ngclient: Make DownloadErrors more consistent

examples/client_example/client_example.py

@@ -10,7 +10,7 @@
 import shutil
 from pathlib import Path
 
-from tuf.api.exceptions import RepositoryError
+from tuf.api.exceptions import DownloadError, RepositoryError
 from tuf.ngclient import Updater
 
 # constants
@@ -73,8 +73,8 @@ def download(target: str) -> bool:
         path = updater.download_target(info)
         print(f"Target downloaded and available in {path}")
 
-    except (OSError, RepositoryError) as e:
-        print(str(e))
+    except (OSError, RepositoryError, DownloadError) as e:
+        print(f"Failed to download target {target}: {e}")
         return False
 
     return True

tests/repository_simulator.py

@@ -56,7 +56,7 @@
 from securesystemslib.keys import generate_ed25519_key
 from securesystemslib.signer import SSlibSigner
 
-from tuf.api.exceptions import FetcherHTTPError
+from tuf.api.exceptions import DownloadHTTPError
 from tuf.api.metadata import (
     SPECIFICATION_VERSION,
     TOP_LEVEL_ROLE_NAMES,
@@ -205,7 +205,7 @@ def publish_root(self) -> None:
         self.signed_roots.append(self.md_root.to_bytes(JSONSerializer()))
         logger.debug("Published root v%d", self.root.version)
 
-    def fetch(self, url: str) -> Iterator[bytes]:
+    def _fetch(self, url: str) -> Iterator[bytes]:
         """Fetches data from the given url and returns an Iterator (or yields
         bytes).
         """
@@ -238,7 +238,7 @@ def fetch(self, url: str) -> Iterator[bytes]:
 
             yield self.fetch_target(target_path, prefix)
         else:
-            raise FetcherHTTPError(f"Unknown path '{path}'", 404)
+            raise DownloadHTTPError(f"Unknown path '{path}'", 404)
 
     def fetch_target(
         self, target_path: str, target_hash: Optional[str]
@@ -251,12 +251,12 @@ def fetch_target(
 
         repo_target = self.target_files.get(target_path)
         if repo_target is None:
-            raise FetcherHTTPError(f"No target {target_path}", 404)
+            raise DownloadHTTPError(f"No target {target_path}", 404)
         if (
             target_hash
             and target_hash not in repo_target.target_file.hashes.values()
         ):
-            raise FetcherHTTPError(f"hash mismatch for {target_path}", 404)
+            raise DownloadHTTPError(f"hash mismatch for {target_path}", 404)
 
         logger.debug("fetched target %s", target_path)
         return repo_target.data
@@ -273,7 +273,7 @@ def fetch_metadata(self, role: str, version: Optional[int] = None) -> bytes:
         if role == Root.type:
             # return a version previously serialized in publish_root()
             if version is None or version > len(self.signed_roots):
-                raise FetcherHTTPError(f"Unknown root version {version}", 404)
+                raise DownloadHTTPError(f"Unknown root version {version}", 404)
             logger.debug("fetched root version %d", version)
             return self.signed_roots[version - 1]
 
@@ -289,7 +289,7 @@ def fetch_metadata(self, role: str, version: Optional[int] = None) -> bytes:
             md = self.md_delegates.get(role)
 
         if md is None:
-            raise FetcherHTTPError(f"Unknown role {role}", 404)
+            raise DownloadHTTPError(f"Unknown role {role}", 404)
 
         md.signatures.clear()
         for signer in self.signers[role].values():

tests/test_fetcher_ng.py

@@ -101,7 +101,7 @@ def test_url_parsing(self) -> None:
 
     # File not found error
     def test_http_error(self) -> None:
-        with self.assertRaises(exceptions.FetcherHTTPError) as cm:
+        with self.assertRaises(exceptions.DownloadHTTPError) as cm:
             self.url = f"{self.url_prefix}/non-existing-path"
             self.fetcher.fetch(self.url)
         self.assertEqual(cm.exception.status_code, 404)

tuf/api/exceptions.py

@@ -52,7 +52,7 @@ class SlowRetrievalError(DownloadError):
     """Indicate that downloading a file took an unreasonably long time."""
 
 
-class FetcherHTTPError(DownloadError):
+class DownloadHTTPError(DownloadError):
     """
     Returned by FetcherInterface implementations for HTTP errors.
 

tuf/ngclient/_internal/requests_fetcher.py

@@ -51,7 +51,7 @@ def __init__(self) -> None:
         self.socket_timeout: int = 4  # seconds
         self.chunk_size: int = 400000  # bytes
 
-    def fetch(self, url: str) -> Iterator[bytes]:
+    def _fetch(self, url: str) -> Iterator[bytes]:
         """Fetches the contents of HTTP/HTTPS url from a remote server
 
         Arguments:
@@ -60,8 +60,7 @@ def fetch(self, url: str) -> Iterator[bytes]:
         Raises:
             exceptions.SlowRetrievalError: A timeout occurs while receiving
                 data.
-            exceptions.FetcherHTTPError: An HTTP error code is received.
-            exceptions.DownloadError: When there is a problem parsing the url.
+            exceptions.DownloadHTTPError: An HTTP error code is received.
 
