This implements a basic --check mode which for now is only implemented on template & copy operations. More detail will be shared with the list

shortly.

bin/ansible

@@ -46,7 +46,7 @@ class Cli(object):
         ''' create an options parser for bin/ansible '''
 
         parser = utils.base_parser(constants=C, runas_opts=True, subset_opts=True, async_opts=True,
-            output_opts=True, connect_opts=True, usage='%prog <host-pattern> [options]')
+            output_opts=True, connect_opts=True, check_opts=True, usage='%prog <host-pattern> [options]')
         parser.add_option('-a', '--args', dest='module_args',
             help="module arguments", default=C.DEFAULT_MODULE_ARGS)
         parser.add_option('-m', '--module-name', dest='module_name',
@@ -109,7 +109,8 @@ class Cli(object):
             pattern=pattern,
             callbacks=self.callbacks, sudo=options.sudo,
             sudo_pass=sudopass,sudo_user=options.sudo_user,
-            transport=options.connection, subset=options.subset
+            transport=options.connection, subset=options.subset,
+            check=options.check
         )
 
         if options.seconds:

bin/ansible-playbook

@@ -52,11 +52,13 @@ def main(args):
 
     # create parser for CLI options
     usage = "%prog playbook.yml"
-    parser = utils.base_parser(constants=C, usage=usage, connect_opts=True, runas_opts=True, subset_opts=True)
+    parser = utils.base_parser(constants=C, usage=usage, connect_opts=True, 
+        runas_opts=True, subset_opts=True, check_opts=True)
     parser.add_option('-e', '--extra-vars', dest="extra_vars", default=None,
         help="set additional key=value variables from the CLI")
     parser.add_option('-t', '--tags', dest='tags', default='all',
         help="only run plays and tasks tagged with these values")
+    # FIXME: list hosts is a common option and can be moved to utils/__init__.py
     parser.add_option('--list-hosts', dest='listhosts', action='store_true',
         help="dump out a list of hosts, each play will run against, does not run playbook!")
     parser.add_option('--syntax-check', dest='syntax', action='store_true',
@@ -120,6 +122,7 @@ def main(args):
             extra_vars=extra_vars,
             private_key_file=options.private_key_file,
             only_tags=only_tags,
+            check=options.check
         )
         
         if options.listhosts:

lib/ansible/module_common.py

@@ -133,7 +133,7 @@ class AnsibleModule(object):
 
     def __init__(self, argument_spec, bypass_checks=False, no_log=False,
         check_invalid_arguments=True, mutually_exclusive=None, required_together=None,
-        required_one_of=None, add_file_common_args=False):
+        required_one_of=None, add_file_common_args=False, supports_check_mode=False):
 
         '''
         common code for quickly building an ansible module in Python
@@ -142,6 +142,8 @@ class AnsibleModule(object):
         '''
 
         self.argument_spec = argument_spec
+        self.supports_check_mode = supports_check_mode
+        self.check_mode = False
 
         if add_file_common_args:
             self.argument_spec.update(FILE_COMMON_ARGUMENTS)
@@ -149,7 +151,7 @@ class AnsibleModule(object):
         os.environ['LANG'] = MODULE_LANG
         (self.params, self.args) = self._load_params()
 
-        self._legal_inputs = []
+        self._legal_inputs = [ 'CHECKMODE' ]
         self._handle_aliases()
 
         if check_invalid_arguments:
@@ -301,6 +303,8 @@ class AnsibleModule(object):
                 new_context[i] = cur_context[i]
         if cur_context != new_context:
             try:
+                if self.check_mode:
+                    return True
                 rc = selinux.lsetfilecon(path, ':'.join(new_context))
             except OSError:
                 self.fail_json(path=path, msg='invalid selinux context', new_context=new_context, cur_context=cur_context, input_was=context)
@@ -319,6 +323,8 @@ class AnsibleModule(object):
                 uid = pwd.getpwnam(owner).pw_uid
             except KeyError:
                 self.fail_json(path=path, msg='chown failed: failed to look up user %s' % owner)
+            if self.check_mode:
+                return True
             try:
                 os.chown(path, uid, -1)
             except OSError:
@@ -332,6 +338,8 @@ class AnsibleModule(object):
             return changed
         old_user, old_group = self.user_and_group(path)
         if old_group != group:
+            if self.check_mode:
+                return True
             try:
                 gid = grp.getgrnam(group).gr_gid
             except KeyError:
@@ -357,6 +365,8 @@ class AnsibleModule(object):
         prev_mode = stat.S_IMODE(st[stat.ST_MODE])
 
