curds: Split out the body of the playlist __next__() function

This lets us have peek() advance a counter without actually changing the
state of the playlist and avoids a potential deadlock from calling
next() with the lock held.

Signed-off-by: Anna Schumaker <Anna@NoWheyCreamery.com>

curds/playlist/playlist.py

@@ -47,6 +47,19 @@ class Playlist(node.PlaylistNode):
         with self.node_lock:
             return len(self.list)
 
+    def __next__(self, cur):
+        if (max := len(self.list) - 1) == -1:
+            return None
+        elif max == 0:
+            return None if (self.loop == False and cur == 0) else 0
+        elif self.random == True:
+            return (cur + random.randint(1, max)) % max
+        elif cur < max:
+            return cur + 1
+        elif self.loop == True:
+            return 0
+        return None
+
     def __setstate__(self, state):
         node.PlaylistNode.__setstate__(self, state)
         self.list       = [ trak.lookup(path) for path in state["list"] ]
@@ -105,22 +118,10 @@ class Playlist(node.PlaylistNode):
 
     def next(self):
         with self.node_lock:
-            max = len(self) - 1
-            if max == -1:
-                return None
-            elif max == 0:
-                if self.loop == False and self.current == 0:
-                    return None
-                self.current = 0
-            elif self.random == True:
-                self.current = (self.current + random.randint(1, max)) % max
-            elif self.current < max:
-                self.current += 1
-            elif self.loop == True:
-                self.current = 0
-            else:
-                return None
-            return self.list[self.current]
+            if (n := self.__next__(self.current)) != None:
+                self.current = n
+                return self.list[self.current]
+            return None
 
     def peek(self, n):
         with self.node_lock:
@@ -128,12 +129,9 @@ class Playlist(node.PlaylistNode):
             res = [ ]
 
             for i in range(n):
-                track = self.next()
-                if track == None:
+                if (cur := self.__next__(cur)) == None:
                     break
-                res.append(track)
-
-            self.current = cur
+                res.append(self.list[cur])
             return res
 
     def remove(self, track):