agent: log events and exit code to the log file

This patch logs the signals, events, errors and the exit
code to the log file instead of printing it on the console.
This should provide a more complete picture for debugging.

command/agent/command.go

@@ -4,6 +4,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"io"
+	"log"
 	"net"
 	"os"
 	"os/signal"
@@ -45,6 +46,7 @@ type Command struct {
 	args              []string
 	logFilter         *logutils.LevelFilter
 	logOutput         io.Writer
+	logger            *log.Logger
 }
 
 // readConfig is responsible for setup of our configuration using
@@ -664,6 +666,14 @@ func (cmd *Command) registerWatches(config *Config) error {
 }
 
 func (cmd *Command) Run(args []string) int {
+	code := cmd.run(args)
+	if cmd.logger != nil {
+		cmd.logger.Println("[INFO] Exit code: ", code)
+	}
+	return code
+}
+
+func (cmd *Command) run(args []string) int {
 	cmd.UI = &cli.PrefixedUi{
 		OutputPrefix: "==> ",
 		InfoPrefix:   "    ",
@@ -690,6 +700,7 @@ func (cmd *Command) Run(args []string) int {
 	}
 	cmd.logFilter = logFilter
 	cmd.logOutput = logOutput
+	cmd.logger = log.New(logOutput, "", log.LstdFlags)
 
 	if err := startupTelemetry(config); err != nil {
 		cmd.UI.Error(err.Error())
@@ -769,7 +780,7 @@ func (cmd *Command) Run(args []string) int {
 		case <-cmd.ShutdownCh:
 			sig = os.Interrupt
 		case err := <-agent.RetryJoinCh():
-			cmd.UI.Error(err.Error())
+			cmd.logger.Println("[ERR] Retry join failed: ", err)
 			return 1
 		case <-agent.ShutdownCh():
 			// Agent is already down!
@@ -781,34 +792,34 @@ func (cmd *Command) Run(args []string) int {
 			continue
 
 		case syscall.SIGHUP:
-			cmd.UI.Output(fmt.Sprintf("Caught signal: %v", sig))
+			cmd.logger.Println("[INFO] Caught signal: ", sig)
 
 			conf, err := cmd.handleReload(agent, config)
 			if conf != nil {
 				config = conf
 			}
 			if err != nil {
-				cmd.UI.Error(err.Error())
+				cmd.logger.Println("[ERR] Reload config failed: ", err)
 			}
 			// Send result back if reload was called via HTTP
 			if reloadErrCh != nil {
 				reloadErrCh <- err
 			}
 
 		default:
-			cmd.UI.Output(fmt.Sprintf("Caught signal: %v", sig))
+			cmd.logger.Println("[INFO] Caught signal: ", sig)
 
 			graceful := (sig == os.Interrupt && !(*config.SkipLeaveOnInt)) || (sig == syscall.SIGTERM && (*config.LeaveOnTerm))
 			if !graceful {
-				cmd.UI.Output("Graceful shutdown disabled. Exiting")
+				cmd.logger.Println("[INFO] Graceful shutdown disabled. Exiting")
 				return 1
 			}
 
-			cmd.UI.Output("Gracefully shutting down agent...")
+			cmd.logger.Println("[INFO] Gracefully shutting down agent...")
 			gracefulCh := make(chan struct{})
 			go func() {
 				if err := agent.Leave(); err != nil {
-					cmd.UI.Error(fmt.Sprintf("Error on leave: %s", err))
+					cmd.logger.Println("[ERR] Error on leave:", err)
 					return
 				}
 				close(gracefulCh)
@@ -817,13 +828,13 @@ func (cmd *Command) Run(args []string) int {
 			gracefulTimeout := 15 * time.Second
 			select {
 			case <-signalCh:
-				cmd.UI.Output(fmt.Sprintf("Caught second signal: %v. Exiting", sig))
+				cmd.logger.Printf("[INFO] Caught second signal %v. Exiting\n", sig)
 				return 1
 			case <-time.After(gracefulTimeout):
-				cmd.UI.Output("Timeout on graceful leave. Exiting")
+				cmd.logger.Println("[INFO] Timeout on graceful leave. Exiting")
 				return 1
 			case <-gracefulCh:
-				cmd.UI.Output("Graceful exit completed")
+				cmd.logger.Println("[INFO] Graceful exit completed")
 				return 0
 			}
 		}
@@ -832,7 +843,7 @@ func (cmd *Command) Run(args []string) int {
 
 // handleReload is invoked when we should reload our configs, e.g. SIGHUP
 func (cmd *Command) handleReload(agent *Agent, cfg *Config) (*Config, error) {
-	cmd.UI.Output("Reloading configuration...")
+	cmd.logger.Println("[INFO] Reloading configuration...")
 	var errs error
 	newCfg := cmd.readConfig()
 	if newCfg == nil {