How to use Data.Compact.Serialize

[Edward Z. Yang]

Yes this looks good to me.

It is NOT necessary to statically link libc, since the only pointers
embedded in compact regions are only ever info table pointers, e.g.,
stuff that GHC generated, not arbitrary functions in libc.

Edward

Excerpts from Christopher Done's message of 2018-12-19 10:31:28 +0000:
> Hi,
> 
> On the docs for Data.Compact.Serialize it says:
> 
> http://hackage.haskell.org/package/compact-0.1.0.1/docs/Data-Compact-Serialize.html
> > Our binary representation contains direct pointers to the info tables
> > of objects in the region. This means that the info tables of the
> > receiving process must be laid out in exactly the same way as from the
> > original process; in practice, this means using static linking, using
> > the exact same binary and turning off ASLR.
> 
> It seems to me that in order to use this module in any practical way
> (i.e. write to file from one process, and then a later run of the
> process reads it), you need to know a special way to build your binary
> which isn't fully described here. What flags, for example, should be
> passed to GHC to make this viable?
> 
> * Turning off ASLR on Linux is done by writing 0 to
>   /proc/sys/kernel/randomize_va_space, which applies to all
>   programs. That's not the most isolated way to deploy an app, but I
>   discovered that you can set this per process here:
>   https://askubuntu.com/a/507954
> * To compile GHC programs statically, use -optl-static -optl-pthread.
> 
> So in total the example would be:
> 
> $ stack build compact
> compact-0.1.0.1: download
> compact-0.1.0.1: configure
> compact-0.1.0.1: build
> compact-0.1.0.1: copy/register
> 
> Example file from docs:
> 
> $ cat main.hs
> {-# LANGUAGE TypeApplications #-}
> import System.Environment
> import Data.Compact
> import Data.Compact.Serialize
> main = do
>   arg:_ <- getArgs
>   case arg of
>     "write" -> do
>       orig_c <- compact ("I want to serialize this", True)
>       writeCompact @(String, Bool) "somefile" orig_c
>     "read" -> do
>       res <- unsafeReadCompact @(String, Bool) "somefile"
>       case res of
>         Left err -> fail err
>         Right c -> print (getCompact c)
> 
> Compiling statically:
> 
> $ stack ghc -- -optl-static -optl-pthread main.hs
> [1 of 1] Compiling Main             ( main.hs, main.o )
> Linking main ...
> [...hundred more warnings like this ...]
> 
> Check that it's static:
> 
> $ ldd main
> not a dynamic executable
> 
> Write the file:
> 
> $ setarch `uname -m` -R ./main write
> 
> Read the file:
> 
> $ setarch `uname -m` -R ./main read
> ("I want to serialize this",True)
> 
> Can a GHC dev confirm that this is the proper way to do this? If so,
> I'll contribute this little example as documentation to the compact package.
> 
> Cheers