consensus/ethash: close mmap before rename, windows limitation #14329
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Our current ethash memory mapped caches generated the cache in a temp file, then renamed it into its final location. Unfortunately this is forbidden under Windows, as it's not capable of renaming open files opposed to unix systems. The PR changes the logic so that instead of flush/rename, we now do a close/rename/open cycle. It is more costly but I didn't want to add Windows specific logic so low level as ethash.
It also works around #3815 . The underlying issue there is that the
mmap-go
library calls flush on the memory map handle, not the file handle. This is an upstream bug, also spotted by someone else (edsrzf/mmap-go#14). However since we close the generated cache down before rename, we don't care that theFlush
method doesn't work as the OS will flush it on close anyway.