feat(itho-wpu): cache results for getnodeid, getserial, getdatatype in a json file

The results for these calls are static. If we cache them we don't have
to retrieve it from the WPU every time we need it.

To ignore the cache (for reading), provide the --no-cache argument.

.gitignore

@@ -0,0 +1 @@
+itho-wpu-cache.json

itho-wpu.py

@@ -9,6 +9,7 @@ import sys
 import time
 import os
 import datetime
+import json
 from collections import namedtuple
 
 consolelogformatter = logging.Formatter("%(asctime)-15s %(levelname)s: %(message)s")
@@ -62,6 +63,8 @@ def parse_args():
     parser.add_argument('--slave-only', action='store_true', help="Only run I2C slave")
     parser.add_argument('--slave-timeout', nargs='?', type=int, default=60,
                         help="Slave timeout in seconds when --slave-only")
+    parser.add_argument('--no-cache', action='store_true',
+                        help="Don't use local cache")
     parser.add_argument('--export-to-influxdb', action='store_true',
                         help="Export results to InfluxDB")
 
@@ -163,14 +166,22 @@ class I2CMaster:
 
 
 class IthoWPU():
-    def __init__(self, master_only, slave_only, slave_timeout):
+    def __init__(self, master_only, slave_only, slave_timeout, no_cache):
         self.master_only = master_only
         self.slave_only = slave_only
         self.slave_timeout = slave_timeout
         self._q = queue.Queue()
+        self.no_cache = no_cache
+        self.cache = IthoWPUCache()
 
     def get(self, action):
-        reponse = None
+        if not self.no_cache:
+            response = self.cache.get(action.replace('get', ''))
+            if response is not None:
+                logger.debug(f"Response (from cache): {response}")
+                return response
+
+        response = None
 
         if not self.master_only:
             slave = I2CSlave(address=0x40, queue=self._q)
@@ -188,9 +199,56 @@ class IthoWPU():
         if not self.master_only:
             slave.close()
 
+        self.cache.set(action.replace('get', ''), response)
+
         return response
 
 
+class IthoWPUCache:
+    def __init__(self):
+        self._cache_file = "itho-wpu-cache.json"
+        self._cache_data = {
+            'nodeid': None,
+            'serial': None,
+            'datatype': None,
+            'schema_version': '1',
+        }
+        self._read_cache()
+
+    def _read_cache(self):
+        if not os.path.exists(self._cache_file):
+            logger.debug(f"Not loading cache file: {self._cache_file} does not exist")
+            return
+        with open(self._cache_file) as cache_file:
+            cache_data = json.load(cache_file)
+            logger.debug(f"Loading local cache: {json.dumps(cache_data)}")
+            for key in ['nodeid', 'serial', 'datatype']:
+                if key in cache_data:
+                    self._cache_data[key] = cache_data[key]
+
+    def _write_cache(self):
+        with open(self._cache_file, 'w') as cache_file:
+            logger.debug(f"Writing to local cache: {json.dumps(self._cache_data)}")
+            json.dump(self._cache_data, cache_file)
+
+    def get(self, action):
+        if action not in ['nodeid', 'serial', 'datatype']:
+            logger.debug(f"Cache for '{action}' is not supported")
+            return None
+        logger.debug(f"Reading '{action}' from local cache")
+        if self._cache_data[action] is None:
+            logger.debug(f"Action '{action}' is not present in local cache")
+        return self._cache_data[action]
+
+    def set(self, action, value):
+        if action not in ['nodeid', 'serial', 'datatype']:
+            logger.debug(f"Cache for '{action}' is not supported")
+            return None
+        logger.debug(f"Writing '{action}' to local cache: {value}")
+        self._cache_data[action] = value
+        self._write_cache()
+
+
 def is_messageclass_valid(action, response):
     if int(response[1], 0) != actions[action][0] and int(response[2], 0) != actions[action][1]:
         logger.error(f"Response MessageClass != {actions[action][0]} {actions[action][1]} "
@@ -304,7 +362,7 @@ if __name__ == "__main__":
     if args.loglevel:
         logger.setLevel(args.loglevel.upper())
 
-    wpu = IthoWPU(args.master_only, args.slave_only, args.slave_timeout)
+    wpu = IthoWPU(args.master_only, args.slave_only, args.slave_timeout, args.no_cache)
     response = wpu.get(args.action)
     if response is not None:
         process_response(args.action, response, args)