On Mon, 10 Apr 2023, 08:57 Andrey Semashev via Std-Discussion, <std-discussion@lists.isocpp.org> wrote:
On 4/10/23 11:42, Edward Catmur via Std-Discussion wrote:
>
>
> On Sun, 9 Apr 2023 at 11:35, Andrey Semashev via Std-Discussion
> <std-discussion@lists.isocpp.org
> <mailto:std-discussion@lists.isocpp.org>> wrote:
>
>     On 4/9/23 06:18, Edward Catmur via Std-Discussion wrote:
>     >
>     > On Sat, 8 Apr 2023 at 19:47, Andrey Semashev via Std-Discussion
>     > <std-discussion@lists.isocpp.org
>     <mailto:std-discussion@lists.isocpp.org>
>     > <mailto:std-discussion@lists.isocpp.org
>     <mailto:std-discussion@lists.isocpp.org>>> wrote:
>     >
>     >     On 4/9/23 02:23, Edward Catmur wrote:
>     >     >
>     >     > Hm, there's something I'm failing to understand here. Let's
>     say a
>     >     > coroutine is constructed and its frame constructs a
>     scope_failure
>     >     object
>     >     > while some number - say 3 - exceptions are in flight. The
>     coroutine is
>     >     > then suspended, and one of the exceptions is cleaned up,
>     leaving 2
>     >     > exceptions in flight. The coroutine is then resumed, and
>     throws an
>     >     > exception out, so that again there are 3 exceptions in flight.
>     >     Wouldn't
>     >     > that result in the uncaught exception count comparison
>     incorrectly
>     >     > returning true?
>     >
>     >     That's the point of saving the unhandled exception counter to the
>     >     coroutine state. When the coroutine is resumed,
>     >     co_unhandled_exceptions() would still return 3, so long as no
>     exception
>     >     is thrown from it. When the exception is thrown, it would
>     increment the
>     >     coroutine-specific counter to 4, which would allow scope_fail
>     to detect
>     >     the exception. When the exception propagates to the caller and
>     then
>     >     rethrown, then the caller's counter is incremented from 2 to 3
>     before
>     >     the stack unwinding proceeds.
>     >
>     > Thanks, I think I understand. But how would throwing an exception
>     > increment that counter? Are we talking extra per-thread state,
>     possibly
>     > a thread-local linked list of coroutines (bear in mind that there
>     can be
>     > multiple active coroutine frames on a thread, if they call each other
>     > rather than awaiting) or is the unwinder detecting coroutine frames to
>     > find the corresponding counter?
>
>     It doesn't really matter, whatever approach is suitable to compiler
>     implementers would be fine.
>
>
> It does matter, because a lot hinges on whether this is implementable
> efficiently. If the overhead is updating a thread-local linked list
> whenever a coroutine is resumed or suspended - even if they don't use
> this facility - I don't think that would be acceptable.

I don't think that updating a pointer during a context switch would be a
deal breaker. The co_unhandled_exceptions() mechanism only needs to know
the current coroutine state of the current thread, it doesn't need a
list. If a linked list is needed to be able to return from one coroutine
to another then such a list must be already in place.

Now that I think of it, co_unhandled_exceptions() might not require any
new pointers. The coroutine already has to keep a pointer to its state
somewhere to be able to reference its local variables. I don't see why
co_unhandled_exceptions() couldn't use that pointer.

But how can co_unhandled_exceptions() *find* that pointer? The current coroutine state is not stored in per thread memory; it's solely on the stack. (The linked list that's "already in place" is the stack itself.)

That said, you might be able to find it when unwinding through a coroutine stack frame. I'm still thinking about it, but it seems like it should work. In that case you've only got overhead added to coroutine creation, coroutine footprint, unwinding through a coroutine, and you've doubled the stack footprint and memory access of the success/failure scope guards, compared to a C++17 implementation using unhandled_exceptions only. 

But doesn't adding the information for co_unhandled_exceptions() to coroutine state break abi? 

>     > Can this be implemented without breaking
>     > backwards compatibility (linking new object files against old
>     runtime)?
>
>     I don't think linking a new object against an older runtime (meaning the
>     runtime that is older than the headers that were used by the object) is
>     really an option. There's no problem with newer objects requiring a
>     newer runtime.
>
> It's not a problem per se, but it is a barrier to adoption.

No, because that's the status quo. At least with some major compilers.

Even with those compilers, it's usually possible to use some new features with the old runtime. Anyway, I guess it's fine if this isn't one of those features. I'm still worried about breaking coroutine abi, though. 

