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From Greg Stein <gst...@gmail.com>
Subject Re: [serf-dev] [serf] r2489 committed - In preparation of serf 1.4.0, remove the get_remaining function from t...
Date Wed, 07 Oct 2015 21:48:16 GMT
On Wed, Oct 7, 2015 at 9:46 AM, Bert Huijben <bert@qqmail.nl> wrote:

> > -----Original Message-----
> > From: Greg Stein [mailto:gstein@gmail.com]
> > Sent: zaterdag 12 september 2015 14:09
> > To: dev@serf.apache.org
> > Subject: Re: [serf-dev] [serf] r2489 committed - In preparation of serf
> 1.4.0,
> > remove the get_remaining function from t...
> >
> > [redirected to dev@serf]
> >
> > On Mon, Apr 6, 2015 at 8:32 AM, Bert Huijben <bert@qqmail.nl> wrote:
> >
> <snip>
>
> > So you're saying that a pointer to a specific function might have *two*
> > values under Windows? Within a single executable? That A could see a
> > different value from B?
>
> Yes, see the other mail explaining the cases when you can see this.
> (Proxy functions calling into shared libraries, etc.)
>

Gotcha.


>
> And I even found a case where two functions with different names but
> identical behavior returned the same value. (When using link time code
> generation; part of full program optimization)
>

Heh. I wonder if it knows function pointer comparisons occur, and will
avoid that optimization?


>
>
> > Assuming that is the case (or we wouldn't be in this situation), then
> let's
> > change how v2 is API-compatibly detected. Again, let's use read_bucket()
> > since it gives us a lot of flexibility and nobody has ever used it. Let's
> > define a "V2_API_BUCKET" type and then ask for bucket FOO to return one
> > of
> > those. If FOO's type has a NULL read_bucket pointer, or returns NULL for
> > that bucket type, then it is V1. If FOO's type has been updated, then it
> > *can* return a V2 bucket (which does nothing; could be a static const
> data
> > struct).
>
> Currently all the vtable implementations are static. How are we going to
> publish the 'type' for this, to pass to the function as argument?
>

No, they're "extern const". See serf_bucket_types.h. That's how the
SERF_BUCKET_IS_FOO() macros work.


> Sharing data from shared libraries introduces new problems... perhaps
> using a helper function to obtain the value to pass?
>

We've been sharing data from shared libraries since day one :-)


>
> I don't think we want to have that implementation hidden in a macro any
> more if there is so much processing inserted.
>

I wouldn't worry about the amount of code, but simply that it would be hard
to express as a macro. So I guess we could have a simple test function:

int serf_bucket_is_v2(const serf_bucket_t *bucket);

.. and use the logic I describe above.

> That will skip any function pointer comparisons. We *do* switch to a data
> > value (serf_bucket_type_v2_api), but I'm guessing Windows *does* have a
> > singular constant value for that within the program space.
>
> This really depends on how you declare it. With the right .def and/or
> __declspec() magic this can work, but it still feels like a hack.
>

We've been doing data pointer comparisons the whole time. Our code depends
upon it.

Note: we cannot use the __declspec(dllimport) magic to unify function
pointers. There are binaries out there which have *not* been compiled that
way. We'd break binary compatibility if we switched to demand that
declaration form.


>
> Can't we just extend serf_bucket_t with a type2 variable at the end?
> (It looks like we have only one places where we allocate these, when I
> ignore the trunk log bucket hacks... which would already break if some
> other bucket did the same trick)
>
> We can then just rev serf_bucket_create() to set type and typ2 to the same
> variable.
>

Nope. We've never said buckets MUST be constructed with that function.
Thus, users may be constructing their own buckets, and they might not be
large enough.

...

I'd be happy to redo our v2 testing, if we agree on the _is_v2() approach
and using a special bucket type for detection.

Cheers,
-g

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