Cache: fix race condition

When checking a cache entry with no input files for a hit, if
`createFile` returned `error.WouldBlock` we would forget about the fact
that the file has been created, and all future checks will assume that a
cache hit has happened, even though one never has or does, leading to
rare `FileNotFound` errors trying the access the protected files.

This fix works by writing an extra byte to the manifest file to
distinguish hits and misses when there no input files to write.

0 files changed, 0 insertions, 0 deletions