worker: use container/list as job queue

The worker uses a buffered channel to queue tasks. Buffered channels
are effective at FIFO, but are prone to blocking. The design of aerc is
such that the UI must always accept a response from the backends, and
the backends must always accept a request from the UI. By using buffered
channels for both of these communication channels, a deadlock will
occur.

Break the chain by using a doubly linked list (container/list from the
standard library) to queue tasks for the worker. Essentially, this is an
infinitely buffered channel - but more memory efficient as it can change
size dynamically.

Signed-off-by: Tim Culverhouse <tim@timculverhouse.com>
Signed-off-by: Robin Jarry <robin@jarry.cc>

worker/types/worker.go

@@ -1,6 +1,7 @@
 package types
 
 import (
+	"container/list"
 	"sync"
 	"sync/atomic"
 
@@ -21,16 +22,19 @@ type Worker struct {
 
 	actionCallbacks  map[int64]func(msg WorkerMessage)
 	messageCallbacks map[int64]func(msg WorkerMessage)
+	actionQueue      *list.List
+	status           int32
 
 	sync.Mutex
 }
 
 func NewWorker() *Worker {
 	return &Worker{
-		Actions:          make(chan WorkerMessage, 50),
+		Actions:          make(chan WorkerMessage),
 		Messages:         make(chan WorkerMessage, 50),
 		actionCallbacks:  make(map[int64]func(msg WorkerMessage)),
 		messageCallbacks: make(map[int64]func(msg WorkerMessage)),
+		actionQueue:      list.New(),
 	}
 }
 
@@ -39,6 +43,40 @@ func (worker *Worker) setId(msg WorkerMessage) {
 	msg.setId(id)
 }
 
+const (
+	idle int32 = iota
+	busy
+)
+
+// Add a new task to the action queue without blocking. Start processing the
+// queue in the background if needed.
+func (worker *Worker) queue(msg WorkerMessage) {
+	worker.Lock()
+	defer worker.Unlock()
+	worker.actionQueue.PushBack(msg)
+	if atomic.LoadInt32(&worker.status) == idle {
+		atomic.StoreInt32(&worker.status, busy)
+		go worker.processQueue()
+	}
+}
+
+// Start processing the action queue and write all messages to the Actions
+// channel, one by one. Stop when the action queue is empty.
+func (worker *Worker) processQueue() {
+	for {
+		worker.Lock()
+		e := worker.actionQueue.Front()
+		if e == nil {
+			atomic.StoreInt32(&worker.status, idle)
+			worker.Unlock()
+			return
+		}
+		msg := worker.actionQueue.Remove(e).(WorkerMessage)
+		worker.Unlock()
+		worker.Actions <- msg
+	}
+}
+
 // PostAction posts an action to the worker. This method should not be called
 // from the same goroutine that the worker runs in or deadlocks may occur
 func (worker *Worker) PostAction(msg WorkerMessage, cb func(msg WorkerMessage)) {
@@ -49,7 +87,8 @@ func (worker *Worker) PostAction(msg WorkerMessage, cb func(msg WorkerMessage))
 	} else {
 		logging.Debugf("PostAction %T", msg)
 	}
-	worker.Actions <- msg
+	// write to Actions channel without blocking
+	worker.queue(msg)
 
 	if cb != nil {
 		worker.Lock()