py2 vs py3: zlib.adler32/crc32

[Karsten Hilbert]

Hello MRAB,

yes, like that :-)

Thanks,
Karsten

On Thu, Nov 14, 2019 at 09:11:37PM +0000, MRAB wrote:
> Date: Thu, 14 Nov 2019 21:11:37 +0000
> From: MRAB <python at mrabarnett.plus.com>
> To: python-list at python.org
> Subject: Re: py2 vs py3: zlib.adler32/crc32
> User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:60.0) Gecko/20100101
>  Thunderbird/60.9.1
>
> On 2019-11-14 19:22, Karsten Hilbert wrote:
> > Hi all,
> >
> > I am unsure how to solve: I use adler32/crc32 to generate integer values from data
> > for setting up an advisory lock in PostgreSQL.
> >
> > The PG function pg_try_advisory_lock()
> >
> >    https://www.postgresql.org/docs/12/functions-admin.html#FUNCTIONS-ADVISORY-LOCKS
> >
> > takes two PG ints which are defined as:
> >
> >     integer 	4 bytes 	typical choice for integer 	-2147483648 to +2147483647
> >
> > Now, in Py > 2.5 zlib.adler32/crc32 will return suitable integers.
> >
> > However, in Py3 the return range has been changed to
> >
> >     The return value is unsigned and in the range [0, 2**32-1] regardless of platform.
> >
> > which will overflow the PostgreSQL function.
> >
> > Is there a way to convert/modify/shift the py3 value such that it shows the
> > same representation as the py2 value ?   What I am looking for is:
> >
> >    v2 = py2.zlib.adler32(data)
> >    v3 = some_magic(py3.zlib.adler32(data))
> >    if v2 == v3:
> >       print('same')
> >
> > I am sure there's something obvious with struct/<</& and
> > such like which I am overlooking.
> >
> > Note that I can't simply do
> >
> >     v2 = py2.zlib.adler32(data) & 0xffffffff
> >
> > because that can overflow the PostgreSQL integer. I think I need
> > the reverse, in a sense, but I am too dense, ATM.
> >
> Unsigned 32-bit to signed 32-bit:
>
>     unsigned - (unsigned & 0x80000000) * 2
>
> Signed 32-bit to unsigned 32-bit:
>
>     signed & 0xFFFFFFFF
> --
> https://mail.python.org/mailman/listinfo/python-list

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