Pix[o], img.Pix[o+1], img.Pix[o+2]
 			brightness := (int(r)*299 + int(g)*587 + int(bl)*114) / 255 / 10
-			index := rampIndex(brightness, options.brightnessThreshold, total, options.invert)
+			colorChanged := first || r != lastR || g != lastG || bl != lastB
-			if first || r != lastR || g != lastG || bl != lastB {
+			if tier == colorTier256 {
-				if tier == colorTierTrueColor {
+				index := quantize256(r, g, bl)
-					fmt.Fprintf(&b, "\x1b[38;2;%d;%d;%dm", r, g, bl)
+				colorChanged = first || index != last256
-				} else {
+				last256 = index
-					fmt.Fprintf(&b, "\x1b[38;5;%dm", quantize256(r, g, bl))
+			}
-				}
+			if colorChanged {
+				buf = appendColor(buf, r, g, bl, tier)
 				lastR, lastG, lastB = r, g, bl
 				first = false
 			}
-			b.WriteRune(ramp[index])
+			buf = append(buf, rampLUT[brightness]...)
+			o += 4
 		}
 		if y < bounds.Max.Y-1 {
-			b.WriteString(ansiReset + "\r\n")
+			buf = append(buf, "\r\n"...)
-			first = true
 		}
 	}
-	b.WriteString(ansiReset)
+	buf = append(buf, ansiReset...)
-	return b.String()
+	output := bytes.Clone(buf)
+	putOutBuf(buf)
+	return output
 }
 
-type FrameRenderer struct {
+type cacheKey struct {
-	colorFrames [][]byte
+	setID           int
-	options     asciiOptions
+	index           int
-	maxEntries  int
+	width           int
+	height          int
+	keepAspectRatio bool
+	tier            colorTier
+}
 
-	mu    sync.Mutex
+type renderCacheShard struct {
-	cache map[string]*list.Element
+	mu       sync.Mutex
-	order *list.List
+	maxBytes int64
+	size     int64
+	entries  map[cacheKey]*list.Element
+	order    *list.List
+}
+
+type renderCache struct {
+	shards     []renderCacheShard
+	size       atomic.Int64
+	hits       atomic.Uint64
+	misses     atomic.Uint64
+	evictions  atomic.Uint64
+	rejections atomic.Uint64
+	renders    atomic.Uint64
+	renderNs   atomic.Uint64
 }
 
 type cacheEntry struct {
-	key   string
+	key   cacheKey
-	ascii string
+	ascii []byte
+	cost  int64
 }
 
-func newFrameRenderer(colorFrames [][]byte, options asciiOptions) *FrameRenderer {
+func entryCost(_ cacheKey, ascii []byte) int64 {
+	return int64(cap(ascii)) + 160
+}
+
+func newRenderCache(maxBytes int64) *renderCache {
+	if maxBytes <= 0 {
+		return nil
+	}
+	shardCount := int(min(int64(16), max(int64(1), maxBytes/(1<<20))))
+	cache := &renderCache{shards: make([]renderCacheShard, shardCount)}
+	for i := range cache.shards {
+		cache.shards[i] = renderCacheShard{
+			maxBytes: maxBytes / int64(shardCount),
+			entries:  make(map[cacheKey]*list.Element),
+			order:    list.New(),
+		}
+	}
+	return cache
+}
+
+func (c *renderCache) shard(key cacheKey) *renderCacheShard {
+	hash := uint64(key.setID)*0x9e3779b185ebca87 ^ uint64(key.index)*0xc2b2ae3d27d4eb4f
+	hash ^= uint64(key.width)<<32 | uint64(uint32(key.height))
+	hash ^= uint64(key.tier)<<1 | uint64(boolToInt(key.keepAspectRatio))
+	return &c.shards[hash%uint64(len(c.shards))]
+}
+
+func boolToInt(value bool) int {
+	if value {
+		return 1
+	}
+	return 0
+}
+
+func (c *renderCache) get(key cacheKey) ([]byte, bool) {
+	if c == nil {
+		return nil, false
+	}
+	shard := c.shard(key)
+	shard.mu.Lock()
+	defer shard.mu.Unlock()
+	el, ok := shard.entries[key]
+	if !ok {
+		c.misses.Add(1)
+		return nil, false
+	}
+	c.hits.Add(1)
+	shard.order.MoveToBack(el)
+	return el.Value.(*cacheEntry).ascii, true
+}
+
+func (c *renderCache) put(key cacheKey, ascii []byte) {
+	if c == nil {
+		return
+	}
+	shard := c.shard(key)
+	cost := entryCost(key, ascii)
+	if cost > shard.maxBytes {
+		c.rejections.Add(1)
+		return
+	}
+	shard.mu.Lock()
+	defer shard.mu.Unlock()
+	if _, ok := shard.entries[key]; ok {
+		return
+	}
+	shard.entries[key] = shard.order.PushBack(&cacheEntry{key: key, ascii: ascii, cost: cost})
+	shard.size += cost
+	c.size.Add(cost)
+	for shard.size > shard.maxBytes {
+		oldest := shard.order.Front()
+		shard.order.Remove(oldest)
+		evicted := oldest.Value.(*cacheEntry)
+		delete(shard.entries, evicted.key)
+		shard.size -= evicted.cost
+		c.size.Add(-evicted.cost)
+		c.evictions.Add(1)
+	}
+}
+
+type renderCacheStats struct {
+	SizeBytes  int64
+	Hits       uint64
+	Misses     uint64
+	Evictions  uint64
+	Rejections uint64
+	Renders    uint64
+	RenderTime time.Duration
+}
+
+func (c *renderCache) stats() renderCacheStats {
+	if c == nil {
+		return renderCacheStats{}
+	}
+	return renderCacheStats{
+		SizeBytes:  c.size.Load(),
+		Hits:       c.hits.Load(),
+		Misses:     c.misses.Load(),
+		Evictions:  c.evictions.Load(),
+		Rejections: c.rejections.Load(),
+		Renders:    c.renders.Load(),
+		RenderTime: time.Duration(c.renderNs.Load()),
+	}
+}
+
+type FrameRenderer struct {
+	setID       int
+	colorFrames [][]byte
+	options     asciiOptions
+	rampLUT     *[101][]byte
+	cache       *renderCache
+
+	inflightMu sync.Mutex
+	inflight   map[cacheKey]*renderCall
+}
+
+type renderCall struct {
+	done  chan struct{}
+	value []byte
+	err   error
+}
+
+func newFrameRenderer(setID int, colorFrames [][]byte, options asciiOptions, cache *renderCache) *FrameRenderer {
+	ramp := []rune(resolveCharset(options.charset))
 	return &FrameRenderer{
+		setID:       setID,
 		colorFrames: colorFrames,
 		options:     options,
-		maxEntries:  4096,
+		rampLUT:     buildRampLUT(ramp, options),
-		cache:       make(map[string]*list.Element),
+		cache:       cache,
-		order:       list.New(),
+		inflight:    make(map[cacheKey]*renderCall),
 	}
 }
 
