Check version compatibility with local agent

Compatibility with agent is necessary because the ESM client makes
health check update using txn request to the local agent first, which
then is forwarded to the servers.

agent.go

@@ -390,35 +390,50 @@ func (a *Agent) watchHealthChecks(nodeListCh chan map[string]bool) {
 	}
 }
 
-// VerifyConsulCompatibility queries Consul for all server versions to verify
+// VerifyConsulCompatibility queries Consul for local agent and all server versions to verify
 // compatibility with ESM.
 func (a *Agent) VerifyConsulCompatibility() error {
 	if a.client == nil {
 		return fmt.Errorf("unable to check version compatibility without Consul client initialized")
 	}
 
-VERIFYCONSULCOMPAT:
+	// Fetch local agent version
+	agentInfo, err := a.client.Agent().Self()
+	if err != nil {
+		// ESM blocks in NewAgent() until agent is available. At this point
+		// /agent/self endpoint should be available and an error would not be useful
+		// to retry the request.
+		return fmt.Errorf("unable to check version compatibility with Consul agent: %s", err)
+	}
+	agentVersionRaw, ok := agentInfo["Config"]["Version"]
+	if !ok {
+		return fmt.Errorf("unable to check version compatibility with Consul agent")
+	}
+	agentVersion := agentVersionRaw.(string)
+
+VERIFYCONSULSERVER:
 	select {
 	case <-a.shutdownCh:
 		return nil
 	default:
 	}
 
+	// Fetch server versions
 	resp, err := a.client.Operator().AutopilotServerHealth(nil)
 	if err != nil {
 		if strings.Contains(err.Error(), "429") {
 			// 429 is a warning that something is unhealthy. This may occur when ESM
-			// races with Consul servers first starting up.
+			// races with Consul servers first starting up, so this is safe to retry.
 			a.logger.Printf("[ERR] Failed to query for Consul server versions (will retry): %v", err)
 			time.Sleep(retryTime)
-			goto VERIFYCONSULCOMPAT
+			goto VERIFYCONSULSERVER
 		}
 
-		return fmt.Errorf("unable to check the version compatibility with Consul: %s", err)
+		return fmt.Errorf("unable to check version compatibility with Consul servers: %s", err)
 	}
 
-	var versions []string
-	uniqueVersions := make(map[string]bool)
+	versions := []string{agentVersion}
+	uniqueVersions := map[string]bool{agentVersion: true}
 	for _, s := range resp.Servers {
 		if !uniqueVersions[s.Version] {
 			uniqueVersions[s.Version] = true
@@ -432,6 +447,6 @@ VERIFYCONSULCOMPAT:
 		return err
 	}
 
-	a.logger.Printf("[DEBUG] All Consul servers are running compatible versions with ESM")
+	a.logger.Printf("[DEBUG] Consul agent and all servers are running compatible versions with ESM")
 	return nil
 }

version/version.go

@@ -108,10 +108,9 @@ func NewConsulVersionError(consulVersions []string) error {
 }
 
 const versionErrorTmpl = `
-Consul ESM version %s is incompatible with the running versions of your
-Consul servers (%s), please refer to the Consul documentation to safely
-upgrade the servers or change to a version of ESM that is compatible with your
-servers.
+Consul ESM version %s is incompatible with the running versions of the local
+Consul agent or Consul servers (%s), please refer to the documentation
+to safely upgrade Consul or change to a version of ESM that is compatible.
 
 https://www.consul.io/docs/upgrading.html
 `