Multiple GHC sessions

[Simon Marlow]

On 18 January 2016 at 11:18, Alan & Kim Zimmerman <alan.zimm at gmail.com>
wrote:

> From a tool writer perspective I would be happy to have GHC/Linker
> sessions be completely separate from each other. But I guess in
> practical terms a shared address space needs to be managed, making
> this impossible.
>
> The external interpreter for GHCI effectively solves the actual
> loading problem, the other things needing a GHC session are loading a
> module as far as type checking, for error reporting. I know this gets
> complicated if there is FFI involved and the loading has to go via
> object file generation.
>

Yes, I think -fexternal-interpreter is the basis of the right solution
here.  You do also need to move the PersistentLinkerState to HscEnv, but
that should be a fairly straightforward refactoring.  (actually I almost
did this as part of the remote GHCi work, but some of the comments next to
PersistentLinkerState suggested that there might be some real use cases for
having the linker state be shared between multiple sessions, so I decided
not to attack that problem right away.)

Cheers,
Simon

Alan
>
> On Mon, Jan 18, 2016 at 12:53 PM, Ben Gamari <ben at smart-cactus.org> wrote:
> > Simon Peyton Jones <simonpj at microsoft.com> writes:
> >
> >> I don't think the state-hack tail should wag the dog here. The nub of
> >> the problem in practice is the shared linker state isn't it?
> >>
> > I'm not sure; it's not clear to me what would break if the linker were
> > used by more than one session at a time, even with the current
> > GLOBAL_VAR. In fact, it arguably makes a fair bit of sense for all
> > sessions to use the same linker state given that they are also sharing
> > an address space.
> >
> > The only issue that may require a bit of care is code unloading.
> > Consider that you have two sessions A and B,
> >
> >   1. A loads MyModule
> >   2. B loads MyModule
> >   3. They both use MyModule for a while
> >   4. A unloads MyModule
> >   5. B continues
> >   6. B unloads MyModule
> >
> > I suspect that things will work fine up to step 6 since we don't
> > actually unload code until the garbage collector has marked the it as
> > dead. That being said, I suspect you would see an error from unloadObj
> > upon attempting to unload the object the second time (and even then, the
> > error is merely a message on stderr). Note, however, that the above
> > conclusions have not been tested.
> >
> > Otherwise, it appears that things should behave fair well as the
> > PersistentLinkerState is already wrapped in an MVar which should
> > serve to serialize linker calls.
> >
> > Cheers,
> >
> > - Ben
> >
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