Fix "GetOverlappedResult: More data available" on windows 10

[adambor]

I've ran into an issue on windows 10, with the ledger-agent crashing with the following error:

  File "f:\trezor-agent\libagent\win_server.py", line 178, in recv
    chunk_size = win32pipe.GetOverlappedResult(self.handle, self.overlapped, False)
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
pywintypes.error: (234, 'GetOverlappedResult', 'More data is available.')

This PR is a fix for it, essentially just catching ERROR_MORE_DATA and returning the whole read buffer in that case.

[adambor]

Fixed the pycodestyle issue, it should now pass the CI.

[romanz]

Please squash and rebase over latest master (the last CI failure should be fixed by #480).

[romanz]

@gtbuchanan @SlugFiller could you please take a look?
I don't use Windows, so I would appreciate your feedback :)

[SlugFiller]

I haven't encountered the error myself, which makes me not completely sure on when ERROR_MORE_DATA is received. I am uncertain if returning the full buffer is correct, here, as it might actually be returning an uninitialized buffer with garbage data.

At the very least, if the buffer has been completely filled, the operation should be over with NO_ERROR. It might actually be more correct to check if the task was cancelled, and if not, to try loop, wait, and attempt to read the result again.

[adambor]

IMO (there also isn't much clarity on the error from the microsoft side), the ERROR_MORE_DATA means that the read buffer was completely filled with data and there is still more data to be read, this is in line with my observation (the agent doesn't work without this fix on win10 19045.5131).

Moreover there already was an attempt to catch the ERROR_MORE_DATA error on lines 165-169, however it is not returned as an error code there, but instead thrown by the later win32pipe.GetOverlappedResult call.

[adambor]

Please squash and rebase over latest master (the last CI failure should be fixed by #480).

Squashed & rebased

[gtbuchanan]

Here is the overlapped IO example from Microsoft. It does loop to call ReadFile and GetOverlappedResult again in this case.