Code review comment for lp:~mandel/ubuntu-download-manager/adapt-network-changes

I tested this on the krillin/devel channel (i.e. vivid) with the built .debs by doing the following:

Flashed the device to r37 and then hacked channel.ini to 36 as per:

https://wiki.ubuntu.com/Process/Merges/TestPlan/ubuntu-system-image

adb shell & sudo, then

$ system-image-cli -vv

As soon as the console indicated that the data files were starting to download, I shut off my wifi. I saw the download pause. Then I waited 15 seconds and turned the wifi back on. About another 30 seconds later, the device reconnected and the download resumed.

However, the download failed with the following traceback:

[systemimage] Nov 18 20:57:50 2014 (15059) uncaught exception in state machine
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/systemimage/state.py", line 123, in __next__
    step()
  File "/usr/lib/python3/dist-packages/systemimage/state.py", line 491, in _download_files
    self.downloader.get_files(downloads, pausable=True)
  File "/usr/lib/python3/dist-packages/systemimage/download.py", line 289, in get_files
    raise FileNotFoundError(reactor.error)
FileNotFoundError: http://system-image.ubuntu.com/pool/device-5533ed5d544bfe1c12e3e756907fa3258e8f03023ea1bfe4536f0fc08122673a.tar.xz:
[systemimage] Nov 18 20:57:50 2014 (15059) system-image-cli exception
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/systemimage/main.py", line 340, in main
    list(state)
  File "/usr/lib/python3/dist-packages/systemimage/state.py", line 123, in __next__
    step()
  File "/usr/lib/python3/dist-packages/systemimage/state.py", line 491, in _download_files
    self.downloader.get_files(downloads, pausable=True)
  File "/usr/lib/python3/dist-packages/systemimage/download.py", line 289, in get_files
    raise FileNotFoundError(reactor.error)
FileNotFoundError: http://system-image.ubuntu.com/pool/device-5533ed5d544bfe1c12e3e756907fa3258e8f03023ea1bfe4536f0fc08122673a.tar.xz:

I wonder if the file whose download got paused was corrupted when the download resumed?