What is the proper way to check if the remote repository is available?

[RasmusN]

What is the proper way to check if the remote repository is available?

(Yes I know, questions are supposed to be posted to stackoverflow with the "tufup" tag. However, since the tag doesn't exist it's not possible to use it unless you already are a stackoverflow guru with 1500 reputation points.)

I tried refreshing the client and catching the DownloadHTTPError before running the update:

from tufup.client import Client
from tuf.api.exceptions import DownloadHTTPError
...
client = Client(...)
try:
  client.refresh()
except DownloadHTTPError as e:
        ...
if client.check_for_updates(pre=None):
        client.download_and_apply_update(...)
...

but this causes an error when calling the client.check_for_updates method:

RuntimeError: Cannot update timestamp after snapshot

System info (please complete the following information):

• OS: Windows
• OS Version: 10
• Python version: 3.11.3

[dennisvang]

@RasmusN Good question.

In the tufup source, you'll see that check_for_updates() already calls refresh() and catches tuf.api.exceptions.DownloadError, so you don't have to (or shouldn't?) call refresh() yourself.

You'll find a usage example in the tufup-example source.

The refresh_required argument to Client() determines what happens when a download error occurs:

If False, the error is ignored and your app proceeds without updates, if True, tufup forces the app to exit (via sys.exit()).

If you want to handle the download error yourself, instead of automatically exiting the interpreter, you could catch the SystemExit from the check_for_updates() call.

For example (off the top of my head):

...
client = Client(
    ...,
    refresh_required=True,
)
try:
    updates = client.check_for_updates()
except SystemExit:
    updates = None
    ...  # <-- do your thing here
if updates:
    client.download_and_apply_update(...)
...

I haven't tested this, but I suppose it should work.

[RasmusN]

@dennisvang Thanks this works!

Is there a specific reason why the client exists the entire process rather than raising a (custom) exception?

[dennisvang]

@RasmusN Good to hear it works.

Basically, the intention of refresh_required was to offer the choice between either continuing or exiting, in case of "network" errors.

The idea was to offer a simple interface, so the app creator would not need to worry about catching the right exceptions, although you could argue it's a bit restrictive.

Another alternative was to add a user callback argument which defaults to sys.exit, but the option to catch the SystemExit was considered sufficient at the time.

[RasmusN]

@dennisvang Alright I understand, makes sense