Re: [PATCH jn/fast-import-blob-access] t9300: avoid short reads from dd

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Jonathan Nieder <jrnieder@xxxxxxxxx> writes:

> dd is a thin wrapper around read(2).  As open group Issue 7 explains:
>
> 	It shall read the input one block at a time, using the specified
> 	input block size; it shall then process the block of data
> 	actually returned, which could be smaller than the requested
> 	block size.
>
> Any short read --- for example from a pipe whose capacity cannot fill
> a block --- results in that block being truncated.  As a result, the
> first cat-blob test (9300.114) fails on Mac OS X, where the pipe
> capacity is around 8 KiB.

I saw a similar breakage on my FBSD 8 bochs.  It is unfortunate and feels
yucky that we have to issue 8k+ read(2) of one byte, but I don't think of
a better way.  I thought it might be possible to specify cbs and/or conv
to have the input buffered to a size to defeat the short read issue, but
count specifies in terms of input blocks, so there doesn't seem to be a
way to do so...  Oh well...

Thanks.

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