>     > For example:
>     >
>     > void f(bool b) {
>     >   unique_ptr<scope_fail> psf1;
>     >   {
>     >     unique_ptr<scope_fail> psf2;
>     >     try {
>     >       unique_ptr<scope_fail> psf = ...;
>     >       scope_success ss([&] { psf1 = std::move(psf); });
>     >       scope_fail sf([&] { psf2 = std::move(psf); });
>     >       if (b)
>     >         throw 1;
>     >     } catch (...) {
>     >     }
>     >   }
>     >   throw 2;
>     > }
>     >
>     > If `b` is true, the dynamic lifetime scope_fail is transferred to
>     psf2,
>     > so its destruction is immediately caused by the
>     > nonexceptional destruction of psf2, but that was only possible because
>     > of the exceptional destruction of sf, so which counts as "causing" the
>     > destruction?
>
>     psf2 destruction is not caused by an exception, so the moved scope_fail
>     guard should not execute its action. It doesn't matter how the program
>     arrived to this state.
>
> And *psf2 destruction is not caused by an exception; it's caused by the
> destructor of psf2 calling `delete` on a pointer. Why is the causation
> transitive through the destructor of psf2 but not through the action of
> sf transferring *psf to psf2?

Why would psf2 destruction depend on anything that happened in the past?
I think you're stretching the "causation" term way too thin. You could
say that everything happening in the program was caused by user hitting
Enter when he ran the program, but that wouldn't be useful for the sake
of the discussion re. scope guards.

Right. But in that case I think your concept of causation is also too indirect. 

When I say "the destruction of the scope guard is caused by a stack
unwinding procedure" I mean literally that the stack unwinding procedure
is present higher up in the call stack from the scope guard destructor.
I'm choosing this criteria because that's what makes most sense to me in
relation to the expected behavior of the scope guards.

That doesn't work. Having an unwind caused destructor above in the stack from a scope guard destructor does not always mean that the scope guard is in a failure state. It feels like we're going round in circles here. 

>     >     The answer to that is
>     >     pretty clear, as the abstract machine is either in the process of
>     >     unwinding the stack, or it isn't; there is no third option, AFAIK.
>     >
>     > But you can do anything you like while stack unwinding is
>     occurring; you
>     > can call whatever code you like from a destructor, as long as you
>     don't
>     > leak another exception out. So stack unwinding is a necessary but not
>     > sufficient condition.
>
>     Yes, you can run arbitrary code during stack unwinding, but that is not
>     the default. Most of the code that is run in destructors is aimed at
>     destroying objects and freeing resources. Indeed, this code is rather
>     special as it is not allowed to fail or return values. There are
>     exceptions to this, of course, and scope guards are a good example of
>     that. Scope guard actions can be considered as "arbitrary code" that is
>     not necessarily destroying anything. I'd even say, the fact that this
>     code is called in the context of a destructor is a hack, an abuse of the
>     core language feature to ensure that a piece of code is called upon
>     leaving, no matter what. Other languages provide constructs distinct
>     from destructors for this purpose (e.g. `finally` blocks in Java). In a
>     perfect world, I would prefer to have scope guard-like functionality in
>     the core language (but not in the form of try/finally from Java).
>
> Even if we had scope guard in the core language, its actions would be
> executed during stack unwinding, no?

Yes, but a core language feature could have better syntax and better
defined semantics as it wouldn't have to rely on unhandled_eceptions().

As could a library feature with compiler support, and that wouldn't require new syntax. 

>     Anyway, first, let's consider only the "destroying code" that is typical
>     and intended for destructors. The fact whether the code is ultimately
>     being called by runtime unwinding the stack is unambiguous and is a good
>     criteria for detecting failure for the purpose of scope_fail. That is,
>     it doesn't matter, whether scope_fail is allocated as a complete object
>     on the stack, or dynamically allocated and pointed to by unique_ptr, or
>     a subobject of a class that is an element of a container, or whatever
>     else - if its destruction is directly or indirectly called by the stack
>     unwinding procedure then the scope guard can assume failure.
>
> Sorry, but no. It can't assume failure, since the scope could have been
> entered when stack unwinding was already in progress.
>
> Let's consider the archetypal example: a scope guard that protects a
> database transaction, committing it on success and cancelling it on
> failure. It is entirely legitimate to open a database transaction during
> stack unwinding - saving work in progress, for example. It would be very
> much untoward if the scope guard were to perform the wrong action in
> that case. This is why C++17 replaced uncaught_exception() with
> uncaught_exceptions().

I'm explicitly talking about "destroying code" in this paragraph, and
initiating a database transaction doesn't count as one, IMO. 

Ah, I understand now, sorry for being slow. I think you're right in as much as in my experience it's very rare for scope guards used in resource freeing code (in the RAII sense) to care whether the scope exit event was normal or exceptional. Still, I wouldn't want to rule it out entirely. 

