all: depend on zstd unconditionally, remove plumbing to make it optional

All platforms use it at this point, including iOS which was the
original hold out for memory reasons. No more reason to make it
optional.

Updates #9332

Change-Id: I743fbc2f370921a852fbcebf4eb9821e2bdd3086
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

control/controlclient/direct.go

@@ -43,6 +43,7 @@ import (
 	"tailscale.com/net/tlsdial"
 	"tailscale.com/net/tsdial"
 	"tailscale.com/net/tshttpproxy"
+	"tailscale.com/smallzstd"
 	"tailscale.com/syncs"
 	"tailscale.com/tailcfg"
 	"tailscale.com/tka"
@@ -68,7 +69,6 @@ type Direct struct {
 	serverURL             string // URL of the tailcontrol server
 	clock                 tstime.Clock
 	lastPrintMap          time.Time
-	newDecompressor       func() (Decompressor, error)
 	logf                  logger.Logf
 	netMon                *netmon.Monitor // or nil
 	discoPubKey           key.DiscoPublic
@@ -119,7 +119,6 @@ type Options struct {
 	Clock                tstime.Clock
 	Hostinfo             *tailcfg.Hostinfo // non-nil passes ownership, nil means to use default using os.Hostname, etc
 	DiscoPublicKey       key.DiscoPublic
-	NewDecompressor      func() (Decompressor, error)
 	Logf                 logger.Logf
 	HTTPTestClient       *http.Client                 // optional HTTP client to use (for tests only)
 	NoiseTestClient      *http.Client                 // optional HTTP client to use for noise RPCs (tests only)
@@ -254,7 +253,6 @@ func NewDirect(opts Options) (*Direct, error) {
 		serverURL:             opts.ServerURL,
 		clock:                 opts.Clock,
 		logf:                  opts.Logf,
-		newDecompressor:       opts.NewDecompressor,
 		persist:               opts.Persist.View(),
 		authKey:               opts.AuthKey,
 		discoPubKey:           opts.DiscoPublicKey,
@@ -891,9 +889,7 @@ func (c *Direct) sendMapRequest(ctx context.Context, isStreaming bool, nu Netmap
 		old := request.DebugFlags
 		request.DebugFlags = append(old[:len(old):len(old)], extraDebugFlags...)
 	}
-	if c.newDecompressor != nil {
-		request.Compress = "zstd"
-	}
+	request.Compress = "zstd"
 
 	bodyData, err := encode(request, serverKey, serverNoiseKey, machinePrivKey)
 	if err != nil {
@@ -1177,19 +1173,14 @@ func (c *Direct) decodeMsg(msg []byte, v any, mkey key.MachinePrivate) error {
 	} else {
 		decrypted = msg
 	}
-	var b []byte
-	if c.newDecompressor == nil {
-		b = decrypted
-	} else {
-		decoder, err := c.newDecompressor()
-		if err != nil {
-			return err
-		}
-		defer decoder.Close()
-		b, err = decoder.DecodeAll(decrypted, nil)
-		if err != nil {
-			return err
-		}
+	decoder, err := smallzstd.NewDecoder(nil)
+	if err != nil {
+		return err
+	}
+	defer decoder.Close()
+	b, err := decoder.DecodeAll(decrypted, nil)
+	if err != nil {
+		return err
 	}
 	if debugMap() {
 		var buf bytes.Buffer