control/controlclient: use lite map request handler to avoid aborting streams

Previously, any change to endpoints or hostinfo (or hostinfo's
netinfo) would result in the long-running map request HTTP stream
being torn down and restarted, losing all compression context along
with it.

This change makes us instead send a lite map request (OmitPeers: true,
Stream: false) that doesn't subscribe to anything, and then the
coordination server knows to not close other streams for that node
when it recives a lite request.

Fixes tailscale/corp#797

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

control/controlclient/direct.go

@@ -521,6 +521,18 @@ func inTest() bool { return flag.Lookup("test.v") != nil }
 // 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 {
+	return c.sendMapRequest(ctx, maxPolls, cb)
+}
+
+// SendLiteMapUpdate makes a /map request to update the server of our latest state,
+// but does not fetch anything. It returns an error if the server did not return a
+// successful 200 OK response.
+func (c *Direct) SendLiteMapUpdate(ctx context.Context) error {
+	return c.sendMapRequest(ctx, 1, nil)
+}
+
+// cb nil means to omit peers.
+func (c *Direct) sendMapRequest(ctx context.Context, maxPolls int, cb func(*NetworkMap)) error {
 	c.mu.Lock()
 	persist := c.persist
 	serverURL := c.serverURL
@@ -555,6 +567,7 @@ func (c *Direct) PollNetMap(ctx context.Context, maxPolls int, cb func(*NetworkM
 		Stream:     allowStream,
 		Hostinfo:   hostinfo,
 		DebugFlags: c.debugFlags,
+		OmitPeers:  cb == nil,
 	}
 	if hostinfo != nil && ipForwardingBroken(hostinfo.RoutableIPs) {
 		old := request.DebugFlags
@@ -607,6 +620,11 @@ func (c *Direct) PollNetMap(ctx context.Context, maxPolls int, cb func(*NetworkM
 	}
 	defer res.Body.Close()
 
+	if cb == nil {
+		io.Copy(ioutil.Discard, res.Body)
+		return nil
+	}
+
 	// If we go more than pollTimeout without hearing from the server,
 	// end the long poll. We should be receiving a keep alive ping
 	// every minute.
@@ -724,6 +742,14 @@ func (c *Direct) PollNetMap(ctx context.Context, maxPolls int, cb func(*NetworkM
 			lastParsedPacketFilter = c.parsePacketFilter(pf)
 		}
 
+		// Get latest localPort. This might've changed if
+		// a lite map update occured meanwhile. This only affects
+		// the end-to-end test.
+		// TODO(bradfitz): remove the NetworkMap.LocalPort field entirely.
+		c.mu.Lock()
+		localPort = c.localPort
+		c.mu.Unlock()
+
 		nm := &NetworkMap{
 			NodeKey:      tailcfg.NodeKey(persist.PrivateNodeKey.Public()),
 			PrivateKey:   persist.PrivateNodeKey,