tailcfg, control/controlclient: add MapResponse.PingRequest

So the control server can test whether a client's actually present.

Most clients are over HTTP/2, so these pings (to the same host) are
super cheap.

This mimics the earlier goroutine dump mechanism.

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

control/controlclient/direct.go

@@ -676,6 +676,10 @@ func (c *Direct) sendMapRequest(ctx context.Context, maxPolls int, cb func(*netm
 			return err
 		}
 
+		if pr := resp.PingRequest; pr != nil {
+			go answerPing(c.logf, c.httpc, pr)
+		}
+
 		if resp.KeepAlive {
 			vlogf("netmap: got keep-alive")
 		} else {
@@ -1204,3 +1208,29 @@ func ipForwardingBroken(routes []netaddr.IPPrefix) bool {
 
 	return false
 }
+
+func answerPing(logf logger.Logf, c *http.Client, pr *tailcfg.PingRequest) {
+	if pr.URL == "" {
+		logf("invalid PingRequest with no URL")
+		return
+	}
+	ctx, cancel := context.WithTimeout(context.Background(), 15*time.Second)
+	defer cancel()
+
+	req, err := http.NewRequestWithContext(ctx, "HEAD", pr.URL, nil)
+	if err != nil {
+		logf("http.NewRequestWithContext(%q): %v", pr.URL, err)
+		return
+	}
+	if pr.Log {
+		logf("answerPing: sending ping to %v ...", pr.URL)
+	}
+	t0 := time.Now()
+	_, err = c.Do(req)
+	d := time.Since(t0).Round(time.Millisecond)
+	if err != nil {
+		logf("answerPing error: %v to %v (after %v)", err, pr.URL, d)
+	} else if pr.Log {
+		logf("answerPing complete to %v (after %v)", pr.URL, d)
+	}
+}

tailcfg/tailcfg.go

@@ -35,6 +35,7 @@ import (
 //     9: 2020-12-30: client doesn't auto-add implicit search domains from peers; only DNSConfig.Domains
 //    10: 2021-01-17: client understands MapResponse.PeerSeenChange
 //    11: 2021-03-03: client understands IPv6, multiple default routes, and goroutine dumping
+//    12: 2021-03-04: client understands PingRequest
 const CurrentMapRequestVersion = 11
 
 type StableID string
@@ -714,8 +715,30 @@ type DNSConfig struct {
 	Proxied bool
 }
 
+// PingRequest is a request to send an HTTP request to prove the
+// long-polling client is still connected.
+type PingRequest struct {
+	// URL is the URL to send a HEAD request to.
+	// It will be a unique URL each time. No auth headers are necessary.
+	URL string
+
+	// Log is whether to log about this ping in the success case.
+	// For failure cases, the client will log regardless.
+	Log bool `json:",omitempty"`
+}
+
 type MapResponse struct {
-	KeepAlive bool `json:",omitempty"` // if set, all other fields are ignored
+	// KeepAlive, if set, represents an empty message just to keep
+	// the connection alive. When true, all other fields except
+	// PingRequestURL are ignored.
+	KeepAlive bool `json:",omitempty"`
+
+	// PingRequest, if non-empty, is a request to the client to
+	// prove it's still there by sending an HTTP request to the
+	// provided URL. No auth headers are necessary.
+	// PingRequest may be sent on any MapResponse (ones with
+	// KeepAlive true or false).
+	PingRequest *PingRequest `json:",omitempty"`
 
 	// Networking
 	Node    *Node