Very basic --diff option for showing what happens when templates change.

Probably output is not useful if not used with --limit

Works well with --check mode

lib/ansible/runner/__init__.py

@@ -118,11 +118,13 @@ class Runner(object):
         is_playbook=False,                  # running from playbook or not?
         inventory=None,                     # reference to Inventory object
         subset=None,                        # subset pattern
-        check=False                         # don't make any changes, just try to probe for potential changes
+        check=False,                        # don't make any changes, just try to probe for potential changes
+        diff=False
         ):
 
         # storage & defaults
         self.check            = check
+        self.diff             = diff
         self.setup_cache      = utils.default(setup_cache, lambda: collections.defaultdict(dict))
         self.basedir          = utils.default(basedir, lambda: os.getcwd())
         self.callbacks        = utils.default(callbacks, lambda: DefaultRunnerCallbacks())
@@ -192,7 +194,7 @@ class Runner(object):
     # *****************************************************
 
     def _execute_module(self, conn, tmp, module_name, args,
-        async_jid=None, async_module=None, async_limit=None, inject=None):
+        async_jid=None, async_module=None, async_limit=None, inject=None, persist_files=False):
 
         ''' runs a module that has already been transferred '''
 
@@ -233,7 +235,7 @@ class Runner(object):
             raise errors.AnsibleError("module is missing interpreter line")
 
         cmd = shebang.replace("#!","") + " " + cmd
-        if tmp.find("tmp") != -1 and C.DEFAULT_KEEP_REMOTE_FILES != '1':
+        if tmp.find("tmp") != -1 and C.DEFAULT_KEEP_REMOTE_FILES != '1' and not persist_files:
             cmd = cmd + "; rm -rf %s >/dev/null 2>&1" % tmp
         res = self._low_level_exec_command(conn, cmd, tmp, sudoable=True)
         data = utils.parse_json(res['stdout'])

lib/ansible/runner/action_plugins/template.py

@@ -20,6 +20,7 @@ import os
 from ansible import utils
 from ansible import errors
 from ansible.runner.return_data import ReturnData
+import base64
 
 class ActionModule(object):
 
@@ -46,6 +47,7 @@ class ActionModule(object):
 
         # if we have first_available_file in our vars
         # look up the files and use the first one we find as src
+
         if 'first_available_file' in inject:
             found = False
             for fn in self.runner.module_vars.get('first_available_file'):
@@ -79,18 +81,35 @@ class ActionModule(object):
 
              # template is different from the remote value
 
-             xfered = self.runner._transfer_str(conn, tmp, 'source', resultant)
+             # if showing diffs, we need to get the remote value
+             dest_contents = None    
+             if self.runner.diff:
+                 # using persist_files to keep the temp directory around to avoid needing to grab another
+                 dest_result = self.runner._execute_module(conn, tmp, 'slurp', "path=%s" % dest, inject=inject, persist_files=True)
+                 dest_contents = dest_result.result['content']
+                 if dest_result.result['encoding'] == 'base64':
+                     dest_contents = base64.b64decode(dest_contents)
+                 else:
+                     raise Exception("unknown encoding, failed: %s" % dest_result.result)
+
+             xfered = self.runner._transfer_str(conn, tmp, source, resultant)
+
              # fix file permissions when the copy is done as a different user
              if self.runner.sudo and self.runner.sudo_user != 'root':
                  self.runner._low_level_exec_command(conn, "chmod a+r %s" % xfered, tmp)
 
              # run the copy module
              module_args = "%s src=%s dest=%s" % (module_args, xfered, dest)
-
+ 
              if self.runner.check:
+                 if self.runner.diff:
+                     self.runner.callbacks.on_file_diff(conn.host, dest_contents, resultant)
                  return ReturnData(conn=conn, comm_ok=True, result=dict(changed=True))
              else:
-                 return self.runner._execute_module(conn, tmp, 'copy', module_args, inject=inject)
+                 res =  self.runner._execute_module(conn, tmp, 'copy', module_args, inject=inject)
+                 if self.runner.diff:
+                     self.runner.callbacks.on_file_diff(conn.host, dest_contents, resultant)
+                 return res
         else:
              return ReturnData(conn=conn, comm_ok=True, result=dict(changed=False))