>     Now let's consider the "arbitrary code" that is run during stack
>     unwinding. For the sake of simplicity, let's assume this code is a scope
>     guard action, but the same argument can be made regardless of the method
>     you use to get such code to run. For such arbitrary code, it doesn't
>     make sense to indicate whether it is run as a result of an exception.
>     For example:
>
>       try
>       {
>         scope_fail sg1{[]
>         {
>           scope_fail sg2{[]
>           {
>             std::cout << "2" << std::endl;
>           }};
>
>           std::cout << "1" << std::endl;
>         }};
>
>         throw 1;
>       }
>       catch (...) {}
>
>     Here, it would be reasonable to expect "1" to be printed, but not "2",
>     because while the outmost code has failed with an exception, the scope
>     guard sg1 action hasn't, so sg2 is not supposed to trigger. In other
>     words, for a scope guard action, the fact that the scope guard's
>     destructor is called due to an exception must be concealed.
>
> Agreed.
>
>     Currently, the language does not offer a way to differentiate the
>     "destroying code" and "arbitrary code", and I don't see a way for the
>     compiler to infer this information from the code. Which means the code
>     will probably need to express this information explicitly.
>
> Prior to coroutines, it was possible to infer this from
> uncaught_exceptions(). If at construction of sg1 uncaught_exceptions()
> was n, then at the destruction of sg1 it would be n+1, whereas at the
> construction and destruction of sg2 it would be n+1 both times.

Of course.

>     Furthermore,
>     there needs to be a way to query whether the code is being called as a
>     result of stack unwinding, and if the code indicates that it wants to
>     act as the "arbitrary code" that query needs to return false. For
>     example, a scope_fail destructor could look something like this then:
>
>       ~scope_fail()
>       {
>         bool& is_exception = std::is_exception_unwinding();
>         if (is_exception)
>         {
>           bool was_exception = std::exchange(is_exception, false);
>           m_action();
>           is_exception = was_exception;
>         }
>       }
>
>
> Shouldn't you be using a scope guard to restore is_exception? :p

I was simplifying this pseudocode for brevity. I could have posted a
code with a scope guard, and a noexcept specification to boot, but that
would take more time and space in an already long post.

And this is truly just a pseudocode to illustrate the idea. The actual
implementation might not have is_exception_unwinding() function but
something else, with a similar effect.

> It's an interesting idea, but I think I can see some issues. Firstly,
> when is is_exception_unwinding() reset to false? It can't be necessarily
> on a catch, since there could be an outer exception unwinding in
> process. I suppose you could consult uncaught_exceptions().

If we don't consider the ability for the user's code to set it to false,
is_exception_unwinding() would return true if co_uncaught_exceptions() >
0. I.e., the flag is set to true by the runtime when an exception is
thrown and reset to false when no exceptions are pending.

When we take modification into the picture, it gets more complicated.
Essentially, when user's code sets the flag to false, it means that any
uncaught exceptions are concealed from the following code. The first
exception thrown by the following code will set the flag to true again,
and catching that exception will reset it to false.

Perhaps, it would make more sense to just allow modifying the uncaught
exceptions counter. Then the implementation would look like this:

  ~scope_fail()
  {
    int count = co_uncaught_exceptions();
    if (count > m_original_count)
    {
      co_set_uncaught_exceptions(0);
      scope_exit guard([&]
      {
        co_set_uncaught_exceptions(
          co_uncaught_exceptions() + count);
      });
      m_action();
    }
  }

Here, m_original_count would be initialized from
co_uncaught_exceptions() on scope guard construction. We could assume it
to be always zero in a program that consistently sets it the uncaught
exceptions count to zero in scope guard-like classes' destructors, like
above. But to be on the safe side, capturing the value on construction
also works.

> More
> importantly, it looks like it would be difficult to get adoption, since
> libraries won't be written to use the facility until it's present, but
> if some use the facility and others don't you would have a problem:
> anyone using an existing scope guard library (or just themselves calling
> code from destructors) would find that your scope_fail thinks that there
> is a failure condition when they would consider there not to be, because
> they haven't cleared the is_exception flag. 

This would be fixed if we allowed to modify the uncaught exceptions
counter, like above.

>     I don't think coupling destructors with storage types and discriminating
>     complete objects and subobjects is useful because I don't think these
>     properties should influence scope guards behavior.
>
> But if that were necessary to being able to implement them at all? 

I don't think that should be necessary.

Well, yes; you've suggested an implementation technique that allows success and failure scope guards as subobjects and in dynamic storage. I agree that it looks like it should work, but the overhead to code that does not use those scope guards, in runtime (and not just in the exceptional case), storage and possible abi break looks to me to be excessive. 

I've suggested an implementation technique that should be highly performant, but at the expense of being able to use success and failure scope guards anywhere other than as complete automatic objects. I'm not saying this technique has to be used, but I would suggest that it should be permitted by specifying the behavior only in the case the scope guard destructor is invoked directly from normal scope exit or directly from unwinding. What happens in other cases would be implementation defined. 

>     I would say, using scope guards in contexts other than a complete
>     automatic objects is not forbidden currently, and I don't see why it
>     shouldn't work.
>
> Because we don't know how to implement them otherwise. Using dynamic
> information was fine before coroutines, but now there doesn't appear to
> be a dynamic implementation technique that won't impose  considerable
> overhead or require simultaneous mass adoption.
>
> Whereas using static information (whether the destructor call of a
> complete automatic object was emitted for normal scope exit or for
> unwinding) will always give predictable answers, surely has negligible
> overhead, and the downsides (being able to use scope_success and
> scope_failure only as complete automatic objects) may not actually matter. 

At this point, if that's our only option, I would rather have a
dedicated core language feature for scope guards.

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