control/controlclient: set MapRequest.ReadOnly on initial empty endpoint request

On startup, clients do a MapRequest with empty endpoints while they
learn the DERP map to discover the STUN servers they then query to
learn their endpoints.

Set MapRequest.ReadOnly on those initial queries to not broadcast the
empty endpoints out to peers. The read results will come a half second
later (or less).

Updates tailscale/corp#557

control/controlclient/direct.go

@@ -13,6 +13,7 @@ import (
 	"encoding/binary"
 	"encoding/json"
 	"errors"
+	"flag"
 	"fmt"
 	"io"
 	"io/ioutil"
@@ -116,6 +117,7 @@ type Direct struct {
 	// hostinfo is mutated in-place while mu is held.
 	hostinfo      *tailcfg.Hostinfo // always non-nil
 	endpoints     []string
+	everEndpoints bool   // whether we've ever had non-empty endpoints
 	localPort     uint16 // or zero to mean auto
 }
 
@@ -476,6 +478,9 @@ func (c *Direct) newEndpoints(localPort uint16, endpoints []string) (changed boo
 	c.logf("client.newEndpoints(%v, %v)", localPort, endpoints)
 	c.localPort = localPort
 	c.endpoints = append(c.endpoints[:0], endpoints...)
+	if len(endpoints) > 0 {
+		c.everEndpoints = true
+	}
 	return true // changed
 }
 
@@ -488,6 +493,13 @@ func (c *Direct) SetEndpoints(localPort uint16, endpoints []string) (changed boo
 	return c.newEndpoints(localPort, 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.
+//
+// maxPolls is how many network maps to download; common values are 1
+// or -1 (to keep a long-poll query open to the server).
 func (c *Direct) PollNetMap(ctx context.Context, maxPolls int, cb func(*NetworkMap)) error {
 	c.mu.Lock()
 	persist := c.persist
@@ -497,6 +509,7 @@ func (c *Direct) PollNetMap(ctx context.Context, maxPolls int, cb func(*NetworkM
 	backendLogID := hostinfo.BackendLogID
 	localPort := c.localPort
 	ep := append([]string(nil), c.endpoints...)
+	everEndpoints := c.everEndpoints
 	c.mu.Unlock()
 
 	if backendLogID == "" {
@@ -504,7 +517,7 @@ func (c *Direct) PollNetMap(ctx context.Context, maxPolls int, cb func(*NetworkM
 	}
 
 	allowStream := maxPolls != 1
-	c.logf("PollNetMap: stream=%v :%v %v", maxPolls, localPort, ep)
+	c.logf("PollNetMap: stream=%v :%v ep=%v", allowStream, localPort, ep)
 
 	vlogf := logger.Discard
 	if Debug.NetMap {
@@ -525,6 +538,17 @@ func (c *Direct) PollNetMap(ctx context.Context, maxPolls int, cb func(*NetworkM
 	if c.newDecompressor != nil {
 		request.Compress = "zstd"
 	}
+	// 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 hyperspecific about the
+	// ordering of things. The e2e tests need love.
+	if len(ep) == 0 && !everEndpoints && !inTest() {
+		request.ReadOnly = true
+	}
 
 	bodyData, err := encode(request, &serverKey, &c.machinePrivKey)
 	if err != nil {
@@ -532,16 +556,17 @@ func (c *Direct) PollNetMap(ctx context.Context, maxPolls int, cb func(*NetworkM
 		return err
 	}
 
+	ctx, cancel := context.WithCancel(ctx)
+	defer cancel()
+
 	machinePubKey := tailcfg.MachineKey(c.machinePrivKey.Public())
 	t0 := time.Now()
 	u := fmt.Sprintf("%s/machine/%s/map", serverURL, machinePubKey.HexString())
-	req, err := http.NewRequest("POST", u, bytes.NewReader(bodyData))
+
+	req, err := http.NewRequestWithContext(ctx, "POST", u, bytes.NewReader(bodyData))
 	if err != nil {
 		return err
 	}
-	ctx, cancel := context.WithCancel(ctx)
-	defer cancel()
-	req = req.WithContext(ctx)
 
 	res, err := c.httpc.Do(req)
 	if err != nil {