Added a check for max_age being greater than 0 (#172)

* Added a check for max_age being greater than 0

* Fixed flake8 by adding missing pydocstyle dependency

* Added the dependency to decrypt_oracle as well

* Added test for max_age<=0 ValueError

* Updated test for max_age<=0.0 ValueError

* Added negative test case

Author: caitlin-tibbetts

decrypt_oracle/tox.ini

@@ -156,6 +156,7 @@ basepython = python3
 deps =
     flake8
     flake8-docstrings
+    pydocstyle<4.0.0
     # https://github.com/JBKahn/flake8-print/pull/30
     flake8-print>=3.1.0
 commands =

src/aws_encryption_sdk/materials_managers/caching.py

@@ -108,6 +108,9 @@ def __attrs_post_init__(self):
         if self.max_bytes_encrypted > MAX_BYTES_PER_KEY:
             raise ValueError("max_bytes_encrypted cannot exceed {}".format(MAX_BYTES_PER_KEY))
 
+        if self.max_age <= 0.0:
+            raise ValueError("max_age cannot be less than or equal to 0")
+
         if self.backing_materials_manager is None:
             if self.master_key_provider is None:
                 raise TypeError("Either backing_materials_manager or master_key_provider must be defined")

test/unit/test_material_managers_caching.py

@@ -117,6 +117,8 @@ def test_mkp_to_default_cmm(mocker):
             dict(max_bytes_encrypted=MAX_BYTES_PER_KEY + 1),
             r"max_bytes_encrypted cannot exceed {}".format(MAX_BYTES_PER_KEY),
         ),
+        (dict(max_age=0.0), r"max_age cannot be less than or equal to 0"),
+        (dict(max_age=-1.0), r"max_age cannot be less than or equal to 0"),
     ),
 )
 def test_invalid_values(invalid_kwargs, error_message):

tox.ini

@@ -117,6 +117,7 @@ basepython = python3
 deps =
     flake8
     flake8-docstrings
+    pydocstyle<4.0.0
     # https://github.com/JBKahn/flake8-print/pull/30
     flake8-print>=3.1.0
     flake8-bugbear