         Returns:
             A bytes iterator
@@ -89,7 +88,7 @@ def fetch(self, url: str) -> Iterator[bytes]:
         except requests.HTTPError as e:
             response.close()
             status = e.response.status_code
-            raise exceptions.FetcherHTTPError(str(e), status)
+            raise exceptions.DownloadHTTPError(str(e), status)
 
         return self._chunks(response)
 
@@ -122,7 +121,7 @@ def _get_session(self, url: str) -> requests.Session:
         parsed_url = parse.urlparse(url)
 
         if not parsed_url.scheme or not parsed_url.hostname:
-            raise exceptions.DownloadError("Failed to parse URL {url}")
+            raise exceptions.DownloadError(f"Failed to parse URL {url}")
 
         session_index = f"{parsed_url.scheme}+{parsed_url.hostname}"
         session = self._sessions.get(session_index)

tuf/ngclient/fetcher.py

@@ -28,38 +28,65 @@ class FetcherInterface:
     __metaclass__ = abc.ABCMeta
 
     @abc.abstractmethod
-    def fetch(self, url: str) -> Iterator[bytes]:
+    def _fetch(self, url: str) -> Iterator[bytes]:
         """Fetches the contents of HTTP/HTTPS url from a remote server.
 
+        Implementations must raise DownloadHTTPError if they receive an
+        HTTP error code.
+
+        Implementations may raise any errors but the ones that are not
+        DownloadErrors will be wrapped in a DownloadError by fetch().
+
         Arguments:
             url: A URL string that represents a file location.
 
         Raises:
-            exceptions.SlowRetrievalError: A timeout occurs while receiving
-                data.
-            exceptions.FetcherHTTPError: An HTTP error code is received.
+            exceptions.DownloadHTTPError: An HTTP error code was received.
 
         Returns:
             A bytes iterator
         """
         raise NotImplementedError  # pragma: no cover
 
+    def fetch(self, url: str) -> Iterator[bytes]:
+        """Fetches the contents of HTTP/HTTPS url from a remote server.
+
+        Arguments:
+            url: A URL string that represents a file location.
+
+        Raises:
+            exceptions.DownloadError: An error occurred during download.
+            exceptions.DownloadHTTPError: An HTTP error code was received.
+
+        Returns:
+            A bytes iterator
+        """
+        # Ensure that fetch() only raises DownloadErrors, regardless of the
+        # fetcher implementation
+        try:
+            return self._fetch(url)
+        except exceptions.DownloadError as e:
+            raise e
+        except Exception as e:
+            raise exceptions.DownloadError(f"Failed to download {url}") from e
+
     @contextmanager
     def download_file(self, url: str, max_length: int) -> Iterator[IO]:
         """Opens a connection to 'url' and downloads the content
         up to 'max_length'.
 
         Args:
-          url: a URL string that represents the location of the file.
-          max_length: an integer value representing the length of
-              the file or an upper bound.
+            url: a URL string that represents the location of the file.
+            max_length: upper bound of file size in bytes.
 
         Raises:
-          exceptions.DownloadLengthMismatchError: downloaded bytes exceed
-            'max_length'.
+            exceptions.DownloadError: An error occurred during download.
+            exceptions.DownloadLengthMismatchError: downloaded bytes exceed
+                'max_length'.
+            exceptions.DownloadHTTPError: An HTTP error code was received.
 
         Yields:
-          A TemporaryFile object that points to the contents of 'url'.
+            A TemporaryFile object that points to the contents of 'url'.
         """
         logger.debug("Downloading: %s", url)
 
@@ -96,8 +123,10 @@ def download_bytes(self, url: str, max_length: int) -> bytes:
             max_length: upper bound of data size in bytes.
 
         Raises:
+            exceptions.DownloadError: An error occurred during download.
             exceptions.DownloadLengthMismatchError: downloaded bytes exceed
                 'max_length'.
+            exceptions.DownloadHTTPError: An HTTP error code was received.
 
         Returns:
             The content of the file in bytes.

tuf/ngclient/updater.py

@@ -315,7 +315,7 @@ def _load_root(self) -> None:
                 self._trusted_set.update_root(data)
                 self._persist_metadata(Root.type, data)
 
-            except exceptions.FetcherHTTPError as exception:
+            except exceptions.DownloadHTTPError as exception:
                 if exception.status_code not in {403, 404}:
                     raise
                 # 404/403 means current root is newest available