PSA: `cabal update` command needs manual unsticking

[Gershom B]

Dear Haskellers,

A recent update to hackage, which fixed up the 01-index.tar.gz file,
revealed a bug in existing versions of cabal-install, when index files
are cleaned up. This bug means that the `cabal update` command, which
updates the hackage index file, will fail silently and leave the old
file in place. It is easy to get things working again, but it requires
manual intervention. Here is the most straightforward way to get
things working again:

On gnu/linux or mac: rm ~/.cabal/packages/hackage.haskell.org/01-index.*
On windows: remove the same files, but from
%appdata%\cabal\packages\hackage.haskell.org

rerun `cabal update` to fetch a new 01-index.

Apologies all for the inconvenience and any confusion this may have
caused. There is a PR to fix this behavior in the works, but since the
problem is on the client side, the fix will only improve the
situations for new versions of `cabal`.

Happy New Years to all, and cheers to a very functional 2018,
Gershom

p.s.: since the problem comes in unpacking the 01-index.tar.gz into
the 01-index.tar, you can also just untar that file in place manually,
or force cabal into doing the same by running `touch
~/.cabal/packages/hackage.haskell.org/01-index.tar`