-func (r *FrameRenderer) render(index, width, height int, keepAspectRatio bool, tier colorTier) (string, error) {
+func (r *FrameRenderer) render(index, width, height int, keepAspectRatio bool, tier colorTier) ([]byte, error) {
-	key := fmt.Sprintf("%d:%dx%d:%t:%d", index, width, height, keepAspectRatio, tier)
+	key := cacheKey{r.setID, index, width, height, keepAspectRatio, tier}
-
+	if ascii, ok := r.cache.get(key); ok {
-	r.mu.Lock()
-	if el, ok := r.cache[key]; ok {
-		r.order.MoveToBack(el)
-		ascii := el.Value.(*cacheEntry).ascii
-		r.mu.Unlock()
 		return ascii, nil
 	}
-	r.mu.Unlock()
 
-	var ascii string
+	r.inflightMu.Lock()
+	if call, ok := r.inflight[key]; ok {
+		r.inflightMu.Unlock()
+		<-call.done
+		return call.value, call.err
+	}
+	call := &renderCall{done: make(chan struct{})}
+	r.inflight[key] = call
+	r.inflightMu.Unlock()
+	defer func() {
+		r.inflightMu.Lock()
+		delete(r.inflight, key)
+		r.inflightMu.Unlock()
+		close(call.done)
+	}()
+
+	if ascii, ok := r.cache.get(key); ok {
+		call.value = ascii
+		return ascii, nil
+	}
+
+	started := time.Now()
+	pix := getPixBuf(4 * width * height)
+	img, err := resizeFrame(r.colorFrames[index], pix, width, height, keepAspectRatio)
+	if err != nil {
+		putPixBuf(pix)
+		call.err = err
+		return nil, err
+	}
+	var ascii []byte
 	if tier == colorTierNone {
-		img, err := resizeFrame(r.colorFrames[index], width, height, keepAspectRatio, tier)
+		ascii = frameToAscii(img, r.rampLUT)
-		if err != nil {
-			return "", err
-		}
-		ascii = frameToAscii(img.(*image.Gray).Pix, r.options)
 	} else {
-		img, err := resizeFrame(r.colorFrames[index], width, height, keepAspectRatio, tier)
+		ascii = frameToAnsi(img, r.rampLUT, tier)
-		if err != nil {
-			return "", err
-		}
-		ascii = frameToAnsi(img.(*image.NRGBA), r.options, tier)
 	}
+	putPixBuf(pix)
 
