net/dns/resolver: remove maxDoHInFlight

It was originally added to control memory use on iOS (#2490), but then
was relaxed conditionally when running on iOS 15 (#3098). Now that we
require iOS 15, there's no need for the limit at all, so simplify back
to the original state.

Signed-off-by: Mihai Parparita <mihai@tailscale.com>

net/dns/resolver/doh_test.go

@@ -45,9 +45,7 @@ func TestDoH(t *testing.T) {
 		t.Fatal("no known DoH")
 	}
 
-	f := &forwarder{
+	f := &forwarder{}
-		dohSem: make(chan struct{}, 10),
-	}
 
 	for _, urlBase := range prefixes {
 		t.Run(urlBase, func(t *testing.T) {

net/dns/resolver/forwarder.go

@@ -15,16 +15,13 @@ import (
 	"net/http"
 	"net/netip"
 	"net/url"
-	"runtime"
 	"sort"
-	"strconv"
 	"strings"
 	"sync"
 	"time"
 
 	dns "golang.org/x/net/dns/dnsmessage"
 	"tailscale.com/envknob"
-	"tailscale.com/hostinfo"
 	"tailscale.com/net/dns/publicdns"
 	"tailscale.com/net/dnscache"
 	"tailscale.com/net/neterror"
@@ -181,7 +178,6 @@ type forwarder struct {
 	linkMon *monitor.Mon
 	linkSel ForwardLinkSelector // TODO(bradfitz): remove this when tsdial.Dialer absorbs it
 	dialer  *tsdial.Dialer
-	dohSem  chan struct{}
 
 	ctx       context.Context    // good until Close
 	ctxCancel context.CancelFunc // closes ctx
@@ -209,36 +205,12 @@ func init() {
 	rand.Seed(time.Now().UnixNano())
 }
 
-func maxDoHInFlight(goos string) int {
-	if goos != "ios" {
-		return 1000 // effectively unlimited
-	}
-	// iOS <  15 limits the memory to 15MB for NetworkExtensions.
-	// iOS >= 15 gives us 50MB.
-	// See: https://tailscale.com/blog/go-linker/
-	ver := hostinfo.GetOSVersion()
-	if ver == "" {
-		// Unknown iOS version, be cautious.
-		return 10
-	}
-	major, _, ok := strings.Cut(ver, ".")
-	if !ok {
-		// Unknown iOS version, be cautious.
-		return 10
-	}
-	if m, err := strconv.Atoi(major); err != nil || m < 15 {
-		return 10
-	}
-	return 1000
-}
-
 func newForwarder(logf logger.Logf, linkMon *monitor.Mon, linkSel ForwardLinkSelector, dialer *tsdial.Dialer) *forwarder {
 	f := &forwarder{
 		logf:    logger.WithPrefix(logf, "forward: "),
 		linkMon: linkMon,
 		linkSel: linkSel,
 		dialer:  dialer,
-		dohSem:  make(chan struct{}, maxDoHInFlight(runtime.GOOS)),
 	}
 	f.ctx, f.ctxCancel = context.WithCancel(context.Background())
 	return f
@@ -433,22 +405,7 @@ func (f *forwarder) getKnownDoHClientForProvider(urlBase string) (c *http.Client
 
 const dohType = "application/dns-message"
 
-func (f *forwarder) releaseDoHSem() { <-f.dohSem }
-
 func (f *forwarder) sendDoH(ctx context.Context, urlBase string, c *http.Client, packet []byte) ([]byte, error) {
-	// Bound the number of HTTP requests in flight. This primarily
-	// matters for iOS where we're very memory constrained and
-	// HTTP requests are heavier on iOS where we don't include
-	// HTTP/2 for binary size reasons (as binaries on iOS linked
-	// with Go code cost memory proportional to the binary size,
-	// for reasons not fully understood).
-	select {
-	case f.dohSem <- struct{}{}:
-	case <-ctx.Done():
-		return nil, ctx.Err()
-	}
-	defer f.releaseDoHSem()
-
 	metricDNSFwdDoH.Add(1)
 	req, err := http.NewRequestWithContext(ctx, "POST", urlBase, bytes.NewReader(packet))
 	if err != nil {

net/dns/resolver/forwarder_test.go

@@ -12,7 +12,6 @@ import (
 	"time"
 
 	dns "golang.org/x/net/dns/dnsmessage"
-	"tailscale.com/hostinfo"
 	"tailscale.com/types/dnstype"
 )
 
@@ -143,33 +142,6 @@ func TestGetRCode(t *testing.T) {
 	}
 }
 
-func TestMaxDoHInFlight(t *testing.T) {
-	tests := []struct {
-		goos string
-		ver  string
-		want int
-	}{
-		{"ios", "", 10},
-		{"ios", "1532", 10},
-		{"ios", "9.3.2", 10},
-		{"ios", "14.3.2", 10},
-		{"ios", "15.3.2", 1000},
-		{"ios", "20.3.2", 1000},
-		{"android", "", 1000},
-		{"darwin", "", 1000},
-		{"linux", "", 1000},
-	}
-	for _, tc := range tests {
-		t.Run(fmt.Sprintf("%s-%s", tc.goos, tc.ver), func(t *testing.T) {
-			hostinfo.SetOSVersion(tc.ver)
-			got := maxDoHInFlight(tc.goos)
-			if got != tc.want {
-				t.Errorf("got %d; want %d", got, tc.want)
-			}
-		})
-	}
-}
-
 var testDNS = flag.Bool("test-dns", false, "run tests that require a working DNS server")
 
 func TestGetKnownDoHClientForProvider(t *testing.T) {