Refactor pvcnoded to reduce Daemon.py size

This branch commit refactors the pvcnoded component to better adhere to
good programming practices. The previous Daemon.py was a massive file
which contained almost 2000 lines of direct, root-level code which was
directly imported. Not only was this poor practice, but this resulted
in a nigh-unmaintainable file which was hard even for me to understand.

This refactoring splits a large section of the code from Daemon.py into
separate small modules and functions in the `util/` directory. This will
hopefully make most of the functionality easy to find and modify without
having to dig through a single large file.

Further the existing subcomponents have been moved to the `objects/`
directory which clearly separates them.

Finally, the Daemon.py code has mostly been moved into a function,
`entrypoint()`, which is then called from the `pvcnoded.py` stub.

An additional item is that most format strings have been replaced by
f-strings to make use of the Python 3.6 features in Daemon.py and the
utility files.

node-daemon/pvcnoded/objects/VMConsoleWatcherInstance.py

@@ -0,0 +1,101 @@
+#!/usr/bin/env python3
+
+# VMConsoleWatcherInstance.py - Class implementing a console log watcher for PVC domains
+# Part of the Parallel Virtual Cluster (PVC) system
+#
+#    Copyright (C) 2018-2021 Joshua M. Boniface <joshua@boniface.me>
+#
+#    This program is free software: you can redistribute it and/or modify
+#    it under the terms of the GNU General Public License as published by
+#    the Free Software Foundation, version 3.
+#
+#    This program is distributed in the hope that it will be useful,
+#    but WITHOUT ANY WARRANTY; without even the implied warranty of
+#    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+#    GNU General Public License for more details.
+#
+#    You should have received a copy of the GNU General Public License
+#    along with this program.  If not, see <https://www.gnu.org/licenses/>.
+#
+###############################################################################
+
+import os
+import time
+
+from threading import Thread, Event
+from collections import deque
+
+
+class VMConsoleWatcherInstance(object):
+    # Initialization function
+    def __init__(self, domuuid, domname, zkhandler, config, logger, this_node):
+        self.domuuid = domuuid
+        self.domname = domname
+        self.zkhandler = zkhandler
+        self.config = config
+        self.logfile = '{}/{}.log'.format(config['console_log_directory'], self.domname)
+        self.console_log_lines = config['console_log_lines']
+        self.logger = logger
+        self.this_node = this_node
+
+        # Try to append (create) the logfile and set its permissions
+        open(self.logfile, 'a').close()
+        os.chmod(self.logfile, 0o600)
+
+        try:
+            self.logdeque = deque(open(self.logfile), self.console_log_lines)
+        except UnicodeDecodeError:
+            # There is corruption in the log file; overwrite it
+            self.logger.out('Failed to decode console log file; clearing existing file', state='w', prefix='Domain {}'.format(self.domuuid))
+            with open(self.logfile, 'w') as lfh:
+                lfh.write('\n')
+            self.logdeque = deque(open(self.logfile), self.console_log_lines)
+
+        self.stamp = None
+        self.cached_stamp = None
+
+        # Set up the deque with the current contents of the log
+        self.last_loglines = None
+        self.loglines = None
+
+        # Thread options
+        self.thread = None
+        self.thread_stopper = Event()
+
+    # Start execution thread
+    def start(self):
+        self.thread_stopper.clear()
+        self.thread = Thread(target=self.run, args=(), kwargs={})
+        self.logger.out('Starting VM log parser', state='i', prefix='Domain {}'.format(self.domuuid))
+        self.thread.start()
+
+    # Stop execution thread
+    def stop(self):
+        if self.thread and self.thread.is_alive():
+            self.logger.out('Stopping VM log parser', state='i', prefix='Domain {}'.format(self.domuuid))
+            self.thread_stopper.set()
+            # Do one final flush
+            self.update()
+
+    # Main entrypoint
+    def run(self):
+        # Main loop
+        while not self.thread_stopper.is_set():
+            self.update()
+            time.sleep(0.5)
+
+    def update(self):
+        self.stamp = os.stat(self.logfile).st_mtime
+        if self.stamp != self.cached_stamp:
+            self.cached_stamp = self.stamp
+            self.fetch_lines()
+        # Update Zookeeper with the new loglines if they changed
+        if self.loglines != self.last_loglines:
+            self.zkhandler.write([
+                (('domain.console.log', self.domuuid), self.loglines)
+            ])
+            self.last_loglines = self.loglines
+
+    def fetch_lines(self):
+        self.logdeque = deque(open(self.logfile), self.console_log_lines)
+        self.loglines = ''.join(self.logdeque)