plugin/util.py: Add JSONPromise to log JSON data

After subclassing JSONPromise, promises can log python objects in JSON
format with:
```
  self.json_logger.info(message, extra={'data': object})
```

and retrieve previously logged objects over a time interval with:
```
  object_list = self.getJsonLogDataInterval(interval)
```
Co-authored-by: Xavier Thompson <xavier.thompson@nexedi.com>

slapos/promise/plugin/util.py

+import itertools
+import json
+import logging
+import os
+import textwrap
+
+from dateutil import parser as dateparser
+
+from slapos.grid.promise.generic import GenericPromise
+
+
+def iter_reverse_lines(f):
+  """
+    Read lines from the end of the file
+  """
+  f.seek(0, os.SEEK_END)
+  while True:
+    try:
+      while f.seek(-2, os.SEEK_CUR) and f.read(1) != b'\n':
+        pass
+    except OSError:
+      return
+    pos = f.tell()
+    yield f.readline()
+    f.seek(pos, os.SEEK_SET)
+
+
+def iter_logrotate_file_handle(path, mode='r'):
+  """
+    Yield successive file handles for rotated logs
+    (XX.log, XX.log.1, XX.log.2, ...)
+  """
+  for i in itertools.count():
+    path_i = path + str(i or '')
+    try:
+      with open(path_i, mode) as f:
+        yield f
+    except OSError:
+      break
+
+
+class JSONPromise(GenericPromise):
+  def __init__(self, config):
+    self.__name = config.get('name', None)
+    self.__log_folder = config.get('log-folder', None)
+
+    super(JSONPromise, self).__init__(config)
+    json_log_name = os.path.splitext(self.__name)[0] + '.json.log'
+    self.__json_log_file = os.path.join(self.__log_folder, json_log_name)
+    self.json_logger = self.__makeJsonLogger(self.__json_log_file)
+
+  def __makeJsonLogger(self, json_log_file):
+    logger = logging.getLogger('json-logger')
+    logger.setLevel(logging.INFO)
+    handler = logging.FileHandler(json_log_file)
+    formatter = logging.Formatter(
+      '{"time": "%(asctime)s", "log_level": "%(levelname)s"'
+      ', "message": "%(message)s", "data": %(data)s}'
+    )
+    handler.setFormatter(formatter)
+    logger.addHandler(handler)
+    return logger
+
+  def getJsonLogDataInterval(self, interval):
+    """
+      Get all data in the last "interval" seconds from JSON log
+      Reads rotated logs too (XX.log, XX.log.1, XX.log.2, ...)
+    """
+    oldest_timestamp = None
+    data_list = []
+    for f in iter_logrotate_file_handle(self.__json_log_file, 'rb'):
+      for line in iter_reverse_lines(f):
+        l = json.loads(line.decode().replace("'", '"'))
+        timestamp = dateparser.parse(l['time'])
+        data_list.append(l['data'])
+        oldest_timestamp = oldest_timestamp or timestamp
+        if (oldest_timestamp - timestamp).total_seconds() > interval:
+          return data_list
+    return data_list
+
+  def getJsonLogLatestTimestamp(log):
+    """
+      Get latest timestamp from JSON log
+      Reads rotated logs too (XX.log, XX.log.1, XX.log.2, ...)
+    """
+    for f in iter_logrotate_file_handle(self.__json_log_file, 'rb'):
+      for line in iter_reverse_lines(f):
+        l = json.loads(line.decode().replace("'", '"'))
+        return dateparser.parse(l['time'])
+    return 0
 
 
 def tail_file(file_path, line_count=10):
   """
   Returns the last lines of file.
   """
-
   line_list = []
   with open(file_path) as f:
     BUFSIZ = 1024
@@ -25,5 +114,4 @@ def tail_file(file_path, line_count=10):
       size -= line_len
       bytes -= BUFSIZ
       block -= 1
-
   return '\n'.join(''.join(line_list).splitlines()[-line_count:])