Implement Sigstore signing and verification of models

model_signing/manifest/manifest.py

@@ -208,6 +208,32 @@ def __str__(self) -> str:
         """
         return f"{str(self.path)}:{self.start}:{self.end}"
 
+    @classmethod
+    def from_str(cls, s: str) -> Self:
+        """Builds a file shard from the string representation.
+
+        It is guaranteed that for a shard `shard` and a (valid) string `s` the
+        following two round-trip properties hold:
+
+        ```
+        str(Shard.from_str(s)) == s
+        Shard.from_str(str(shard)) == shard
+        ```
+
+        Raises:
+            ValueError: if the string argument does not represent a valid shard
+            serialization (is not in the format `path:start:end`).
+        """
+        parts = s.split(":")
+        if len(parts) != 3:
+            raise ValueError(f"Expected 3 components separated by `:`, got {s}")
+
+        path = pathlib.PurePosixPath(parts[0])
+        start = int(parts[1])
+        end = int(parts[2])
+
+        return cls(path, start, end)
+
 
 @dataclasses.dataclass
 class ShardedFileManifestItem(ManifestItem):

model_signing/manifest/manifest_test.py

@@ -89,6 +89,43 @@ def test_manifest_has_the_correct_resource_descriptors(self):
         assert descriptors[1].digest.digest_value == b"hash2"
 
 
+class TestShard:
+
+    def test_round_trip_from_shard(self):
+        shard = manifest.Shard(pathlib.PurePosixPath("file"), 0, 42)
+        shard_str = str(shard)
+        assert manifest.Shard.from_str(shard_str) == shard
+
+    def test_round_trip_from_string(self):
+        shard_str = "file:0:42"
+        shard = manifest.Shard.from_str(shard_str)
+        assert str(shard) == shard_str
+
+    def test_invalid_shard_str_too_few_components(self):
+        shard_str = "file"
+
+        with pytest.raises(ValueError, match="Expected 3 components"):
+            manifest.Shard.from_str(shard_str)
+
+    def test_invalid_shard_str_too_many_components(self):
+        shard_str = "file:0:1:2"
+
+        with pytest.raises(ValueError, match="Expected 3 components"):
+            manifest.Shard.from_str(shard_str)
+
+    def test_invalid_shard_bad_type_for_start_offset(self):
+        shard_str = "file:zero:4"
+
+        with pytest.raises(ValueError, match="invalid literal for int"):
+            manifest.Shard.from_str(shard_str)
+
+    def test_invalid_shard_bad_type_for_endart_offset(self):
+        shard_str = "file:0:four"
+
+        with pytest.raises(ValueError, match="invalid literal for int"):
+            manifest.Shard.from_str(shard_str)
+
+
 class TestShardLevelManifest:
 
     def test_insert_order_does_not_matter(self):