amazon/ssm_driver: Update log polling logic

This change checks for closed iochans related to the log polling
function and will break out of the loop when both channels are closed.
Which is an indicator that the SSM session has been terminated by some
external process. This fixes an issue where Packer hangs, due to the
infinite loop, when an SSM session is killed outside of Packer.

Related to #9442

builder/amazon/common/ssm_driver.go

@@ -46,7 +46,7 @@ func NewSSMDriver(config SSMDriverConfig) *SSMDriver {
 // not wish to manage the session manually calling StopSession on a instance of this driver will terminate the active session
 // created from calling StartSession.
 func (d *SSMDriver) StartSession(ctx context.Context, input ssm.StartSessionInput) (*ssm.StartSessionOutput, error) {
-	log.Printf("Starting PortForwarding session to instance %q with following params %v", aws.StringValue(input.Target), input.Parameters)
+	log.Printf("Starting PortForwarding session to instance %q", aws.StringValue(input.Target))
 
 	var output *ssm.StartSessionOutput
 	err := retry.Config{
@@ -110,15 +110,30 @@ func (d *SSMDriver) openTunnelForSession(ctx context.Context) error {
 			select {
 			case <-ctx.Done():
 				return
-			case output := <-stderrCh:
+			case output, ok := <-stderrCh:
+				if !ok {
+					stderrCh = nil
+					break
+				}
+
 				if output != "" {
 					log.Printf("[ERROR] %s: %s", prefix, output)
 				}
-			case output := <-stdoutCh:
+			case output, ok := <-stdoutCh:
+				if !ok {
+					stdoutCh = nil
+					break
+				}
+
 				if output != "" {
 					log.Printf("[DEBUG] %s: %s", prefix, output)
 				}
 			}
+
+			if stdoutCh == nil && stderrCh == nil {
+				log.Printf("[DEBUG] %s: %s", prefix, "active session has been terminated; stopping all log polling processes.")
+				return
+			}
 		}
 	}(ctx, sessionManagerPluginName)