Add missing lock file

chaind/lock.py

@@ -0,0 +1,34 @@
+# standard imports
+import time
+
+# local imports
+from .error import BackendError
+
+BASE_DELAY = 0.01
+BASE_DELAY_LIMIT = 3.0
+
+
+class StoreLock:
+
+    def __init__(self, delay=BASE_DELAY, delay_limit=BASE_DELAY_LIMIT, error=BackendError, description=None):
+        self.base_delay = delay
+        self.delay = delay
+        self.delay_limit = delay_limit
+        self.error = error
+        self.description = description
+
+    
+    def again(self, e=None):
+        if self.delay > self.delay_limit:
+            err = None
+            if e != None:
+                err = str(e)
+            else:
+                err = self.description
+            raise self.error(err)
+        time.sleep(self.delay)
+        self.delay *= 2
+
+
+    def reset(self):
+        self.delay = self.base_delay