Make buildout redownload in case of having wrong local md5sum.

src/zc/buildout/download.py

@@ -132,9 +132,14 @@ class Download(object):
                     pass
 
             if not check_md5sum(cached_path, md5sum):
-                raise ChecksumError(
-                    'MD5 checksum mismatch for cached download '
-                    'from %r at %r' % (url, cached_path))
+                try:
+                    _, is_temp = self.download(url, md5sum, cached_path)
+                except ChecksumError:
+                    raise ChecksumError(
+                        'MD5 checksum mismatch for cached download '
+                        'from %r at %r' % (url, cached_path))
+                except Exception:
+                    pass
             self.logger.debug('Using cache file %s' % cached_path)
         else:
             self.logger.debug('Cache miss; will cache %s as %s' %

src/zc/buildout/download.txt

@@ -168,11 +168,28 @@ This is a foo text.
 If we specify an MD5 checksum for a file that is already in the cache, the
 cached copy's checksum will be verified:
 
->>> download(server_url+'foo.txt', md5('The wrong text.').hexdigest())
+>>> download(server_url+'foo.txt', md5('The totally wrong text.').hexdigest())
 Traceback (most recent call last):
 ChecksumError: MD5 checksum mismatch for cached download
                from 'http://localhost/foo.txt' at '/download-cache/foo.txt'
 
+But if MD5 checksum matches remote resource download utility will try to
+download it once to match MD5 checksum:
+
+>>> path, is_temp = download(server_url+'foo.txt', md5('The wrong text.').hexdigest())
+>>> print path
+/download-cache/foo.txt
+>>> cat(path)
+The wrong text.
+
+Now it is time to write again the correct text and download it for next test:
+>>> write(server_data, 'foo.txt', 'This is a foo text.')
+>>> path, is_temp = download(server_url+'foo.txt', md5('This is a foo text.').hexdigest())
+>>> print path
+/download-cache/foo.txt
+>>> cat(path)
+This is a foo text.
+
 Trying to access another file at a different URL which has the same base name
 will result in the cached copy being used: