Re: write() _will_ fail on Mac OS X/XNU if nbytes > INT_MAX

["Torsten Bögershausen" <tboegi@web.de>]

Hej,
I think the patch went in and out in git.git, please see below.

(I coudn't the following  in msysgit,
 but if it was there, the clipped_write() for Windows could go away.

/Torsten



commit 0b6806b9e45c659d25b87fb5713c920a3081eac8
Author: Steffen Prohaska <prohaska@zib.de>
Date:   Tue Aug 20 08:43:54 2013 +0200

    xread, xwrite: limit size of IO to 8MB
    
    Checking out 2GB or more through an external filter (see test) fails
    on Mac OS X 10.8.4 (12E55) for a 64-bit executable with:
    
        error: read from external filter cat failed
        error: cannot feed the input to external filter cat
        error: cat died of signal 13
        error: external filter cat failed 141
        error: external filter cat failed
    
    The reason is that read() immediately returns with EINVAL when asked
    to read more than 2GB.  According to POSIX [1], if the value of
    nbyte passed to read() is greater than SSIZE_MAX, the result is
    implementation-defined.  The write function has the same restriction
    [2].  Since OS X still supports running 32-bit executables, the
    32-bit limit (SSIZE_MAX = INT_MAX = 2GB - 1) seems to be also
    imposed on 64-bit executables under certain conditions.  For write,
    the problem has been addressed earlier [6c642a].
    
    Address the problem for read() and write() differently, by limiting
    size of IO chunks unconditionally on all platforms in xread() and
    xwrite().  Large chunks only cause problems, like causing latencies
    when killing the process, even if OS X was not buggy.  Doing IO in
    reasonably sized smaller chunks should have no negative impact on
    performance.
    
    The compat wrapper clipped_write() introduced earlier [6c642a] is
    not needed anymore.  It will be reverted in a separate commit.  The
    new test catches read and write problems.
    
    Note that 'git add' exits with 0 even if it prints filtering errors
    to stderr.  The test, therefore, checks stderr.  'git add' should
    probably be changed (sometime in another commit) to exit with
    nonzero if filtering fails.  The test could then be changed to use
    test_must_fail.


On 2013-11-20 11.15, Erik Faye-Lund wrote:
> I know I'm extremely late to the party, and this patch has already
> landed, but...
> 
> On Sat, May 11, 2013 at 1:05 AM, Junio C Hamano <gitster@pobox.com> wrote:
>> Filipe Cabecinhas <filcab@gmail.com> writes:
>>
>>> Due to a bug in the Darwin kernel, write() calls have a maximum size of
>>> INT_MAX bytes.
>>>
>>> This patch introduces a new compat function: clipped_write
>>> This function behaves the same as write() but will write, at most, INT_MAX
>>> characters.
>>> It may be necessary to include this function on Windows, too.
> 
> We are already doing something similar for Windows in mingw_write (see
> compat/mingw.c), but with a much smaller size.
> 
> It feels a bit pointless to duplicate this logic.
> 
>> diff --git a/compat/clipped-write.c b/compat/clipped-write.c
>> new file mode 100644
>> index 0000000..9183698
>> --- /dev/null
>> +++ b/compat/clipped-write.c
>> @@ -0,0 +1,13 @@
>> +#include <limits.h>
>> +#include <unistd.h>
>> +
>> +/*
>> + * Version of write that will write at most INT_MAX bytes.
>> + * Workaround a xnu bug on Mac OS X
>> + */
>> +ssize_t clipped_write(int fildes, const void *buf, size_t nbyte)
>> +{
>> +       if (nbyte > INT_MAX)
>> +               nbyte = INT_MAX;
>> +       return write(fildes, buf, nbyte);
>> +}
> 
> If we were to reuse this logic with Windows, we'd need to have some
> way of overriding the max-size of the write.
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