Support should_verify_existence on strategies

This prevents extra IO. Different defaults are used for async backends
since we can’t assume that `existence_required` resulted in existence
synchronously.

imagekit/cachefiles/__init__.py

@@ -128,7 +128,14 @@ class ImageCacheFile(BaseIKFile, ImageFile):
         # Dispatch the existence_required signal before checking to see if the
         # file exists. This gives the strategy a chance to create the file.
         existence_required.send(sender=self, file=self)
-        return self.cachefile_backend.exists(self)
+
+        try:
+            check = self.cachefile_strategy.should_verify_existence(self)
+        except AttributeError:
+            # All synchronous backends should have created the file as part of
+            # `existence_required` if they wanted to.
+            check = getattr(self.cachefile_backend, 'is_async', False)
+        return self.cachefile_backend.exists(self) if check else True
 
     def __getstate__(self):
         state = copy(self.__dict__)

imagekit/cachefiles/backends.py

@@ -121,6 +121,8 @@ class BaseAsync(Simple):
     """
     Base class for cache file backends that generate files asynchronously.
     """
+    is_async = True
+
     def generate(self, file, force=False):
         # Schedule the file for generation, unless we know for sure we don't
         # need to. If an already-generated file sneaks through, that's okay;

imagekit/cachefiles/strategies.py

@@ -29,6 +29,9 @@ class Optimistic(object):
     def on_source_saved(self, file):
         file.generate()
 
+    def should_verify_existence(self, file):
+        return False
+
 
 class DictStrategy(object):
     def __init__(self, callbacks):

tests/utils.py

@@ -77,5 +77,7 @@ class DummyAsyncCacheFileBackend(Simple):
     A cache file backend meant to simulate async generation.
 
     """
+    is_async = True
+
     def generate(self, file, force=False):
         pass