Predicate FileWidget state on File.is_decrypted, not is_downloaded

[rmol]

Description

To export or print, a file must have been decrypted, so present FileWidget affordances based on that, not just whether it's been downloaded.

Also change how FileWidget's download animation happens: now it starts immediately, and the timer is used when stopping instead, to ensure it always runs for at least 300ms. Credit to @eloquence for that idea.

Finally, start a DownloadException hierarchy, adding DownloadDecryptionException to the existing
DownloadChecksumMismatchException, so we can better diagnose retrieval problems.

Fixes #756.

Also fixes #754 as I had to fix the animation to test.

Test Plan

• Start a SecureDrop development server instance with make dev in a local working copy.

• Run the client with run.sh

• In a web browser, upload a file through the dev server's source interface.

• In the client, find the source and the file and click the download button to retrieve it. The downloading animation should start, run briefly, and stop, resulting in the export/print buttons becoming available.

• Stop the client, and add a line like raise CryptoError("BOOM!") at the beginning of FileDownloadJob.call_decrypt.

• Stop and restart the dev server.

• Upload another file.

• Restart the client.

• Try to download the file. The download animation should start, an error should appear in the status bar, the download animation should stop, and you should be able to retry the download. (This is also the test for downloading animation never ends if the file cannot be decrypted #754.)

Checklist

If these changes modify code paths involving cryptography, the opening of files in VMs or network (via the RPC service) traffic, Qubes testing in the staging environment is required. For fine tuning of the graphical user interface, testing in any environment in Qubes is required. Please check as applicable:

• I have tested these changes in the appropriate Qubes environment
• I do not have an appropriate Qubes OS workstation set up (the reviewer will need to test these changes)
• These changes should not need testing in Qubes

If these changes add or remove files other than client code, packaging logic (e.g., the AppArmor profile) may need to be updated. Please check as applicable:

• I have submitted a separate PR to the packaging repo
• No update to the packaging logic (e.g., AppArmor profile) is required for these changes
• I don't know and would appreciate guidance

[sssoleileraaa]

first pass things look pretty good

if a CryptoError is raised during decryption of a file, which I need to be able to test in order to verify that this fixes the 2 named bugs, the client crashes with the following error:

QWidget::setTabOrder: 'first' and 'second' must be in the same window
Traceback (most recent call last):
  File "/home/creviera/workspace/freedomofpress/securedrop-client/securedrop_client/logic.py", line 718, in on_file_download_failure
    self.file_missing.emit(exception.uuid)
AttributeError: 'InterfaceError' object has no attribute 'uuid'

[rmol]

That was happening because it didn't receive a DownloadException. A CryptoError raised in any call_decrypt method of a DownloadJob should be chained into a DownloadDecryptionException, preventing this. Not sure how the InterfaceError wound up there, but I've changed Controller.on_file_download_failure to only signal file_missing when the exception makes the file's UUID available.

[sssoleileraaa]

✔️ i see that we are now always showing the animation in order to avoid a flicker if our operation duration happens to be just a little bit higher than our delay

✔️ This fixes #756

❌ i'm not seeing this fix #754, if there is a crypto error raised in decrypt_submission_or_reply, you'll see the follow animation:

This needs a rebase, most notably you need to make sure to update:

@pyqtSlot(str)
def _on_file_downloaded(self, file_uuid: str) -> None:

to:

    @pyqtSlot(str, str, str)
    def _on_file_downloaded(self, source_uuid: str, file_uuid: str, filename: str) -> None:

which matches what the file_ready signal emits and this is what fixed #774

[sssoleileraaa]

My PR comment was addressed and this issue was fixed: #754

While testing this I noticed a new issue that is on master as well where the the "DOWNLOAD" text doesn't change on hover anymore, only the download icon does. And now that this PR fixed the issue with the never-ending download spinner, we can see that the download icon remains highlighted even when your not on mouseover if the file was deleted and needs to be redownloaded:

[sssoleileraaa]

lgtm

Follow-up issues:

• Filename Hover State #795
• Fix file widget layout #694
• Download icon flickers when mousing over spinner #791