Fix deadlock in Stop

tomo.go

@@ -1,15 +1,37 @@
 package tomo
 
+import "sync"
 import "image"
 import "git.tebibyte.media/tomo/tomo/canvas"
 
+// TODO this really sucks. It might be a good idea to have Do be the entry point
+// for every off-thread call, and Stop should just call backend.Stop within
+// backend.Do. This is because Do is a queue and is not vulnerable to recursive
+// locking.
+
+var stopping bool
+var stoppingLock sync.Mutex
+func isStopping () bool {
+	stoppingLock.Lock()
+	defer stoppingLock.Unlock()
+	return stopping
+}
+func setStopping (is bool) {
+	stoppingLock.Lock()
+	defer stoppingLock.Unlock()
+	stopping = is
+}
+
 // Stop stops the currently running backend.
 func Stop () {
+	if isStopping() { return }
+	setStopping(true)
 	backendLock.Lock()
 	defer backendLock.Unlock()
 	if backend == nil { return }
 	backend.Stop()
 	backend = nil
+	setStopping(false)
 }
 
 // Do performs a callback function in the event loop thread as soon as possible.