Always return checksums for all files even if empty, add a new filetype flag `is_empty`

[pombredanne]

Following #459 we need to always return checksums such as SHA1 and add a new is_empty flag for empty files.

[sschuberth]

While working on this, it probably makes sense to consider adding more flags that also are orthogonal to the current flags like is_link, is_file etc. I'm mainly thinking of is_binary (and / or is_source) and is_archive to have direct mappings to the SPDX FileType field.

[kartiksibal]

@pombredanne Since, you have explicitly mentioned in #457, the reason for not using a hash, for an empty file. Could you please elucidate your expectations with this?

Thanks :-)

[pombredanne]

are you sure this was #457 ? that does not sounds related?

[kartiksibal]

@pombredanne I might have interpreted it wrong. I had based my conclusion on,

[arnav-mandal1234]

@pombredanne for this issue, I have to calculate all checksums such as sha1 and md5 for an empty file as well keep one extra boolean is_empty to keep track of empty files. So, should I assign a constant hash for the empty files? (all empty files would have same hash value) or we can change the hash function and it would automatically assign the hash value to the empty files as well and all the hashes would be some combination of other features or what should I do?
Please guide me if I am wrong. :)

[pombredanne]

@arnav-mandal1234 correct, but I think this is no longer an issue. That would be worth checking on a --info scan that would have some empty files

[pombredanne]

Note that this is a possible issue in the SPDX output that demands to have a checksum even for an empty file.

So, should I assign a constant hash for the empty files? (all empty files would have same hash value)

That's likely the best approach

[sschuberth]

Note that at the example of SHA-1, the hash for an empty string / no content is da39a3ee5e6b4b0d3255bfef95601890afd80709.