make magic-folder (downloader) polling interval configurable

[meejah]

It's not currently possible to configure the polling interval for the magic-folder downloader.

Additionally, we should think about what a good default is (3 seconds was used during development, but that's way too short).

We also should document this behavior and say something about how to choose a default (e.g. maybe a *rough* estimate of how much traffic it means).

[meejah]

For a first cut, at least making "a" config-option is a must for 0.1.12

Longer term, it might be good to have other ways of scheduling magic-folder syncs. (e.g. even "only when told to" or schedule times of low-freq/high-freq, etc).

[dawuud]

I checked and currently MagicFolder? defaults to a 1 second polling interval.

Here's I've changed it to 5 seconds default and made it configureable via a magic folder polling interval config file option: ​https://github.com/david415/tahoe-lafs/tree/2829.magic_folder_polling_interval.0 ​https://github.com/david415/tahoe-lafs/commit/63e52c276ba3d2e13dd55b74d2c88e4456b168f4

[dawuud]

upon looking at the code more closely i don't understand why we have two delay variables in the queue mixin:

1. turn delay
2. scan interval

the scan interval only controls how often when_queue_is_empty is called which is only used by the Downloader which performs a remote collective dir scan every scan interval. having the scan interval and the turn delay be different for the Downloader does nothing special.

the uploader doesn't even use when_queue_is_empty. it's a no-op.

can we get rid of scan_interval? I certainly do not like shadowing variable in mixins and in the inheriting class.

[dawuud]

i added more commits to that dev branch experimenting with removing scan_interval. it did not work out so well. the unit tests... many of them fail. not sure why.

[meejah]

See ​https://github.com/tahoe-lafs/tahoe-lafs/pull/371

[warner]

Landed in [233d0e2].

[meejah]

Closed as fixed since the PR was merged.