Subject: Re: [PATCH v2 5/5] convert: add filter.<driver>.process option

From: Jakub Narębski <[email-masked]>
Date: Sat, 30 Jul 2016 01:11:24 +0200

W dniu 2016-07-29 o 19:35, Junio C Hamano pisze:
> Lars Schneider <[email-masked]> writes:
> 
>> I think sending it upfront is nice for buffer allocations of big files
>> and it doesn't cost us anything to do it.
> 
> While I do NOT think "total size upfront" MUST BE avoided at all costs,
> I do not think the above statement to justify it makes ANY sense.
> 
> Big files are by definition something you cannot afford to hold its
> entirety in core, so you do not want to be told that you'd be fed 40GB
> and ask xmalloc to allocate that much.

I don't know much how filter driver work internally, but in some cases
Git reads or writes from file (file descriptor), in other cases it reads
or writes from str+len pair (it probably predates strbuf) - I think in
those cases file needs to fit in memory (in size_t).  So in some cases
Git reads file into memory.  Whether it uses xmalloc or mmap, I don't
know.

> 
> It allows the reader to be lazy for buffer allocations as long as
> you know the file fits in-core, at the cost of forcing the writer to
> somehow come up with the total number of bytes even before sending a
> single byte (in other words, if the writer cannot produce and hold
> the data in-core, it may even have to spool the data in a temporary
> file only to count, and then play it back after showing the total
> size).

For some types of filters you can know the size upfront:
 - for filters such as rot13, with 1-to-1 transformation, you know
   that the output size is the same as the input size
 - for block encodings, and for constant-width to constant-width
   encoding conversion, filter can calculate output size from the
   input size (e.g. <output size> = 2*<input size>)
 - filter may have get size from somewhere, for example LFS filter
   stub is constant size, and files are stored in artifactory with
   their length 

> 
> It is good that you allow both mode of operations and the size of
> the data can either be given upfront (which allows a single fixed
> allocation upfront without realloc, as long as the data fits in
> core), or be left "(atend)".

I think the protocol should be either: <size> + <contents>, or
<size unknown> + <contents> + <flush>, that is do not use flush
packet if size is known upfront -- it would be a second point
of truth (SPOT principle).
 
> I just don't want to see it oversold as a "feature" that the size
> has to come before data.  That is a limitation, not a feature.
> 
> Thanks.
>