         if prev_mode != mode:
+            if self.check_mode:
+                return True
             # FIXME: comparison against string above will cause this to be executed
             # every time
             try:
@@ -451,6 +461,11 @@ class AnsibleModule(object):
 
     def _check_invalid_arguments(self):
         for (k,v) in self.params.iteritems():
+            if k == 'CHECKMODE':
+                if not self.supports_check_mode:
+                    self.exit_json(skipped=True, msg="remote module does not support check mode")
+                if self.supports_check_mode:
+                    self.check_mode = True
             if k not in self._legal_inputs:
                 self.fail_json(msg="unsupported parameter for module: %s" % k)
 

lib/ansible/playbook/__init__.py

@@ -61,7 +61,8 @@ class PlayBook(object):
         extra_vars       = None,
         only_tags        = None,
         subset           = C.DEFAULT_SUBSET,
-        inventory        = None):
+        inventory        = None,
+        check            = False):
 
         """
         playbook:         path to a playbook file
@@ -79,6 +80,7 @@ class PlayBook(object):
         stats:            holds aggregrate data about events occuring to each host
         sudo:             if not specified per play, requests all plays use sudo mode
         inventory:        can be specified instead of host_list to use a pre-existing inventory object
+        check:            don't change anything, just try to detect some potential changes
         """
 
         self.SETUP_CACHE = SETUP_CACHE
@@ -91,6 +93,7 @@ class PlayBook(object):
         if only_tags is None:
             only_tags = [ 'all' ]
 
+        self.check            = check
         self.module_path      = module_path
         self.forks            = forks
         self.timeout          = timeout
@@ -267,7 +270,8 @@ class PlayBook(object):
             setup_cache=self.SETUP_CACHE, basedir=task.play.basedir,
             conditional=task.only_if, callbacks=self.runner_callbacks,
             sudo=task.sudo, sudo_user=task.sudo_user,
-            transport=task.transport, sudo_pass=task.sudo_pass, is_playbook=True
+            transport=task.transport, sudo_pass=task.sudo_pass, is_playbook=True,
+            check=self.check
         )
 
         if task.async_seconds == 0:
@@ -373,6 +377,7 @@ class PlayBook(object):
             remote_pass=self.remote_pass, remote_port=play.remote_port, private_key_file=self.private_key_file,
             setup_cache=self.SETUP_CACHE, callbacks=self.runner_callbacks, sudo=play.sudo, sudo_user=play.sudo_user,
             transport=play.transport, sudo_pass=self.sudo_pass, is_playbook=True, module_vars=play.vars,
+            check=self.check
         ).run()
         self.stats.compute(setup_results, setup=True)
 

lib/ansible/runner/__init__.py

@@ -114,10 +114,12 @@ class Runner(object):
         module_vars=None,                   # a playbooks internals thing
         is_playbook=False,                  # running from playbook or not?
         inventory=None,                     # reference to Inventory object
-        subset=None                         # subset pattern
+        subset=None,                        # subset pattern
+        check=False                         # don't make any changes, just try to probe for potential changes
         ):
 
         # storage & defaults
+        self.check            = check
         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())
@@ -207,6 +209,11 @@ class Runner(object):
 
         cmd = ""
         if not is_new_style:
+            if 'CHECKMODE=True' in args:
+                # if module isn't using AnsibleModuleCommon infrastructure we can't be certain it knows how to
+                # do --check mode, so to be safe we will not run it.
+                return ReturnData(conn=conn, result=dict(skippped=True, msg="cannot run check mode against old-style modules"))
+
             args = utils.template(self.basedir, args, inject)
             argsfile = self._transfer_str(conn, tmp, 'arguments', args)
             if async_jid is None:

lib/ansible/runner/action_plugins/add_host.py

@@ -35,6 +35,10 @@ class ActionModule(object):
         self.runner = runner
 
     def run(self, conn, tmp, module_name, module_args, inject):
+
+        if self.runner.check:
+            return ReturnData(conn=conn, comm_ok=True, result=dict(skipped=True, msg='check mode not supported for this module')
+
         args = parse_kv(module_args)
         if not 'hostname' in args:
             raise ae("'hostname' is a required argument.")

lib/ansible/runner/action_plugins/async.py

@@ -15,6 +15,8 @@
 # You should have received a copy of the GNU General Public License
 # along with Ansible.  If not, see <http://www.gnu.org/licenses/>.
 
