cabal was failing with:

$ cabal --config-file=/home/hackage-docs/hackage-docs/build-cache/cabal-config install --enable-documentation --enable-tests --haddock-html-location='http://192.168.142.1:8080/package/$pkg-$version/doc' --haddock-option=--built-in-themes --haddock-contents-location='http://192.168.142.1:8080/package/$pkg-$version' --haddock-hyperlink-source --prefix=/home/hackage-docs/hackage-docs/build-cache HUnit-1.2.5.1
Resolving dependencies...
/home/hackage-docs/hackage-docs/build-cache/local.conf.d/package.cache: openBinaryFile: does not exist (No such file or directory)
cabal: /home/hackage-docs/.cabal/logs/HUnit-1.2.5.1.log: resource busy

as it tried to open the log file for reading when it already had it open
for writing:

open("/home/hackage-docs/.cabal/logs/HUnit-1.2.5.1.log", O_WRONLY|O_CREAT|O_NOCTTY|O_APPEND|O_NONBLOCK, 0666) = 3
open("/home/hackage-docs/.cabal/logs/HUnit-1.2.5.1.log", O_RDONLY|O_NOCTTY|O_NONBLOCK) = 5
close(5)                                = 0
close(3)                                = 0