Add class FetcherSettings

Move tuf settings related to network download to a separate
class which can be used by a FetcherInterface implementation
or replaced with its own settings.

Signed-off-by: Teodora Sechkova <tsechkova@vmware.com>

Author: sechkova

tuf/fetcher.py

@@ -23,7 +23,6 @@
 
 import urllib3.exceptions
 import tuf.exceptions
-import tuf.settings
 
 logger = logging.getLogger(__name__)
 
@@ -62,7 +61,25 @@ def fetch(self, url, required_length):
 
 
 
-class RequestsFetcher(FetcherInterface):
+class FetcherSettings():
+  """
+  <Purpose>
+  Data class. Defines fetcher-specific settings and their default values
+  """
+
+  # The maximum chunk of data, in bytes, we would download in every round.
+  CHUNK_SIZE = 400000 #bytes
+  # Set a timeout value in seconds (float) for non-blocking socket operations.
+  SOCKET_TIMEOUT = 4 #seconds
+  # The client's update procedure (contained within a while-loop) can potentially
+  # hog the CPU.  The following setting can be used to force the update sequence
+  # to suspend execution for a specified amount of time.  See
+  # theupdateframework/tuf/issue#338.
+  SLEEP_BEFORE_ROUND = None
+
+
+
+class RequestsFetcher(FetcherInterface, FetcherSettings):
   """
   <Purpose>
     A concrete implementation of FetcherInterface based on the Requests
@@ -97,7 +114,7 @@ def fetch(self, url, required_length):
     #  - connect timeout (max delay before first byte is received)
     #  - read (gap) timeout (max delay between bytes received)
     with session.get(url, stream=True,
-        timeout=tuf.settings.SOCKET_TIMEOUT) as response:
+        timeout=self.SOCKET_TIMEOUT) as response:
 
       # Check response status.
       response.raise_for_status()
@@ -110,11 +127,11 @@ def fetch(self, url, required_length):
           # to download an extremely large file in one shot.
           # Before beginning the round, sleep (if set) for a short amount of time
           # so that the CPU is not hogged in the while loop.
-          if tuf.settings.SLEEP_BEFORE_ROUND:
-            time.sleep(tuf.settings.SLEEP_BEFORE_ROUND)
+          if self.SLEEP_BEFORE_ROUND:
+            time.sleep(self.SLEEP_BEFORE_ROUND)
 
           read_amount = min(
-              tuf.settings.CHUNK_SIZE, required_length - bytes_received)
+              self.CHUNK_SIZE, required_length - bytes_received)
 
           # NOTE: This may not handle some servers adding a Content-Encoding
           # header, which may cause urllib3 to misbehave:

tuf/settings.py

@@ -74,12 +74,6 @@
 # download Targets metadata.
 DEFAULT_TARGETS_REQUIRED_LENGTH = 5000000 #bytes
 
-# Set a timeout value in seconds (float) for non-blocking socket operations.
-SOCKET_TIMEOUT = 4 #seconds
-
-# The maximum chunk of data, in bytes, we would download in every round.
-CHUNK_SIZE = 400000 #bytes
-
 # The minimum average download speed (bytes/second) that must be met to
 # avoid being considered as a slow retrieval attack.
 MIN_AVERAGE_DOWNLOAD_SPEED = 50 #bytes/second
@@ -97,12 +91,6 @@
 # The hashing algorithms used to compute file hashes
 FILE_HASH_ALGORITHMS = ['sha256', 'sha512']
 
-# The client's update procedure (contained within a while-loop) can potentially
-# hog the CPU.  The following setting can be used to force the update sequence
-# to suspend execution for a specified amount of time.  See
-# theupdateframework/tuf/issue#338.
-SLEEP_BEFORE_ROUND = None
-
 # Maximum number of root metadata file rotations we should perform in order to
 # prevent a denial-of-service (DoS) attack.
 MAX_NUMBER_ROOT_ROTATIONS = 2**5