wgengine/magicsock: fix DERP reader hang regression during concurrent reads

Fixes #1282

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

wgengine/magicsock/magicsock.go

@@ -12,6 +12,7 @@ import (
 	crand "crypto/rand"
 	"encoding/binary"
 	"errors"
+	"expvar"
 	"fmt"
 	"hash/fnv"
 	"math"
@@ -155,13 +156,10 @@ type Conn struct {
 	// derpRecvCh is used by ReceiveIPv4 to read DERP messages.
 	derpRecvCh chan derpReadResult
 
-	// derpRecvCountAtomic is atomically incremented by runDerpReader whenever
+	// derpRecvCountAtomic is how many derpRecvCh sends are pending.
-	// a DERP message arrives. It's incremented before runDerpReader is interrupted.
+	// It's incremented by runDerpReader whenever a DERP message
+	// arrives and decremented when they're read.
 	derpRecvCountAtomic int64
-	// derpRecvCountLast is used by ReceiveIPv4 to compare against
-	// its last read value of derpRecvCountAtomic to determine
-	// whether a DERP channel read should be done.
-	derpRecvCountLast int64 // owned by ReceiveIPv4
 
 	// ippEndpoint4 and ippEndpoint6 are owned by ReceiveIPv4 and
 	// ReceiveIPv6, respectively, to cache an IPPort->endpoint for
@@ -1358,6 +1356,8 @@ type derpReadResult struct {
 	// copyBuf is called to copy the data to dst.  It returns how
 	// much data was copied, which will be n if dst is large
 	// enough. copyBuf can only be called once.
+	// If copyBuf is nil, that's a signal from the sender to ignore
+	// this message.
 	copyBuf func(dst []byte) int
 }
 
@@ -1458,6 +1458,11 @@ func (c *Conn) runDerpReader(ctx context.Context, derpFakeAddr netaddr.IPPort, d
 	}
 }
 
+var (
+	testCounterZeroDerpReadResultSend expvar.Int
+	testCounterZeroDerpReadResultRecv expvar.Int
+)
+
 // sendDerpReadResult sends res to c.derpRecvCh and reports whether it
 // was sent. (It reports false if ctx was done first.)
 //
@@ -1465,7 +1470,7 @@ func (c *Conn) runDerpReader(ctx context.Context, derpFakeAddr netaddr.IPPort, d
 // ReceiveIPv4's blocking UDP read.
 func (c *Conn) sendDerpReadResult(ctx context.Context, res derpReadResult) (sent bool) {
 	// Before we wake up ReceiveIPv4 with SetReadDeadline,
-	// note that a DERP packet has arrived.  ReceiveIPv4
+	// note that a DERP packet has arrived. ReceiveIPv4
 	// will read this field to note that its UDP read
 	// error is due to us.
 	atomic.AddInt64(&c.derpRecvCountAtomic, 1)
@@ -1473,6 +1478,23 @@ func (c *Conn) sendDerpReadResult(ctx context.Context, res derpReadResult) (sent
 	c.pconn4.SetReadDeadline(aLongTimeAgo)
 	select {
 	case <-ctx.Done():
+		select {
+		case <-c.donec:
+			// The whole Conn shut down. The reader of
+			// c.derpRecvCh also selects on c.donec, so it's
+			// safe to abort now.
+		case c.derpRecvCh <- (derpReadResult{}):
+			// Just this DERP reader is closing (perhaps
+			// the user is logging out, or the DERP
+			// connection is too idle for sends). Since we
+			// already incremented c.derpRecvCountAtomic,
+			// we need to send on the channel (unless the
+			// conn is going down).
+			// The receiver treats a derpReadResult zero value
+			// message as a skip.
+			testCounterZeroDerpReadResultSend.Add(1)
+
+		}
 		return false
 	case c.derpRecvCh <- res:
 		return true
@@ -1568,20 +1590,20 @@ func (c *Conn) ReceiveIPv6(b []byte) (int, conn.Endpoint, error) {
 }
 
 func (c *Conn) derpPacketArrived() bool {
-	rc := atomic.LoadInt64(&c.derpRecvCountAtomic)
+	return atomic.LoadInt64(&c.derpRecvCountAtomic) > 0
-	if rc != c.derpRecvCountLast {
-		c.derpRecvCountLast = rc
-		return true
-	}
-	return false
 }
 
 // ReceiveIPv4 is called by wireguard-go to receive an IPv4 packet.
 // In Tailscale's case, that packet might also arrive via DERP. A DERP packet arrival
 // aborts the pconn4 read deadline to make it fail.
 func (c *Conn) ReceiveIPv4(b []byte) (n int, ep conn.Endpoint, err error) {
+	var pAddr net.Addr
 	for {
-		n, pAddr, err := c.pconn4.ReadFrom(b)
+		// Drain DERP queues before reading new UDP packets.
+		if c.derpPacketArrived() {
+			goto ReadDERP
+		}
+		n, pAddr, err = c.pconn4.ReadFrom(b)
 		if err != nil {
 			// If the pconn4 read failed, the likely reason is a DERP reader received
 			// a packet and interrupted us.
@@ -1589,18 +1611,21 @@ func (c *Conn) ReceiveIPv4(b []byte) (n int, ep conn.Endpoint, err error) {
 			// and for there to have also had a DERP packet arrive, but that's fine:
 			// we'll get the same error from ReadFrom later.
 			if c.derpPacketArrived() {
-				c.pconn4.SetReadDeadline(time.Time{}) // restore
+				goto ReadDERP
-				n, ep, err = c.receiveIPv4DERP(b)
-				if err == errLoopAgain {
-					continue
-				}
-				return n, ep, err
 			}
 			return 0, nil, err
 		}
 		if ep, ok := c.receiveIP(b[:n], pAddr.(*net.UDPAddr), &c.ippEndpoint4); ok {
 			return n, ep, nil
+		} else {
+			continue
 		}
+	ReadDERP:
+		n, ep, err = c.receiveIPv4DERP(b)
+		if err == errLoopAgain {
+			continue
+		}
+		return n, ep, err
 	}
 }
 
