control/controlclient: improve handling of concurrent lite map requests

This reverts commit 6eca47b16c and fixes forward.

Previously the first ever streaming MapRequest that a client sent would also
set ReadOnly to true as it didn't have any endpoints and expected/relied on the
map poll to restart as soon as it got endpoints. However with 48f6c1eba4,
we would no longer restart MapRequests as frequently as we used to, so control
would only ever get the first streaming MapRequest which had ReadOnly=true.

Control would treat this as an uninteresting request and would not send it
any further netmaps, while the client would happily stay in the map poll forever
while litemap updates happened in parallel.

This makes it so that we never set `ReadOnly=true` when we are doing a streaming
MapRequest. This is no longer necessary either as most endpoint discovery happens
over disco anyway.

Co-authored-by: Andrew Dunham <andrew@du.nham.ca>
Signed-off-by: Maisem Ali <maisem@tailscale.com>

cmd/tailscaled/depaware.txt

@@ -414,7 +414,7 @@ tailscale.com/cmd/tailscaled dependencies: (generated by github.com/tailscale/de
         encoding/xml                                                 from github.com/tailscale/goupnp+
         errors                                                       from bufio+
         expvar                                                       from tailscale.com/derp+
-        flag                                                         from tailscale.com/control/controlclient+
+        flag                                                         from net/http/httptest+
         fmt                                                          from compress/flate+
         hash                                                         from crypto+
         hash/adler32                                                 from tailscale.com/ipn/ipnlocal

control/controlclient/auto.go

@@ -66,6 +66,8 @@ type Auto struct {
 	synced               bool               // true if our netmap is up-to-date
 	inPollNetMap         bool               // true if currently running a PollNetMap
 	inLiteMapUpdate      bool               // true if a lite (non-streaming) map request is outstanding
+	liteMapUpdateCancel  context.CancelFunc // cancels a lite map update, may be nil
+	liteMapUpdateCancels int                // how many times we've canceled a lite map update
 	inSendStatus         int                // number of sendStatus calls currently in progress
 	state                State
 
@@ -168,28 +170,56 @@ func (c *Auto) Start() {
 func (c *Auto) sendNewMapRequest() {
 	c.mu.Lock()
 
-	// If we're not already streaming a netmap, or if we're already stuck
-	// in a lite update, then tear down everything and start a new stream
-	// (which starts by sending a new map request)
-	if !c.inPollNetMap || c.inLiteMapUpdate || !c.loggedIn {
+	// If we're not already streaming a netmap, then tear down everything
+	// and start a new stream (which starts by sending a new map request)
+	if !c.inPollNetMap || !c.loggedIn {
 		c.mu.Unlock()
 		c.cancelMapSafely()
 		return
 	}
 
+	// If we are already in process of doing a LiteMapUpdate, cancel it and
+	// try a new one. If this is the 10th time we have done this
+	// cancelation, tear down everything and start again.
+	const maxLiteMapUpdateAttempts = 10
+	if c.inLiteMapUpdate {
+		// Always cancel the in-flight lite map update, regardless of
+		// whether we cancel the streaming map request or not.
+		c.liteMapUpdateCancel()
+		c.inLiteMapUpdate = false
+
+		if c.liteMapUpdateCancels >= maxLiteMapUpdateAttempts {
+			// Not making progress
+			c.mu.Unlock()
+			c.cancelMapSafely()
+			return
+		}
+
+		// Increment our cancel counter and continue below to start a
+		// new lite update.
+		c.liteMapUpdateCancels++
+	}
+
 	// Otherwise, send a lite update that doesn't keep a
 	// long-running stream response.
 	defer c.mu.Unlock()
 	c.inLiteMapUpdate = true
 	ctx, cancel := context.WithTimeout(c.mapCtx, 10*time.Second)
+	c.liteMapUpdateCancel = cancel
 	go func() {
 		defer cancel()
 		t0 := time.Now()
 		err := c.direct.SendLiteMapUpdate(ctx)
 		d := time.Since(t0).Round(time.Millisecond)
+
 		c.mu.Lock()
 		c.inLiteMapUpdate = false
+		c.liteMapUpdateCancel = nil
+		if err == nil {
+			c.liteMapUpdateCancels = 0
+		}
 		c.mu.Unlock()
+
 		if err == nil {
 			c.logf("[v1] successful lite map update in %v", d)
 			return
@@ -197,10 +227,13 @@ func (c *Auto) sendNewMapRequest() {
 		if ctx.Err() == nil {
 			c.logf("lite map update after %v: %v", d, err)
 		}
+		if !errors.Is(ctx.Err(), context.Canceled) {
 			// Fall back to restarting the long-polling map
 			// request (the old heavy way) if the lite update
-		// failed for any reason.
+			// failed for reasons other than the context being
+			// canceled.
 			c.cancelMapSafely()
+		}
 	}()
 }
 
