fix data race

Since state.Checks() returns a shallow copy
its elements must not be modified. Copying
the elements in the handler does not guarantee
consistency since that list is guarded by a different
lock. Therefore, the only solution is to have state.Checks()
return a deep copy.

agent/agent_endpoint.go

@@ -152,6 +152,7 @@ func (s *HTTPServer) AgentChecks(resp http.ResponseWriter, req *http.Request) (i
 	}
 
 	// Use empty list instead of nil
+	// checks needs to be a deep copy for this not be racy
 	for _, c := range checks {
 		if c.ServiceTags == nil {
 			c.ServiceTags = make([]string, 0)

agent/local.go

@@ -341,12 +341,14 @@ func (l *localState) UpdateCheck(checkID types.CheckID, status, output string) {
 // Checks returns the locally registered checks that the
 // agent is aware of and are being kept in sync with the server
 func (l *localState) Checks() map[types.CheckID]*structs.HealthCheck {
-	checks := make(map[types.CheckID]*structs.HealthCheck)
 	l.RLock()
 	defer l.RUnlock()
 
-	for checkID, check := range l.checks {
-		checks[checkID] = check
+	checks := make(map[types.CheckID]*structs.HealthCheck)
+	for id, c := range l.checks {
+		c2 := new(structs.HealthCheck)
+		*c2 = *c
+		checks[id] = c2
 	}
 	return checks
 }