Fail early, when we discover no tests at all, or filter out all of them.
There is also `--allow-empty-runs` to disable test to allow workflows
like `LIT_FILTER=abc ninja check-all`. Apparently `check-all` invokes
lit multiple times if certain projects are enabled, which would produce
unwanted "empty runs". Specify via `LIT_OPTS=--allow-empty-runs`.
There are 3 causes for empty runs:
1) No tests discovered. This is always an error. Fix test suite config
or command line.
2) All tests filtered out. This is an error by default, but can be
suppressed via `--alow-empty-runs`. Should prevent accidentally
passing empty runs, but allow the workflow above.
3) The number of shards is greater than the number of tests. Currently,
this is never an error. Personally, I think we should consider
making this an error by default; if this happens, you are doing
something wrong. I added a warning but did not change the behavior,
since this warrants more discussion.
Reviewed By: atrick, jdenny
Differential Revision: https://reviews.llvm.org/D70105
95339f1d20