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Subject: Re: [boost] [system] Would it be possible to trial a breaking change to Boost.System and see what happens?
From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2018-01-13 13:09:19

On 01/13/18 02:58, Niall Douglas via Boost wrote:
>>> 1. Stop treating code zero as always meaning success irrespective of
>>> category. This remedies a defect where some custom error code domains
>>> cannot be represented by error_code due to using value 0 for an error.
>>> In actual code, I've never seen anyone ever use comparison to anything
>>> but a default constructed error code, so I think this will be safe.
>> In my code I'm using contextual conversion to bool extensively, e.g. "if
>> (err)" or "if (!err)". This is currently equivalent to comparing against
>> 0, so I assume it will be broken by this change. So, you can count me
>> right now as the one (loudly) complaining, among many others, I think.
> Sorry, I forgot to mention that the category would be asked whether the
> error code's current value is success or failure (its's Friday, and my
> current contract is many hours drive from home. Sorry)
> This is unavoidable because system_category (on POSIX and Win32) and
> generic_category both define value = 0 as success. In some other error
> code domain, -1 might mean success. Or 9999. Up to the domain. No
> requirement that 0 be set aside as special, as currently.

I appreciate that the special meaning of 0 may be undesirable, but
having a virtual function call overhead whenever one wants to test if
the error_code has a success value seems a too high price for that. I
don't want to be paying that price in my Linux programs, sorry.

>>> 2. Success becomes solely the default constructed error code, which is
>>> code zero and a new category of "null_category". This is internally
>>> represented by { 0, nullptr }, and thus makes the default constructor
>>> trivial which is highly desirable as it eliminates any compiler magic
>>> static fencing for the default constructor. Right now the default
>>> constructor uses system_category with value 0, and I suspect no code
>>> will notice this change either.
>> This breaks "category()" as it returns a reference now.
> It would still return a reference, just as now. You would check for
> internal nullptr, if so return a static null_category.

Why check in the first place if you could initialize the pointer to the
global instance of the category?

>> Also, "message()" has to check for a null category now.
> I don't think that much cost relative to constructing a std::string.

True, but it looks like an unnecessary cost nevertheless. (Note that a
small string construction is very cheap and can fit in a few
instructions, including the string contents initialization.)

>>> 3, Make the default constructor constexpr, now possible. No code should
>>> notice a difference, except less runtime code will be generated.
>> Not sure there will be any difference in the generated code, since the
>> constructor still has to initialize the int and the pointer. Optimizers
>> are already able to propagate constants even if the code is not constexpr.
> But it cannot elide the potential call to an extern function, the
> function which retrieves the error_category&. And therefore must emit
> code, just in case the system_category might be fetched.

I don't think the current standard requires an external function call.
The default constructor can obtain the pointer to the global category
instance directly, if it is exported. `system_category()` can be an
inline function as well.

> The present design of error_code generates code bloat when used by
> Outcome or Expected. Changing it as I describe stops forcing the
> compiler to emit code, and thus allows the compiler to fold more code
> during optimisation, thus emitting less assembler. Tighter, less bloaty
> code is the result.

I assume, you mean the code that calls `system_category()` in runtime? I
think, this is a quesion of QoI that can be solved as I described above
without the need to break users' code or adding runtime overhead.

>>> 4. error_code::message() returns a std::string_view instead of
>>> std::string if running C++ 17 or better. This lets us remove the
>>> <string> dependency, and thus stop dragging in half the STL with
>>> <system_error> which in turn makes <system_error> actually useful to the
>>> embedded systems crowd. The guess here is that std::string_view has such
>>> excellent interoperation with std::string that I think 99% of code will
>>> compile and work perfectly without anybody noticing a thing.
>> This will only work if "error_category::message()" returns a string from
>> a static storage. It will not allow relying on strerror_r or similar
>> API, for example. This will make migration problematic if users define
>> their own error categories that rely on external API like that.
> You are correct that the no 4 change is by far the most consequential
> proposed. I do not believe it affects end user code except for those who
> inherit from error_code and override message() - surely a very, very
> tiny number of people - but it sure as hell breaks everybody who
> implements a custom error code category if you change the category's
> message() signature.

I don't have an estimate on how widespread custom error categories are,
but I surely have a few. I know that several Boost libraries define
their categories (namely, Boost.Filesystem, Boost.ASIO, Boost.Beast,
Boost.Thread come to mind). Searching on GitHub[1] finds a few examples
of custom categories (with a lot of noise from the comments though).
Judging by that, I wouldn't say that custom categories are rare.


> The number of people worldwide currently implementing their own error
> categories is likely low, and me personally speaking feel that their
> mild inconvenience is worth losing <string> from the header
> dependencies. But if backwards compatibility were felt to be super
> important, in error_code::message() you could try dynamic_cast to an
> error_category2 first in order to utilise an
> error_category2::message_view() function. If the dynamic_cast fails, you
> statically cache the strings returned by error_category::message() and
> return string_views of them.

The `dynamic_cast` is yet more expensive than a null check, and it may
be more expensive than constructing a small `std::string`. It may also
be problematic on the embedded systems, which often have RTTI disabled.
Caching the string in a static storage means thread safety issues, which
will probably require TLS. This is a lot of complication and runtime
overhead just to remove one #include.

> This is an API breaking change being discussed. Code _might_ break
> beyond custom error categories.
> But I currently believe that code other than custom error categories
> _won't_ break. We are changing the API in ways that I would be very,
> very surprised if much real world code out there even notices.

Code that relies on a specific error category in the default-constructed
`error_code` will break.

> Finally, Boost has broken API much more severely than this in the past
> as part of natural evolution (not always intentionally). If it's
> preannounced that we're doing this, and we do it, I don't see a problem
> here. Plenty of old APIs have been retired before. And besides, this is
> for testing a proposed change to a future C++ standard. That's a great
> reason to break API, if you're going to do it.

I don't think I remember an intentional major breakage of API without a
deprecation period and a clear migration path. Such breakages are
typically introduced as new libraries that live alongside with the old ones.

Some parts of your proposal (namely, switching to `string_view`) do not
have a clear migration path, especially for C++03 users. Other parts
seem to be targeted at a rather niche use case but have high associated
overhead that is going to be applied to all uses of `error_code`. And,
on top of that, users' code may break, including silently. Sorry, but to
me this doesn't look like a safe change that can be done to a widely
used core library. Boost.System2, that will live side by side with the
current Boost.System, seems more appropriate. I realize that you won't
know how much of the world will be affected by the proposed changes.

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