@@ -1668,6 +1693,13 @@ func (c *Conn) receiveIPv4DERP(b []byte) (n int, ep conn.Endpoint, err error) {
 	case dm = <-c.derpRecvCh:
 		// Below.
 	}
+	if atomic.AddInt64(&c.derpRecvCountAtomic, -1) == 0 {
+		c.pconn4.SetReadDeadline(time.Time{})
+	}
+	if dm.copyBuf == nil {
+		testCounterZeroDerpReadResultRecv.Add(1)
+		return 0, nil, errLoopAgain
+	}
 
 	var regionID int
 	n, regionID = dm.n, dm.regionID

wgengine/magicsock/magicsock_test.go

@@ -11,7 +11,6 @@ import (
 	"crypto/tls"
 	"encoding/binary"
 	"encoding/json"
-	"flag"
 	"fmt"
 	"io/ioutil"
 	"net"
@@ -1435,8 +1434,6 @@ func newNonLegacyTestConn(t testing.TB) *Conn {
 	return conn
 }
 
-var testIssue1282 = flag.Bool("test-issue-1282", false, "run test for https://github.com/tailscale/tailscale/issues/1282 on all CPUs")
-
 // Tests concurrent DERP readers pushing DERP data into ReceiveIPv4
 // (which should blend all DERP reads into UDP reads).
 func TestDerpReceiveFromIPv4(t *testing.T) {
@@ -1450,42 +1447,54 @@ func TestDerpReceiveFromIPv4(t *testing.T) {
 	defer sendConn.Close()
 	nodeKey, _ := addTestEndpoint(conn, sendConn)
 
-	var sends int = 500
+	var sends int = 250e3 // takes about a second
-	senders := runtime.NumCPU()
+	if testing.Short() {
-	if !*testIssue1282 {
+		sends /= 10
-		t.Logf("--test-issue-1282 was not specified; so doing single-threaded test (instead of NumCPU=%d) to work around https://github.com/tailscale/tailscale/issues/1282", senders)
-		senders = 1
 	}
+	senders := runtime.NumCPU()
 	sends -= (sends % senders)
 	var wg sync.WaitGroup
 	defer wg.Wait()
 	t.Logf("doing %v sends over %d senders", sends, senders)
-	ctx := context.Background()
+
+	ctx, cancel := context.WithCancel(context.Background())
+	defer conn.Close()
+	defer cancel()
+
+	doneCtx, cancelDoneCtx := context.WithCancel(context.Background())
+	cancelDoneCtx()
 
 	for i := 0; i < senders; i++ {
 		wg.Add(1)
 		regionID := i + 1
 		go func() {
 			defer wg.Done()
-			ch := make(chan bool, 1)
 			for i := 0; i < sends/senders; i++ {
-				if !conn.sendDerpReadResult(ctx, derpReadResult{
+				res := derpReadResult{
 					regionID: regionID,
 					n:        123,
 					src:      key.Public(nodeKey),
-					copyBuf: func(dst []byte) int {
+					copyBuf:  func(dst []byte) int { return 123 },
-						ch <- true
+				}
-						return 123
+				// First send with the closed context. ~50% of
-					},
+				// these should end up going through the
-				}) {
+				// send-a-zero-derpReadResult path, returning
+				// true, in which case we don't want to send again.
+				// We test later that we hit the other path.
+				if conn.sendDerpReadResult(doneCtx, res) {
+					continue
+				}
+
+				if !conn.sendDerpReadResult(ctx, res) {
 					t.Error("unexpected false")
 					return
 				}
-				<-ch
 			}
 		}()
 	}
 
+	zeroSendsStart := testCounterZeroDerpReadResultSend.Value()
+
 	buf := make([]byte, 1500)
 	for i := 0; i < sends; i++ {
 		n, ep, err := conn.ReceiveIPv4(buf)
@@ -1495,6 +1504,20 @@ func TestDerpReceiveFromIPv4(t *testing.T) {
 		_ = n
 		_ = ep
 	}
+
+	t.Logf("did %d ReceiveIPv4 calls", sends)
+
+	zeroSends, zeroRecv := testCounterZeroDerpReadResultSend.Value(), testCounterZeroDerpReadResultRecv.Value()
+	if zeroSends != zeroRecv {
+		t.Errorf("did %d zero sends != %d corresponding receives", zeroSends, zeroRecv)
+	}
+	zeroSendDelta := zeroSends - zeroSendsStart
+	if zeroSendDelta == 0 {
+		t.Errorf("didn't see any sends of derpReadResult zero value")
+	}
+	if zeroSendDelta == int64(sends) {
+		t.Errorf("saw %v sends of the derpReadResult zero value which was unexpectedly high (100%% of our %v sends)", zeroSendDelta, sends)
+	}
 }
 
 // addTestEndpoint sets conn's network map to a single peer expected