@@ -237,6 +270,12 @@ func (c *Auto) cancelMapSafely() {
 	c.mu.Lock()
 	defer c.mu.Unlock()
 
+	// Always reset our lite map cancels counter if we're canceling
+	// everything, since we're about to restart with a new map update; this
+	// allows future calls to sendNewMapRequest to retry sending lite
+	// updates.
+	c.liteMapUpdateCancels = 0
+
 	c.logf("[v1] cancelMapSafely: synced=%v", c.synced)
 
 	if c.inPollNetMap {

control/controlclient/direct.go

@@ -12,7 +12,6 @@ import (
 	"encoding/binary"
 	"encoding/json"
 	"errors"
-	"flag"
 	"fmt"
 	"io"
 	"log"
@@ -97,7 +96,6 @@ type Direct struct {
 	netinfo      *tailcfg.NetInfo
 	endpoints    []tailcfg.Endpoint
 	tkaHead      string
-	everEndpoints bool   // whether we've ever had non-empty endpoints
 	lastPingURL  string // last PingRequest.URL received, for dup suppression
 }
 
@@ -753,9 +751,6 @@ func (c *Direct) newEndpoints(endpoints []tailcfg.Endpoint) (changed bool) {
 	}
 	c.logf("[v2] client.newEndpoints(%v)", epStrs)
 	c.endpoints = append(c.endpoints[:0], endpoints...)
-	if len(endpoints) > 0 {
-		c.everEndpoints = true
-	}
 	return true // changed
 }
 
@@ -768,8 +763,6 @@ func (c *Direct) SetEndpoints(endpoints []tailcfg.Endpoint) (changed bool) {
 	return c.newEndpoints(endpoints)
 }
 
-func inTest() bool { return flag.Lookup("test.v") != nil }
-
 // PollNetMap makes a /map request to download the network map, calling cb with
 // each new netmap.
 func (c *Direct) PollNetMap(ctx context.Context, cb func(*netmap.NetworkMap)) error {
@@ -824,7 +817,6 @@ func (c *Direct) sendMapRequest(ctx context.Context, maxPolls int, readOnly bool
 		epStrs = append(epStrs, ep.Addr.String())
 		epTypes = append(epTypes, ep.Type)
 	}
-	everEndpoints := c.everEndpoints
 	c.mu.Unlock()
 
 	machinePrivKey, err := c.getMachinePrivKey()
@@ -865,15 +857,17 @@ func (c *Direct) sendMapRequest(ctx context.Context, maxPolls int, readOnly bool
 		OmitPeers:     cb == nil,
 		TKAHead:       c.tkaHead,
 
-		// On initial startup before we know our endpoints, set the ReadOnly flag
-		// to tell the control server not to distribute out our (empty) endpoints to peers.
-		// Presumably we'll learn our endpoints in a half second and do another post
-		// with useful results. The first POST just gets us the DERP map which we
-		// need to do the STUN queries to discover our endpoints.
-		// TODO(bradfitz): we skip this optimization in tests, though,
-		// because the e2e tests are currently hyper-specific about the
-		// ordering of things. The e2e tests need love.
-		ReadOnly: readOnly || (len(epStrs) == 0 && !everEndpoints && !inTest()),
+		// Previously we'd set ReadOnly to true if we didn't have any endpoints
+		// yet as we expected to learn them in a half second and restart the full
+		// streaming map poll, however as we are trying to reduce the number of
+		// times we restart the full streaming map poll we now just set ReadOnly
+		// false when we're doing a full streaming map poll.
+		//
+		// TODO(maisem/bradfitz): really ReadOnly should be set to true if for
+		// all streams and we should only do writes via lite map updates.
+		// However that requires an audit and a bunch of testing to make sure we
+		// don't break anything.
+		ReadOnly: readOnly && !allowStream,
 	}
 	var extraDebugFlags []string
 	if hi != nil && c.linkMon != nil && !c.skipIPForwardingCheck &&