-	r.mu.Lock()
+	if r.cache != nil && cap(ascii) > len(ascii)+len(ascii)/4 {
-	if _, ok := r.cache[key]; !ok {
+		ascii = bytes.Clone(ascii)
-		r.cache[key] = r.order.PushBack(&cacheEntry{key, ascii})
-		if r.order.Len() > r.maxEntries {
-			oldest := r.order.Front()
-			r.order.Remove(oldest)
-			delete(r.cache, oldest.Value.(*cacheEntry).key)
-		}
 	}
-	r.mu.Unlock()
+	r.cache.put(key, ascii)
+	if r.cache != nil {
+		r.cache.renders.Add(1)
+		r.cache.renderNs.Add(uint64(time.Since(started)))
+	}
+	call.value = ascii
 	return ascii, nil
 }

src/logger.go

@@ -47,18 +47,20 @@ func sanitizeN(value any, maxLength int) string {
 		str = fmt.Sprint(v)
 	}
 	var b strings.Builder
+	count := 0
 	for _, r := range str {
+		if count >= maxLength {
+			b.WriteRune('…')
+			return b.String()
+		}
 		if r < 0x20 || (r >= 0x7f && r <= 0x9f) {
 			b.WriteRune('�')
 		} else {
 			b.WriteRune(r)
 		}
+		count++
 	}
-	out := []rune(b.String())
+	return b.String()
-	if len(out) > maxLength {
-		return string(out[:maxLength]) + "…"
-	}
-	return string(out)
 }
 
 func emit(level logLevel, name string, stream *os.File, args []any) {

src/main.go

@@ -6,7 +6,9 @@ import (
 	"os/signal"
 	"path/filepath"
 	"runtime"
+	"runtime/debug"
 	"sort"
+	"strconv"
 	"strings"
 	"sync"
 	"syscall"
@@ -16,12 +18,13 @@ import (
 )
 
 type cliArgs struct {
-	generate bool
+	generate   bool
-	video    string
+	video      string
+	resolution int
 }
 
 func parseArgs(argv []string) cliArgs {
-	var args cliArgs
+	args := cliArgs{resolution: 512}
 	for i := 0; i < len(argv); i++ {
 		switch argv[i] {
 		case "--generate", "-g":
@@ -31,6 +34,13 @@ func parseArgs(argv []string) cliArgs {
 				i++
 				args.video = argv[i]
 			}
+		case "--resolution", "-r":
+			if i+1 < len(argv) {
+				i++
+				if n, err := strconv.Atoi(argv[i]); err == nil {
+					args.resolution = max(n, 16)
+				}
+			}
 		}
 	}
 	return args
@@ -52,7 +62,7 @@ func resolveVideoPath(explicitPath string) string {
 	return ""
 }
 
-func generateFrames(config Config, framesDir, videoArg string) {
+func generateFrames(framesDir, videoArg string, resolution int) {
 	if videoArg == "" {
 		fail("No source video given. Pass --video <path>.")
 	}
@@ -68,11 +78,13 @@ func generateFrames(config Config, framesDir, videoArg string) {
 	output := filepath.Join(framesDir, base+".tsf")
 
 	logInfo(fmt.Sprintf("Generating frames from %q -> %s", videoPath, output))
-	if err := processVideo(videoPath, output, config.FrameResolution); err != nil {
+	if err := processVideo(videoPath, output, resolution); err != nil {
 		fail(fmt.Sprintf("Failed to generate frames from %q: %s", videoPath, err.Error()))
 	}
 }
 
+const frameDataWarnBytes = 2 << 30
+
 func loadAllFrames(framesDir string) []*FramesContainer {
 	entries, err := os.ReadDir(framesDir)
 	var files []string
@@ -91,6 +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)))
 
@@ -143,9 +171,33 @@ func loadAllFrames(framesDir string) []*FramesContainer {
 	return results
 }
 
+func applyMemoryLimit() {
+	if os.Getenv("GOMEMLIMIT") != "" {
+		return
+	}
+	for _, path := range []string{
+		"/sys/fs/cgroup/memory.max",
+		"/sys/fs/cgroup/memory/memory.limit_in_bytes",
+	} {
+		raw, err := os.ReadFile(path)
+		if err != nil {
+			continue
+		}
+		n, err := strconv.ParseInt(strings.TrimSpace(string(raw)), 10, 64)
+		if err != nil || n <= 0 || n > 1<<48 {
+			return
+		}
+		limit := n * 9 / 10
+		debug.SetMemoryLimit(limit)
+		logInfo(fmt.Sprintf("Memory limit set to %d MB (90%% of cgroup limit)", limit>>20))
+		return
+	}
+}
+
 func main() {
 	_ = godotenv.Load()
 	logThreshold = resolveThreshold()
+	applyMemoryLimit()
 
 	config := loadConfig()
 	args := parseArgs(os.Args[1:])
@@ -158,7 +210,7 @@ func main() {
 	framesDir := filepath.Join(cwd, "frames")
 
 	if args.generate {
-		generateFrames(config, framesDir, args.video)
+		generateFrames(framesDir, args.video, args.resolution)
 		return
 	}
 
