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List:       rpm-devel
Subject:    Re: Fwd: Re: rsyncable gzdio
From:       Alexey Tourbin <at () altlinux ! ru>
Date:       2008-07-10 5:12:01
Message-ID: 20080710051201.GN6725 () solemn ! turbinal
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On Wed, Jul 09, 2008 at 05:35:46AM -0700, Jeff Johnson wrote:
>2) From a fundamental coding design POV, adding --rsyncable to rpmio  
>code is just
>plain wrong. That's as true for the patched internal zlib as it is  
>for your gzdio
>patches. The issue of when gzflush() is called is fundamentally a  
>compression
>and rsync transport, not a *.rpm payload packaging, issue.
>
>So isn't a pure API/ABI drop-in replacement for gzio(3), with a  
>"gzwrite" rather
>than a "rsyncable_gzwrite" symbol a better approach? There are many  
>applications,
>not just rpm, that _MUST_ participate in an efficient *.rpm  
>distribution framework,
>so patching gzdio in rpmio is just a tiny piece of a much bigger  
>puzzle, where "gzwrite"
>rather than "rsyncable_gzwrite" is likelier to be successful. JMHO,  
>YMMV, as always.

Hmm.  Let me argue here just a bit.  You're saying that indirect
gzflush() calls are somewhat alien to plain gzdio abstrcation.  However,
from gzFile higher-level (logical) perspective, calling gzflush() is
also simply a no-op.  The worst thing that can ever happen is calling
gzflush() too often, which can degrade compression.

Now, rsyncable_gzwrite is simply about calling real gzwrite+gzflush
in a loop.  Provided that pointer arithmetic in rsyncable_gzwrite(),
is valid, gzflush is still a no-op, and gzdio abstraction is preserved,
and nothing bad can ever happen.  The rest of the code is sync_hint(),
which is about finding the right sync points, and also, basically, not
calling gzflush() too often.

So, the code may look weired, but the idea is simple.  I introdice
rsyncable_gzwrite, which is equivalent to gzwrite from the higher-level
(logical) point of view.  There is no possiblity for rsyncable_gzwrite
to produce logically invalid output.  The rest of the code is just
a hint for rsyncable_gzwrite() loop.

BTW, we can have something like %_rsyncable_gzwrite macro, to switch
between plain gzwrite and rsyncable_gzwrite.

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