Use FIFO for storage locking

This prevents starvation of writers in the current process

radicale/storage.py

@@ -524,7 +524,8 @@ class Collection(BaseCollection):
             return "".join([item.serialize() for item in items])
         return ""
 
-    _lock = threading.Condition()
+    _lock = threading.Lock()
+    _waiters = []
     _lock_file = None
     _lock_file_locked = False
     _readers = 0
@@ -544,11 +545,20 @@ class Collection(BaseCollection):
         # Use a primitive lock which only works within one process as a
         # precondition for inter-process file-based locking
         with cls._lock:
-            cls._lock.wait_for(condition)
+            if cls._waiters or not condition():
+                # use FIFO for access requests
+                waiter = threading.Condition(lock=cls._lock)
+                cls._waiters.append(waiter)
+                while True:
+                    waiter.wait()
+                    if condition():
+                        break
+                cls._waiters.pop(0)
             if mode == "r":
                 cls._readers += 1
                 # notify additional potential readers
-                cls._lock.notify()
+                if cls._waiters:
+                    cls._waiters[0].notify()
             else:
                 cls._writer = True
             if not cls._lock_file:
@@ -596,4 +606,5 @@ class Collection(BaseCollection):
                     except OSError:
                         cls.logger.debug("Unlocking not supported")
                 cls._lock_file_locked = False
-            cls._lock.notify()
+            if cls._waiters:
+                cls._waiters[0].notify()