Instead of first checking whether a file is in the repository cache and
then opening it, we just can open the file. This saves one stat call. If
the file is in the cache, everything is fine and otherwise the code
follows its normal fallback path.
Many instances of errors.Wrap in this package would produce messages
like "Open: open <filename>: no such file or directory"; those now omit
the first "Open:" (or "Stat:", or "MkdirAll"). The function readVersion
now appends its own name to the error message, rather than the function
that failed, to make it easier to spot. Other function names (e.g.,
Load) are already added further up in the call chain.
- be explicit when discarding returned errors from .Close(), etc.
- remove named return values from funcs when naked return not used
- fix some "err" shadowing when redeclaration not needed
This commits adds rudimentary support for a cache directory, enabled by
default. The cache directory is created if it does not exist. The cache
is used if there's anything in it, newly created snapshot and index
files are written to the cache automatically.