@@ -192,14 +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/mmap_other.go

@@ -0,0 +1,10 @@
+//go:build !unix
+
+package main
+
+import "os"
+
+func readFrameFile(filename string) (*frameFile, error) {
+	data, err := os.ReadFile(filename)
+	return &frameFile{data: data}, err
+}

src/mmap_unix.go

@@ -0,0 +1,37 @@
+//go:build unix
+
+package main
+
+import (
+	"os"
+	"syscall"
+)
+
+func readFrameFile(filename string) (*frameFile, error) {
+	f, err := os.Open(filename)
+	if err != nil {
+		return nil, err
+	}
+	defer func() { _ = f.Close() }()
+
+	info, err := f.Stat()
+	if err != nil {
+		return nil, err
+	}
+	size := info.Size()
+	if size <= 0 || size != int64(int(size)) {
+		data, err := os.ReadFile(filename)
+		return &frameFile{data: data}, err
+	}
+	data, err := syscall.Mmap(int(f.Fd()), 0, int(size), syscall.PROT_READ, syscall.MAP_SHARED)
+	if err != nil {
+		data, err := os.ReadFile(filename)
+		return &frameFile{data: data}, err
+	}
+	return &frameFile{
+		data: data,
+		cleanup: func() error {
+			return syscall.Munmap(data)
+		},
+	}, 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/render_bench_test.go

@@ -0,0 +1,67 @@
+package main
+
+import (
+	"path/filepath"
+	"sync/atomic"
+	"testing"
+)
+
+func loadBenchSet(b *testing.B) *FramesContainer {
+	b.Helper()
+	matches, _ := filepath.Glob("../frames/*.tsf")
+	if len(matches) == 0 {
+		b.Skip("no .tsf frame set in ../frames")
+	}
+	fc, err := loadTSF(matches[0])
+	if err != nil {
+		b.Skip("failed to load frame set:", err)
+	}
+	b.Cleanup(func() { _ = fc.Close() })
+	return fc
+}
+
+func benchRender(b *testing.B, tier colorTier, w, h int) {
+	fc := loadBenchSet(b)
+	r := newFrameRenderer(0, fc.ColorFrames, asciiOptions{
+		brightnessThreshold: 40,
+		charset:             "detailed",
+	}, nil)
+	b.ReportAllocs()
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		if _, err := r.render(i%len(fc.ColorFrames), w, h, false, tier); err != nil {
+			b.Fatal(err)
+		}
+	}
+}
+
+func BenchmarkRenderTrueColor(b *testing.B) { benchRender(b, colorTierTrueColor, 120, 40) }
+func BenchmarkRender256(b *testing.B)       { benchRender(b, colorTier256, 120, 40) }
+func BenchmarkRenderGray(b *testing.B)      { benchRender(b, colorTierNone, 120, 40) }
+func BenchmarkRenderTrueBig(b *testing.B)   { benchRender(b, colorTierTrueColor, 240, 70) }
+
+func BenchmarkRenderCachedParallel(b *testing.B) {
+	fc := loadBenchSet(b)
+	r := newFrameRenderer(0, fc.ColorFrames, asciiOptions{
+		brightnessThreshold: 40,
+		charset:             "detailed",
+	}, newRenderCache(8<<20))
+	frame, err := r.render(0, 120, 40, false, colorTierTrueColor)
+	if err != nil {
+		b.Fatal(err)
+	}
+	b.SetBytes(int64(len(frame)))
+	b.ReportAllocs()
+	var failures atomic.Uint64
+	b.ResetTimer()
+	b.RunParallel(func(pb *testing.PB) {
+		for pb.Next() {
+			if _, err := r.render(0, 120, 40, false, colorTierTrueColor); err != nil {
+				failures.Add(1)
+			}
+		}
+	})
+	if failures.Load() != 0 {
+		b.Fatalf("render failures: %d", failures.Load())
+	}
+}

src/server.go

@@ -4,6 +4,8 @@ import (
 	"encoding/binary"
 	"errors"
 	"fmt"
+	"io"
+	"math"
 	"math/rand"
 	"net"
 	"strings"
@@ -13,7 +15,84 @@ import (
 	"golang.org/x/crypto/ssh"
 )
 
-const clearScreen = "\x1b[2J\x1b[0f"
+const (
+	clearScreen         = "\x1b[2J\x1b[0f"
+	hideCursor          = "\x1b[?25l"
+	showCursor          = "\x1b[?25h"
+	syncStart           = "\x1b[?2026h"
+	syncEnd             = "\x1b[?2026l"
+	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
@@ -25,15 +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 {
@@ -54,10 +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 {
@@ -69,10 +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
 }
 
@@ -85,28 +203,40 @@ 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 {
 	config := deps.Config
 