+from ansible.runner.return_data import ReturnData
+
 class ActionModule(object):
 
     def __init__(self, runner):
@@ -23,6 +25,9 @@ class ActionModule(object):
     def run(self, conn, tmp, module_name, module_args, inject):
         ''' transfer the given module name, plus the async module, then run it '''
 
+        if self.runner.check:
+            return ReturnData(conn=conn, comm_ok=True, result=dict(skipped=True, msg='check mode not supported for this module'))
+
         # shell and command module are the same
         if module_name == 'shell':
             module_name = 'command'

lib/ansible/runner/action_plugins/copy.py

@@ -69,6 +69,12 @@ class ActionModule(object):
 
         exec_rc = None
         if local_md5 != remote_md5:
+
+            if self.runner.check:
+                # TODO: if the filesize is small, include a nice pretty-printed diff by 
+                # calling a (new) diff callback
+                return ReturnData(conn=conn, result=dict(changed=True))
+
             # transfer the file to a remote tmp location
             tmp_src = tmp + os.path.basename(source)
             conn.put_file(source, tmp_src)
@@ -86,5 +92,7 @@ class ActionModule(object):
 
             tmp_src = tmp + os.path.basename(source)
             module_args = "%s src=%s" % (module_args, tmp_src)
+            if self.runner.check:
+                module_args = "%s CHECKMODE=True" % module_args
             return self.runner._execute_module(conn, tmp, 'file', module_args, inject=inject)
 

lib/ansible/runner/action_plugins/fail.py

@@ -29,6 +29,10 @@ class ActionModule(object):
         self.runner = runner
 
     def run(self, conn, tmp, module_name, module_args, inject):
+
+        # note: the fail module does not need to pay attention to check mode
+        # it always runs.
+
         args = utils.parse_kv(module_args)
         if not 'msg' in args:
             args['msg'] = 'Failed as requested from task'

lib/ansible/runner/action_plugins/fetch.py

@@ -36,6 +36,9 @@ class ActionModule(object):
     def run(self, conn, tmp, module_name, module_args, inject):
         ''' handler for fetch operations '''
 
+        if self.runner.check:
+            return ReturnData(conn=conn, comm_ok=True, result=dict(skipped=True, msg='check mode not (yet) supported for this module'))
+
         # load up options
         options = utils.parse_kv(module_args)
         source = options.get('src', None)

lib/ansible/runner/action_plugins/group_by.py

@@ -33,6 +33,10 @@ class ActionModule(object):
         self.runner = runner
 
     def run(self, conn, tmp, module_name, module_args, inject):
+
+        # the group_by module does not need to pay attention to check mode.
+        # it always runs.
+
         args = parse_kv(self.runner.module_args)
         if not 'key' in args:
             raise ae("'key' is a required argument.")

lib/ansible/runner/action_plugins/normal.py

@@ -36,6 +36,13 @@ class ActionModule(object):
     def run(self, conn, tmp, module_name, module_args, inject):
         ''' transfer & execute a module that is not 'copy' or 'template' '''
 
+        if self.runner.check:
+            if module_name in [ 'shell', 'command' ]:
+                return ReturnData(conn=conn, comm_ok=True, result=dict(skipped=True, msg='check mode not supported for %s' % module_name))
+            # else let the module parsing code decide, though this will only be allowed for AnsibleModuleCommon using
+            # python modules for now
+            module_args += " CHECKMODE=True"
+
         # shell and command are the same module
         if module_name == 'shell':
             module_name = 'command'

lib/ansible/runner/action_plugins/pause.py

@@ -48,6 +48,10 @@ class ActionModule(object):
 
     def run(self, conn, tmp, module_name, module_args, inject):
         ''' run the pause action module '''
+
+        # note: this module does not need to pay attention to the 'check'
+        # flag, it always runs
+
         hosts = ', '.join(self.runner.host_set)
         args = parse_kv(template(self.runner.basedir, module_args, inject))
 

lib/ansible/runner/action_plugins/raw.py

@@ -29,6 +29,11 @@ class ActionModule(object):
         self.runner = runner
 
     def run(self, conn, tmp, module_name, module_args, inject):
+
+        if self.runner.check:
+            # in --check mode, always skip this module execution
+            return ReturnData(conn=conn, comm_ok=True, result=dict(skipped=True))
+
         executable = ''
         # From library/command, keep in sync
         r = re.compile(r'(^|\s)(executable)=(?P<quote>[\'"])?(.*?)(?(quote)(?<!\\)(?P=quote))((?<!\\)\s|$)')

lib/ansible/runner/action_plugins/script.py

@@ -31,6 +31,10 @@ class ActionModule(object):
     def run(self, conn, tmp, module_name, module_args, inject):
         ''' handler for file transfer operations '''
 
