Last step in cabal-nix sometimes takes a loooooooooong time #3019
Description
Building hackage-server:
cabal-nix new-build -v3
...
link: linkables are ...
LinkableM (2016-01-02 08:32:56 UTC) main:Main
[DotO /Users/e/wt/projects/tuf/hackage-server/dist-newstyle/build/hackage-server-0.5.1/build/PackageTests/PackageTests-tmp/Main.o]
LinkableM (2016-01-02 08:32:55 UTC) main:Distribution.Server.Packages.Unpack
[DotO /Users/e/wt/projects/tuf/hackage-server/dist-newstyle/build/hackage-server-0.5.1/build/PackageTests/PackageTests-tmp/Distribution/Server/Packages/Unpack.o]
LinkableM (2016-01-02 08:32:56 UTC) main:Distribution.Server.Packages.UnpackTest
[DotO /Users/e/wt/projects/tuf/hackage-server/dist-newstyle/build/hackage-server-0.5.1/build/PackageTests/PackageTests-tmp/Distribution/Server/Packages/UnpackTest.o]
LinkableM (2015-12-30 15:01:50 UTC) main:Distribution.Server.Util.Parse
[DotO /Users/e/wt/projects/tuf/hackage-server/dist-newstyle/build/hackage-server-0.5.1/build/PackageTests/PackageTests-tmp/Distribution/Server/Util/Parse.o]
LinkableM (2015-12-30 15:01:49 UTC) main:Distribution.Server.Util.GZip
[DotO /Users/e/wt/projects/tuf/hackage-server/dist-newstyle/build/hackage-server-0.5.1/build/PackageTests/PackageTests-tmp/Distribution/Server/Util/GZip.o]
/Users/e/wt/projects/tuf/hackage-server/dist-newstyle/build/hackage-server-0.5.1/build/PackageTests/PackageTests is up to date, linking not required.
*** Deleting temp files:
Deleting:
*** Deleting temp dirs:
Deleting: /var/folders/94/vkm1c1l17cddyvzk2252hnp40000gs/T/ghc81573_0
and then.. nothing.. The cabal-nix process is using some 25% CPU at this point, and I have no idea what it's doing. That temp directory mentioned on the last line is already gone, so it's not just deleting. What it is doing is anyone's guess.. I CTRL-C'ed it after 12 minutes. Retried, same thing. Wiped dist-newbuild, same thing. I've seen long times before but never something like this. No idea what's happening. Will reboot machine and see if that resolved anything. (Incidentally, I thought maybe it was busy waiting for the hackage-server executable, but the server was not running at this point.)