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Subject: Re: [boost] [process] Formal Review starts today, 27 October
From: Lee Clagett (forum_at_[hidden])
Date: 2016-11-09 21:50:24

On Wed, 9 Nov 2016 14:50:19 +0100
Klemens Morgenstern <klemens.morgenstern_at_[hidden]> wrote:

> [...]
> [...]
> > Some properties do things before and after the process is launched.
> > Others try to manipulate the implementation defined `executor`
> > object. Then another (`shell`) changes what process is launched via
> > argument manipulation. Does it make sense for all of these to be
> > taken in the same variadic argument (especially the last case)?
> Yes
> > I know there is some difficulty because the Posix fork + exec design
> > does not work the same as way as the Windows API. But I think there
> > is some value in specifying some basic concept of an `executor` and
> > its relationship with a `property`. For instance, the executor will
> > have some function (call operator) that starts a new process using
> > internally stored string arguments and returns a handle to the new
> > process. The string arguments should arguably be non-modifiable
> > once the executor is created, etc.
> The string are only set after the executor is created, I don't know
> how that would work. Also the flags need to be accumulated through
> disjunction, I don't see how it would be better to this on
> construction time, and not in on_setup. It's internal, so why bother
> with a strange workaround there?

What I noticed in the source is that some of the properties require
special logic because they are manipulating other properties instead of
the "executor". For example, the `shell` property is manipulating the
path and arg properties but nothing else.

Earlier I mentioned on working out a loose way of specifying a
`property`. I think such a specification would not consider properties
that only manipulate other properties to be a property (since it could
do that separately from the `child` constructor).

> [...]
> [...]
> > I was asking whether you could store a handle to the `group` in some
> > object whose call operator starts the process (like Niall originally
> > mentioned). So the handle would be present, but potentially invalid
> > depending on how it was implemented.
> Yes it could, but I stated enought times why this would be a horrible
> idea.

Yes, and I disagreed with your analysis. I recall the primary objection
was poor performance - specifically that space for each type of possible
property was needed. This is not true, the binding approach (at least
how I saw it), would return a new callable type after each binding. In
other words, the binding was conceptually calling `push_back` on a
fusion like tuple. Moves + ref-qualifiers meant that dynamic memory
would not copied in many cases. So
`launcher("foo").group(...).stdout(...)` would return the equivalent of
`tuple<launcher, group, stdout>` while moving `std::string` memory as

After thinking about it, all that I (and I think Bjorn and Niall) were
trying to accomplish is separating the path and arguments from the
remainder of the properties. If the path and arguments were in a single
property, `shell` and `cmd` could return that property instead of
requiring custom logic internally.

> >
> [...]
> [...]
> [...]
> > `launch` returns a callable that accepts 0 or more `properties` to
> > launch the process. Example:
> >
> > class launcher {
> > std::string exe_;
> > std::vector<std::string> args_;
> > public:
> > launcher(std::string exe, std::vector<std::string> args)
> > : exe_(std::move(exe)), args_(std::move(args)) {
> > }
> >
> > template<typename... Props>
> > child operator()(Props&&... props) const {
> > return detail::exec(exe_, args_,
> > std::forward<Props>(props)...); }
> > };
> > struct launch_ {
> > template<typename... Args>
> > launcher operator()(std::string exe, Args&&... args) const {
> > return {std::move(exe), {std::forward<Args>(args)...}};
> > }
> > };
> > constexpr launch_ launch{};
> >
> > // syntax:
> > // launch("ls", "-la", "/")(stdout(my_pipe));
> Yeah, that still doesn't work. When do you launcht process? Please
> think those Ideas through, if you want me to take them serious. If
> you launch at the call of "launch("ls", "-la", "/") than you couldn't
> bind stdout. If you launch after the binding of stdout, how would you
> append another paremeter? It's broken.

