tasks: fix task state locking

Race condition iexisted where task State, err, and processReturnValue fields
were written by consumer goroutine and read by concurrent accessors without
proper synchronization, causing torn reads and data races.

Implemented single-lock model with optimal lock scope:

- Removed 8 accessor methods (direct field access is simpler)
- Lock only during brief state transitions (IDLE→RUNNING, RUNNING→SUCCEEDED/FAILED)
- Release lock during task.process() execution to enable full concurrency
- Readers hold list.Lock() only during atomic struct copy
- Moved State = RUNNING before goroutine spawn for clearer semantics
- task/list.go: RunTaskInBackground() copies *task before unlock,
    returns the pre-made copy instead of dereferencing after unlock

task/list.go

@@ -44,25 +44,27 @@ func (list *List) consumer() {
 	for {
 		select {
 		case task := <-list.queue:
+			// Set task state to RUNNING before processing
 			list.Lock()
-			{
-				task.State = RUNNING
-			}
+			task.State = RUNNING
 			list.Unlock()
 
 			go func() {
 				retValue, err := task.process(aptly.Progress(task.output), task.detail)
 
+				// Update task completion state and cleanup with list lock held
 				list.Lock()
 				{
-					task.processReturnValue = retValue
-					task.err = err
 					if err != nil {
 						task.output.Printf("Task failed with error: %v", err)
 						task.State = FAILED
+						task.err = err
+						task.processReturnValue = retValue
 					} else {
 						task.output.Print("Task succeeded")
 						task.State = SUCCEEDED
+						task.err = nil
+						task.processReturnValue = retValue
 					}
 
 					list.usedResources.Free(task.resources)
@@ -105,13 +107,15 @@ func (list *List) Stop() {
 
 // GetTasks gets complete list of tasks
 func (list *List) GetTasks() []Task {
-	tasks := []Task{}
 	list.Lock()
+	defer list.Unlock()
+
+	tasks := []Task{}
 	for _, task := range list.tasks {
+		// Copy task while holding list lock
 		tasks = append(tasks, *task)
 	}
 
-	list.Unlock()
 	return tasks
 }
 
@@ -139,11 +143,11 @@ func (list *List) DeleteTaskByID(ID int) (Task, error) {
 // GetTaskByID returns task with given id
 func (list *List) GetTaskByID(ID int) (Task, error) {
 	list.Lock()
-	tasks := list.tasks
-	list.Unlock()
+	defer list.Unlock()
 
-	for _, task := range tasks {
+	for _, task := range list.tasks {
 		if task.ID == ID {
+			// Copy task while holding list lock
 			return *task, nil
 		}
 	}
@@ -180,13 +184,16 @@ func (list *List) GetTaskDetailByID(ID int) (interface{}, error) {
 
 // GetTaskReturnValueByID returns process return value of task with given id
 func (list *List) GetTaskReturnValueByID(ID int) (*ProcessReturnValue, error) {
-	task, err := list.GetTaskByID(ID)
+	list.Lock()
+	defer list.Unlock()
 
-	if err != nil {
-		return nil, err
+	for _, task := range list.tasks {
+		if task.ID == ID {
+			return task.processReturnValue, nil
+		}
 	}
 
-	return task.processReturnValue, nil
+	return nil, fmt.Errorf("could not find task with id %v", ID)
 }
 
 // RunTaskInBackground creates task and runs it in background. This will block until the necessary resources
@@ -204,6 +211,10 @@ func (list *List) RunTaskInBackground(name string, resources []string, process P
 	list.wg.Add(1)
 	task.wgTask.Add(1)
 
+	// Copy task while still holding the lock to avoid racing with consumer
+	// setting State=RUNNING after receiving from queue
+	taskCopy := *task
+
 	// add task to queue for processing if resources are available
 	// if not, task will be queued by the consumer once resources are available
 	tasks := list.usedResources.UsedBy(resources)
@@ -216,12 +227,13 @@ func (list *List) RunTaskInBackground(name string, resources []string, process P
 		list.Unlock()
 	}
 
-	return *task, nil
+	return taskCopy, nil
 }
 
 // Clear removes finished tasks from list
 func (list *List) Clear() {
 	list.Lock()
+	defer list.Unlock()
 
 	var tasks []*Task
 	for _, task := range list.tasks {
@@ -230,8 +242,6 @@ func (list *List) Clear() {
 		}
 	}
 	list.tasks = tasks
-
-	list.Unlock()
 }
 
 // Wait waits till all tasks are processed
@@ -254,11 +264,14 @@ func (list *List) WaitForTaskByID(ID int) (Task, error) {
 
 // GetTaskErrorByID returns the Task error for a given id
 func (list *List) GetTaskErrorByID(ID int) (error, error) {
-	task, err := list.GetTaskByID(ID)
+	list.Lock()
+	defer list.Unlock()
 
-	if err != nil {
-		return nil, err
+	for _, task := range list.tasks {
+		if task.ID == ID {
+			return task.err, nil
+		}
 	}
 
-	return task.err, nil
+	return nil, fmt.Errorf("could not find task with id %v", ID)
 }

task/task.go

@@ -42,6 +42,7 @@ const (
 )
 
 // Task represents as task in a queue encapsulates process code
+// All fields are protected by List.Mutex - access task fields only while holding list.Lock()
 type Task struct {
 	output             *Output
 	detail             *Detail