Timestamp hashes

[fommil]

Because of sbt/sbt#363 I am unable to say anything about the perf characteristics of this PR.

I guess it is addressing the same problem that #371 is addressing and a hacky alternative to #433 (which is probably the correct solution).

What I thought was an I/O bottleneck in sbt/io#79 turned out to be a CPU hashcode bottleneck when hashing classpath files, not source files. YourKit doesn't pick up the hash/crypto work, showing it as InputFilterStream. Given the many many bugs in the entire scala tooling chain when it comes to .jar files mutating on disk, see https://gitlab.com/fommil/class-monkey/blob/master/README.md, I really don't think SHA-1 is needed here. Timestamps (and file size) should be more than enough.

Also, this hashing could be done in parallel. Even if it's as simple as using the terrible .par.

[lightbend-cla-validator]

Hi @fommil,

Thank you for your contribution! We really value the time you've taken to put this together.

We see that you have signed the Lightbend Contributors License Agreement before, however, the CLA has changed since you last signed it.
Please review the new CLA and sign it before we proceed with reviewing this pull request:

http://www.lightbend.com/contribute/cla

[fommil]

BTW, I'm not signing the Lightbend CLA... you can have my contributions under the terms of the BSD-3, the license of this project. Otherwise it is not a symmetric and fair agreement.

[fommil]

I'd like to do some more work on this, but I'm basically blocked until I can try out my changes locally without having to build the entire sbt ecosystem from scratch... I just want to be able to stick a jar at the right place in the sbt classpath.

[fommil]

closing in preference of #433

[romanowski]

@fommil We also have problems with classpath hashing (you can probably imagine) and caching was best approach for us.
I've open PR ( #427) that allows client o implement classpath hashing on thier side (and I think it should be possible to override this from sbt level).

[fommil]

Typical @romanowski : "yeah, we already seen this and fixed it months ago... 🤷‍♂️ " 😁