Wrap download functions in FileDownload class

Encapsulate download fucntionality in a FileDownload
class which adds file handling capabilities to fetcher.

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

Author: sechkova

tuf/ngclient/_internal/download.py

@@ -27,76 +27,82 @@
 from urllib import parse
 
 from tuf import exceptions
+from tuf.ngclient._internal.requests_fetcher import RequestsFetcher
 
 logger = logging.getLogger(__name__)
 
 
-def download_file(url, required_length, fetcher):
-    """
-    <Purpose>
-      Given the url and length of the desired file, this function opens a
-      connection to 'url' and downloads the file up to 'required_length'.
-
-    <Arguments>
-      url:
-        A URL string that represents the location of the file.
-
-      required_length:
-        An integer value representing the length of the file or an
-        upper boundary.
-
-    <Side Effects>
-      A file object is created on disk to store the contents of 'url'.
+class FileDownloader:
+    """Provides methods to download the contents of a URL to a file object.
 
-    <Exceptions>
-      exceptions.DownloadLengthMismatchError, if there was a
-      mismatch of observed vs expected lengths while downloading the file.
-
-      Any other unforeseen runtime exception.
-
-    <Returns>
-      A file object that points to the contents of 'url'.
+    Attributes:
+        fetcher: A FetcherInterface implementation which provides
+            the network IO library.
     """
-    # 'url.replace('\\', '/')' is needed for compatibility with Windows-based
-    # systems, because they might use back-slashes in place of forward-slashes.
-    # This converts it to the common format.  unquote() replaces %xx escapes in
-    # a url with their single-character equivalent.  A back-slash may be
-    # encoded as %5c in the url, which should also be replaced with a forward
-    # slash.
-    url = parse.unquote(url).replace("\\", "/")
-    logger.debug("Downloading: %s", url)
-
-    # This is the temporary file that we will return to contain the contents of
-    # the downloaded file.
-    temp_file = tempfile.TemporaryFile()  # pylint: disable=consider-using-with
-
-    number_of_bytes_received = 0
-
-    try:
-        chunks = fetcher.fetch(url, required_length)
-        for chunk in chunks:
-            temp_file.write(chunk)
-            number_of_bytes_received += len(chunk)
-
-        if number_of_bytes_received > required_length:
-            raise exceptions.DownloadLengthMismatchError(
-                required_length, number_of_bytes_received
-            )
-
-    except Exception:
-        # Close 'temp_file'.  Any written data is lost.
-        temp_file.close()
-        raise
-
-    else:
-        temp_file.seek(0)
-        return temp_file
-
-
-def download_bytes(url, required_length, fetcher):
-    """Download bytes from given url
-
-    Returns the downloaded bytes, otherwise like download_file()
-    """
-    with download_file(url, required_length, fetcher) as dl_file:
-        return dl_file.read()
+
+    def __init__(self, fetcher):
+        if fetcher is None:
+            fetcher = RequestsFetcher()
+
+        self._fetcher = fetcher
+
+    def download_file(self, url, required_length):
+        """Opens a connection to 'url' and downloads the content
+        up to 'required_length'.
+
+        Args:
+          url: a URL string that represents the location of the file.
+          required_length: an integer value representing the length of
+             the file or an upper boundary.
+
+        Raises:
+          DownloadLengthMismatchError: a mismatch of observed vs expected
+             lengths while downloading the file.
+
+        Returns:
+          A file object that points to the contents of 'url'.
+        """
+        # 'url.replace('\\', '/')' is needed for compatibility with
+        # Windows-based systems, because they might use back-slashes in place
+        # of forward-slashes. This converts it to the common format.
+        # unquote() replaces %xx escapes in a url with their single-character
+        # equivalent. A back-slash may beencoded as %5c in the url, which
+        # should also be replaced with a forward slash.
+        url = parse.unquote(url).replace("\\", "/")
+        logger.debug("Downloading: %s", url)
+
+        # This is the temporary file that we will return to contain the
+        # contents of the downloaded file.
+        temp_file = (
+            tempfile.TemporaryFile()
+        )  # pylint: disable=consider-using-with
+
+        number_of_bytes_received = 0
+
+        try:
+            chunks = self._fetcher.fetch(url, required_length)
+            for chunk in chunks:
+                temp_file.write(chunk)
+                number_of_bytes_received += len(chunk)
+
+            if number_of_bytes_received > required_length:
+                raise exceptions.DownloadLengthMismatchError(
+                    required_length, number_of_bytes_received
+                )
+
+        except Exception:
+            # Close 'temp_file'.  Any written data is lost.
+            temp_file.close()
+            raise
+
+        else:
+            temp_file.seek(0)
+            return temp_file
+
+    def download_bytes(self, url, required_length):
+        """Download bytes from given url
+
+        Returns the downloaded bytes, otherwise like download_file()
+        """
+        with self.download_file(url, required_length) as dl_file:
+            return dl_file.read()

tuf/ngclient/updater.py

@@ -15,8 +15,8 @@
 from securesystemslib import util as sslib_util
 
 from tuf import exceptions
-from tuf.ngclient._internal import download, metadata_bundle, requests_fetcher
-from tuf.ngclient.fetcher import FetcherInterface
+from tuf.ngclient._internal import metadata_bundle
+from tuf.ngclient._internal.download import FileDownloader
 
 # Globals
 MAX_ROOT_ROTATIONS = 32
@@ -40,7 +40,7 @@ def __init__(
         repository_dir: str,
         metadata_base_url: str,
         target_base_url: Optional[str] = None,
-        fetcher: Optional[FetcherInterface] = None,
+        fetcher: Optional["FetcherInterface"] = None,
     ):
         """
         Args:
@@ -67,10 +67,7 @@ def __init__(
         data = self._load_local_metadata("root")
         self._bundle = metadata_bundle.MetadataBundle(data)
 
-        if fetcher is None:
-            self._fetcher = requests_fetcher.RequestsFetcher()
-        else:
-            self._fetcher = fetcher
+        self.downloader = FileDownloader(fetcher)
 
     def refresh(self) -> None:
         """
@@ -200,8 +197,8 @@ def download_target(
 
         full_url = parse.urljoin(target_base_url, targetinfo["filepath"])
 
-        with download.download_file(
-            full_url, targetinfo["fileinfo"].length, self._fetcher
+        with self.downloader.download_file(
+            full_url, targetinfo["fileinfo"].length
         ) as target_file:
             _check_file_length(target_file, targetinfo["fileinfo"].length)
             _check_hashes_obj(target_file, targetinfo["fileinfo"].hashes)
@@ -220,7 +217,7 @@ def _download_metadata(
         else:
             filename = f"{version}.{rolename}.json"
         url = parse.urljoin(self._metadata_base_url, filename)
-        return download.download_bytes(url, length, self._fetcher)
+        return self.downloader.download_bytes(url, length)
 
     def _load_local_metadata(self, rolename: str) -> bytes:
         with open(os.path.join(self._dir, f"{rolename}.json"), "rb") as f: