Build reports can leave entries in the blob storage without corresponding file on disk about hackage-server HOT 1 CLOSED

The current hypothesis is that these entries were caused by the server crash (not the hackage-server, the entire machine), and the AcidState for the blob storage was written to disk but the files themselves had not. Some evidence for this is provided by the fact that the server log itself is also missing entries for these blob IDs, so the server log didn't get written either.

If we want to fix the blob id writing we need to do fsync(fd) the blob when we write it to disk and fsync(dir_fd) of the directory once we have done the rename/move to atomically move the written blob file into its final location; from the fsync man page:

Calling fsync() does not necessarily ensure that the entry in the directory containing the file has also reached disk. For that an explicit fsync() on a file descriptor for the directory is also needed.

fsync is not in the unix lib sadly, have to ffi import it

foreign import ccall "fsync"
      c_fsync :: CInt -> IO CInt
fsync :: Fd -> IO ()
fsync (Fd fd) =  throwErrnoIfMinus1_ "fsync" $ c_fsync fd

Can steal it from http://hackage.haskell.org/packages/archive/cautious-file/1.0.2/doc/html/src/System-Posix-Fsync.html#fsync.

from hackage-server.