From 333e9e75d40dd1787e69ac281f393f8cd151e451 Mon Sep 17 00:00:00 2001
From: Brad Fitzpatrick <bradfitz@tailscale.com>
Date: Tue, 15 Jun 2021 12:28:34 -0700
Subject: [PATCH] tailcfg, control/controlclient: clarify more, enforce
 PingRequest.URL is unique

Updates #2079

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
---
 control/controlclient/direct.go | 20 +++++++++++++++++++-
 tailcfg/tailcfg.go              |  3 +++
 2 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/control/controlclient/direct.go b/control/controlclient/direct.go
index 05d4086ea..d1cfdf62b 100644
--- a/control/controlclient/direct.go
+++ b/control/controlclient/direct.go
@@ -80,6 +80,7 @@ type Direct struct {
 	endpoints     []tailcfg.Endpoint
 	everEndpoints bool   // whether we've ever had non-empty endpoints
 	localPort     uint16 // or zero to mean auto
+	lastPingURL   string // last PingRequest.URL received, for dup suppresion
 }
 
 type Options struct {
@@ -775,7 +776,7 @@ func (c *Direct) sendMapRequest(ctx context.Context, maxPolls int, cb func(*netm
 			health.GotStreamedMapResponse()
 		}
 
-		if pr := resp.PingRequest; pr != nil {
+		if pr := resp.PingRequest; pr != nil && c.isUniquePingRequest(pr) {
 			go answerPing(c.logf, c.httpc, pr)
 		}
 
@@ -1170,6 +1171,23 @@ func ipForwardingBroken(routes []netaddr.IPPrefix, state *interfaces.State) bool
 	return false
 }
 
+// isUniquePingRequest reports whether pr contains a new PingRequest.URL
+// not already handled, noting its value when returning true.
+func (c *Direct) isUniquePingRequest(pr *tailcfg.PingRequest) bool {
+	if pr == nil || pr.URL == "" {
+		// Bogus.
+		return false
+	}
+
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	if pr.URL == c.lastPingURL {
+		return false
+	}
+	c.lastPingURL = pr.URL
+	return true
+}
+
 func answerPing(logf logger.Logf, c *http.Client, pr *tailcfg.PingRequest) {
 	if pr.URL == "" {
 		logf("invalid PingRequest with no URL")
diff --git a/tailcfg/tailcfg.go b/tailcfg/tailcfg.go
index a4ec682dd..d57a72e8a 100644
--- a/tailcfg/tailcfg.go
+++ b/tailcfg/tailcfg.go
@@ -880,6 +880,9 @@ type DNSConfig struct {
 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.
+	//
+	// If the client sees multiple PingRequests with the same URL,
+	// subsequent ones should be ignored.
 	URL string
 
 	// Log is whether to log about this ping in the success case.