RepositorySimulator: add non-consistent snapshot support

Extend filename partitioning to support serving non-versioned
metadata and non-prefixed target files.

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

tests/repository_simulator.py

@@ -108,7 +108,7 @@ def __init__(self):
         # target downloads are served from this dict
         self.target_files: Dict[str, RepositoryTarget] = {}
 
-        # Whether to compute hashes and legth for meta in snapshot/timestamp
+        # Whether to compute hashes and length for meta in snapshot/timestamp
         self.compute_metafile_hashes_length = False
 
         self.dump_dir = None
@@ -145,6 +145,21 @@ def create_key() -> Tuple[Key, SSlibSigner]:
         sslib_key = generate_ed25519_key()
         return Key.from_securesystemslib_key(sslib_key), SSlibSigner(sslib_key)
 
+    @staticmethod
+    def _partition_filename(filename: str) -> Tuple[str, Optional[str]]:
+        """Partition 'filename' into filename and prefix, if any"""
+        p1, sep, p2 = filename.partition(".")
+        # If the separator is not found, returns a 3-tuple containing
+        # the string itself, followed by two empty strings.
+        if not sep:
+            prefix = None
+            filename = p1
+        else:
+            prefix = p1
+            filename = p2
+
+        return filename, prefix
+
     def add_signer(self, role: str, signer: SSlibSigner):
         if role not in self.signers:
             self.signers[role] = {}
@@ -185,24 +200,19 @@ def publish_root(self):
         logger.debug("Published root v%d", self.root.version)
 
     def fetch(self, url: str) -> Iterator[bytes]:
-        if not self.root.consistent_snapshot:
-            raise NotImplementedError("non-consistent snapshot not supported")
         path = parse.urlparse(url).path
         if path.startswith("/metadata/") and path.endswith(".json"):
             ver_and_name = path[len("/metadata/") :][: -len(".json")]
-            # only consistent_snapshot supported ATM: timestamp is special case
-            if ver_and_name == "timestamp":
-                version = None
-                role = "timestamp"
-            else:
-                version, _, role = ver_and_name.partition(".")
+            role, version = self._partition_filename(ver_and_name)
+            if version:
                 version = int(version)
+
             yield self._fetch_metadata(role, version)
         elif path.startswith("/targets/"):
             # figure out target path and hash prefix
             target_path = path[len("/targets/") :]
             dir_parts, sep, prefixed_filename = target_path.rpartition("/")
-            prefix, _, filename = prefixed_filename.partition(".")
+            filename, prefix = self._partition_filename(prefixed_filename)
             target_path = f"{dir_parts}{sep}{filename}"
 
             yield self._fetch_target(target_path, prefix)
@@ -245,7 +255,7 @@ def _fetch_metadata(
             elif role == "targets":
                 md = self.md_targets
             else:
-                md = self.md_delegates[role]
+                md = self.md_delegates.get(role)
 
             if md is None:
                 raise FetcherHTTPError(f"Unknown role {role}", 404)