When the ghc-paths library is compiled, the paths of the
compiler it is compiled with are being hardcoded in the
library (and can then be queried from other applications
using the library).
But on Nix, packages are compiled with ghc-wrapper, and
subsequently possibly used with a special version of ghc
generated for a particular environment of packages. So
one version of ghc-paths may potentially end up being
used by lots of different instances of ghc. The hardcoding
approach fails.
As a work-around, we now patch ghc-paths so that it allows
setting the paths that can be queried via environment
variables. Specific GHC environments can then set these
environment variables in the wrapper shell script that
invokes GHC.
This should at least partially solve issue #213.
The previos version did a for loop over the output of set, which spits out _all_
defined variables and their contents. This not only is dangerous if there is a
variable starting with CONFIG_ but also can't handle whitespace, as the IFS is
set to any (horizontal _and_ vertical) whitespace by default.
So, imagine (actually don't imagine, something like this is the case in a lot of
kernel configuration files) you have the following variable:
CONFIG_ARCH_HWEIGHT_CFLAGS="-fcall-saved-rdi -fcall-saved-rsi ..."
A loop with for and the default IFS would result in the following variable
pieces:
0: CONFIG_ARCH_HWEIGHT_CFLAGS="-fcall-saved-rdi
1: -fcall-saved-rsi
2: ..."
This obviously leads to the problem that this config variable is being cut off
at the first whitespace.
Another downside of this approach is that set not only returns variables but
functions as well. This could lead to quite a lot of unexpected behaviour and
confusion.
So the new approach doesn't source the kernel configuration anymore but uses
`read` to parse the file line-by line, setting IFS to '=', thus splitting all
configuration lines into key/value pairs.
Using parameter expansion, we ensure that we only read lines starting with
"CONFIG_". This particularily has the advantage of not being bash-specific,
should we choose to change to a different default shell someday.
Now, after we got a correct "CONFIG_" line, we're using a temporary variable to
split off the first quote from the result. Particularily the reason behind this
is shell compatibility again, as ${${foo#"}%"} only works in Bash, Zsh and
whatnot but not in plain SH.
And within the next line we obviously insert the no_firstquote variable without
it's last quote removed.
But, what about escaping?
First of all, if we'd just eval the $val variable, we would correctly unescape
the value, but this has the downside that variables within the content would be
expanded, for example look at this:
CONFIG_DEFCONFIG_LIST="/lib/modules/$UNAME_RELEASE/.config"
Well, obviously this is a bad example at the Nix sense, but just to show that
variables within kernel configuration entries aren't impossible.
And second, which would have been a show stopper if \" would be within $val: It
simply would end up being an invalid Nix expression, because \" would end up as
a ploin " within a double quoted string.
Signed-off-by: aszlig <aszlig@redmoonstudios.org>
