issue #164: precisely emulate Ansible's stdio behaviour.

* Use identical logic to select when stdout/stderr are merged, so
  'stdout', 'stdout_lines', 'stderr', 'stderr_lines' contain the same
  output before/after the extension.

* When stdout/stderr are merged, synthesize carriage returns just like
  the TTY layer.

* Mimic the SSH connection multiplexing message on stderr. Not really
  for user code, but so compare_output_test.sh needs fewer fixups.

ansible_mitogen/connection.py

@@ -303,7 +303,7 @@ class Connection(ansible.plugins.connection.ConnectionBase):
             LOG.debug('Call %s%r took %d ms', func.func_name, args,
                       1000 * (time.time() - t0))
 
-    def exec_command(self, cmd, in_data='', sudoable=True):
+    def exec_command(self, cmd, in_data='', sudoable=True, mitogen_chdir=None):
         """
         Implement exec_command() by calling the corresponding
         ansible_mitogen.helpers function in the target.
@@ -315,8 +315,20 @@ class Connection(ansible.plugins.connection.ConnectionBase):
         :returns:
             (return code, stdout bytes, stderr bytes)
         """
-        return self.call(ansible_mitogen.helpers.exec_command,
-                         cast(cmd), cast(in_data))
+        emulate_tty = (not in_data and sudoable)
+        rc, stdout, stderr = self.call(
+            ansible_mitogen.helpers.exec_command,
+            cmd=cast(cmd),
+            in_data=cast(in_data),
+            chdir=mitogen_chdir,
+            emulate_tty=emulate_tty,
+        )
+
+        stderr += 'Shared connection to %s closed.%s' % (
+            self._play_context.remote_addr,
+            ('\r\n' if emulate_tty else '\n'),
+        )
+        return rc, stdout, stderr
 
     def fetch_file(self, in_path, out_path):
         """

ansible_mitogen/helpers.py

@@ -176,7 +176,7 @@ def get_user_shell():
     return pw_shell or '/bin/sh'
 
 
-def exec_args(args, in_data='', chdir=None, shell=None):
+def exec_args(args, in_data='', chdir=None, shell=None, emulate_tty=False):
     """
     Run a command in a subprocess, emulating the argument handling behaviour of
     SSH.
@@ -185,24 +185,36 @@ def exec_args(args, in_data='', chdir=None, shell=None):
         Argument vector.
     :param bytes in_data:
         Optional standard input for the command.
+    :param bool emulate_tty:
+        If :data:`True`, arrange for stdout and stderr to be merged into the
+        stdout pipe and for LF to be translated into CRLF, emulating the
+        behaviour of a TTY.
     :return:
         (return code, stdout bytes, stderr bytes)
     """
     LOG.debug('exec_args(%r, ..., chdir=%r)', args, chdir)
     assert isinstance(args, list)
 
+    if emulate_tty:
+        stderr = subprocess.STDOUT
+    else:
+        stderr = subprocess.PIPE
+
     proc = subprocess.Popen(
         args=args,
         stdout=subprocess.PIPE,
-        stderr=subprocess.PIPE,
+        stderr=stderr,
         stdin=subprocess.PIPE,
         cwd=chdir,
     )
     stdout, stderr = proc.communicate(in_data)
-    return proc.returncode, stdout, stderr
+
+    if emulate_tty:
+        stdout = stdout.replace('\n', '\r\n')
+    return proc.returncode, stdout, stderr or ''
 
 
-def exec_command(cmd, in_data='', chdir=None, shell=None):
+def exec_command(cmd, in_data='', chdir=None, shell=None, emulate_tty=False):
     """
     Run a command in a subprocess, emulating the argument handling behaviour of
     SSH.
@@ -220,6 +232,7 @@ def exec_command(cmd, in_data='', chdir=None, shell=None):
         in_data=in_data,
         chdir=chdir,
         shell=shell,
+        emulate_tty=emulate_tty,
     )
 
 

ansible_mitogen/mixins.py

@@ -367,11 +367,11 @@ class ActionModuleMixin(ansible.plugins.action.ActionBase):
         if executable:
             cmd = executable + ' -c ' + commands.mkarg(cmd)
 
-        rc, stdout, stderr = self.call(
-            ansible_mitogen.helpers.exec_command,
-            cast(cmd),
-            cast(in_data),
-            chdir=cast(chdir),
+        rc, stdout, stderr = self._connection.exec_command(
+            cmd=cast(cmd),
+            in_data=cast(in_data),
+            sudoable=sudoable,
+            mitogen_chdir=cast(chdir),
         )
         stdout_text = to_text(stdout, errors=encoding_errors)
 

ansible_mitogen/runner.py

@@ -243,6 +243,7 @@ class ProgramRunner(Runner):
         try:
             rc, stdout, stderr = ansible_mitogen.helpers.exec_args(
                 args=self._get_program_args(),
+                emulate_tty=True,
             )
         except Exception, e:
             LOG.exception('While running %s', self._get_program_args())

docs/ansible.rst

@@ -165,28 +165,10 @@ Low Risk
   number of targets. This is a subject of ongoing investigation and
   improvements will appear in time.
 
