build_invalidator and patns cache mismatch causes classes to not get written to pants

[LarryFinn]

Affects 1.3.0 (maybe more).
Requirements: JVM project with zinc (see https://github.com/jsirois/lf-example for example) with multiple branches affecting the same class (in this case Example class)

1. Start off in branch A. Do a ./pants compile ::. See the Example class appear in the cache directory
2. Switch to branch B. Do a ./pants compile ::. See both versions of the Example class in the cache directory
3. Delete the cache directory for just the Example class
4. Switch back to branch A. Do a ./pants compile ::. Pants should recompile Example class but it does NOT write it to the cache.

It seems like the build_invalidator thinks that the class is in the cache, and when it isn't it gets recompiled but nothing gets pushed to cache. I believe the class should be written to the cache. Running pants invalidate between switching branches fixes the problem.

[stuhood]

I explained this in https://pantsbuild.slack.com/archives/C046T6T9U/p1520268937000694 ... it is intended that zinc incremental compiles are not written to the build cache, as incremental compiles with zinc are super buggy (still). See compile tickets under https://github.com/pantsbuild/pants/labels/jvm for an (incomplete) list.

[LarryFinn]

how does pants determine that this is an "incremental" compile? Because it is not in a clean state?
the thing is, if i dont do step #3, everything works fine. It pushes the classes to the cache fine.

[stuhood]

Which cache directory are you deleting here:

Delete the cache directory for just the Example class

?