test artifact caching

Artifact caching is not part of the spec but it is a good idea
* Client CLI document did not require caching before: Add it in the doc
* Add test that verifies that two download commands do not result in
  two downloads
* python test client did not support this so support was added

Signed-off-by: Jussi Kukkonen <jkukkonen@google.com>

Author: jku

CLIENT-CLI.md

@@ -72,8 +72,12 @@ given targets directory. Client must ensure that metadata is up-to-date before d
 Client must use exit code 0 if the download succeeded fully and exit code 1 if any part of
 the download failed.
 
-Client may use any filename or directory structure it wants within `TARGET_DIR`: The expectation is that if a 
-targetpath is downloaded twice it is stored with the same filename (even if the content changed).
+Client should support artifact caching: if client is requested to download an artifact that has
+already been downloaded (with matching hashes), it should not download the artifact again.
+
+Client may use any filename or directory structure it wants within `TARGET_DIR`: The expectation
+is that if an artifact is downloaded twice (because the artifact content changed) it is stored in
+`TARGET_DIR` with the same filename.
 
 This command may be called multiple times in a test.
 

clients/python-tuf/python_tuf.py

@@ -50,7 +50,8 @@ def download_target(
     target_info = updater.get_targetinfo(target_name)
     if not target_info:
         raise RuntimeError(f"{target_name} not found in repository")
-    updater.download_target(target_info)
+    if not updater.find_cached_target(target_info):
+        updater.download_target(target_info)
 
 
 def main() -> int:

tuf_conformance/test_file_download.py

@@ -238,3 +238,30 @@ def test_download_with_unknown_hash_algorithm(
     # and only then realize hash cannot be verified
     assert client.download_target(init_data, target_path) == 1
     assert client.get_downloaded_target_bytes() == []
+
+
+def test_artifact_cache(
+    client: ClientRunner, server: SimulatorServer
+) -> None:
+    """In this test client is asked to download the same artifact twice.
+    The client is expected return the cached content on the second time.
+
+    Artifact caching is not required in the specification: clients
+    that do not support it can mark this test as expected to fail
+    """
+
+    init_data, repo = server.new_test(client.test_name)
+    assert client.init_client(init_data) == 0
+
+    # Create a test artifact, add it to the repository
+    target_path = "target_file.txt"
+    target_content = b"target file contents"
+    repo.add_artifact(Targets.type, target_content, target_path)
+    repo.publish([Targets.type, Snapshot.type, Timestamp.type])
+
+    # Client is asked to download artifact twice
+    assert client.download_target(init_data, target_path) == 0
+    assert client.download_target(init_data, target_path) == 0
+
+    # Expect only one download from repository
+    assert len(repo.artifact_statistics) == 1