therning.org/ magnus

This is another one of those posts that I make mostly for myself, you know for organising and help my memory

There are as far as I can see three ways to deal with sockets in Haskell. There’s the type Socket which is used throughout Network.Socket. From that it’s possible to get to the underlying filedescriptor, and it in turn can be converted to a Handle.

When coupled with fork+exec it’s crucial to make sure the child process can find the socket Leaving it in a predictable place seems to be the easiest way to do that, and as far as I can see that requires using dupTo from System.Posix.IO. So, on the child-side it’s necessary to find a way to turn an integer (CInt) into something that can be treated as a socket (i.e. a Socket, a Handle, or a filedescriptor).

A basic parent-child which obviously won’t work since the child’s socket is represented as a Socket:

import Control.Concurrent
import System.Posix.Process
import Network.Socket
 
childFunc s = send s "Ping from child" >> return ()
 
main = do
    (childSock, parentSock) <- socketPair AF_UNIX Stream defaultProtocol
    print (childSock, parentSock)
    child <- forkProcess $ childFunc childSock
    recv parentSock 10 >>= print

Let the child take a CInt and turn it into a filedescriptor:

import Control.Concurrent
import Control.Concurrent.MVar
import System.Posix.Process
import System.Posix.IO
import System.Posix.Types
import Network.Socket
 
childFunc sInt = do
    let fd = Fd sInt
    fdWrite fd "Ping from child" >> return ()
 
main = do
    (childSock, parentSock) <- socketPair AF_UNIX Stream defaultProtocol
    let childInt = fdSocket childSock
    print (childInt, parentSock)
    child <- forkProcess $ childFunc childInt
    recv parentSock 10 >>= print

Let the child take a CInt and turn it into a Handle:

import Control.Concurrent
import System.Posix.Process
import System.Posix.IO
import System.Posix.Types
import Network.Socket
import System.IO
 
childFunc sInt = do
    h <- fdToHandle $ Fd sInt
    hPutStr h "Ping from child"
    hFlush h
 
main = do
    (childSock, parentSock) <- socketPair AF_UNIX Stream defaultProtocol
    let childInt = fdSocket childSock
    print (childSock, parentSock)
    child <- forkProcess $ childFunc childInt
    recv parentSock 10 >>= print

Let the child take a CInt and turn it into a Socketⁱ:

import Control.Concurrent
import Control.Concurrent.MVar
import System.Posix.Process
import System.Posix.IO
import System.Posix.Types
import Network.Socket
 
childFunc sInt = do
    s <- mkSocket sInt AF_UNIX Stream defaultProtocol Connected
    send s "Ping from child" >> return ()
 
main = do
    (childSock, parentSock) <- socketPair AF_UNIX Stream defaultProtocol
    let childInt = fdSocket childSock
    print (childInt, parentSock)
    child <- forkProcess $ childFunc childInt
    recv parentSock 10 >>= print

1. It seems the socket is in the Connected state after socketPair succeeds.[back]

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With Python being dropped as a language for extensions in epiphany 2.28 I needed to replace epilicious. I tried writing it in JavaScript (seed was integrated in 2.28), but I gave up due to hitting too many hurdles on the way. Instead I decided to rewrite epilicious using Vala and a minimal layer of C.

It turned out to be very doable, despite epiphany’s extension API lacking Vala bindings (Cosimo Cecchi, I’m looking at you ). Basically I did the following setup:

1. Add the extension following the instructions in Writing Epiphany Extensions by Adam Hooper.
2. Add a check for valac in configure.ac.
3. Add a rule in the extension’s Makefile.am to generate a C header file for all the Vala code, for use from C.

Then I started writing the actual extension. I did the minimal amount of work in C, trying to escape as soon as possible into Vala. In the few places I needed to call from Vala into C I would declare a delegate in Vala, and pass a function from Cⁱ.

I call this new version BMS, for bookmark synchronisation. I have a patch for BMS that applies to epiphany 2.28.1. (The file also contain a PKGBUILD in order to delight Arch users )

I should probably point out that while epilicious never could be called polished, BMS is even less so. I might find the time to make it multi-threaded, so that it’s possible to do some sort of progress dialogue, but don’t hold your breath. In the back of my mind is also the thought of rewriting it yet again, in JavaScript/seed.

1. The format of .vapi files are unknown to me, and I haven’t managed to find much documentation. Using function pointers seemed like an easier way, especially given that I needed this in exactly 3 places.[back]

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