implement checksum in terms of reducefileobj.

farmfs/fs.py

@@ -26,7 +26,7 @@
 from os.path import splitext
 from fnmatch import fnmatchcase
 from functools import total_ordering
-from farmfs.util import ingest, safetype, uncurry, first, second, ffilter, copyfileobj
+from farmfs.util import ingest, safetype, uncurry, first, second, ffilter, copyfileobj, reducefileobj
 from future.utils import python_2_unicode_compatible
 from safeoutput import open as safeopen
 from safeoutput import _sameDir as sameDir
@@ -349,15 +349,10 @@ def checksum(self):
         If self points to a missing file or a broken symlink, raises FileDoesNotExist.
         If self points to a directory or a symlink facing directory, raises IsADirectory.
         """
-        hasher = md5()
         with self.open('rb') as fd:
-            buf = fd.read(_BLOCKSIZE)
-            while len(buf) > 0:
-                # TODO Could cancel work here.
-                hasher.update(buf)
-                buf = fd.read(_BLOCKSIZE)
-            digest = safetype(hasher.hexdigest())
-            return digest
+            hash = reducefileobj(lambda hasher, buf: hasher.update(buf) or hasher, fd, md5(), _BLOCKSIZE)
+        digest = safetype(hash.hexdigest())
+        return digest
 
     def __cmp__(self, other):
         return (self > other) - (self < other)

farmfs/util.py

@@ -288,3 +288,14 @@ def copyfileobj(fsrc, fdst, length=16 * 1024):
             break
         fdst.write(buf)
 
+def reducefileobj(function, fsrc, initial=None, length=16 * 1024):
+    if initial is None:
+        acc = fsrc.read(length)
+    else:
+        acc = initial
+    while 1:
+        buf = fsrc.read(length)
+        if not buf:
+            break
+        acc = function(acc, buf)
+    return acc