drivers: update ordering of events in StartTask to fix executor leak

If an error occurs before a task is started, but after this executor
process is created, the executor must be explicity stopped.  This change
updates the logic in StartTask so launching the task happens immediately
after creating the executor.

drivers/exec/driver.go

@@ -492,13 +492,6 @@ func (d *Driver) StartTask(cfg *drivers.TaskConfig) (*drivers.TaskHandle, *drive
 		Compute:     d.compute,
 	}
 
-	exec, pluginClient, err := executor.CreateExecutor(
-		d.logger.With("task_name", handle.Config.Name, "alloc_id", handle.Config.AllocID),
-		d.nomadConfig, executorConfig)
-	if err != nil {
-		return nil, nil, fmt.Errorf("failed to create executor: %v", err)
-	}
-
 	user := cfg.User
 	if cfg.DNS != nil {
 		dnsMount, err := resolvconf.GenerateDNSMount(cfg.TaskDir().Dir, cfg.DNS)
@@ -516,6 +509,15 @@ func (d *Driver) StartTask(cfg *drivers.TaskConfig) (*drivers.TaskHandle, *drive
 	}
 	d.logger.Debug("task capabilities", "capabilities", caps)
 
+	// If any error scenarios occur in StartTask and after creating the executor process,
+	// the executor process must be killed.
+	exec, pluginClient, err := executor.CreateExecutor(
+		d.logger.With("task_name", handle.Config.Name, "alloc_id", handle.Config.AllocID),
+		d.nomadConfig, executorConfig)
+	if err != nil {
+		return nil, nil, fmt.Errorf("failed to create executor: %v", err)
+	}
+
 	execCmd := &executor.ExecCommand{
 		Cmd:              driverConfig.Command,
 		Args:             driverConfig.Args,

drivers/java/driver.go

@@ -467,13 +467,6 @@ func (d *Driver) StartTask(cfg *drivers.TaskConfig) (*drivers.TaskHandle, *drive
 		Compute:     d.nomadConfig.Topology.Compute(),
 	}
 
-	exec, pluginClient, err := executor.CreateExecutor(
-		d.logger.With("task_name", handle.Config.Name, "alloc_id", handle.Config.AllocID),
-		d.nomadConfig, executorConfig)
-	if err != nil {
-		return nil, nil, fmt.Errorf("failed to create executor: %v", err)
-	}
-
 	user := cfg.User
 	if user == "" {
 		user = "nobody"
@@ -495,6 +488,15 @@ func (d *Driver) StartTask(cfg *drivers.TaskConfig) (*drivers.TaskHandle, *drive
 	}
 	d.logger.Debug("task capabilities", "capabilities", caps)
 
+	// If any error scenarios occur in StartTask and after creating the executor process,
+	// the executor process must be killed.
+	exec, pluginClient, err := executor.CreateExecutor(
+		d.logger.With("task_name", handle.Config.Name, "alloc_id", handle.Config.AllocID),
+		d.nomadConfig, executorConfig)
+	if err != nil {
+		return nil, nil, fmt.Errorf("failed to create executor: %v", err)
+	}
+
 	execCmd := &executor.ExecCommand{
 		Cmd:              absPath,
 		Args:             args,