+	cache := newRenderCache(int64(config.RenderCacheMB) << 20)
 	sets := make([]frameSet, len(deps.VideoSets))
 	for i, data := range deps.VideoSets {
 		sets[i] = frameSet{
 			data: data,
-			renderer: newFrameRenderer(data.ColorFrames, asciiOptions{
+			renderer: newFrameRenderer(i, data.ColorFrames, asciiOptions{
 				brightnessThreshold: config.BrightnessThreshold,
 				charset:             config.Charset,
 				invert:              config.Invert,
-			}),
+			}, cache),
 		}
 	}
 
@@ -147,9 +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{}),
 	}
 }
 
@@ -166,7 +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()
@@ -176,29 +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))
@@ -221,35 +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()
 }
 
@@ -296,20 +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)
@@ -323,7 +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)
@@ -340,14 +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 {
@@ -355,6 +569,10 @@ func (s *Server) handleSession(
 			}
 		}
 	}
+	_ = channel.Close()
+	if playDone != nil {
+		<-playDone
+	}
 }
 
 func (s *Server) pickNextSetIndex(exclude int) int {
@@ -383,9 +601,16 @@ func (s *Server) playVideo(
 	w, h := size.get()
 	logDebug(fmt.Sprintf("Terminal size %dx%d for %s", w, h, ip))
 
+	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{})
@@ -429,9 +654,17 @@ 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
+	}
+
+	if err := writePartsWithTimeout(sshConn, channel, outputStallTimeout, hideCursor); err != nil {
 		return
 	}
 
@@ -444,7 +677,7 @@ func (s *Server) playVideo(
 
 	currentFrame := 0
 	loopCount := 0
-
+	lastW, lastH := 0, 0
 	for {
 		select {
 		case <-done:
@@ -457,6 +690,7 @@ func (s *Server) playVideo(
 			setIndex = (setIndex + delta + len(s.sets)) % len(s.sets)
 			current = s.sets[setIndex]
 			currentFrame = 0
+			lastW, lastH = 0, 0
 			logDebug(fmt.Sprintf("%s switched to %q", ip, current.data.Name))
 			ticker.Reset(frameInterval())
 
@@ -469,7 +703,14 @@ func (s *Server) playVideo(
 				return
 			}
 
-			if _, err := channel.Write([]byte(clearScreen + ascii)); err != nil {
+			prefix := homeCursor
+			if w != lastW || h != lastH {
+				prefix = clearScreen
+				lastW, lastH = w, h
+			}
+			if err := writeFrameWithTimeout(
+				sshConn, channel, outputStallTimeout, prefix, ascii,
+			); err != nil {
 				closeSession()
 				return
 			}
@@ -482,11 +723,27 @@ func (s *Server) playVideo(
 			currentFrame = 0
 			loopCount++
 			if config.MaxLoop > 0 && loopCount >= config.MaxLoop {
-				_, _ = channel.Write([]byte(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()

src/videoprocessor.go

@@ -1,15 +1,20 @@
 package main
 
 import (
+	"bufio"
 	"bytes"
+	"encoding/binary"
 	"encoding/json"
 	"errors"
 	"fmt"
 	"io"
 	"math"
+	"os"
 	"os/exec"
+	"path/filepath"
 	"strconv"
 	"strings"
+	"time"
 )
 
 var (
@@ -17,29 +22,177 @@ var (
 	jpegEOI = []byte{0xff, 0xd9}
 )
 
+const (
+	maxJPEGFrameBytes = 64 << 20
+	maxFFmpegLogBytes = 64 << 10
+)
+
 type jpegFrameSplitter struct {
 	buffer []byte
+	scan   int
+	inJPEG bool
 }
 
