clean solution for concurrence in session.go by serializing all external calls (apart from initialization) through a channel.

pkg/agent/help.txt

@@ -1,10 +1,5 @@
-Session is set to expire at %v
 
-The session expires automatically after %v.
-If there are no more sessions after logging out, the agent
-terminates.
-
-You can extend this time using
+You can extend expiry of the session using
 
   {{ if eq .os "windows" -}}
   echo > %agentdir%\.hold
@@ -13,7 +8,7 @@ You can extend this time using
   {{- end }}
 
 The expiry time is equal to the modification time of the .hold
-file with the expiry duration added.
+file with the expiry duration (%v) added.
 
 To prevent the agent from exiting after the last session is gone,
 also use the above command in any shell.

pkg/agent/session.go

@@ -2,6 +2,7 @@ package agent
 
 import (
 	"bytes"
+	"converge/pkg/async"
 	"fmt"
 	"github.com/fsnotify/fsnotify"
 	"github.com/gliderlabs/ssh"
@@ -38,6 +39,9 @@ type AgentState struct {
 	// session expiry time
 	agentExpriryTime time.Duration
 
+	// Last expiry time reported to the user.
+	lastExpiryTimmeReported time.Time
+
 	// ticker
 	tickerInterval time.Duration
 	ticker         *time.Ticker
@@ -63,6 +67,11 @@ const holdFilename = ".hold"
 var helpMessageTemplate string
 var helpMessage = formatHelpMessage()
 
+// Events channel for asynchronous events.
+var events = make(chan func(), 10)
+
+// External interface, asynchronous, apart from the initialization.
+
 func ConfigureAgent(advanceWarningTime, agentExpiryTime, tickerInterval time.Duration) {
 	if fileExists(holdFilename) {
 		log.Printf("Removing hold file '%s'", holdFilename)
@@ -75,6 +84,7 @@ func ConfigureAgent(advanceWarningTime, agentExpiryTime, tickerInterval time.Dur
 		startTime:               time.Now(),
 		advanceWarningTime:      advanceWarningTime,
 		agentExpriryTime:        agentExpiryTime,
+		lastExpiryTimmeReported: time.Time{},
 		tickerInterval:          tickerInterval,
 		ticker:                  time.NewTicker(tickerInterval),
 		sessions:                make(map[int]*AgentSession),
@@ -84,12 +94,29 @@ func ConfigureAgent(advanceWarningTime, agentExpiryTime, tickerInterval time.Dur
 	go func() {
 		for {
 			<-state.ticker.C
-			check()
+			events <- async.Async(check)
 		}
 	}()
 	go monitorHoldFile()
+	go func() {
+		for {
+			event := <-events
+			event()
+		}
+	}()
+
 }
 
+func Login(sessionId int, sshSession ssh.Session) {
+	events <- async.Async(login, sessionId, sshSession)
+}
+
+func LogOut(sessionId int) {
+	events <- async.Async(logOut, sessionId)
+}
+
+// Internal interface synchronous
+
 func monitorHoldFile() {
 	watcher, err := fsnotify.NewWatcher()
 	if err != nil {
@@ -100,7 +127,6 @@ func monitorHoldFile() {
 	if err != nil {
 		log.Printf("Cannot watch old file %s, user notifications for change in expiry time will be unavailable: %v", holdFilename, err)
 	}
-	expiryTime := state.expiryTime(holdFilename)
 	for {
 		select {
 		case event, ok := <-watcher.Events:
@@ -109,14 +135,7 @@ func monitorHoldFile() {
 			}
 			base := filepath.Base(event.Name)
 			if base == holdFilename {
-				newExpiryTIme := state.expiryTime(holdFilename)
-				if newExpiryTIme != expiryTime {
-					message := fmt.Sprintf("Expiry time of session is now %s\n",
-						newExpiryTIme.Format(time.DateTime))
-					message += holdFileMessage()
-					messageUsers(message)
-					expiryTime = newExpiryTIme
-				}
+				events <- async.Async(holdFileChange)
 			}
 
 		case err, ok := <-watcher.Errors:
@@ -142,7 +161,7 @@ func holdFileMessage() string {
 	return message
 }
 
-func Login(sessionId int, sshSession ssh.Session) {
+func login(sessionId int, sshSession ssh.Session) {
 	log.Println("New login")
 	hostname, _ := os.Hostname()
 
