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Date:	Mon, 7 Mar 2011 10:37:08 -0500
From:	Christopher Faylor <cgf-use-the-mailinglist-please AT cygwin DOT com>
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Subject:	Re: 1.7.8: write fails with EAGAIN
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On Mon, Mar 07, 2011 at 11:39:51AM +0100, Corinna Vinschen wrote:
>On Mar  5 21:12, Robert Wruck wrote:
>> Hi,
>> 
>> recently, I found that cygwin-git was not able to 'cat-file' files
>> that exceeded some size (in my case about 80MB).
>> I tracked this down to the cygwin implementation of write() that
>> behaves quite odd in some cases.
>> 
>> I wrote a small program (source attached) that mmaps a given file
>> and tries to write it to another file or stdout.
>> 
>> The results vary:
>> 
>> If the destination is a file (`writetest infile outfile` or
>> `writetest infile > outfile`), the write succeeds in a single call.
>> 
>> If the destination is a pipe (`writetest infile | cat > outfile`),
>> the write succeeds in most cases. BUT:
>> 
>> Under WinXP (XP Service Pack 2, 32bit), the call returns -1 and
>> errno=EAGAIN. Nevertheless, SOME data is written to the pipe (in my
>> case 4096 byte for each call).
>> This breaks git since it does an infinite loop while errno=EAGAIN.
>
>Hang on, you are saying that a *blocking* write(2) to a pipe returns
>with EAGAIN?  Are you sure?  It would be quite a surprise if git would
>actually do that.  EAGAIN is only an expected error for non-blocking
>I/O, so applications which use blocking I/O usually only test for EINTR.

I can barely convince myself that there's a pathological case where an
EAGAIN could leak out.  I'm investigating now.

cgf

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