Thanks for the Feedback, here are my thoughts on the raised points.
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.
I do actually agree with that - boost.iostreams has an obsolete design. But using it's file_descriptors and streams makes it so much easier for now, which would be a lot of reimplementation. So as long as boost.iostreams is not marked as obsolete, I wouldn't think it's necessary to do so.
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.
Actually I did not look at boost.afio, which might be a good idea.
Currently you only have the async_pipe representation, which wraps
around a either ordinary posix-pipe or a named pipe on windows.
Now I do not think, that I need to make everything async, because I can
think of enough scenarios where this is a complete overkill. For
example, I might just want to pipe from one process to another:
pipe p;
auto c1 = execute("e1", std_out>p);
auto c2 = execute("e2", std_in 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. I don't get what you want to tell me here, sry. Thing is: I need a
custom object, beacuse the current asio-objets
(windows::stream_handle/posix::stream_descriptor) are bidirectional for
one handle, but I need an object that writes on one and reads on the other. 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. If I understand what you say here correctly, that this is what I
originally planned. But I decided that against that, because it is much
easier the current way and you can store all needed information in a
simple child. And actually you can detach the child. 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. Well again: that would require the usage of asio all the time, I don't
think that's very sensible. Currently you can just write a simple
process call and wait for it, and it's as simple as it gets:
execute("something");
Also I need to have a callback on_exit to cancel the read operations on
async pipes. I thought about doing this automatically, but then again
one could use one pipe with several subprocesses.
BUT: allowing a std::future<int> to be set on exit seems like a neat
feature, I should add that. 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. Ok now I am confused: you want me to make everything asio, so that
you'll always need boost.asio, but you want to eliminate the dependency
on boost.fusion? Why? That's definitely not going to happen, since I do
a lot of meta-programming there, this would become too much work for a
library which clearly needs other boost libraries. The only way I see
that happening is, when I propose a similar library for the C++
Standard, and that won't happen very soon... 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". Uhm, what so is this a bad thing, to use boost.asio that much? I don't
really get your point here. 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. Well I think we have different philosophies on what the library should
do. If you want a pure async implementation of boost.process, one could
maybe built it atop boost.process and call it apio or something.
The reasons I integrated the asio stuff are the following:
- on windows you need named pipes for async stuff (should be
distinguished be the pipe library)
- notification of on_exit must be integrated, so it is launched
immediatly after executing the subprocess.
- io_service must get notified on fork
If that weren't the case I would prefere boost.process to be much
simpler, and have not async functionality whatsoever. Though
theoretically the whole pipe-functionality could be moved into another
library, which is just used by boost.process. Hope this is helpful. Sure it was, thank you. I hope my reation doesn't seem to stubborn. 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. I really don't think this is the case: boost.process should first of all
be a wrapper around the syscalls, so I can use it on different platforms
without and #ifdefs.
The async stuff is second, and is a set of features, extending it. At
least that's the reason I need a process library, which is the reason
I'm working on that.