@@ -160,21 +179,21 @@ func Login(sessionId int, sshSession ssh.Session) {
 		}
 	}
 
-	PrintMessage(sshSession, fmt.Sprintf("You are now on %s\n", hostname))
-	PrintHelpMessage(sshSession)
-
 	agentSession := AgentSession{
 		startTime:  time.Now(),
 		sshSession: sshSession,
 	}
 	state.sessions[sessionId] = &agentSession
 	state.agentUsed = true
-	LogStatus()
+	logStatus()
+
+	printMessage(sshSession, fmt.Sprintf("You are now on %s\n", hostname))
+	holdFileChange()
+	printHelpMessage(sshSession)
 }
 
-func PrintHelpMessage(sshSession ssh.Session) {
-	PrintMessage(sshSession, fmt.Sprintf(helpMessage,
-		state.expiryTime(holdFilename).Format(time.DateTime),
+func printHelpMessage(sshSession ssh.Session) {
+	printMessage(sshSession, fmt.Sprintf(helpMessage,
 		state.agentExpriryTime))
 }
 
@@ -194,21 +213,21 @@ func formatHelpMessage() string {
 	return helpFormatted
 }
 
-func LogOut(sessionId int) {
+func logOut(sessionId int) {
 	log.Println("User logged out")
 	delete(state.sessions, sessionId)
-	LogStatus()
+	logStatus()
 	check()
 }
 
-func PrintMessage(sshSession ssh.Session, message string) {
+func printMessage(sshSession ssh.Session, message string) {
 	for _, line := range strings.Split(message, "\n") {
 		io.WriteString(sshSession.Stderr(), "### "+line+"\n\r")
 	}
 	io.WriteString(sshSession.Stderr(), "\n\r")
 }
 
-func LogStatus() {
+func logStatus() {
 	fmt := "%-20s %-20s %-20s"
 	log.Println()
 	log.Printf(fmt, "UID", "START_TIME", "TYPE")
@@ -242,6 +261,17 @@ func (state *AgentState) expiryTime(filename string) time.Time {
 	return stats.ModTime().Add(state.agentExpriryTime)
 }
 
+func holdFileChange() {
+	newExpiryTIme := state.expiryTime(holdFilename)
+	if newExpiryTIme != state.lastExpiryTimmeReported {
+		message := fmt.Sprintf("Expiry time of session is now %s\n",
+			newExpiryTIme.Format(time.DateTime))
+		message += holdFileMessage()
+		messageUsers(message)
+		state.lastExpiryTimmeReported = newExpiryTIme
+	}
+}
+
 // Behavior to implement
 //  1. there is a global timeout for all agent sessions together: state.agentExpirtyTime
 //  2. The expiry time is relative to the modification time of the .hold file in the
@@ -251,7 +281,6 @@ func (state *AgentState) expiryTime(filename string) time.Time {
 //  4. If the last user logs out, the aagent will exit immediately if no .hold file is
 //     present. Otherwise it will exit after the epxiry time. This allows users to
 //     reconnect later.
-
 func check() {
 	now := time.Now()
 
@@ -268,7 +297,7 @@ func check() {
 		messageUsers(
 			fmt.Sprintf("Session will expire at %s", expiryTime.Format(time.DateTime)))
 		for _, session := range state.sessions {
-			PrintHelpMessage(session.sshSession)
+			printHelpMessage(session.sshSession)
 		}
 	}
 
@@ -281,6 +310,6 @@ func check() {
 func messageUsers(message string) {
 	log.Printf("=== Notification to users: %s", message)
 	for _, session := range state.sessions {
-		PrintMessage(session.sshSession, message)
+		printMessage(session.sshSession, message)
 	}
 }

pkg/async/async.go

@@ -0,0 +1,18 @@
+package async
+
+import "reflect"
+
+func Async(fn interface{}, args ...interface{}) func() {
+	fnValue := reflect.ValueOf(fn)
+
+	// Prepare the arguments
+	params := make([]reflect.Value, len(args))
+	for i, arg := range args {
+		params[i] = reflect.ValueOf(arg)
+	}
+
+	// Return a function that, when called, will invoke the original function
+	return func() {
+		fnValue.Call(params)
+	}
+}