support pymongo connecting in background

[chuckliu1979]

hi,

in pymongo https://github.com/mongodb/mongo-python-driver/blob/master/pymongo/mongo_client.py: "```python

Starting with version 3.0 the MongoClient constructor no longer blocks while connecting

to the server or servers, and it no longer raises ConnectionFailure if they are unavailable.

Instead, the constructor returns immediately and launches the connection process on background

threads.

so log4mongo handler could continue initialize when mongodb not running. I made a patch for this:
> import threading
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< try:
<     from pymongo import MongoClient as Connection
< except ImportError:
<     from pymongo import Connection
< 
---
> from pymongo import MongoClient
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< from pymongo.errors import OperationFailure, PyMongoError
< import pymongo
< if pymongo.version_tuple[0] >= 3:
<     from pymongo.errors import ServerSelectionTimeoutError
<     write_method = 'insert_one'
<     write_many_method = 'insert_many'
< else:
<     write_method = 'save'
<     write_many_method = 'insert'
---
> from pymongo.errors import ConfigurationError, OperationFailure, ServerSelectionTimeoutError
> 
> # pylint: disable=pointless-string-statement,invalid-name
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> # pylint: disable=too-many-instance-attributes,too-many-arguments,too-many-locals
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<     DEFAULT_PROPERTIES = logging.LogRecord(
<         '', '', '', '', '', '', '', '').__dict__.keys()
---
>     DEFAULT_PROPERTIES = logging.LogRecord("", "", "", "", "", "", "", "").__dict__.keys()  # type: ignore
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<             'timestamp': dt.datetime.utcnow(),
<             'level': record.levelname,
<             'thread': record.thread,
<             'threadName': record.threadName,
<             'message': record.getMessage(),
<             'loggerName': record.name,
<             'fileName': record.pathname,
<             'module': record.module,
<             'method': record.funcName,
<             'lineNumber': record.lineno
---
>             "timestamp": dt.datetime.utcnow(),
>             "level": record.levelname,
>             "thread": record.thread,
>             "threadName": record.threadName,
>             "message": record.getMessage(),
>             "loggerName": record.name,
>             "fileName": record.pathname,
>             "module": record.module,
>             "method": record.funcName,
>             "lineNumber": record.lineno,
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<             document.update({
<                 'exception': {
<                     'message': str(record.exc_info[1]),
<                     'code': 0,
<                     'stackTrace': self.formatException(record.exc_info)
<                 }
<             })
---
>             document.update(
>                 {"exception": {"message": str(record.exc_info[1]), "code": 0, "stackTrace": self.formatException(record.exc_info)}}
>             )
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<             contextual_extra = set(record.__dict__).difference(
<                 set(self.DEFAULT_PROPERTIES))
---
>             contextual_extra = set(record.__dict__).difference(set(self.DEFAULT_PROPERTIES))
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< 
<     def __init__(self, level=logging.NOTSET, host='localhost', port=27017,
<                  database_name='logs', collection='logs',
<                  username=None, password=None, authentication_db='admin',
<                  fail_silently=False, formatter=None, capped=False,
<                  capped_max=1000, capped_size=1000000, reuse=True, **kwargs):
---
>     def __init__(
>         self,
>         level=logging.NOTSET,
>         host="localhost",
>         port=27017,
>         database_name="logs",
>         collection="logs",
>         username=None,
>         password=None,
>         authentication_db="admin",
>         fail_silently=False,
>         formatter=None,
>         capped=False,
>         capped_max=1000,
>         capped_size=1000000,
>         reuse=True,
>         ttl=3600,
>         **kwargs,
>     ):
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>         self.ttl = ttl
>         self.ttl_index = ""
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<         global _connection
---
>         global _connection  # pylint: disable=global-statement
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<             if pymongo.version_tuple[0] < 3:
<                 try:
<                     self.connection = Connection(host=self.host,
<                                                  port=self.port, **kwargs)
<                 # pymongo >= 3.0 does not raise this error
<                 except PyMongoError:
<                     if self.fail_silently:
<                         return
<                     else:
<                         raise
<             else:
<                 self.connection = Connection(host=self.host, port=self.port,
<                                              **kwargs)
<                 try:
<                     self.connection.is_primary
<                 except ServerSelectionTimeoutError:
<                     if self.fail_silently:
<                         return
<                     else:
<                         raise
---
>             self.connection = MongoClient(host=self.host, port=self.port, **kwargs)
>             try:
>                 self.connection.is_primary
>             except ServerSelectionTimeoutError:
>                 if self.fail_silently:
>                     return
>                 raise
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>         self._setup()
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<         self.db = self.connection[self.database_name]
---
>     def _setup(self):
>         self.db = self.connection[self.database_name]  # type: ignore
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<             auth_db = self.connection[self.authentication_database_name]
<             self.authenticated = auth_db.authenticate(self.username,
<                                                       self.password)
---
>             auth_db = self.connection[self.authentication_database_name]  # type: ignore
>             self.authenticated = auth_db.authenticate(self.username, self.password)
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<                 self.collection = Collection(self.db, self.collection_name,
<                                              capped=True, max=self.capped_max,
<                                              size=self.capped_size)
---