-func (s *jpegFrameSplitter) push(chunk []byte) [][]byte {
+func (s *jpegFrameSplitter) push(chunk []byte, emit func([]byte) error) (int, error) {
 	s.buffer = append(s.buffer, chunk...)
-	var frames [][]byte
+	emitted := 0
 	for {
-		start := bytes.Index(s.buffer, jpegSOI)
+		if !s.inJPEG {
-		if start == -1 {
+			start := bytes.Index(s.buffer[s.scan:], jpegSOI)
-			break
+			if start == -1 {
+				// Retain only a possible marker prefix spanning two reads.
+				if len(s.buffer) > 0 && s.buffer[len(s.buffer)-1] == jpegSOI[0] {
+					s.buffer = s.buffer[len(s.buffer)-1:]
+				} else {
+					s.buffer = s.buffer[:0]
+				}
+				s.scan = 0
+				return emitted, nil
+			}
+			start += s.scan
+			s.buffer = s.buffer[start:]
+			s.scan = len(jpegSOI)
+			s.inJPEG = true
 		}
-		end := bytes.Index(s.buffer[start+len(jpegSOI):], jpegEOI)
+
+		end := bytes.Index(s.buffer[s.scan:], jpegEOI)
 		if end == -1 {
-			break
+			if len(s.buffer) > maxJPEGFrameBytes {
+				return emitted, fmt.Errorf("JPEG frame exceeds %d MiB limit", maxJPEGFrameBytes>>20)
+			}
+			s.scan = max(len(jpegSOI), len(s.buffer)-1)
+			return emitted, nil
 		}
-		frameEnd := start + len(jpegSOI) + end + len(jpegEOI)
+		frameEnd := s.scan + end + len(jpegEOI)
-		frame := make([]byte, frameEnd-start)
+		if frameEnd > maxJPEGFrameBytes {
-		copy(frame, s.buffer[start:frameEnd])
+			return emitted, fmt.Errorf("JPEG frame exceeds %d MiB limit", maxJPEGFrameBytes>>20)
-		frames = append(frames, frame)
+		}
+		if err := emit(s.buffer[:frameEnd]); err != nil {
+			return emitted, err
+		}
+		emitted++
 		s.buffer = s.buffer[frameEnd:]
+		s.scan = 0
+		s.inJPEG = false
+	}
+}
+
+func (s *jpegFrameSplitter) finish() error {
+	if s.inJPEG {
+		return fmt.Errorf("ffmpeg produced a truncated JPEG frame")
+	}
+	return nil
+}
+
+type boundedLog struct {
+	buffer bytes.Buffer
+	limit  int
+}
+
+func (w *boundedLog) Write(p []byte) (int, error) {
+	n := len(p)
+	if remaining := w.limit - w.buffer.Len(); remaining > 0 {
+		_, _ = w.buffer.Write(p[:min(len(p), remaining)])
+	}
+	return n, nil
+}
+
+func (w *boundedLog) String() string {
+	return strings.TrimSpace(w.buffer.String())
+}
+
+type streamingTSF struct {
+	file   *os.File
+	writer *bufio.Writer
+	path   string
+	count  uint32
+}
+
+func newStreamingTSF(output string, fps float64) (*streamingTSF, error) {
+	if math.IsNaN(fps) || math.IsInf(fps, 0) || fps <= 0 || fps > maxTSFFPS {
+		return nil, fmt.Errorf("cannot write .tsf: fps must be finite and between 0 and %d", maxTSFFPS)
+	}
+	dir := filepath.Dir(output)
+	f, err := os.CreateTemp(dir, "."+filepath.Base(output)+"-*.tmp")
+	if err != nil {
+		return nil, err
+	}
+	s := &streamingTSF{file: f, writer: bufio.NewWriterSize(f, 1<<20), path: f.Name()}
+	if err := f.Chmod(0o644); err != nil {
+		s.abort()
+		return nil, err
+	}
+	if _, err := s.writer.WriteString(tsfMagic); err != nil {
+		s.abort()
+		return nil, err
+	}
+	var hdr [14]byte
+	binary.LittleEndian.PutUint16(hdr[0:], tsfVersion)
+	binary.LittleEndian.PutUint64(hdr[2:], math.Float64bits(fps))
+	if _, err := s.writer.Write(hdr[:]); err != nil {
+		s.abort()
+		return nil, err
+	}
+	return s, nil
+}
+
+func (s *streamingTSF) addFrame(frame []byte) error {
+	if s.count >= maxTSFFrameCount {
+		return fmt.Errorf("too many video frames")
+	}
+	if uint64(len(frame)) > math.MaxUint32 {
+		return fmt.Errorf("JPEG frame is too large")
+	}
+	var size [4]byte
+	binary.LittleEndian.PutUint32(size[:], uint32(len(frame)))
+	if _, err := s.writer.Write(size[:]); err != nil {
+		return err
+	}
+	if _, err := s.writer.Write(frame); err != nil {
+		return err
+	}
+	s.count++
+	return nil
+}
+
+func (s *streamingTSF) commit(output string) error {
+	if s.count == 0 {
+		return fmt.Errorf("no frames were decoded from the video")
+	}
+	if err := s.writer.Flush(); err != nil {
+		return err
+	}
+	if _, err := s.file.Seek(14, io.SeekStart); err != nil {
+		return err
+	}
+	var count [4]byte
+	binary.LittleEndian.PutUint32(count[:], s.count)
+	if _, err := s.file.Write(count[:]); err != nil {
+		return err
+	}
+	if err := s.file.Sync(); err != nil {
+		return err
+	}
+	if err := s.file.Close(); err != nil {
+		return err
+	}
+	s.file = nil
+	if err := os.Rename(s.path, output); err != nil {
+		return err
+	}
+	s.path = ""
+	return nil
+}
+
+func (s *streamingTSF) abort() {
+	if s.file != nil {
+		_ = s.file.Close()
+		s.file = nil
+	}
+	if s.path != "" {
+		_ = os.Remove(s.path)
+		s.path = ""
 	}
-	return frames
 }
 
 type ffprobeOutput struct {
@@ -72,59 +225,82 @@ func parseFrameRate(rate string) float64 {
 	return num
 }
 