How is it broken? Look at the code snippet, the second function call
takes a variadic argument list like the `child` constructor. The string
arguments are the only thing "bound" to an object. So appending another
property would be done like:

  launch("ls, "-la", "/")(stdout(my_pipe1), stdin(my_pipe2));

That style is simply separating string arguments into a separate
variadic function call. Or another way of looking at it - there is a
single `property` for specifying the path and args that has a fixed
positional location.

> > I don't see a good reason for the `child` class to actually launch
> > the process. A global or static function that returns a `child`
> > object should do it instead. This would remove the need for the
> > `shell` property, which would be a distinct function.
> Well we had a global function for that in previous versions, but that
> wasn't like that much; especially since people got reminded of
> std::async. I have to agree, it's not the common C++ idiom to use
> functions to construct object, you also don't write
> std::thread t = std::launch_thread([]{});
> Also this would be bad, and probably a common mistake. It would
> launch the process and immediatly terminate it.

Why would it immediately terminate the process? Or do you mean a
non-joined `child` should terminate the process like `std::thread`?
Admittedly, I did forget about that case - its probably best to follow
`std::thread` closely and terminate the current process if not `join`ed
or `detach`ed. I don't there is anything wrong with a function
returning a `child`, provided the behavior of ignoring the `child`
is well documented and defined. However, there is little difference to
the code snippet above and:

  bp::child c1(bp::exec("ls", "-la", "/"), bp::stdout(my_pipe1));
  bp::child c2(bp::shell("ls", "-la", "/"), bp::stdout(my_pipe2));
  bp::child c3("ls", {"-la, "/"}, bp::stdout(my_pipe3));

Where the constructor in `c3` forwards to the constructor in `c1`; `c2`
uses the `bp::exec` object returned by `bp::shell`; `c1` uses the
`bp::exec` property to set the path and args of the executor.

> bp::launch("foo");
> Secondly, I'm not sure why shell would be a distrinct function - that
> doesn make sense, you can use it in all three launch-functions. A
> shell ist just another process, why would I handly it any different?

This should already be covered, but the shell function would just
return an object storing the path and arguments.

> > Also, is there an advantage to allowing string arguments anywhere in
> > the function call? A single `std::vector<std::string>` argument can
> > be initialized implicitly with a `{...}`. This would easily
> > separate the arguments from the rest of the properties without the
> > lazy binding provided above:
> >
> > shell(std::string cmd, Props&&...)
> > launch(std::string exe, std::vector<std::string> args,
> > Props&&...)
> >
> > Which can be invoked like:
> >
> > bp::shell("ls -la /", bp::stdout(my_pipe));
> > bp::launch("ls", {"-la", "/"}, bp::stdout(my_pipe));
> > bp::launch("foo", {}, bp::stdout(my_pipe));
> >
> > Is requiring a `{}` pair less desirable than the variadic syntax?
> Yes there is, bp::system("gcc", "main.cpp", "-o", "main.o"); looks
> awesome. Also keep in mind, that you have the cmd-syntax, which is a
> single string and that you can path a boost::filesystem path. What
> you are basically proposing are positional arguments, which might
> make sense for exe/args or cmd, but not for the test of that. There
> is no logical order of the other arguments and secondly it is
> variadic to allow the user to skip some arguments.

Requiring a `{}` hardly seems detrimental to me, but I guess people
have different tastes. I hope you at least consider positional
arguments for the path and args. For the remainder - I agree the
variadic constructor is probably preferable since its behavior can be
closer to std::thread.

> Also I don't think arguments are any different from the other
> properties, or at least I don't consider them to be. It's a value
> that determines how a process behaves that I pass at launch, so why
> would that be any different from the environment? And yes, args args
> are optional.

I agree, its just that I do not think `cmd` and `shell` are like the
other properties. I also think there is benefit to having a single
property for path and args that is taken positionally instead of
anywhere in the argument list.

> Since you're coming up with so many ideas, I would encourage you to
> start your own library, which would allow us to talk about some tests
> way of doing things. Currently you're only
> Feel free to fork mine or the older version from Boris. That might be
> a good proof-of-concept.


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