Support client_max_window_bits and server_max_window_bits compression options

accept.go

@@ -15,6 +15,7 @@ import (
 	"net/textproto"
 	"net/url"
 	"path"
+	"strconv"
 	"strings"
 
 	"github.com/coder/websocket/internal/errd"
@@ -298,15 +299,34 @@ func acceptDeflate(ext websocketExtension, mode CompressionMode) (*compressionOp
 		case "server_no_context_takeover":
 			copts.serverNoContextTakeover = true
 			continue
-		case "client_max_window_bits",
-			"server_max_window_bits=15":
+		case "client_max_window_bits":
+			copts.clientMaxWindowBits = 15 // default
+			continue
+		case "server_max_window_bits":
+			copts.serverMaxWindowBits = 15 // default
 			continue
 		}
 
 		if strings.HasPrefix(p, "client_max_window_bits=") {
-			// We can't adjust the deflate window, but decoding with a larger window is acceptable.
+			// We don't need to change decoder settings; larger window decoder can read smaller windows.
+			if v, err := strconv.Atoi(strings.TrimPrefix(p, "client_max_window_bits=")); err == nil {
+				if v >= 8 && v <= 15 {
+					copts.clientMaxWindowBits = v
+				}
+			}
 			continue
 		}
+
+		if strings.HasPrefix(p, "server_max_window_bits=") {
+			vstr := strings.TrimPrefix(p, "server_max_window_bits=")
+			v, err := strconv.Atoi(vstr)
+			if err != nil || v < 8 || v > 15 {
+				return nil, false // invalid per RFC
+			}
+			copts.serverMaxWindowBits = v
+			continue
+		}
+
 		return nil, false
 	}
 	return copts, true

accept_test.go

@@ -515,6 +515,22 @@ func Test_selectDeflate(t *testing.T) {
 			expCopts: &compressionOptions{
 				clientNoContextTakeover: true,
 				serverNoContextTakeover: true,
+
+				clientMaxWindowBits: 15,
+				serverMaxWindowBits: 0,
+			},
+			expOK: true,
+		},
+		{
+			name:   "permessage-deflate/custom-client-window-bits",
+			mode:   CompressionNoContextTakeover,
+			header: "permessage-deflate; client_max_window_bits=12",
+			expCopts: &compressionOptions{
+				clientNoContextTakeover: true,
+				serverNoContextTakeover: true,
+
+				clientMaxWindowBits: 12,
+				serverMaxWindowBits: 0,
 			},
 			expOK: true,
 		},
@@ -531,6 +547,9 @@ func Test_selectDeflate(t *testing.T) {
 			expCopts: &compressionOptions{
 				clientNoContextTakeover: true,
 				serverNoContextTakeover: true,
+
+				clientMaxWindowBits: 15,
+				serverMaxWindowBits: 0,
 			},
 			expOK: true,
 		},

compress.go

@@ -3,7 +3,10 @@
 package websocket
 
 import (
-	"compress/flate"
+	"strconv"
+
+	"github.com/klauspost/compress/flate"
+
 	"io"
 	"sync"
 )
@@ -53,12 +56,18 @@ func (m CompressionMode) opts() *compressionOptions {
 	return &compressionOptions{
 		clientNoContextTakeover: m == CompressionNoContextTakeover,
 		serverNoContextTakeover: m == CompressionNoContextTakeover,
+
+		serverMaxWindowBits: 0,
+		clientMaxWindowBits: 0,
 	}
 }
 
 type compressionOptions struct {
 	clientNoContextTakeover bool
 	serverNoContextTakeover bool
+
+	serverMaxWindowBits int
+	clientMaxWindowBits int
 }
 
 func (copts *compressionOptions) String() string {
@@ -69,6 +78,11 @@ func (copts *compressionOptions) String() string {
 	if copts.serverNoContextTakeover {
 		s += "; server_no_context_takeover"
 	}
+
+	if copts.clientMaxWindowBits != 0 {
+		s += "; client_max_window_bits=" + strconv.Itoa(copts.clientMaxWindowBits)
+	}
+
 	return s
 }
 
@@ -147,20 +161,24 @@ func putFlateReader(fr io.Reader) {
 	flateReaderPool.Put(fr)
 }
 
-var flateWriterPool sync.Pool
+var flateWriterPool [16]sync.Pool
 
-func getFlateWriter(w io.Writer) *flate.Writer {
-	fw, ok := flateWriterPool.Get().(*flate.Writer)
+func getFlateWriter(w io.Writer, bits int) *flate.Writer {
+	fw, ok := flateWriterPool[bits].Get().(*flate.Writer)
 	if !ok {
-		fw, _ = flate.NewWriter(w, flate.BestSpeed)
+		if bits == 0 {
+			fw, _ = flate.NewWriter(w, flate.BestCompression)
+		} else {
+			fw, _ = flate.NewWriterWindow(w, 1<<bits)
+		}
 		return fw
 	}
 	fw.Reset(w)
 	return fw
 }
 
-func putFlateWriter(w *flate.Writer) {
-	flateWriterPool.Put(w)
+func putFlateWriter(w *flate.Writer, bits int) {
+	flateWriterPool[bits].Put(w)
 }
 
 type slidingWindow struct {

go.mod

@@ -1,3 +1,5 @@
 module github.com/coder/websocket
 
 go 1.23
+
+require github.com/klauspost/compress v1.18.0 // indirect

go.sum

@@ -0,0 +1,2 @@
+github.com/klauspost/compress v1.18.0 h1:c/Cqfb0r+Yi+JtIEq73FWXVkRonBlf0CRNYc8Zttxdo=
+github.com/klauspost/compress v1.18.0/go.mod h1:2Pp+KzxcywXVXMr50+X0Q/Lsb43OQHYWRCY2AiWywWQ=

write.go

@@ -4,7 +4,6 @@ package websocket
 
 import (
 	"bufio"
-	"compress/flate"
 	"context"
 	"crypto/rand"
 	"encoding/binary"
@@ -14,6 +13,8 @@ import (
 	"net"
 	"time"
 
+	"github.com/klauspost/compress/flate"
+
 	"github.com/coder/websocket/internal/errd"
 	"github.com/coder/websocket/internal/util"
 )
@@ -79,7 +80,7 @@ func (mw *msgWriter) ensureFlate() {
 	}
 
 	if mw.flateWriter == nil {
-		mw.flateWriter = getFlateWriter(mw.trimWriter)
+		mw.flateWriter = getFlateWriter(mw.trimWriter, mw.c.copts.serverMaxWindowBits)
 	}
 	mw.flate = true
 }
@@ -137,7 +138,7 @@ func (mw *msgWriter) reset(ctx context.Context, typ MessageType) error {
 
 func (mw *msgWriter) putFlateWriter() {
 	if mw.flateWriter != nil {
-		putFlateWriter(mw.flateWriter)
+		putFlateWriter(mw.flateWriter, mw.c.copts.serverMaxWindowBits)
 		mw.flateWriter = nil
 	}
 }