-func extractFrames(path, vf, label string, maxDimension, totalFrames int) ([][]byte, error) {
+func extractFrames(path, vf, label string, totalFrames int, emit func([]byte) error) (int, error) {
 	cmd := exec.Command(
-		"ffmpeg", "-i", path,
+		"ffmpeg", "-hide_banner", "-loglevel", "error", "-nostats",
+		"-i", path,
 		"-c:v", "mjpeg",
 		"-q:v", "3",
 		"-vf", vf,
 		"-f", "image2pipe",
 		"pipe:1",
 	)
-	var stderr bytes.Buffer
+	stderr := &boundedLog{limit: maxFFmpegLogBytes}
-	cmd.Stderr = &stderr
+	cmd.Stderr = stderr
 	stdout, err := cmd.StdoutPipe()
 	if err != nil {
-		return nil, fmt.Errorf("ffmpeg failed: %w", err)
+		return 0, fmt.Errorf("ffmpeg failed: %w", err)
 	}
 	if err := cmd.Start(); err != nil {
-		return nil, fmt.Errorf("ffmpeg failed: %w", err)
+		return 0, fmt.Errorf("ffmpeg failed: %w", err)
 	}
 
-	var frames [][]byte
 	splitter := &jpegFrameSplitter{}
-	reportProgress := func(count int) {
+	frameCount := 0
-		if totalFrames > 0 {
+	lastReport := time.Now()
-			pct := min(100, int(math.Round(float64(count)/float64(totalFrames)*100)))
+	reportProgress := func(force bool) {
-			fmt.Printf("\rGenerating %s frames: %d/%d (%d%%)", label, count, totalFrames, pct)
+		if !force && time.Since(lastReport) < 250*time.Millisecond {
-		} else {
+			return
-			fmt.Printf("\rGenerating %s frames: %d", label, count)
 		}
+		if totalFrames > 0 {
+			pct := min(100, int(math.Round(float64(frameCount)/float64(totalFrames)*100)))
+			fmt.Printf("\rGenerating %s frames: %d/%d (%d%%)", label, frameCount, totalFrames, pct)
+		} else {
+			fmt.Printf("\rGenerating %s frames: %d", label, frameCount)
+		}
+		lastReport = time.Now()
+	}
+	failStream := func(err error) (int, error) {
+		_ = cmd.Process.Kill()
+		_ = cmd.Wait()
+		return frameCount, err
 	}
 
 	buf := make([]byte, 256*1024)
 	for {
-		n, err := stdout.Read(buf)
+		n, readErr := stdout.Read(buf)
 		if n > 0 {
-			frames = append(frames, splitter.push(buf[:n])...)
+			emitted, splitErr := splitter.push(buf[:n], emit)
-			reportProgress(len(frames))
+			frameCount += emitted
+			if splitErr != nil {
+				return failStream(fmt.Errorf("ffmpeg stream error: %w", splitErr))
+			}
+			if emitted > 0 {
+				reportProgress(false)
+			}
 		}
-		if err == io.EOF {
+		if readErr == io.EOF {
 			break
 		}
-		if err != nil {
+		if readErr != nil {
-			_ = cmd.Wait()
+			return failStream(fmt.Errorf("ffmpeg stream error: %w", readErr))
-			return nil, fmt.Errorf("ffmpeg stream error: %s", err.Error())
 		}
 	}
 	if err := cmd.Wait(); err != nil {
-		return nil, fmt.Errorf("ffmpeg failed: %s", strings.TrimSpace(stderr.String()))
+		if msg := stderr.String(); msg != "" {
+			return frameCount, fmt.Errorf("ffmpeg failed: %s", msg)
+		}
+		return frameCount, fmt.Errorf("ffmpeg failed: %w", err)
 	}
-	if len(frames) == 0 {
+	if err := splitter.finish(); err != nil {
-		return nil, fmt.Errorf("no frames were decoded from the video")
+		return frameCount, err
 	}
+	if frameCount == 0 {
+		return 0, fmt.Errorf("no frames were decoded from the video")
+	}
+	reportProgress(true)
 	fmt.Println()
-	return frames, nil
+	return frameCount, nil
 }
 
 func processVideo(path, output string, maxDimension int) error {
@@ -175,15 +351,19 @@ func processVideo(path, output string, maxDimension int) error {
 		maxDimension, maxDimension,
 	)
 
-	colorFrames, err := extractFrames(path, scaleFilter, "color", maxDimension, totalFrames)
+	outputFile, err := newStreamingTSF(output, fps)
 	if err != nil {
 		return err
 	}
+	defer outputFile.abort()
 