+        if self.runner.check:
+            # in check mode, always skip this module
+            return ReturnData(conn=conn, comm_ok=True, result=dict(skipped=True, msg='check mode not supported for this module'))
+
         tokens  = shlex.split(module_args)
         source  = tokens[0]
         # FIXME: error handling

lib/ansible/runner/action_plugins/template.py

@@ -29,6 +29,9 @@ class ActionModule(object):
     def run(self, conn, tmp, module_name, module_args, inject):
         ''' handler for template operations '''
 
+        # note: since this module just calls the copy module, the --check mode support
+        # can be implemented entirely over there
+
         if not self.runner.is_playbook:
             raise errors.AnsibleError("in current versions of ansible, templates are only usable in playbooks")
 
@@ -62,21 +65,32 @@ class ActionModule(object):
             base = os.path.basename(source)
             dest = os.path.join(dest, base)
 
-        # template the source data locally & transfer
+        # template the source data locally & get ready to transfer
         try:
             resultant = utils.template_from_file(self.runner.basedir, source, inject)
         except Exception, e:
             result = dict(failed=True, msg=str(e))
             return ReturnData(conn=conn, comm_ok=False, result=result)
 
+        local_md5 = utils.md5s(resultant)
+        remote_md5 = self.runner._remote_md5(conn, tmp, dest)
+
+        if local_md5 != remote_md5:
+
+             # template is different from the remote value
+
              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, 
+                 self.runner._low_level_exec_command(conn, "chmod a+r %s" % xfered, tmp)
-                tmp)
 
              # run the copy module
              module_args = "%s src=%s dest=%s" % (module_args, xfered, dest)
-        return self.runner._execute_module(conn, tmp, 'copy', module_args, inject=inject)
 
+             if self.runner.check:
+                 return ReturnData(conn=conn, comm_ok=True, result=dict(changed=True))
+             else:
+                 return self.runner._execute_module(conn, tmp, 'copy', module_args, inject=inject)
+        else:
+             return ReturnData(conn=conn, comm_ok=True, result=dict(changed=False))
 

lib/ansible/utils/__init__.py

@@ -392,7 +392,7 @@ def increment_debug(option, opt, value, parser):
     VERBOSITY += 1
 
 def base_parser(constants=C, usage="", output_opts=False, runas_opts=False,
-    async_opts=False, connect_opts=False, subset_opts=False):
+    async_opts=False, connect_opts=False, subset_opts=False, check_opts=False):
     ''' create an options parser for any ansible script '''
 
     parser = SortedOptParser(usage, version=version("%prog"))
@@ -449,6 +449,11 @@ def base_parser(constants=C, usage="", output_opts=False, runas_opts=False,
         parser.add_option('-B', '--background', dest='seconds', type='int', default=0,
             help='run asynchronously, failing after X seconds (default=N/A)')
 
+    if check_opts:
+        parser.add_option("-C", "--check", default=False, dest='check', action='store_true',
+            help="don't make any changes, instead try to predict some of the changes that may occur"
+        )
+
     return parser
 
 def do_encrypt(result, encrypt, salt_size=None, salt=None):

library/copy

@@ -69,7 +69,7 @@ def main():
             dest=dict(required=True),
             backup=dict(default=False, choices=BOOLEANS),
         ),
-        add_file_common_args=True
+        add_file_common_args=True,
     )
 
     src  = os.path.expanduser(module.params['src'])

library/file

@@ -134,7 +134,8 @@ def main():
             state = dict(choices=['file','directory','link','absent'], default='file'),
             path  = dict(aliases=['dest', 'name'], required=True),
         ),
-        add_file_common_args=True
+        add_file_common_args=True,
+        supports_check_mode=True
     )
 
     params = module.params

library/ping

@@ -36,7 +36,8 @@ author: Michael DeHaan
 
 def main():
     module = AnsibleModule(
-        argument_spec = dict()
+        argument_spec = dict(),
+        supports_check_mode = True
     )
     module.exit_json(ping='pong')
 

library/setup

@@ -914,7 +914,8 @@ def run_setup(module):
 def main():
     global module
     module = AnsibleModule(
-        argument_spec = dict()
+        argument_spec = dict(),
+        supports_check_mode = True,
     )
     data = run_setup(module)
     module.exit_json(**data)