Add flush to the file write functions

[ilikecake]

Sorry for the churn. Further testing of the logging file handlers showed that they needed a flush() after the write() command in order for the contents of the log file to be updated in a reasonable time.

[dhalbert]

In the CPython version of this library, StreamHandler provides a flush() operation, and so do its subclasses, such as FileHandler.

I think the user should be able to use flush() as needed to guarantee that a log entry has been written to a file, but it should be optional. There are use cases where you don't want to always do a flush(). This change will cause more wear on the flash chips.

[ilikecake]

Added a flush() function to the StreamHandler. It should be inherited by the file handler classes below it. There does not seem to be a good way to actually call this function, however. Do we also need to impelment the shutdown() function?

https://docs.python.org/3/library/logging.html#logging.shutdown

[ilikecake]

How about this? I added a flushHandlers() function to the logger. This is not in the default python implementation of the logger, but I can't see any other way to force a flush of the handlers without accessing the semi-private _handlers list from user code.

[dhalbert]

I am sorry! I have led you astray. It turns out the that in the CPython impl, StreamHandler flushes by default: https://github.com/python/cpython/blob/3bf7a5079c8d1845af98c145056e7ae1e102be43/Lib/logging/__init__.py#L1164.

If you didn't want this to happen, you can create your own handlers and have it implements a particular strategy. There is a superclass BufferingHandler, which has a shouldFlush() operation: https://github.com/python/cpython/blob/3bf7a5079c8d1845af98c145056e7ae1e102be43/Lib/logging/handlers.py#L1304

The CPython library has a .handlers public property which returns all the registered handlers. But apparently it's discouraged to use this to flush: https://stackoverflow.com/questions/13176173/python-how-to-flush-the-log-django.

We try not to add features to the CPython library API, so I'd be reluctant to add flushHandlers(). I'm thiking we should go back to your original auto-flush() in 337770e.

If desired, a separate library could be added that would define a non-flushing file handler, not based on StreamHandler. I am not suggesting it is worth doing that now.

[ilikecake]

Sounds good, I have reverted it back to calling flush() after write().

[FoamyGuy]

This looks good to me.

Thanks for working on this followup @ilikecake!