-	videoData := FramesContainer{FPS: fps, ColorFrames: colorFrames}
+	frameCount, err := extractFrames(path, scaleFilter, "color", totalFrames, outputFile.addFrame)
-	if err := writeTSF(output, &videoData); err != nil {
+	if err != nil {
 		return err
 	}
-	logInfo(fmt.Sprintf("Saved %d frames to %s", len(videoData.ColorFrames), output))
+	if err := outputFile.commit(output); err != nil {
+		return err
+	}
+	logInfo(fmt.Sprintf("Saved %d frames to %s", frameCount, output))
 	return nil
 }

src/videoprocessor_test.go

@@ -0,0 +1,122 @@
+package main
+
+import (
+	"bytes"
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestJPEGFrameSplitterAcrossChunks(t *testing.T) {
+	splitter := &jpegFrameSplitter{}
+	var frames [][]byte
+	emit := func(frame []byte) error {
+		frames = append(frames, bytes.Clone(frame))
+		return nil
+	}
+
+	chunks := [][]byte{
+		{0x01, 0x02, 0xff},
+		{0xd8, 0x10, 0xff},
+		{0xd9, 0xff, 0xd8, 0x20},
+		{0x30, 0xff},
+		{0xd9, 0x03},
+	}
+	for _, chunk := range chunks {
+		if _, err := splitter.push(chunk, emit); err != nil {
+			t.Fatalf("push: %v", err)
+		}
+	}
+	if err := splitter.finish(); err != nil {
+		t.Fatalf("finish: %v", err)
+	}
+
+	want := [][]byte{
+		{0xff, 0xd8, 0x10, 0xff, 0xd9},
+		{0xff, 0xd8, 0x20, 0x30, 0xff, 0xd9},
+	}
+	if len(frames) != len(want) {
+		t.Fatalf("got %d frames, want %d", len(frames), len(want))
+	}
+	for i := range want {
+		if !bytes.Equal(frames[i], want[i]) {
+			t.Errorf("frame %d = %x, want %x", i, frames[i], want[i])
+		}
+	}
+}
+
+func TestJPEGFrameSplitterRejectsTruncatedFrame(t *testing.T) {
+	splitter := &jpegFrameSplitter{}
+	if _, err := splitter.push([]byte{0xff, 0xd8, 0x01}, func([]byte) error { return nil }); err != nil {
+		t.Fatalf("push: %v", err)
+	}
+	if err := splitter.finish(); err == nil {
+		t.Fatal("finish accepted a truncated JPEG")
+	}
+}
+
+func TestBoundedLog(t *testing.T) {
+	log := &boundedLog{limit: 4}
+	if n, err := log.Write([]byte("abcdefgh")); err != nil || n != 8 {
+		t.Fatalf("Write = %d, %v", n, err)
+	}
+	if got := log.String(); got != "abcd" {
+		t.Fatalf("String = %q, want %q", got, "abcd")
+	}
+}
+
+func TestStreamingTSFCommitAndAbort(t *testing.T) {
+	dir := t.TempDir()
+	output := filepath.Join(dir, "frames.tsf")
+
+	stream, err := newStreamingTSF(output, 24)
+	if err != nil {
+		t.Fatalf("newStreamingTSF: %v", err)
+	}
+	for _, frame := range [][]byte{{1, 2, 3}, {4, 5}} {
+		if err := stream.addFrame(frame); err != nil {
+			t.Fatalf("addFrame: %v", err)
+		}
+	}
+	if err := stream.commit(output); err != nil {
+		t.Fatalf("commit: %v", err)
+	}
+	stream.abort()
+
+	got, err := loadTSF(output)
+	if err != nil {
+		t.Fatalf("loadTSF: %v", err)
+	}
+	defer func() { _ = got.Close() }()
+	if got.FPS != 24 || len(got.ColorFrames) != 2 || !bytes.Equal(got.ColorFrames[1], []byte{4, 5}) {
+		t.Fatalf("unexpected streamed TSF: %+v", got)
+	}
+
+	original := []byte("existing destination")
+	if err := os.WriteFile(output, original, 0o644); err != nil {
+		t.Fatalf("WriteFile: %v", err)
+	}
+	failed, err := newStreamingTSF(output, 24)
+	if err != nil {
+		t.Fatalf("newStreamingTSF: %v", err)
+	}
+	if err := failed.addFrame([]byte{9}); err != nil {
+		t.Fatalf("addFrame: %v", err)
+	}
+	failed.abort()
+
+	contents, err := os.ReadFile(output)
+	if err != nil {
+		t.Fatalf("ReadFile: %v", err)
+	}
+	if !bytes.Equal(contents, original) {
+		t.Fatalf("destination changed after abort: %q", contents)
+	}
+	matches, err := filepath.Glob(filepath.Join(dir, ".frames.tsf-*.tmp"))
+	if err != nil {
+		t.Fatalf("Glob: %v", err)
+	}
+	if len(matches) != 0 {
+		t.Fatalf("temporary files remain after abort: %v", matches)
+	}
+}