controlclient/netmap: write our own b.ConciseDiffFrom(a) function.

This removes the need for go-cmp, which is extremely bloaty so we had
to leave it out of iOS. As a result, we had also left it out of macOS,
and so we didn't print netmap diffs at all on darwin-based platforms.
Oops.

As a bonus, the output format of the new function is way better.

Minor oddity: because I used the dumbest possible diff algorithm, the
sort order is a bit dumb. We print all "removed" lines and then print
all "added" lines, rather than doing the usual diff-like thing of
interspersing them. This probably doesn't matter (maybe it's an
improvement).

cmd/relaynode/relaynode.go

@@ -23,7 +23,6 @@ import (
 	"time"
 
 	"github.com/apenwarr/fixconsole"
-	"github.com/google/go-cmp/cmp"
 	"github.com/klauspost/compress/zstd"
 	"github.com/pborman/getopt/v2"
 	"github.com/tailscale/wireguard-go/wgcfg"
@@ -103,9 +102,7 @@ func main() {
 
 		if m := new.NetMap; m != nil {
 			if lastNetMap != nil {
-				s1 := strings.Split(lastNetMap.Concise(), "\n")
-				s2 := strings.Split(new.NetMap.Concise(), "\n")
-				logf("netmap diff:\n%v\n", cmp.Diff(s1, s2))
+				logf("netmap diff:\n%v\n", new.NetMap.ConciseDiffFrom(lastNetMap))
 			}
 			lastNetMap = m
 

control/controlclient/netmap.go

@@ -115,6 +115,34 @@ func (nm *NetworkMap) Concise() string {
 	return buf.String()
 }
 
+func (b *NetworkMap) ConciseDiffFrom(a *NetworkMap) string {
+	out := []string{}
+	ra := strings.Split(a.Concise(), "\n")
+	rb := strings.Split(b.Concise(), "\n")
+
+	ma := map[string]struct{}{}
+	for _, s := range ra {
+		ma[s] = struct{}{}
+	}
+
+	mb := map[string]struct{}{}
+	for _, s := range rb {
+		mb[s] = struct{}{}
+	}
+
+	for _, s := range ra {
+		if _, ok := mb[s]; !ok {
+			out = append(out, "-"+s)
+		}
+	}
+	for _, s := range rb {
+		if _, ok := ma[s]; !ok {
+			out = append(out, "+"+s)
+		}
+	}
+	return strings.Join(out, "\n")
+}
+
 func (nm *NetworkMap) JSON() string {
 	b, err := json.MarshalIndent(*nm, "", "  ")
 	if err != nil {

ipn/ipnserver/server.go

@@ -18,7 +18,6 @@ import (
 	"syscall"
 	"time"
 
-	"github.com/google/go-cmp/cmp"
 	"github.com/klauspost/compress/zstd"
 	"tailscale.com/control/controlclient"
 	"tailscale.com/ipn"
@@ -112,7 +111,6 @@ func Run(rctx context.Context, logf logger.Logf, logid string, opts Options, e w
 	b.SetDecompressor(func() (controlclient.Decompressor, error) {
 		return zstd.NewReader(nil)
 	})
-	b.SetCmpDiff(func(x, y interface{}) string { return cmp.Diff(x, y) })
 
 	var s net.Conn
 	serverToClient := func(b []byte) {

ipn/local.go

@@ -34,7 +34,6 @@ type LocalBackend struct {
 	backendLogID    string
 	portpoll        *portlist.Poller // may be nil
 	newDecompressor func() (controlclient.Decompressor, error)
-	cmpDiff         func(x, y interface{}) string
 
 	// The mutex protects the following elements.
 	mu           sync.Mutex
@@ -111,17 +110,6 @@ func (b *LocalBackend) SetDecompressor(fn func() (controlclient.Decompressor, er
 	b.newDecompressor = fn
 }
 
-// SetCmpDiff sets a comparison function used to generate logs of what
-// has changed in the network map.
-//
-// Typically the comparison function comes from go-cmp.
-// We don't wire it in directly here because the go-cmp package adds
-// 1.77mb to the binary size of the iOS NetworkExtension, which takes
-// away from its precious RSS limit.
-func (b *LocalBackend) SetCmpDiff(cmpDiff func(x, y interface{}) string) {
-	b.cmpDiff = cmpDiff
-}
-
 func (b *LocalBackend) Start(opts Options) error {
 	if opts.Prefs == nil && opts.StateKey == "" {
 		return errors.New("no state key or prefs provided")
@@ -224,13 +212,9 @@ func (b *LocalBackend) Start(opts Options) error {
 		}
 		if newSt.NetMap != nil {
 			b.mu.Lock()
-			if b.netMapCache != nil && b.cmpDiff != nil {
-				s1 := strings.Split(b.netMapCache.Concise(), "\n")
-				s2 := strings.Split(newSt.NetMap.Concise(), "\n")
-				diff := b.cmpDiff(s1, s2)
-				if strings.TrimSpace(diff) != "" {
-					b.logf("netmap diff:\n%v\n", diff)
-				}
+			if b.netMapCache != nil {
+				diff := newSt.NetMap.ConciseDiffFrom(b.netMapCache)
+				b.logf("netmap diff:\n%v\n", diff)
 			}
 			b.netMapCache = newSt.NetMap
 			b.mu.Unlock()