Subject: Re: [boost] [boost.process] 0.6 Redesign
From: Niall Douglas (s_sourceforge_at_[hidden])
Date: 2016-04-18 17:49:44
On 18 Apr 2016 at 17:35, Klemens Morgenstern wrote:
> At the current state, the tests pass on linux as well as windows (gcc-5
> & MSVC-14). Requirements are C++, boost.fusion, boost.asio,
> boost.iostreams, boost.filesystem and boost.system.
>
> I really could use some feedback and hope you're interested.
Firstly, well done for working on this. Last contract I got annoyed
with Boost.Process and ended up replacing it with a hacked shoe in
based on ASIO. More recently, I needed a child process spawner for my
ACCU presentation I'm giving this week and ended up again reinventing
Boost.Process, only this edition is a "bare metal" reinvention based
on Outcomes [1]. So I'm all for this in principle.
However I'm not sure if I'm for your specific formulation. Here are
my top issues:
1. You should rip out all usage of Boost.Iostreams. It's been without
maintainer for years now, and has a fundamentally broken design for
async i/o. Nobody should be encouraged to use it in new code.
2. You should completely eliminate all synchronous i/o as that is
also fundamentally broken for speaking to child processes. Everything
needs to be async 100% of the time, it's the only sane design choice
[2]. You can absolutely present publicly a device which appears to
quack and waddle like a synchronous pipe for compatibility purposes,
indeed AFIO v2 presents asynchronous i/o devices as synchronous ones
if you use the synchronous APIs even though underneath the i/o
service's run() loop gets pumped during i/o blocks. But underneath it
needs to be 100% async, and therefore probably ASIO.
3. Instead of inventing your own i/o objects, I think you need to
provide:
(a) Async and sync objects extending ASIO's base objects with
child_stdin, child_stdout and child_stderr - or whatever your
preferred naming.
(b) std::istream and std::ostream wrappers. These are not hard, ASIO
helps you a lot with these once you have the native ASIO objects.
4. Child processes are not like threads and should not be represented
as a first order object. They should instead be an opaque object
represented by an abstract base class publicly and managed by a RAII
managing class from whom the opaque object can be detached, assigned,
transferred etc.
5. Replace all the on_exit() machinery with future continuations i.e.
launching a process *always* returns a future. If someone wants to
hook code onto when the process exits, a future continuation is the
right tool. Similarly for fetching return codes, or detaching oneself
from the child. Python's new subprocess.run() returns exactly the
struct your future also needs to return.
6. Looking through your source code, I see references to
boost::fusion and lots of other stuff. Great, but most people wanting
a Process management library don't want to drag in a copy of Boost to
get one. It's easier to just roll your own. So drop the Boost
dependency.
7. Looking through your source code, I am struck about how much
functionality is done elsewhere by other libraries, especially ASIO.
I think less is more for Boost.Process, I always personally greatly
preferred the pre-peer-review Boost.Process even with its warts over
the post-peer-review one which had become too "flowery" and "ornate"
if that makes sense. The latter became unintuitive to program
against, I kept having to look up the documentation and that annoys
me. This stuff should be blindingly obvious to use. It should "just
work".
I conclude my mini-review by suggesting "less is more" for
Boost.Process. 99% of users want the absolute *minimum* featureset.
Look at Python 3.5's new subprocess module, that is a very good API
design and featureset to follow. It's intuitive, it gets the job done
quickly, but it exposes enough depth if you really need it to write a
really custom solution. I'd *strongly* recommend you copy that API
design for Boost.Python and dispense with the current API design
entirely. The absolute clincher in Python's subprocess is you can
never, ever race nor deadlock stdout and stderr. That makes an
underlying async i/o implementation unavoidable. I'd personally
suggest save yourself a ton of hassle and use ASIO's pipe/unix socket
support facilities, it's becoming the Networking TS anyway.
Hope this is helpful.
Niall
[1]:
https://github.com/ned14/boost.afio/blob/master/include/boost/afio/v2/
detail/child_process.hpp
[2]: I refer to the stdout/stderr deadlock problem which is the
biggest reason anyone reaches for a process management library
instead of just using the
syscalls directy. The internals of the child i/o needs to be 100%
async to
prevent deadlocking. You can absolutely present publicly a device
which appears
to quack and waddle like a synchronous pipe for compatibility
purposes, indeed AFIO v2 presents asynchronous i/o devices as
synchronous ones if you use the synchronous APIs even though
underneath the i/o service's run() loop gets pumped during i/o
blocks.
-- ned Productions Limited Consulting http://www.nedproductions.biz/ http://ie.linkedin.com/in/nialldouglas/