>                 self.collection = Collection(self.db, self.collection_name, capped=True, max=self.capped_max, size=self.capped_size)
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>     def _ensure_log4mongo_index(self):
>         if not self.ttl_index and self.ttl > 0:
>             try:
>                 self.ttl_index = self.collection.create_index(  # type: ignore
>                     "timestamp", expireAfterSeconds=self.ttl, background=True
>                 )
>             except (TypeError, ConfigurationError, ServerSelectionTimeoutError):
>                 pass
> 
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<             self.db.logout()
---
>             self.db.logout()  # type: ignore
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> 
>         if self.collection is None:
>             try:
>                 self._setup()
>             except ServerSelectionTimeoutError:
>                 pass
>         self._ensure_log4mongo_index()
> 
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<                 getattr(self.collection, write_method)(self.format(record))
<             except Exception:
---
>                 self.collection.insert_one(self.format(record))
>             except Exception:  # pylint: disable=broad-except
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<     def __exit__(self, type, value, traceback):
---
>     def __exit__(self, type, value, traceback):  # pylint: disable=redefined-builtin
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< 
<     def __init__(self, level=logging.NOTSET, host='localhost', port=27017,
<                  database_name='logs', collection='logs',
<                  username=None, password=None, authentication_db='admin',
<                  fail_silently=False, formatter=None, capped=False,
<                  capped_max=1000, capped_size=1000000, reuse=True,
<                  buffer_size=100, buffer_periodical_flush_timing=5.0,
<                  buffer_early_flush_level=logging.CRITICAL, **kwargs):
---
>     def __init__(
>         self,
>         level=logging.NOTSET,
>         host="localhost",
>         port=27017,
>         database_name="logs",
>         collection="logs",
>         username=None,
>         password=None,
>         authentication_db="admin",
>         fail_silently=False,
>         formatter=None,
>         capped=False,
>         capped_max=1000,
>         capped_size=1000000,
>         reuse=True,
>         ttl=3600,
>         buffer_size=100,
>         buffer_periodical_flush_timing=5.0,
>         buffer_early_flush_level=logging.CRITICAL,
>         **kwargs,
>     ):
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<         MongoHandler.__init__(self, level=level, host=host, port=port, database_name=database_name, collection=collection,
<                               username=username, password=password, authentication_db=authentication_db,
<                               fail_silently=fail_silently, formatter=formatter, capped=capped, capped_max=capped_max,
<                               capped_size=capped_size, reuse=reuse, **kwargs)
---
>         MongoHandler.__init__(
>             self,
>             level=level,
>             host=host,
>             port=port,
>             database_name=database_name,
>             collection=collection,
>             username=username,
>             password=password,
>             authentication_db=authentication_db,
>             fail_silently=fail_silently,
>             formatter=formatter,
>             capped=capped,
>             capped_max=capped_max,
>             capped_size=capped_size,
>             reuse=reuse,
>             ttl=ttl,
>             **kwargs,
>         )
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<         self.last_record = None #kept for handling the error on flush
---
>         self.last_record = None  # kept for handling the error on flush
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<         self._buffer_lock = None
---
>         self.buffer_lock = threading.RLock()
> 
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<             import atexit
<             atexit.register(self.destroy)
---
>             import atexit  # pylint: disable=import-outside-toplevel
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<             import threading
<             self._buffer_lock = threading.RLock()
---
>             atexit.register(self.destroy)
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<             self._timer_stopper, self.buffer_timer_thread = call_repeatedly(self.buffer_periodical_flush_timing, self.flush_to_mongo)
---
>             self._timer_stopper, self.buffer_timer_thread = call_repeatedly(
>                 self.buffer_periodical_flush_timing, self.flush_to_mongo
>             )
262,263c306,315
< 
<         self.add_to_buffer(record)
---
>         if self.collection is None:
>             try:
>                 MongoHandler._setup(self)
>             except ServerSelectionTimeoutError:
>                 pass
>         MongoHandler._ensure_log4mongo_index(self)
> 
>         with self.buffer_lock:
>             self.last_record = record
>             self.buffer.append(self.format(record))
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<         return
< 
<     def buffer_lock_acquire(self):
<         """Acquire lock on buffer (only if periodical flush is set)."""
<         if self._buffer_lock:
<             self._buffer_lock.acquire()
< 
<     def buffer_lock_release(self):
<         """Release lock on buffer (only if periodical flush is set)."""
<         if self._buffer_lock:
<             self._buffer_lock.release()
< 
<     def add_to_buffer(self, record):
<         """Add a formatted record to buffer."""
< 
<         self.buffer_lock_acquire()
< 
<         self.last_record = record
<         self.buffer.append(self.format(record))
< 
<         self.buffer_lock_release()
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<             self.buffer_lock_acquire()
<             try:
< 
<                 getattr(self.collection, write_many_method)(self.buffer)
<                 self.empty_buffer()
< 
<             except Exception as e:
<                 if not self.fail_silently:
<                     self.handleError(self.last_record) #handling the error on flush
<             finally:
<                 self.buffer_lock_release()
---
>             with self.buffer_lock:
>                 try:
>                     self.collection.insert_many(self.buffer)
>                     self.empty_buffer()
>                 except Exception:  # pylint: disable=broad-except
>                     if not self.fail_silently:
>                         self.handleError(self.last_record)  # type: ignore
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< 
<