cask: skip sha256 generation on directories

[bevanjkay]

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This is an idea for fixing - Homebrew/homebrew-cask-fonts#5855

Because svn is used to download the fonts, the Digest::SHA256 is failing on the contents because it is a directory not a file.
I have added a check to not return a value in these cases so that no shasum is stored.
The downside of running the check there is that I believe that we may still be fetching the download when the cask is version :latest.
We could potentially add a check for URLs using :svn instead, however there may be other edge cases?

I am not aware of a way to gather a shasum for a directory, if there is then that could be another approach.

CC: @apainintheneck

[BrewTestBot]

Review period will end on 2022-06-01 at 14:04:32 UTC.

[BrewTestBot]

Review period skipped due to critical label.

[apainintheneck]

Another thing I overlooked. 😰

Yeah, there doesn't seem to be a default way to get a checksum for a directory. You could always tarball it but that's probably more trouble than it's worth. I think this is a good band-aid for now but ideally we'd want to check if it's :svn before downloading like you already mentioned. I can look into that.

[MikeMcQuaid]

Might want to make sure whatever solution is adopted here can be used for #13232 too.

[apainintheneck]

Would it be possible to check for :svn with cask.url.using?

Edit: Yes, that is possible but wouldn't work with the new git sparse checkout download strategy mentioned above.

[bevanjkay]

Closing in favour of #13358 - as it avoids fetching any downloads before making a decision on shasum generation.