>> Why not simply increase the amount of data written instead of limiting
>> the pipe size?
>
> Hmm, indeed :-)
>
>> By the way, there's support.PIPE_MAX_SIZE for that purpose.
>
> Hardwired to 3 MB. I wonder if it may broken soon.

On Linux, for non root users, it's limited to 1048576, and can be set
to /proc/sys/fs/pipe-max-size.
After a quick look at the kernel code, there's apparently no max value
(it must be a multiple of the page size, though).

I think 3MB should be more than enough, so I suggest to update the
test to use support.PIPE_MAX_SIZE.
If this breaks up again, then we could set PIPE_MAX_SIZE dynamically, like:

r, w = os.pipe()
PIPE_MAX_SIZE = 2 * fcntl.fcntl(w, 1032)

But F_GETPIPE_SZ is Linux only, and quite recent (since 2.6.35 apparently).