Re: Concurrency via isolated process/thread

[Ihor Radchenko]

Eli Zaretskii <eliz@gnu.org> writes:

>> > I don't think you can design on this base.  Security and all that.
>> 
>> But why is it a problem?
>
> I'm not an expert, but AFAIK reading from, and writing to, the memory
> of another process is something allowed basically only for debuggers.
>
> And how would you know the address in another process anyway, given
> today's ASLR techniques?

I am looking at
https://stackoverflow.com/questions/5656530/how-to-use-shared-memory-with-linux-in-c

AFAIU, it is possible to create shared memory only readable by child
processes.

Then, exchanging data between the two Emacs processes may be done using
memcpy to/from shared memory.

It may be dumb (I have no experience with processes in C), but I have
something like the following in mind:

1. Main Emacs process has a normal Elisp thread that watches for async
   Emacs process requests.
2. Once a request arrives, asking to get/modify main Emacs process data,
   the request is fulfilled synchronously and signaled back by writing
   to memory accessible by the async process.

>> > Also, complex structures include pointers and references, which you
>> > cannot safely copy as-is anyway.
>> 
>> May you please elaborate?
>
> For example, a list whose member is a string includes a pointer to
> that string's data.

I imagine that instead of trying to copy Lisp objects recursively, there
will need to be a special "remote" Lisp object type. Getting/setting its
value will involve talking to other Emacs process.

>> > I wish this were the only problem with threads.
>> 
>> Maybe. But I haven't seen other problems preventing threads from being used.
>
> I have, too many of them.  Some are semi-fixed, but I'm afraid we only
> don't hear about them because threads are not used in serious,
> production-quality programs.

You are talking about bugs? If nobody goes far enough to discover those,
they are probably not the real reason why people do not use Elisp threads.

-- 
Ihor Radchenko // yantar92,
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