Adds support for validate_only to the transaction

Issues:
Fixes #653

Problem:
The validateOnly functionality of transactions was not exposed to
the end user. Therefore, the user could never know if the transaction
would pass or fail if they ran it without actually running it

Analysis:
This patch adds support to the TransactionContextManager so that
it allows for the validate_only parameter to be provided.

By default, this parameter is set to False to maintain backwards
compatibility.

Tests:
tests/functional/tm/transaction/test_transaction.py::TestTransaction::test_validate_only_true

Author: caphrim007

f5/bigip/contexts.py

@@ -27,8 +27,33 @@ class TransactionSubmitException(F5SDKError):
 
 
 class TransactionContextManager(object):
-    def __init__(self, transaction):
+    def __init__(self, transaction, validate_only=False):
+        """Initialize a new Transaction context
+
+        Args:
+            validate_only (bool): Will not commit the transaction, but only
+                validate that it will succeed or not.
+
+        Attributes:
+            transaction (Transaction): The transaction object that was sent
+                to the context manager
+            validate_only (bool): Specifies whether the transaction should
+                commit itself upon `__exit__` or whether the commands in the
+                transaction should just be checked to make sure they don't
+                raise an error.
+            bigip (dict): A reference to the dictionary containing the BIG-IP
+                mgmt_root
+            icr (iControlRESTSession): A reference to the dictionary
+                containing the iControl REST session
+            original_headers (dict): A deep copy of all the headers that were
+                originally part of the iControl REST session. A copy is needed
+                so that we can revert back to them after the transaction has
+                been committed, since the only way to commit the transaction
+                is to set the X-F5-REST-Coordination-Id to the value of the
+                transaction ID of the transaction.
+        """
         self.transaction = transaction
+        self.validate_only = validate_only
         self.bigip = transaction._meta_data['bigip']
         self.icr = self.bigip._meta_data['icr_session']
         self.original_headers = copy.deepcopy(self.icr.session.headers)
@@ -68,10 +93,12 @@ def __exit__(self, exc_type, exc_value, exc_tb):
         parameters will be None
         :returns: void
         """
+
         self.icr.session.headers = dict()
         if exc_tb is None:
             try:
-                self.transaction.update(state="VALIDATING")
+                self.transaction.modify(state="VALIDATING",
+                                        validateOnly=self.validate_only)
             except Exception as e:
                 logging.debug(e)
                 raise TransactionSubmitException(e)

test/functional/tm/transaction/test_transaction.py

@@ -15,6 +15,7 @@
 
 import pytest
 
+from distutils.version import LooseVersion
 from f5.bigip.contexts import TransactionContextManager
 from f5.bigip.contexts import TransactionSubmitException
 
@@ -34,4 +35,42 @@ def test_create_empty(self, mgmt_root):
             with TransactionContextManager(tx) as api:  # NOQA
                 pass
         assert "there is no command to commit in the transaction" \
-               in str(ex.value.message)
+               in str(ex.value)
+
+    @pytest.mark.skipif(
+        LooseVersion(pytest.config.getoption('--release'))
+        < LooseVersion('12.0.0'),
+        reason='Needs v12.0.0 TMOS or greater to pass.'
+    )
+    def test_validate_only_true(self, mgmt_root):
+        s1 = mgmt_root.tm.sys.dbs.db.load(name='setup.run')
+        s1.update(value=False)
+
+        tx = mgmt_root.tm.transactions.transaction
+        with TransactionContextManager(tx, validate_only=True) as api:  # NOQA
+            s2 = api.tm.sys.dbs.db.load(name='setup.run')
+            s2.update(value=True)
+
+        s3 = mgmt_root.tm.sys.dbs.db.load(name='setup.run')
+        result = s3.value
+
+        assert result == 'false'
+
+    @pytest.mark.skipif(
+        LooseVersion(pytest.config.getoption('--release'))
+        < LooseVersion('12.0.0'),
+        reason='Needs v12.0.0 TMOS or greater to pass.'
+    )
+    def test_validate_only_false(self, mgmt_root):
+        s1 = mgmt_root.tm.sys.dbs.db.load(name='setup.run')
+        s1.update(value=False)
+
+        tx = mgmt_root.tm.transactions.transaction
+        with TransactionContextManager(tx, validate_only=False) as api:  # NOQA
+            s2 = api.tm.sys.dbs.db.load(name='setup.run')
+            s2.update(value=True)
+
+        s3 = mgmt_root.tm.sys.dbs.db.load(name='setup.run')
+        result = s3.value
+
+        assert result == 'true'