kvm: Fix endless promise failing loop

The problem has been introduced in 5da9c082
"kvm: Improve boot image url handling".

Note the internal usage of md5sum value as image name in the download config
creator.

software/kvm/buildout.hash.cfg

@@ -79,11 +79,11 @@ md5sum = 438192aab9f11e40dc521b46a4854dcf
 
 [image-download-controller]
 filename = template/image-download-controller.py
-md5sum = 4d48b3da5bc611fc6533335b5953c840
+md5sum = 3cc10323fd4d2db4cfbac536b66eae7c
 
 [image-download-config-creator]
 filename = template/image-download-config-creator.py
-md5sum = 8fbe05c4175a7f31b6bffced9ad4e91d
+md5sum = 22ed19d9b8f7b983c97c52caa686bcd7
 
 [whitelist-firewall-download-controller]
 filename = template/whitelist-firewall-download-controller.py

software/kvm/template/image-download-config-creator.py

@@ -49,6 +49,10 @@ if __name__ == "__main__":
         image_list.append({
           'md5sum': md5sum,
           'url': url,
+          # Note: The destination here it's the waneted md5sum on purpose, as
+          #       it allows to assume, that correctly downloaded and hashed
+          #       image stored at this filename matches the md5sum, so it does
+          #       not have to be hashed on each download run.
           'destination': md5sum,
           'destination-tmp': md5sum + '_tmp',
           'image-number': '%03i' % (image_number,),

software/kvm/template/image-download-controller.py

@@ -68,13 +68,17 @@ if __name__ == "__main__":
     destination = os.path.join(
       config['destination-directory'], image['destination'])
     if os.path.exists(destination):
-      if md5Checksum(destination) == image['md5sum']:
+      # Note: There is no need to recheck md5sum here
+      #       The image name is its md5sum, so if it exists, it means it has
+      #       correct md5sum
+      #       Calculating md5sum of big images takes more time than processing
+      #       of the partition and running promises and this leads to endless
+      #       loop of never ending promise failures
+      #       Of course, someone nasty can come to the partition and damage
+      #       this image, but it's another story, and shall not be fixed
+      #       during download phase.
       print('INF: %s : already downloaded' % (image['url'],))
       continue
-      else:
-        print('INF: %s : Removed, as expected checksum does not match %s' % (
-          image['url'], image['md5sum']))
-        os.remove(destination)
     # key is str, as the dict is dumped to JSON which does not accept tuples
     md5sum_state_key = '%s#%s' % (image['url'], image['md5sum'])
     md5sum_state_amount = md5sum_state_dict.get(md5sum_state_key, 0)