DNF: fix logging, now it will work from Ansible

The Ansible dnf module uses the python dnf bindings.  In contexts like
these, stdout/stderr is owned by the host app (Ansible).  dnf should not
mess with stdout/stderr, unless the host app asks it to log there.

Specifically, it was breaking the JSON output of the Ansible module.
This was only noticeable when the etckeeper message began with a "["
character.  Ansible has a mechanism to try and skip header messages like
login banners.  However, "[" is a valid character to start a JSON document.

https://unix.stackexchange.com/questions/511210/ansible-dnf-module-module-failure/

This requires decoding the etckeeper messages into unicode.  For gory
details, see the code comment.

Also enable unicode string literals.  This reduces differences between py2
and py3 semantics; it is universal inside the dnf code.

Also when I tested the failure case, I noticed the exit code was not
printed correctly.  Fix it.

etckeeper-dnf/etckeeper.py

@@ -7,30 +7,74 @@
 # Distutils code below was copied from etckeeper-bzr distributed with v1.15
 #
 
+from __future__ import unicode_literals
 from dnfpluginscore import logger
 
-import os
+import subprocess
+import locale
 import dnf
 
 
 class Etckeeper(dnf.Plugin):
 
     name = 'etckeeper'
 
-    def _out(self, msg):
+    def _debug(self, msg):
         logger.debug('Etckeeper plugin: %s', msg)
 
+    def _log_pipe(self, pipe):
+        # etckeeper & git messages should be encoded using the default locale
+        # (or us-ascii, which is a strict subset).
+        #
+        # Normally py2 breaks if you print arbitrary unicode when stdout is
+        # not a tty (UnicodeEncodeError).  However the dnf cli has a
+        # workaround; it will survive regardless of what we do.
+        #
+        # Then we have logging.FileHandler.  In py3 it will use
+        # locale.getpreferredencoding(False) by default.  This should match
+        # the default locale, unless we are on py < 3.2, AND the program
+        # forgot to call setlocale(LC_ALL, "").  dnf already calls
+        # setlocale(LC_ALL, ""), so it will be nice and consistent.
+        # In fact it is the dnf *library* that calls setlocale, this is not
+        # really recommended, but it makes me pretty confident here.
+        #
+        # errors='replace' means that decode errors give us '\ufffd', which
+        # causes UnicodeEncodeError in some character encodings.  Let us
+        # simulate a round-trip through errors='replace', by replacing them
+        # with a question mark.
+        #
+        # The story for py2 is more complex.  In libdnf 2.6.3, the logfile
+        # is equivalent to hardcoding a utf8 encoding.  That is survivable
+        # (and if it changes to match py3, it will also be fine).
+        #
+        encoding = locale.getpreferredencoding(False)
+        for line in pipe:
+            line = line.decode(encoding, 'replace')
+            line.replace('\ufffd', '?')
+            line = line.rstrip('\n')
+            logger.info('%s', line)
+
     def resolved(self):
-        self._out('pre transaction commit')
-        command = '%s %s' % ('etckeeper', " pre-install")
-        ret = os.system(command)
+        self._debug('pre transaction commit')
+        proc = subprocess.Popen(("etckeeper", "pre-install"),
+                                stdout=subprocess.PIPE,
+                                stderr=subprocess.STDOUT,
+                                close_fds=True)
+        self._log_pipe(proc.stdout)
+        ret = proc.wait()
         if ret != 0:
-            raise dnf.exceptions.Error('etckeeper returned %d' % (ret >> 8))
+            raise dnf.exceptions.Error('etckeeper returned %d' % ret)
 
     def transaction(self):
-        self._out('post transaction commit')
-        command = '%s %s > /dev/null' % ('etckeeper', "post-install")
-        os.system(command)
+        self._debug('post transaction commit')
+        proc = subprocess.Popen(("etckeeper", "post-install"),
+                                stdout=None,
+                                stderr=subprocess.PIPE,
+                                close_fds=True)
+        self._log_pipe(proc.stderr)
+        ret = proc.wait()
+        if ret != 0:
+            logger.err('etckeeper returned %d' % ret)
 
 if __name__ == "__main__":
     from distutils.core import setup