-* Ansible defaults to requiring pseudo TTYs for most SSH invocations, in order
-  to allow it to handle ``sudo`` with ``requiretty`` enabled, however it
-  disables pseudo TTYs for certain commands where standard input is required or
-  ``sudo`` is not in use. Mitogen does not require this, as it can simply call
-  :py:func:`pty.openpty` from the SSH user account during ``sudo`` setup.
-
-  A major downside to Ansible's default is that stdout and stderr of any
-  resulting executed command are merged, with additional carriage return
-  characters synthesized in the output by the TTY layer. Neither of these
-  problems are apparent using the Mitogen extension, which may break some
-  playbooks.
-
-  A future version will emulate Ansible's behaviour, once it is clear precisely
-  what that behaviour is supposed to be. See `Ansible#14377`_ for related
-  discussion.
-
 * "Module Replacer" style modules are not yet supported. These rarely appear in
   practice, and light Github code searches failed to reveal many examples of
   them.
 
-.. _Ansible#14377: https://github.com/ansible/ansible/issues/14377
-
 
 Behavioural Differences
 -----------------------
@@ -212,10 +194,6 @@ Behavioural Differences
   captured and returned to the host machine, where it can be viewed as desired
   with ``-vvv``.
 
-* Ansible with SSH multiplexing enabled causes a string like ``Shared
-  connection to host closed`` to appear in ``stderr`` output of every executed
-  command. This never manifests with the Mitogen extension.
-
 * Local commands are executed in a reuseable Python interpreter created
   identically to interpreters used on remote hosts. At present only one such
   interpreter per ``become_user`` exists, and so only one action may be
@@ -357,6 +335,30 @@ plug-ins are unlikely to attempt similar patches, so the risk to an established
 configuration should be minimal.
 
 
+Standard IO
+~~~~~~~~~~~
+
+Ansible uses pseudo TTYs for most invocations, to allow it to handle typing
+passwords interactively, however it disables pseudo TTYs for certain commands
+where standard input is required or ``sudo`` is not in use. Additionally when
+SSH multiplexing is enabled, a string like ``Shared connection to localhost
+closed\r\n`` appears in ``stderr`` of every invocation.
+
+Mitogen does not naturally require either of these, as command output is
+embedded within the SSH stream, and it can simply call :py:func:`pty.openpty`
+in every location an interactive password must be typed.
+
+A major downside to Ansible's behaviour is that ``stdout`` and ``stderr`` are
+merged together into a single ``stdout`` variable, with carriage returns
+inserted in the output by the TTY layer. However ugly, the extension emulates
+all of this behaviour precisely, to avoid breaking playbooks that expect
+certain text to appear in certain variables with certain linefeed characters.
+
+See `Ansible#14377`_ for related discussion.
+
+.. _Ansible#14377: https://github.com/ansible/ansible/issues/14377
+
+
 Flag Emulation
 ~~~~~~~~~~~~~~
 

tests/ansible/integration/action__low_level_execute_command.yml

@@ -28,6 +28,7 @@
     - debug: msg={{raw}}
     - name: Verify raw module output.
       assert:
-        that:
-          - 'raw.rc == 0'
-          - 'raw.stdout_lines == ["root"]'
+        that: |
+          raw.rc == 0 and
+          raw.stdout == "root\r\n" and
+          raw.stdout_lines == ["root"]

tests/ansible/integration/runner__custom_binary_single_null.yml

@@ -15,4 +15,10 @@
           out.failed and
           out.results[0].failed and
           out.results[0].msg == 'MODULE FAILURE' and
-          out.results[0].rc == 126
+          out.results[0].module_stdout.startswith('/bin/sh: ') and
+          out.results[0].module_stdout.endswith('/custom_binary_single_null: cannot execute binary file\r\n')
+
+
+# Can't test this: Mitogen returns 126, 2.5.x returns 126, 2.4.x discarded the
+# return value and always returned 0.
+# out.results[0].rc == 126