Some refactoring of the updater class.

In order to make the process of downloading metadata files more flexible,
finding mirrors and the actual download were moved to a so-called handler class.
With that change, it is possible to load a file from a specific commit of
a git repository for example

Signed-off-by: Renata <vrenata8@gmail.com>

tuf/client/handlers.py

@@ -0,0 +1,158 @@
+import tuf
+import logging
+import tuf.exceptions
+
+logger = logging.getLogger('tuf.client.updater')
+
+
+class MetadataUpdater(object):
+  """
+  <Purpose>
+    Provide a way to redefine certain parts of the process of updating metadata.
+    To be more specific, this class should enable redefinition of how metadata
+    is downloaded.
+
+
+  <Arguments>
+    mirrors:
+      A dictionary holding repository mirror information, conformant to
+      'tuf.formats.MIRRORDICT_SCHEMA'.
+
+    repository_directory:
+      Client's repository directory. Specified via tuf.settings.repositories_directory.
+
+
+  <Exceptions>
+    None.
+
+  <Side Effects>
+    None.
+
+  <Returns>
+    None.
+  """
+  def __init__(self, mirrors, repository_directory):
+    self.mirrors = mirrors
+    self.repository_directory = repository_directory
+
+class RemoteMetadataUpdater(MetadataUpdater):
+  """
+  Subclass of 'MetadataUpdater' which handles the case of
+  downloading metadata files from remote mirrors.
+  """
+
+
+  def get_mirrors(self, remote_filename):
+    """
+    <Purpose>
+      Finds mirrors from which the specified file can be downloaded.
+
+
+    <Arguments>
+      remote_filename:
+        The relative file path (on the remote repository) of a metadata role.
+
+
+    <Exceptions>
+      None.
+
+    Side Effects>
+      None.
+
+    <Returns>
+      A list of mirrors from which the specified file can be downloaded.
+    """
+    return tuf.mirrors.get_list_of_mirrors('meta', remote_filename,
+      self.mirrors)
+
+
+  def get_metadata_file(self, file_mirror, _filename, _upperbound_filelength):
+    """
+    <Purpose>
+      Downloads the metadata file from the provided mirror. Calls 'unsafe_download', which,
+      given the 'url' and 'required_length' of the desired file downloads the file and
+      returns its contents.
+
+
+    <Arguments>
+      file_mirror:
+        Mirror from which the file should be downloaded.
+
+      _filename:
+        The relative file path (on the remote repository) of a metadata role.
+
+      _upperbound_filelength:
+        An integer value representing the upper limit of the length of the file.
+
+    <Exceptions>
+      tuf.ssl_commons.exceptions.DownloadLengthMismatchError, if there was a
+      mismatch of observed vs expected lengths while downloading the file.
+
+      securesystemslib.exceptions.FormatError, if any of the arguments are
+      improperly formatted.
+
+      Any other unforeseen runtime exception.
+
+    Side Effects>
+      A 'securesystemslib.util.TempFile' object is created on disk to store the
+      contents of 'url'.
+
+    <Returns>
+      A 'securesystemslib.util.TempFile' file-like object that points to the
+      contents of 'url'.
+    """
+    return tuf.download.unsafe_download(file_mirror,
+        _upperbound_filelength)
+
+
+  def on_successful_update(self, filename, mirror):
+    """
+    <Purpose>
+      React to successful update of a metadata file 'filename'. Called
+      after file 'filename' is downloaded from 'mirror' and all
+      validation checks pass. In this case, nothing needs to be done,
+      so the method is empty.
+
+
+    <Arguments>
+      filename:
+        The relative file path (on the remote repository) of a metadata role.
+
+      mirror:
+        The mirror from whih th file was successfully downloaded.
+
+
+    <Exceptions>
+      None.
+
+    Side Effects>
+      None.
+
+    <Returns>
+      None.
+    """
+
+
+
+  def on_unsuccessful_update(self, filename):
+    """
+    <Purpose>
+      React to unsuccessful update of a metadata file 'filename'. Called
+      after all attempts to download file 'filename' fail.
+      In this case, nothing needs to be done, so the method is empty.
+
+
+    <Arguments>
+      filename:
+        The relative file path (on the remote repository) of a metadata role.
+
+
+    <Exceptions>
+      None.
+
+    Side Effects>
+      None.
+
+    <Returns>
+      None
+    """

tuf/client/updater.py

@@ -139,6 +139,7 @@
 import tuf.roledb
 import tuf.sig
 import tuf.exceptions
+import tuf.client.handlers as handlers
 
 import securesystemslib.hash
 import securesystemslib.keys
@@ -629,7 +630,8 @@ class Updater(object):
     http://www.python.org/dev/peps/pep-0008/#method-names-and-instance-variables
   """
 
-  def __init__(self, repository_name, repository_mirrors):
+  def __init__(self, repository_name, repository_mirrors,
+      update_handler_cls=handlers.RemoteMetadataUpdater):
     """
     <Purpose>
       Constructor.  Instantiating an updater object causes all the metadata
@@ -737,6 +739,7 @@ def __init__(self, repository_name, repository_mirrors):
     repositories_directory = tuf.settings.repositories_directory
     repository_directory = os.path.join(repositories_directory, self.repository_name)
     current_path = os.path.join(repository_directory, 'metadata', 'current')
+    self.update_handler = update_handler_cls(repository_mirrors, repository_directory)
 
     # Ensure the current path is valid/exists before saving it.
     if not os.path.exists(current_path):
@@ -1472,17 +1475,17 @@ def _get_metadata_file(self, metadata_role, remote_filename,
       metadata.
     """
 
-    file_mirrors = tuf.mirrors.get_list_of_mirrors('meta', remote_filename,
-        self.mirrors)
+    file_mirrors = self.update_handler.get_mirrors(remote_filename)
 
     # file_mirror (URL): error (Exception)
     file_mirror_errors = {}
     file_object = None
+    successful_mirror = None
 
     for file_mirror in file_mirrors:
       try:
-        file_object = tuf.download.unsafe_download(file_mirror,
-            upperbound_filelength)
+        file_object = self.update_handler.get_metadata_file(file_mirror,
+            remote_filename, upperbound_filelength)
 
         # Verify 'file_object' according to the callable function.
         # 'file_object' is also verified if decompressed above (i.e., the
@@ -1549,12 +1552,15 @@ def _get_metadata_file(self, metadata_role, remote_filename,
         file_object = None
 
       else:
+        successful_mirror = file_mirror
         break
 
     if file_object:
+      self.update_handler.on_successful_update(remote_filename, successful_mirror)
       return file_object
 
     else:
+      self.update_handler.on_unsuccessful_update(remote_filename)
       logger.error('Failed to update ' + repr(remote_filename) + ' from all'
         ' mirrors: ' + repr(file_mirror_errors))
       raise tuf.exceptions.NoWorkingMirrorError(file_mirror_errors)