utils: Take monitorShutdownSignal to take an exitFunc which would executed upon error. (#2378)

This hook approach allows program to keep running but being able to handle exiting
of the program in the dynamic way.

Fixes #2377

main.go

@@ -191,7 +191,7 @@ func main() {
 
 	// Initialize and monitor shutdown signal
 	shutdownSignal = make(chan bool, 1)
-	monitorShutdownSignal()
+	monitorShutdownSignal(os.Exit)
 
 	// Run the app - exit on error.
 	app.RunAndExitOnError()

utils.go

@@ -100,10 +100,13 @@ func registerObjectStorageShutdown(callback func() errCode) {
 	shutdownObjectStorageCallbacks = append(shutdownObjectStorageCallbacks, callback)
 }
 
+// Represents a type of an exit func which will be invoked during shutdown signal.
+type onExitFunc func(code int)
+
 // Start to monitor shutdownSignal to execute shutdown callbacks
-func monitorShutdownSignal() {
+func monitorShutdownSignal(onExitFn onExitFunc) {
 	go func() {
-		// Monitor processus signal
+		// Monitor signals.
 		trapCh := signalTrap(os.Interrupt, syscall.SIGTERM)
 		for {
 			select {
@@ -115,7 +118,7 @@ func monitorShutdownSignal() {
 				for _, callback := range shutdownCallbacks {
 					exitCode := callback()
 					if exitCode != exitSuccess {
-						os.Exit(int(exitCode))
+						onExitFn(int(exitCode))
 					}
 
 				}
@@ -123,11 +126,11 @@ func monitorShutdownSignal() {
 				for _, callback := range shutdownObjectStorageCallbacks {
 					exitCode := callback()
 					if exitCode != exitSuccess {
-						os.Exit(int(exitCode))
+						onExitFn(int(exitCode))
 					}
 
 				}
-				os.Exit(int(exitSuccess))
+				onExitFn(int(exitSuccess))
 			}
 		}
 	}()

utils_test.go

@@ -16,13 +16,10 @@
 
 package main
 
-import (
-	"testing"
-)
+import "testing"
 
-// ShutdownCallback simulates a successful exit here, all registered
-// shutdown callbacks need to return exitSuccess for this test to succeed
-func TestShutdownCallback(t *testing.T) {
+// ShutdownCallback simulates a successful and failure exit here.
+func TestShutdownCallbackSuccess(t *testing.T) {
 	// Register two callbacks that return success
 	registerObjectStorageShutdown(func() errCode {
 		return exitSuccess
@@ -33,10 +30,11 @@ func TestShutdownCallback(t *testing.T) {
 
 	shutdownSignal = make(chan bool, 1)
 	shutdownSignal <- true
-	// Start executing callbacks and quit if everything is fine
-	monitorShutdownSignal()
-
-	// Infinite loop here simulates an infinite running program
-	for {
+	// Start executing callbacks and exitFunc receives a success.
+	dummySuccess := func(code int) {
+		if code != int(exitSuccess) {
+			t.Fatalf("Expected %d, got %d instead.", code, exitSuccess)
+		}
 	}
+	monitorShutdownSignal(dummySuccess)
 }