GHC.Handle.fdToHandle' locks regular files

[bos@…]

The code in GHC.Handle.fdToHandle' locks a regular file. This violates POSIX file semantics, because it means that a process can't open the same file twice simultaneously. The locking should be removed.

[igloo]

 http://haskell.org/onlinereport/io.html section "21.2.3 File locking" says

Implementations should enforce as far as possible, at least locally to the Haskell process, multiple-reader single-writer locking on files. That is, there may either be many handles on the same file which manage input, or just one handle on the file which manages output.

I'm not sure if this was done to combat problems with lazy IO, or in order to make the behaviour the same on all OSes, but GHC is just following the Haskell 98 report here.